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Antibiotic-loaded polymeric microspheres for passive lung targeting after intravenous administration Agnoletti, Monica 2020

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        ANTIBIOTIC-LOADED POLYMERIC MICROSPHERES FOR PASSIVE LUNG TARGETING AFTER INTRAVENOUS ADMINISTRATION  by MONICA AGNOLETTI M.Sc., University of Parma, 2016    A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Pharmaceutical Sciences)    THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  July 2020 © Monica Agnoletti, 2020  U N I V E R S I T Y  O F  C O P E N H A G E N  F A C U L T Y  O F  H E A L T H  A N D  M E D I C A L  S C I E N C E S        PhD Thesis Monica Agnoletti  Antibiotic-loaded polymeric microspheres for passive lung targeting after intravenous administration  This thesis has been submitted to the Graduate School of Health and Medical Sciences, University of Copenhagen on March 15, 2020   i  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled:  Antibiotic-loaded polymeric microspheres for passive lung targeting after intravenous adminiatration  submitted by Monica Agnoletti  in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Pharmaceutical Sciences  Examining Committee: Susan Weng Larsen, Faculty of Health and Medical Sciences, University of Copenhagen University Examiner  Marcel Bally, Faculty of Medicine, University of British Columbia University Examiner Simon Bjerregaard, Ferring Pharmaceuticals External Examiner   Additional Supervisory Committee Members: - Supervisory Committee Member - Supervisory Committee Member     ii  Academic Advisors Urs Otto Häfeli Professor, PhD Department of Pharmacy Faculty of Health and Medical Sciences University of Copenhagen, Denmark and Faculty of Pharmaceutical Sciences University of British Columbia, Canada  Hanne Mørck Nielsen Professor, PhD Department of Pharmacy Faculty of Health and Medical Sciences University of Copenhagen, Denmark  Katayoun Saatchi Research Associate, PhD Faculty of Pharmaceutical Sciences University of British Columbia, Canada   Assessment Committee  Susan Weng Larsen (Chair) Associate Professor, PhD Department of Pharmacy Faculty of Health and Medical Sciences University of Copenhagen, Denmark  Simon Bjerregaard Senior Scientist in Pharmaceutical Development, PhD International PharmaScience Center Ferring Pharmaceuticals, Denmark  Marcel Bally Professor, PhD Department of Pathology and Laboratory Medicine Faculty of Medicine University of British Columbia, Canada   iii  Preface The present thesis entitled “Antibiotic-loaded polymeric microspheres for passive lung targeting after intravenous administration” was submitted to the Faculty of Health and Medical Sciences, University of Copenhagen and the Graduate and Postdoctoral Studies, University of British Columbia. This dissertation is formatted in accordance with the regulations of University of Copenhagen and submitted in partial fulfillment of the requirements for a PhD degree awarded jointly by University of Copenhagen and the University of British Columbia. Versions of this dissertation will exist in the institutional repositories of both institutions. The experimental work was conducted in the Drug Delivery and Biophysics of Biopharmaceuticals group headed by Professor Hanne Mørck Nielsen, PhD, Department of Pharmacy, University of Copenhagen, and in the Nanomedicine, Drug Delivery and Radiopharmaceuticals group headed by Professor Urs Otto Häfeli, PhD, Faculty of Pharmaceutical Sciences, University of British Columbia.  The project has been performed in affiliation with the Drug Research Academy at the University of Copenhagen and the Pharmaceutical Sciences Graduate Program at the University of British Columbia. This project was financially supported by the Lundbeck Foundation, Denmark. The research conducted over the course of the PhD project has resulted in one published scientific manuscript, included in the Appendix and reproduced with permission. Agnoletti M, 5RGUÕJXH]-5RGULJXH] C, .áRG]LQVND SN, Esposito TVF, Saatchi K, Mørck Nielsen H, Häfeli UO0RQRVL]HG3RO\PHULF0LFURVSKHUHVIRU3DVVLYH/XQJTargeting: Biodistribution and Pharmacokinetics after Intravenous Administration. ACS Nano. 2020, 14, 6693-6706. I designed and conducted the experiments, DQDO\]HGDQGLQWHUSUHWed the experimental data, made all figures, and wrote, edited and prepared the manuscript for submission. My supervisors, Professor Hanne Mørck Nielsen and Professor Urs Otto Häfeli, provided guidance in the study design and interpretation of the results, and were involved in the preparation and editing process of the manuscript. Cristina 5RGUÕJXH]-5RGULJXH] contributed to the execution and interpretation of the SPECT/CT, biodistribution and histology studies 6\OYLD 1DWDOLH .áRG]LQVND FRQWULEXWHG WR WKH execution of the antimicrobial activity study and to the writing of the first draft of the manuscript. Tullio Vito Francesco Esposito provided expert advice on the cytotoxicity, radiolabeling and SPECT/CT studies as well as the initial design of the histology study. Kathy Saatchi modified the microspheres with the chelator and oversaw the radiolabeling study. All authors approved the final manuscript. My total contribution to this research was >90%. Etics approval for the animal studies was obtained from the Animal Care Committee of the University of British Columbia under the approved protocol A16-0150.   iv  In addition to the included study, the project has involved collaboration on the preparation of the following manuscripts, which are not discussed in the current thesis. .ORG]LĔska SN†, Esposito TVF†, Agnoletti M, RodriguH]-5RGULJXH]&%ODFNDGDUC, Wu L, Thakur A, Saatchi K, Rades T, Häfeli UO, Mørck Nielsen H. Pharmacokinetics of LL-37 and nanogel-encapsulated LL-37 after pulmonary administration: a SPECT/CT study. In preparation. *HF]\5Agnoletti M, Hansen MF, Kutter JP, Saatchi K, Häfeli UO. Microfluidic Approaches for the Production of Monodisperse, Superparamagnetic Microspheres LQWKH/RZ0LFURPHWHU6L]H5DQJHJ. Magn. Magn. Mater. 2019, 471, 286–293.      v  Acknowledgements I would like to thank my main supervisor Professor Urs Otto Häfeli for the competent mentoring, valuable advice and kind support throughout the duration of the PhD project. Thanks for pushing me to perform at my highest level and for giving me the opportunity to be a Joint PhD at UCPH and UBC. A special thanks goes to my main co-supervisor Professor Hanne Mørck Nielsen for her excellent guidance, constructive feedback on my work, and never ending professional and moral support throughout the three years. I would also like to thank my co-supervisor Research Associate Kathy Saatchi for the positive attitude, encouragement, and the scientific support and guidance with the radiolabeling experiments.  I would like to thank everyone who was involved in the collaboration between UCPH and UBC, especially the Head of the Department of Pharmacy at UCPH Flemming Madsen, the former Vice-Dean at the Faculty of Health and Medical Sciences at UCPH Professor Sven Frøkjær, the Dean at the Faculty of Pharmaceutical Sciences at UBC Professor Michael Coughtrie, and the Associate Dean of Graduate Studies at UBC Professor Tom Chang. You were all crucial in making the collaboration happen, and I am very grateful for the wonderful opportunity I have been given. A special thanks goes to Samra Alam and Marianne Wieslander Jørgensen at UCPH for their patience and help with the financial and administrative issues – your advice has truly been valuable when I needed help navigating the procedures of the University. I would like to thank the Lundbeck Foundation for providing financial support for my project, and the Graduate School of Health and Medical Sciences at UCPH for supporting my stay abroad at UBC in Vancouver.  I owe a special thanks to Cristina RodULJXH]-RodrLJXH] 6\OYLD .ORG]LQVND DQG 7XOOLREsposito. Thanks to Cristina for helping with the in vivo studies, for the great discussions and support during the days at CCM – I really enjoyed working with you in “our” very RUJDQL]HGZD\7KDQNVWR6\OYLDIRUKHOSLQg with the antibacterial activity experiments, for being a fantastic office mate and for the support given from day one. Thanks to Tullio for the extraordinary scientific inputs, help and support in the Hafeli Lab. I would also like to thank Mai Bay Stie for translating the Abstract of my thesis in the Danish Resumé.  A big thanks goes to all the wonderful colleagues I had both at the Drug Delivery and Biophysics of Biopharmaceuticals group at UCPH and at the Hafeli Lab at UBC. Thanks to Sofie Fogh Hedegaard, Sylvia .ORG]LQVND, Ditlev Birch for the lunches, push-up challenges and fun times in the office 417. Thanks to Danai Panou, Mai Bay Stie, Sara Malekkhaiat, Kate Browning, Elisa Parra 2UWL]DQG5HND*HF]\ for the “hyggeligt” working environment and for always providing a good mood. Thanks to Marco van de Weert for the support and for regularly bringing chocolate in the PhD office. Thanks to the technician Karina Juul Vissing for the excellent support in the lab. Thanks to Lennart Bohrmann, Marta Bergamo, Tullio Esposito, and Zeynab Nosrati for creating an enjoyable working environment in the Hafeli Lab, for supporting me in the ups and downs, and for the beautiful hikes in British vi Columbia. Thanks to Lukas Hohenwarter, Roland Böttger, Ravi Gaikwad and all the graduate students of PharGS at UBC for the great social activities.  I owe a special thanks to my friends outside the lab. Thanks to Alejandra, Elia, Marco, Fabio and Fabio, Andrea, Filippo, Riccardo, Viktor, Alessandro, Federico, Nicola for being the family in Copenhagen and bringing a little bit of Italy in Denmark; Michela and Jess for sharing good laughs while learning Danish. Thanks to Thea and Thomas for welcoming me in Copenhagen. Thanks to my friends in Italy, especially Silvia, Giulia, Margherita and Federica for the great friendship and for always listening to my vocal messages. Above all, I would like to thank my boyfriend Oliver for his patience, support and for making me happy every day. A big thanks goes to my aunt Fiorisa and my uncle Pasquale for their wise advice and positive spirit. Finally, I would like to thank my parents, Lorella and Massimo, and my sister Alessandra, for their endless love and encouragement, wherever I am. Monica Agnoletti, Copenhagen, March 2020 vii Table of Contents ACADEMIC ADVISORS ............................................................................................................................. ii ASSESSMENT COMMITTEE .................................................................................................................... ii PREFACE ..................................................................................................................................................... iii ACKNOWLEDGEMENTS .......................................................................................................................... v TABLE OF CONTENTS ............................................................................................................................ vii ABBREVIATIONS .................................................................................................................................... viii ABSTRACT ..................................................................................................................................................  ix RESUMÉ ........................................................................................................................................................ x LAY SUMMARY .......................................................................................................................................... xi 1 SETTING THE SCENE ........................................................................................................................ 1 1.1 Hypothesis and Aims ..................................................................................................................... 2 2 BACKGROUND .................................................................................................................................... 3 2.1 The Respiratory System ................................................................................................................ 3 2.1.1 The Respiratory Zone ................................................................................................................. 3 2.1.2 Protective Mechanisms of the Respiratory System ..................................................................... 5 2.2 Bacterial Infections in the Respiratory Zone .............................................................................. 6 2.2.1 Treatment of Bacteria Lung Infections ..................................................................................... 10 2.2.2 Challenges with Antibiotic Treatments ..................................................................................... 13 2.2.3 New Strategies to Treat Bacterial Infections ............................................................................ 14 2.3 Lung Targeting ............................................................................................................................ 21 2.3.1 Direct Lung Targeting via Inhalation ........................................................................................ 21 2.3.2 Passive Lung Targeting via Intravenous Administration .......................................................... 24 3 SCIENTIFIC OUTCOME .................................................................................................................. 27 3.1 Research Manuscript .................................................................................................................. 27 4 DISCUSSION ....................................................................................................................................... 28 4.1 Levofloxacin-Loaded PLGA Microspheres as Drug Delivery Systems .................................. 28 4.2 Preparation of Microspheres ...................................................................................................... 30 4.3 Physico-Chemical Characterization .......................................................................................... 33 4.4 In Vitro Characterization ............................................................................................................ 34 4.4.1 Levofloxacin Release and Microspheres Degradation .............................................................. 35 4.4.2 Antibacterial Activity ................................................................................................................ 38 4.4.3 Cytotoxicity and Hemocompatibility ........................................................................................ 38 4.5 In Vivo Characterization ............................................................................................................. 41 4.5.1 Microspheres Degradation ........................................................................................................ 41 4.5.2 Lung Targeting after Intravenous Administration .................................................................... 42 5 CONCLUSIONS AND FUTURE PERSPECTIVES ........................................................................ 47 6 REFERENCES ..................................................................................................................................... 50 7 APPENDIX ........................................................................................................................................... 65   viii  Abbreviations 111In 111Indium AMPs Antimicrobial peptides CDC Centers for Disease Control and Prevention COPD Chronic obstructive pulmonary disease  Cu Copper CP Continuous phase DP Dispersed phase DTPA p-SCN-Bn-DTPA,  S-2-(4-LVRWKLRF\DQDWREHQ]\O-diethylenetriamine pentaacetic acid  EDTA Ethylenediaminetetraacetic acid EMA  European Medicines Agency FIB-SEM Focused ion beam-scanning electron microscopy logD Distribution coefficient MDR Multidrug-resistant MIC Minimum inhibitory concentration MRSA Methicillin-resistant Staphylococcus aureus MTT 3-(4,5-GLPHWK\OWKLD]RO-2-yl)-2,5-GLSKHQ\OWHWUD]ROLXPEURPLGH O/W Oil/water PDR Pandrug-resistant PEG Polyethylene glycol PLA Poly(lactic acid) PLGA Poly(lactic-co-glycolic) acid PVA  Polyvinyl alcohol QCP Flow rate of the CP QDP Flow rate of the DP R2 Correlation coefficient RMSE Root mean squared error SEM Scanning electron microscopy SPECT/CT Single-photon emission computed tomography US FDA United States Food and Drug Administration W/O/W Water/oil/water WHO :RUOG+HDOWK2UJDQL]DWLRQ XDR Extensively drug-resistant   ix Abstract Low respiratory tract bacterial infections are currently amongst the leading causes of mortality worldwide. Current treatments consist of oral or intravenous administration of antibiotics. Today´s treatments of pulmonary bacterial infections are often not sufficiently effective due to the difficulty of drugs reaching the infection deep in the lungs, the insufficient drug doses at the site of infection, the development of multi-drug resistance, and the side effects caused by some of the currently used and effective antibiotics. Lung-targeted delivery of antibiotics by using injectable drug-loaded microspheres is a promising alternative to the traditional antibiotic solutions as they achieve local therapeutic concentrations of antibiotics and minimise unwanted off-target effects. This delivery strategy offers potential, especially in the case of patients with compromised lung function or obstruction of the respiratory tract, due to inflammation, infection or significant mucus production, while they still have normal lung perfusion.  The aim of this project was to explore the use of polymeric microspheres as carriers for lung delivery of antibiotics to increase the efficacy of these drugs against bacterial respiratory infections, specifically by selectively targeting the lung capillaries after intravenous administration.  Biodegradable poly(lactic-co-glycolic) acid (PLGA) microspheres encapsulating levofloxacin were prepared with a flow-focusing microfluidic chip and FKDUDFWHUL]HG IRU their physico-chemical properties, and their in vitro and in vivo performance. The PLGA microspheres ZHUH KLJKO\ KRPRJHQHRXV LQ VL]H with a mean diameter of ~12 μm and coefficient of variation <5.2%. The microspheres slowly released the encapsulated levofloxacin in a controlled fashion over five days and slightly reduced its antibacterial activity against Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus. The microspheres degradation studies showed changes in the internal structure and in the surface morphology, and a faster degradation kinetic in vivo than in vitro. The microspheres showed low toxicity for endothelial and alveolar epithelial cell lines, and did not cause lysis of red blood cells. The biodistribution and pharmacokinetics study showed that 111Indium-labeled microspheres distributed almost exclusively and homogenously in the lungs after intravenous administration. Overall, intravenous administration of 12 μm PLGA microspheres is suitable for passive lung targeting and is promising for pulmonary therapy.   x  Resumé  Infektioner, som skyldes bakterier i de nedre luftveje, er blandt de hyppigste dødsårsager på verdensplan. De mest almindelige behandlinger af denne type infektioner består af antibiotika administreret enten oralt eller intravenøst. Nuværende behandlinger af bakterielle infektioner i lungerne er ofte ikke tilstrækkeligt effektive grundet begrænset levering af lægemidlet dybt ned i lungerne, for lav koncentration af lægemiddelstoffet i infektionsområdet, udvikling af multiresistens hos bakterierne samt bivirkninger ved lægemidlet. Levering af antibiotika direkte til lungerne ved hjælp af mikrosfærer doseret som et injicerbart lægemiddel er et lovende alternativ til de traditionelle formuleringer af antibiotika, da det herved er muligt at opnå terapeutiske koncentrationer af antibiotika lokalt og samtidig minimere uønskede bivirkninger i andre dele af kroppen. Denne strategi har lovende potentiale især for patienter med normal lungeperfusion, men med nedsat lungefunktion eller patienter med kronisk obstruktion af luftvejene på grund af inflammation, infektion eller høj produktion af mucus. Formålet med dette projekt var at undersøge polymer mikrosfærer til målrettet levering af antibiotika til blodkapillærer ved alveolerne i lungerne efter intravenøs administration med henblik på at øge virkningen af disse lægemidler mod bakterielle luftvejsinfektioner. Bionedbrydelige poly(lactic-co-glycolic) acid (PLGA) mikrosfærer med levofloxacin blev fremstillet ved hjælp af en flow-fokuseret mikrofluid chip og karakteriseret for deres fysisk-kemiske egenskaber og deres virkning in vitro og in vivo. PLGA-mikrosfærerne udviste en homogen størrelsesfordeling med en gennemsnitlig diameter på ~12 μm og en variationskoefficient <5,2%. Levofloxacin blev frigivet langsomt fra mikrosfærerne på kontrolleret vis over fem dage, og den antibakterielle aktivitet var let reduceret overfor Pseudomonas aeruginosa, Escherichia coli og Staphylococcus aureus i forthold til det ikke indkapslede stof. Nedbrydning af mikrosfærerne blev evalueret og viste ændringer i deres indre struktur og overflademorfologi samt en hurtigere nedbrydningskinetik in vivo end in vitro. Mikrosfærerne udviste lav toksicitet for endotel- og alveolære epitelcellelinjer og forårsagede ikke lysering af røde blodlegemer. En undersøgelse af biodistribusions og farmakokinetik viste, at 111Indium-mærkede mikrosfærer udviste en homogen distribution næsten udelukkende til lungerne efter intravenøs administration. Samlet set, er intravenøs administration af 12 μm PLGA mikrosfærer velegnet til passiv lungemålretning og er lovende for lungeterapi.      xi  Lay Summary Bacterial infections in the respiratory tract are currently one of the leading causes of mortality worldwide. Today´s treatments are often not sufficiently effective due to the difficulty of drugs reaching the infection deep in the lungs, the insufficient drug doses at the site of infection, the development of multi-drug resistance, and the side effects caused by some of the currently used and effective antibiotics.  The aim of this project was to investigate the use of polymeric microspheres as carriers for lung delivery of antibiotics after intravenous administration. The antibiotic-loaded micrRVSKHUHVZHUHSUHSDUHGDQGFKDUDFWHUL]HGwith a wide range of methods and assays to gain a comprehensive understanding of their in vitro and in vivo performance.  The findings of this work showed that injectable drug-loaded microspheres are a promising alternative to the traditional therapies and are expected to improve the treatments of bacterial lung infections and, potentially, other lung diseases.    Setting the Scene  1  1 Setting the Scene Lung infections, such as pneumonia, tuberculosis, bacterial infections associated with cystic fibrosis or non-cystic fibrosis bronchiectasis, are amongst the top ten causes of death worldwide 1,2. Of the responsible pathogens that populate the respiratory tract (e.g., viruses, bacteria, fungi), bacteria, such as Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pneumoniae, have attracted special attention since they are responsible for most of the hospital-acquired infections and are associated with high morbidity and mortality 3,4. The recommended treatment for bacterial infections includes oral or intravenous administration of antibiotics 5. Only in the case of P. aeruginosa infections associated with cystic fibrosis, inhalable antibiotics are applied 6. However, the success of the antibiotic treatment is dependent on the right amount of the drug reaching the target site and is further highly challenged currently by the increasing appearance of pathogens resistant to the traditional antibiotics. Specifically, in the case of systemic administration, distribution to non-infected sites occurs, thus the antibiotics do not always reach therapeutically active concentrations in the lungs and side effects in non-targeted organs may appear.  To overcome the challenges associated with the use of antibiotics, different approaches can be used, including drug delivery systems to selectively target the lungs. Several lipid- and polymer-based nano- and micro-particles are currently investigated to ORFDOL]HDQWLELRWLFVWRinfection sites in the periphery of the lungs after inhalation 7. Other aims include obtaining maximum targeting and thus therapeutic efficacy, while maintaining low drug levels at non-target sites. However, in patients with respiratory tract obstruction (e.g., due to inflammation or mucus plugs) or compromised lung function (e.g., due to bronchial narrowing), the delivery of antibiotics by inhalation is challenging and often not sufficiently effective to eliminate all the pathogens present in the lungs. In these cases, the deposition of inhaled drugs in the lung periphery is in fact limited and heterogeneous.  Therefore, targeted drug delivery to infected lungs tissue from the vascular side by using drug delivery systems is a promising approach that could be a valid alternative to conventional administration routes or a complementary therapy to inhaled antibiotics. After intravenous injection, drug delivery systems with diameters slightly larger than the diameter of lung capillaries (i.e., 7.5 ± 2.3 μm for healthy adults 8,9, 6.6 ± 1.6 μm, and 7.5 ± 1.7 μm in rats and dogs, respectively 10) will be trapped in the lung capillaries. While the matrix of the drug delivery system degrades, the encapsulated antibiotic is released and can freely diffuse into the alveolar space. Amongst the few formulations investigated, polymeric microspheres are of particular interest, specifically those composed of biodegradable and biocompatible polymers.   Setting the Scene  2  1.1 Hypothesis and Aims The overall hypothesis leading to this project is that it is possible to design a suitable drug delivery system for lung targeting of antibiotics through emboOL]DWLRQ RI pulmonary capillaries in the form of drug-loaded microspheres to improve the treatment of bacterial infections in the respiratory tract. In comparison to intravenously administered solutions of antibiotics, the use of particles will hypothetically result in higher drug concentration in the UHVSLUDWRU\]RQH, i.e., at the site of infection, thus lowering administered doses of antibiotic, and reducing risk of systemic toxicity. This drug delivery approach could therefore improve the treatment of low respiratory tract infections (e.g., pneumonia) and/or complement inhaled therapy in patients with pulmonary disease with compromised lung function or respiratory tract obstruction (e.g., cystic fibrosis). To support the hypothesis, the specific aims in this project were: Aim I: To develop monodisperse poly(lactic-co-glycolic acid) (PLGA) microspheres loaded with levofloxacin, with properties suitable for passive lung targeting and with sufficient antibacterial activity. Aim II: To evaluate the pharmacokinetics and biodistribution of the radiolabeled PLGA microspheres to the lungs and other organs by single-photon emission computed tomography (SPECT/CT) imaging.              Background  3  2 Background 2.1 The Respiratory System 7KHUHVSLUDWRU\V\VWHPLVKLJKO\VSHFLDOL]HGDQGGHVLJQHGWRHQDEOHJDVH[FKDQJH between the respiratory tract and the vasculature. Anatomically, it can be divided into upper and lower respiratory tract. The upper respiratory tract includes the nasal cavity, the pharynx and the larynx. The lower respiratory tract comprises the trachea, which bifurcates into the bronchi. The primary bronchi continue to branch into smaller bronchioles, which end with the alveolar sacs 11,12 (Figure 1). Functionally, the respiratory system can be divided into a FRQGXFWLYH ]RQH DQG D UHVSLUDWRU\ ]RQH 7KH FRQGXFWLYH ]RQH is mainly responsible for carrying, filtering, humidifying and warming the incoming air. It includes larynx, trachea, bronchi, bronchioles, and terminal bronchioles. 7KHUHVSLUDWRU\]RQHconsists of respiratory bronchioles, alveolar ducts, alveolar sacs, and is directly responsible for gas exchange 11,12 (Figure 1).  Figure 1. Anatomical and functional division of the respiratory system. Figure created with BioRender.com. 2.1.1 The Respiratory Zone  The key function of the respiratory system is gas exchange, which takes place in the alveoli. There, the respiratory tract and the pulmonary vasculature are in very close proximity over a large surface area allowing carbon dioxide waste to be removed from the blood and to be replaced by inhaled oxygen.   Background  4  Structurally, the alveolar sacs are clusters of many individual alveoli, with each alveolus representing a functional unit of the respiratory tract. The alveolar wall consists of a thin monolayer of type I alveolar epithelial cells (also known as pneumocytes), interspersed with few type II pneumocytes (Figure 2) 9,12–16. Type I pneumocytes are directly involved in gas exchange, while type II pneumocytes V\QWKHVL]H DQG secrete phospholipids and proteins, which constitute the alveolar lining fluid. This alveolar lining fluid is a thin (from 0.2 ȝPon flat alveolar walls to 0.9 ȝP in the alveolar corners 17,18) continuous liquid layer that covers the apical surface of the epithelium (Figure 2), prevents alveolar collapse during breathing by decreasing the surface tension, and contains alveolar macrophages and various complement proteins and antimicrobial proteins, such as defensins 13,15,16. The alveoli have an epithelial surface area of ~140 m2 in humans and ~125 cm2 in mice,  ~ 90% of which is occupied by pulmonary capillaries RUJDQL]HGLQDdense network 9,19. The pulmonary capillaries originate from branches of the pulmonary arteries (also called axial arteries), that run along the side of the bronchi, and from supernumerary arteries, that branch out of axial arteries and enter the lung parenchyma 19,20. The pulmonary capillaries subsequently merge into pulmonary veins, which drain into the left atrium (Figure 2). Structurally, the lung capillaries have an average diameter of 5-8 —P“ȝP IRUhealthy adults 8,9, 6.6 ± 1.6 μm, and 7.5 ± 1.7 ȝPLQUDWVDQGGRJV, respectively 10) and have the basal side of the epithelium surrounded by a thin endothelial cell monolayer (Figure 2).  Taken together, the alveolar epithelium, the monolayer of endothelial cells, and the very thin interstitial compartment inbetween form the alveolar-capillary barrier 19 (Figure 2), which is ~300 nm thick in mice, ~400 nm in rats, ~500 μm in dogs, and  ~600 nm in humans 21. This thin barrier provides the structural basis for exchange of gas, ions and small molecules by simple diffusion. Figure 2. 7KHUHVSLUDWRU\]RQH and the structures responsible for in the gas exchange. Figure created with BioRender.com.   Background  5  2.1.2 Protective Mechanisms of the Respiratory System The respiratory tract has developed protective mechanisms against microorganisms, particulate materials, and toxic gases to which it is exposed with the inhaled air.  ,QWKHFRQGXFWLYH]RQHWKHoropharyngeal area entraps particles larger than 10 μm. Further down the respiratory tract, an important anatomical barrier is represented by the geometry of the respiratory tract, which bifurcates ~16 times and gradually narrow down in diameter before reaching the alveoli. This tree-like structure facilitates the deposition of particles of ~5-10 μm by impaction LQ WKH FRQGXFWLYH ]RQH, which can then be removed either by absorption into the systemic circulation or by the mucociliary clearance 22,23. Insoluble particles are removed by the mucociliary escalator that consists of mucus and cilia that are present on the apical surface of the epithelial cells. The epithelium of the respiratory tract is in fact ciliated from the trachea to the terminal bronchioles and is covered by the epithelial lining fluid (Figure 3) 14,24,25. This protective layer is composed of two sublayers: a low-viscosity fluid layer (known as periciliar layer) and a more viscous mucus layer on top 24. The periciliar layer is ~7 μm thick throughout all the conductive respiratory tract 14, while the mucus layer varies in WKLFNQHVVGHSHQGLQJRQWKHORFDOL]DWLRQfrom 10-30 μm to 70 μm in the trachea, and 2-5 μm in the bronchioles 14,18,24,26,27 (Figure 3). The mucus layer is produced by secretory cells in the epithelium (i.e., goblet cells and Clara cells) and serous cells of the submucosal glands (Figure 3) 14. The mucus traps the inhaled substances and the upward cilia movements carry them out of the lungs before being expelled by coughing or being swallowed and eliminated by the gastrointestinal tract 12,23,28.  Figure 3. The mucosa and the epithelium with the structures contributing to the protection of the respiratory tract. Figure created with BioRender.com with inspiration from 29 with permission.  In the respiratory ]RQHWKH protection is provided by the alveolar macrophages (12-14 per alveolus in healthy non-smokers 30), which phagocytose small (< 5 μm) insoluble particles and pathogens deposited in the alveoli 12,23,28. Macrophages can also initiate an inflammatory response by secreting inflammatory factors and recruiting neutrophils.    Background  6  In contrast to particulate matter, solubLOL]HG molecules may be absorbed from both the FRQGXFWLYH DQG UHVSLUDWRU\ ]RQH into the systemic circulation or the lymphatic system through the epithelium by passive diffusion or by active transport mechanisms 12,22,23,28. The rate and mechanism of absorption depends on the physico-chemical properties of the molecule in relation to the absorption site.  2.2 Bacterial Infections in the Respiratory Zone   The defense mechanisms that protect the respiratory tract can fail, e.g., as a consequence of diseases, of a large invasion of pathogens, of a highly virulent microorganism, or of medical treatments that damage the tissue and/or the immune system. A failure results in respiratory diseases, which can be classified based on the anatomical division of the respiratory system or based on the etiology. In the first case diseases are described as being related to the upper respiratory tract (e.g., acute and chronic infections, nasopharyngeal carcinoma, laryngeal tumors) and of the lower respiratory tract, such as the bronchi (e.g., bronchitis, bronchiectasis), the smaller bronchi and bronchioles (e.g., bronchiolitis), the alveolar ducts and alveoli (e.g., pneumonia) 31. The etiologic classification distinguishes viral infections (e.g., viral pneumonia, inIOXHQ]D), bacterial infections (e.g., bacterial pneumonia, tuberculosis), allergic diseases (e.g., asthma, hay fever), acute diseases of the bronchi (e.g., bronchitis, bronchiolitis, bronchiectasis), airflow obstruction diseases or chronic obstructive pulmonary disease (COPD, e.g., chronic bronchitis, pulmonary emphysema), and lung cancer 31.  Of the pulmonary diseases (Table 1), the lower respiratory tract infections are one of the major clinical challenges, as they represent among the primary causes of mortality worldwide for all ages and the leading cause of death among children under 5 years of age 1,2. Many lung infections are frequently acute self-limiting viral infections and not lethal unless a secondary bacterial infection develops. These co-infections frequently worsen the clinical outcome and increase the severity of the infection 32. In a hospital setting, respiratory bacterial infections are more numerous than viral infections and are usually associated to significant morbidity and mortality in sick patients 33. These bacterial infections are located in the respiratory or conductive ]RQHV ,Q the UHVSLUDWRU\]RQHWKHEDFWHULDDUHHLWKHULQWKHlung parenchyma (e.g., in pneumonia) 34 or in the alveolar macrophages (e.g., in tuberculosis) 35,36. In both cases, their proximity to the thin alveolar barrier makes the two conditions dangerous since the pathogens may cross the barrier and distribute systemically. Bacterial infections (e.g., caused by Pseudomonas spp) can also ORFDOL]H LQVRPHVWUXFWXUHVRIWKHFRQGXFWLYH]RQHe.g., bronchi, bronchioles), and co-occur with or be a consequence of chronic lung disorders with impaired respiratory tract clearance (e.g., cystic fibrosis, bronchiectasis, COPD) or immune compromising conditions (e.g., human immunodeficiency virus, diabetes mellitus). Regarding cystic fibrosis, for   Background  7  example, the genetic mutation of the cystic fibrosis transmembrane conductance regulator gene results in deficiencies in the pulmonary physiology, including alteration of the osmolarity of the respiratory tract surface liquid layer and hypersecretion of thick mucus that cannot be cleared sufficiently by the mucociliary escalator 37. Overall, this environment together with inflammation responses promotes the development of recurring viral and chronic bacterial infections that are difficult to eradicate and can lead to host tissue damage, respiratory failure, or, eventually, death 37. The major responsible bacteria of these secondary infections in cystic fibrosis are S. aureus, P. aeruginosa, Haemophilius influenza, Achromobacter xylosoxidans and Burkholderi cepacia complex. They are particularly dangerous, especially if resistant to antibiotics (e.g., multidrug-resistant P. aeruginosa, methicillin-resistant S. aureus (MRSA)) and if able to switch to a biofilm mode of growth (i.e., mucoid phenotype) 6,38. The mucoid phenotype populates not only WKHFRQGXFWLYH]RQH, EXWDOVRWKHUHVSLUDWRU\]RQH6.        8Background Table 1. Main pulmonary diseases 31,39 .  Disease Anatomic site Major pathologic changes Etiology Infections Acute infections Larynx, trachea Laryngitis, inflammation of the trachea Viruses  (rhinovirus, adenovirus, RSV, LQIOXHQ]DYLUXV Pneumonia Alveolar sacs Alveoli filled with inflammatory exudate  CAP Bacteria (S. pneumoniae, H. influenzae, M. pneumoniae - less frequently S. aureus, P. aeruginosaYLUXVHV569SDUDLQIOXHQ]DYLUXVFKLOGUHQLQIOXHQ]D$DQG%DGXOWV)) HAP Enterobacteriaceae (Klebsiella spp., E. coli), Pseudomonas spp., S. aureus (usually MRSA) Tuberculosis Alveoli (alveolar macrophages) Caseous granulomas and cavitation, as consequences of lung tissue hypersensitivity to tubercular antigens and lung tissue destruction  M. tuberculosis, Mycobacterium avium complex COPD Chronic bronchitis/ bronchiolitis Bronchi/ bronchioles Hypertrophy and hyperplasia of the mucus glands, mucus hypersecretion Tobacco smoke, air pollutants (e.g., sulfur dioxide,  nitrogen dioxide), viruses (e.g., RSV),  and less frequently bacteria (e.g., M. pneumoniae). Bronchiectasis Bronchi Destruction of smooth muscle and the supporting elastic tissue, permanent dilation of bronchi and bronchioles. Secondary to persistent or severe infections Emphysema Acini Enlargement of the air spaces distal to the terminal bronchioles, destruction of the alveolar walls 7REDFFRVPRNHĮ-anti-trypsin deficiency         9Background Allergies Asthma Bronchi Smooth muscle hypertrophy and hyperplasia, excessive mucus, inflammation Outdoor allergens (e.g., pollens from grass, trees, weeds), undefined causes Hay feverNoseEdema of the nasal mucosaOutdoor allergens (e.g., pollens from grass, trees, weeds), indoor allergens (e.g., pet hair, dust mites, mold) Vascular diseases Pulmonary embolism Pulmonary arteries Occlusion of the pulmonary arteries Blood clot (thrombus) that usually arise by the large deep veins of the legs (condition known as deep vein thrombosis), clump of material (e.g., fat from the bone marrow, air bubbles) Pulmonary hypertension Pulmonary vasculature Increase in pulmonary vascular resistance,  SUHVVXUHVDWUHVW•PP+J Diseases that cause decrease in the cross-section of the pulmonary vascular bed or increase of pulmonary vascular blood flow (e.g., COPD, interstitial lung disease, mitral stenosis, systemic sclerosis) Cancers Adenocarcinoma Small airway epithelial and type II pneumocytes Transformation of benign cells in neoplastic cells, in some cases metastasis (e.g., small-cell lung carcinoma) or intrathoracic spread (e.g., squamous cell carcinoma) Tobacco smoke - less frequently: environmental exposure  (e.g., asbestos, tar, soot), genetic polymosphisms of P-450  (e.g., CYP1A1) Squamous cell carcinoma Bronchi Large cell carcinoma Bronchi Small cell carcinoma Small airway epithelial and type II pneumocytes COPD: Chronic obstructive pulmonary diseases, RSV: respiratory syncytial virus, CAP: community-acquired pneumonia, HAP: hospital-acquired (nosocomial) pneumonia, MRSA: Methicillin-resistant Staphylococcus aureus.  Background   10    2.2.1 Treatment of Bacteria Lung Infections Treating the bacterial infections is important to reduce the mortality of respiratory tract infections, and it is currently done with antibiotics. Antibiotics are molecules that via one or more targets on the bacteria cause inhibition of growth (if bacteriostatic) or preferably cell death (if bactericidal). Antibiotics target the cellular membrane (e.g., ȕ-lactams, glycopeptides), intracellular components that are essential for the synthesis of protein (e.g., macrolides, amphenicols, tetracycline, aminoglycosides) or nucleic acids (e.g., quinolones), or metabolic pathways (e.g., sulfonamides) (Figure 4) 40–42.   Figure 4. Antibiotics targets and resistance mechanisms in bacteria cell. Figure created with BioRender.com with inspiration from 43 with permission.      Background   11    For treatment of bacterial respiratory infections, different classes of antibiotics are used alone or in combination. The selection of the treatment regimen is based on several factors, including the severity of the disease (e.g., ORFDOL]DWLRQLQWKHUHVSLUDWRU\WUDFWSUHVHQFHRIbiofilm, re-occurring infection), whether treatment will be done in the community or hospital setting, patient-specific factors (e.g., age, antibiotic allergies), the local epidemiology (e.g., national/local antimicrobial resistance), and the presence of risk factors for infection with drug-resistant organisms.  In general, the guidelines from the European Society of Clinical Microbiology and Infectious Diseases and the European Respiratory Society recommend treatments by oral or intravenous administration of antibiotics, depending on the severity of the disease. Amoxicillin and tetracyclines are first-choice antibiotics, while newer macrolides (e.g., D]LWKURP\FLQ, clarithromicin) and some fluoroquinolones (e.g., levofloxacin, moxifloxacin) are good alternatives when clinically relevant bacterial resistance against all first-choice agents is observed. In case of pseudomonal infections, co-administration of antipseudomonal fluoroquinolones (e.g., levofloxacin, ciprofloxacin) with an DQWLSVHXGRPRQDO ȕ-lactam antibiotic (e.g., D]WUHRQDP, cefepime, FHIWD]LGLPH, imipenem, meropenem) is recommended 5 (Table 2).  The treatment of infections with cystic fibrosis, particularly by P. aeruginosa, includes systemic and inhaled antibiotics (i.e., WREUDP\FLQOHYRIOR[DFLQD]WUHRQDPFROLVWLQ 6,38,44,45), either alone or combined, in order to reach high concentrations both in the lung lining fluid LQWKHFRQGXFWLYH]RQHvia LQKDODWLRQDQGLQWKHUHVSLUDWRU\]RQHvia oral or intravenous administration) 6.      12 Background Table 2. Antibiotic treatments for bacterial infections in the lower respiratory tract, according to European and American guidelines. LRTI type Severity/sub-group TreatmentPreferred Alternative * Community setting LRTI 5,46 All Oral amoxicillin + doxycycline Oral co-amoxiclav, clarithromycin/D]LWKURP\FLQ, levofloxacin/moxifloxacin Hospital setting Pneumonia 5,46–48  Moderate/severe Oral co-amoxiclav + D]LWKURP\FLQFODULWKURP\FLQ Oral levofloxacin, moxifloxacin + risk factors for  P. aeruginosa IV meropenem/imipenem/cefepime + ciprofloxacin/levofloxacin + risk factors for MRSA Oral or IV vDQFRP\FLQOLQH]ROLGOHYRIOR[DFLQPR[LIOR[DFLQ Infections in bronchiectasis 5,49,50  No risk factors for  P. aeruginosa Oral co-amoxiclav, levofloxacin, moxifloxacin + risk factors for  P. aeruginosa Oral ciprofloxacin, IV beta-lactam+aminoglycoside, inhaled colistin/tobramycin/gentamicin Infections (primarily P. aeruginosa) in cystic fibrosis 51–54Moderate/severe Inhaled toEUDP\FLQD]WUHRQDPFROLVWLQ“oral ciprofloxacin ,9DPLNDFLQFHIWD]LGLPHWREUDP\FLQFROLVWLQPHURSHQHP COPD 5  Mild Oral amoxicillin, doxycycline Oral co-amoxiclav, FODULWKURP\FLQD]LWKURP\FLQOHYRIOR[DFLQmoxifloxacin Moderate/severe Oral co-amoxiclav Oral levofloxacin, moxifloxacin + risk factors for P. aeruginosa Oral ciprofloxacin Tuberculosis 55,56  Severe Oral or IV rifampin + oral S\UD]LQDPLGH oral, IV or IM LVRQLD]LGoral ethambutol Oral or IV levofloxacin/moxifloxacin/gatifloxacin + IV or IM amikacin/capreomycin + 2 amongst ethionamide/oral F\FORVHULQHOLQH]ROLGFORID]LPLQH“EHGDTXLOLQHLPLSHQHPPHURSHQHP COPD: chronic obstructive pulmonary disease, IV: intravenous, LRTIs: lower respiratory tract infections, MRSA: methicillin-resistant Staphylococcus aureus.  *In case of hypersensitivity to preferred drugs or national/local prevalence of clinically relevant resistance    Background   13    2.2.2 Challenges with Antibiotic Treatments Today´s treatments of bacterial infections, including pulmonary infections, are often not sufficiently effective. The above mentioned treatments are all mainly based on intravenous or oral administration of antibiotics dissolved in aqueous medium or formulated in oral dosage forms (e.g., capsules, tablets), respectively. Even though these routes of administration are FKDUDFWHUL]HGE\KLJKV\VWHPLFELRDYDLODELOLW\, this does not ensure high enough concentrations of the antibiotic at the actual target site as required to efficiently treat the infections 33,44.  If the antibiotic concentration is not sufficient to kill the bacteria at the site(s) of infection, the pathogens may instead be able to develop mechanisms to resist the effect of the specific antibiotic. These mechanisms include: (i) antibiotic efflux (e.g., expression of efflux pumps, such as the ATP-binding cassette transporters or the major facilitator superfamily transporters that transport tetracyclines, glycopeptides, and macrolides out of cells), (ii) inhibition of drug entry in the cell (e.g., decrease in the expression of outer membrane porins of Gram-negative bacteria that usually facilitate the entry of hydrophilic antibiotics such as VRPHȕ-lactams), (iii) SURGXFWLRQRIHQ]\PHVWKDWGHJUDGHDQGLQDFWLYDWHDQWLELRWLFVe.g., ȕ-lactamases that K\GURO\VH WKH ȕ-ODFWDP ULQJ RI ȕ-lactams, aminoglycoside modifying HQ]\PHVWKDWacetylate, phosphorylate or adenylate aminoglycosides) and (iv) modification of the target site (e.g., mutations of penicillin-binding proteins that are responsible for bacterial cell wall synthesis and are usually bound and inactivated by ȕ-lactams, production of modified peptidoglycan precursor with lower affinity for glycopeptides, rRNA methylation at sites that usually bind macrolides, lincosamides, streptogamins and aminoglycosides) (Figure 4) 41–43,57,58. These genetic resistance modifications can also be transferred from a resistant to a susceptible bacterium, resulting in  the distribution between genera and species 57,58. As a consequence, many pathogens responsible for serious diseases have become resistant to various classes of antibiotics, and some evolving into multidrug-resistant (MDR), extensively drug-resistant (XDR), or even pandrug-resistant (PDR) types, in which cases they are resistant to more than one, almost all, or all approved antibiotics, respectively 59. Clinically, high doses (250-750 mg) of antibiotics are administered orally or intravenously to obtain the local drug concentrations to effectively treat the infection. As a consequence of the systemic levels of antibiotics, part of the dose may reach non-infected tissues, alter the growth of harmless and useful organisms (e.g., bacteria in the intestine) 33,44, and/or be responsible for toxicity at not-targeted site(s) 33,44. For example, prolonged administration of high doses of the aminoglycoside tobramycin can cause acute and chronic nephrotoxicity, by decreasing glomerular filtration rate and altering excretion of electrolytes 60,61. Overall, this leads to inefficiency of existing approaches to treat infection. Antimicrobial resistance has now become a global public health concern and has directed the focus of PDQ\RUJDQL]DWLRQVVXFKDVWKH&HQWHUVIRU'LVHDVH&RQWURODQG3UHYHQWLRQCDC) and the   Background   14    :RUOG +HDOWK 2UJDQL]DWLRQ :+2 to identify the bacteria responsible for serious infections and prone to develop resistance, in order to focus the attention and develop strategies to treat these pathogens. These bacteria are classified as urgent, serious and concerning threats by the CDC 62 and, similarly, as pathogens with critical, high, medium priority by the WHO 63. Urgent threats include carbapenem-resistant Acinetobacter, carbapenem-resistant Enterobacteriaceae, and Clostridioides difficile. Serious threats include extended-spectrum beta-lactamase-producing Enterobacteriaceae, MDR P. aeruginosa, MRSA, drug-resistant S. pneumoniae, MDR and XDR Mycobacterium tuberculosis, vancomycin-resistant Enterococci  62. 2.2.3 New Strategies to Treat Bacterial Infections The challenges and obstacles that are associated with antibiotic treatments have directed the attention to new strategies to treat bacterial infections in the future, such as the discovery of new classes of antibiotics, and the improvement of the therapeutic effect of existing compounds 33,44.  Discovery and development of new compounds that are potent and effective especially against MDR bacteria is expected to likely be one of the best options to fight infections worldwide, according to the Global Action Plan against antimicrobial resistance proposed by WHO in 2015 64. Since 2000, 29 new antibiotic drugs (including three multidrug combinations) have been approved by United States Food and Drug Administration (US FDA) and European Medicines Agency (EMA) 65 (Table 3) and 42 are currently in clinical trials or under regulatory evaluation for treating bacterial infections (as of September 2019) 66. Unfortunately, even though the total number of antibiotics has increased, the majority are analogues of currently marketed drugs and have shown to be susceptible to similar mechanisms of resistance, already seen for the known classes of antibiotics. Moreover, of the antibiotics currently in the development pipeline only 11 have the potential activity to act against at least one of the pathogens that are resistant to carbapenems classified as urgent threats according to the CDC 67. Only ~ one in five drugs in the pipeline has an innovative component, such as being a novel class (new scaffold, new pharmacophore) and/or hitting a novel target (new binding site, and, therefore, novel mechanism of action).          Background 15Table 3. Antibiotics and combinations US FDA (https://www.accessdata.fda.gov/scripts/cder/daf/) and EMA (https://www.ema.europa.eu/en/medicines) approved from 2000 to 2019. Adapted from  65,66,68–70 with permission.  Year approved Drug name Drug class Route of administration Indication(s) Innovation?* 2019 Pretomanid 1LWURLPLGD]ROH Oral MDR- and XDR-TB - 2018 Omadacycline Tetracycline IV, oral CABP, ABSSSI - 2018 Eravacycline Tetracycline IV cUTI ض 2018 3OD]RPLFLQ Aminoglycoside IV cUTI - 2017 Meropenem+ vaborbactam ȕ-ODFWDPF\FOLFERURQDWHȕ-lactamase inhibitor IV cUTI ض 2017 Delafloxacin Fluoroquinolone IV, oral ABSSSI - 2015 2]HQR[DFLQ Quinolone Cutaneous Impetigo - 2015 &HIWD]LGLPHDYLEDFWDP ȕ-lactam + GLD]DELF\FORRFWDQH ȕ-lactamase inhibitor IV cIAI, cUTI, HABP, VABP ض 2014 Finafloxacin Fluoroquinolone Otic AOE - 2014 Nemonoxacin Quinolone IV CABP - 2014 &HIWROR]DQHWD]REDFWDP ȕ-ODFWDPȕ-lactamase inhibitor IV cUTI, cIAI - 2014 7HGL]ROLGSKRVSKDWH 2[D]ROLGLQRQH IV, oral ABSSSI - 2014 Oritavancin Glycopeptide IV ABSSSI - 2014 Dalbavancin Glycopeptide IV ABSSSI - 2014 Delamanid 1LWURLPLGD]ROH Oral MDR-TB - 2012 Bedaquiline Diarylquinoline Oral MDR-TB ض 2012 Fidaxomicin Tiacumicin Oral C. difficile-associated diarrhea ض 2010 Ceftaroline fosamil Cephalosporin IV ABSSSI, CABP - 2009 Besifloxacin Fluoroquinolone Ophthalmic Bacterial conjunctivitis - 2009TelavancinGlycopeptideIVcSSSI-      Background 16ABSSSI: acute bacterial skin and skin structure infections, AECB: acute bacterial exacerbation of chronic bronchitis,  AOE: acute otitis externa, CABP: community-acquired bacterial pneumonia, HABP: hospital-acquired bacterial pneumonia, cSSSI: complicated skin and skin structure infections, cUTI: complicated urinary tract infections, cAIA: complicated intra-abdominal infections, IV: intravenous, IM: intramuscular, MDR- and XDR-TB: multi-drug resistant and extensively drug-resistant tuberculosis, uSSSI: uncomplicated skin and skin structure infections, VABP: ventilation-acquired bacterial pneumonia, VRE: vancomycin-resistant Enterococci. * intended as being a novel class (new scaffold, new pharmacophore) and/or hitting a novel target (new binding site, novel mechanism of action). # withdrawn by EMA and US FDA.   2007 Garenoxacin #  Quinolone IV, oral CABP, cSSSI, cIAI,  acute pelvic infections - 2007 Retapamulin Pleuromutilin Cutaneous Impetigo ض 2005 Tigecycline Tetracycline IV cSSSI, cIAI, CABP - 2005 Doripenem Carbapenem IV cIAI, cUTI - 2004 Gemifloxacin Fluoroquinolone Oral CABP, AECB - 2003 Daptomycin Lipopeptide IV cSSSSI, S. aureus bloodstream infections ض 2002 Ertapenem Carbapenem IV, IM cIAI, cSSSI, CABP, cUTI, acute pelvic infections - 2001 Telithromycin Macrolide Oral CABP - 2000 /LQH]ROLG 2[D]ROLGLQRQH IV, oral VRE infections, CABP, HABP, VABP, cSSSI, uSSSI -   Background  17  Among the compounds in the clinical development with a certain degree of novelty  (Table 4), antimicrobial peptides (AMPs) are promising alternatives to traditional antibiotics. They have different mechanisms of action (e.g., disruption of the cell membrane, immunomodulation, intracellular penetration and damage to intracellular biomolecules 71) and often a broad spectrum of antimicrobial activity, not only against bacteria, but also against viruses, fungi and parasites 72,73. For bacterial infections, they have been studied as monotherapies, giving their effective bacterial growth killing 72,74–77, but also as adjuvants to traditional antibiotics to provide a synergistic effect 72,74,76. To date, only one (i.e., daptomycin) AMP has been approved clinically as antibiotic for monotherapy 73.  The antibiotics currently used in the clinic have different mechanisms of action against vital processes or structures of the bacteria. However, as mentioned above, their action against a single target frequently causes the development of resistance in bacteria. Using them in combination therapy instead would result in hitting different targets at the same time, and overcome the antibiotic tolerance that often result in therapeutic failure 78. Not only the combination of antibiotics from different classes (e.g., colistin and tigecycline, rifampin, meropenem79) or antibiotics and inhibitors (e.g., amoxicillin and clavulanate) has already showed synergistic effect, but also their combination with new classes of anti-infectives (e.g., AMPs, quorum sensing inhibitors) 44,76. For example, the synergistic effect of antibiotics and AMPs has recently shown to be effective against infections caused by critical and high-priority pathogens 80 (e.g., D]LWKURP\FLQDQG//-37 for MDR P. aeruginosa and MDR Acinetobacter baumannii 81, ampicillin and nisin for Salmonella enterica 82, chloramphenicol and brevinin-2 CE for MRSA 83, and ampicillin and cryprdin-2 for S. enterica 84). Despite the promising results, currently there are not combinations of classes planned to be introduced in the market in the near future, a part from one (i.e., imipenem/cilastatin + relebactam (MK-7655A))66. Another more recently applied strategy is interfering with the growth of bacteria through targeting the bacterial communication systems (i.e., quorum sensing), used by the bacteria to communicate with each other, to develop biofilm, and to produce secondary metabolites and virulence factors 85,86. Quorum sensing is mediated by species-specific and strain-specific signalling molecules, which are produced and released by the bacteria 85. These are also known as autoinducers, such as acyl-homoserine lactones (used by Pseudomonas spp., Acinobacter spp.), fatty acids (used by Xanthomonas spp., Burkholderia spp.), ketones (used by Legionella spp.) or quinolones (used by P. aeruginosa) 85. By interfering with the production, by scavenging or by degrading the autoinducers, quorum sensing inhibitors (e.g., 5-fluorouracil, eugenol, farnesol), quorum quenching antibodies (e.g., Fab RS2-1G9) and macromolecules (e.g., Į-cyclodextrin) RU TXRUXPTXHQFKLQJ HQ]\PHV (e.g., acylase Pvdq, lactonase AiiA), respectively, disrupt this molecular communication system making the bacteria more sensitive to antibiotics and decrease biofilms 85. These molecules are promising anti-infective agents that could be used to complement antibiotic therapy, especially in the case of biofilm-related infections 85,86. However, to date, no quorum sensing inhibitors have been applied clinically.      18Background Table 4. Antibiotics containing a degree of innovation (novel class or novel target) currently in clinical development or under regulatory evaluation at the beginning of 2020. Adapted from 66,67  with permission. Development phase Drug name Drug class Mechanism of action (target) Route of administration Expected activity against pathogens Potential indication(s) NDA Lefamulin Pleuromutilin Protein synthesis inhibitor  (50S ribosomal subunit at the peptidyl transferase center) IV, oral S. aureus ABSSSI, CABP, HABP, VABP III Murepavidin (POL-7080) AMP mimetic Interference with the biogenesis of lipopolysaccharide of the outer membrane  ȕ-barrel protein LptD) IV, inhalation Carbapenem-resistant P. aeruginosa ** HABP, VABP, ABSSSI, bloodstream infection, cIAI III 5LGLQLOD]ROH (SMT 19969) Bis-EHQ]LPLGD]ROH Inhibition of cell division and reduction of toxin production (drug target unknown) Oral C. difficile** C. difficile infections II SQ-109 Diamine Inhibition of cell wall synthesis  (MmpL3 membrane transporter) Oral M. tuberculosis TB II Telacebec  (Q-203) ,PLGD]RS\ULGLQHamide Inhibition of ATP synthesis  (qcrB subunit of the cytochrome bc1 complex) Oral M. tuberculosis TB II 0DFR]LQRQH %HQ]RWKLD]LQRQH Inhibition of cell wall synthesis IODYRHQ]\PH'SU( Oral M. tuberculosis TB II OPC-167832 Carbostyril Inhibition of cell wall synthesis IODYRHQ]\PH'SU( Oral M. tuberculosis TB II Gepotidacin (GSK-2140944) 7ULD]DDFHQDSKWK\OHQH Topoisomerase II inhibitor  (novel A subunit site) IV, oral S. aureus,   N. gonorrhoeae, Enterobacteriaceae uUTI, cUTI, ABSSSI, urogenital gonorrhea, CABP     19Background ABSSSI: acute bacterial skin and skin structure infections, AMP: antimicrobial peptide, CABP: community-acquired bacterial pneumonia, HABP: hospital-acquired bacterial pneumonia, cUTI: complicated urinary tract infections, cAIA: complicated intra-abdominal infections, IV: intravenous, LptD: lipopolysaccharide transport protein D, MRSA: methicillin-resistant Staphylococcus aureus, NDA: new drug application, TB: tuberculosis, uUTI: uncomplicated urinary tract infections, VABP: ventilation-acquired bacterial pneumonia. **and* indicate the bacteria classified as urgent and serious threats, respectively, according to the Centers for Disease Control and Prevention (CDC) 62.II Zoliflodacin (ETX0914) Spiropyrimidinetrione Topoisomerase II inhibitor  (novel site) Oral S. aureus,   N. gonorrhoeae Uncomplicated gonorrhea IIBrilacidin (PMX-30063) Defensin mimetic Cell membrane IV S. aureus ABSSSI II CG-400549 %HQ]\OS\ULGLQRQH FabI inhibitor Oral MRSA* ABSSSI II MGB-BP-3 Distamycin DNA minor groove binding Oral C. difficile** C. difficile infections II Afabicin (Debio-1450) %HQ]RIXUDQnaphthyridine FabI inhibitor IV, oral S. aureus ABSSSI, bone or joint infections due to S. aureus I GSK-070 (GSK3036656) Oxaborole Inhibition of protein synthesis (Leucyl-tRNA synthetase) Oral M. tuberculosis TB I TBA-7371 $]DLQGROH Inhibition of cell wall synthesis IODYRHQ]\PH'SU( Oral M. tuberculosis TB I CRS3123 Diaryldiamine Methionyl-tRNA synthetase Oral E. faecium, S. aureus, C. difficile** C. difficile infections   Background  20  Another strategy currently under investigation is represented by damaging the bacterial membrane and/or by altering bacteria metabolic pathways (e.g., the respiration system), as a consequence of intracellular oxidative stress 87–89. Normally, in the bacterial cell the production and clearance of reactive oxygen species are in equilibrium, ensuring physiological conditions for survival. However,  excessive oxidative stress can be induced by nanoparticles of various metals (e.g., VLOYHU]LQFFRSSHUJROG WLWDQLXPUHVulting in inhibition of bacterial growth or death 87–89. The research of this class of non-traditional antibiotics is still at the preclinical phase. While new compounds, new combinations, and new targets are heavily investigated, another approach that can contribute to increasing the therapeutic efficacy of antibiotic treatments LV WR HQVXUH RSWLPL]HG HIIHFW DW WKH VSHFLILF VLWH RI LQIHFWLRQ E\ e.g., re-formulation of existing or new antibiotics into novel drug delivery systems, e.g., nano- DQGPLFURQVL]HGparticles. Re-formulation of existing and/or new antibiotics in more advanced drug delivery systems can increase the efficacy of the treatment 44,90 by selective delivery of the antibiotic to the infection site and exposure of the bacteria to a higher dose of the antibiotic, resulting in more effective and rapid bacteria killing, lowered risk of development antibacterial resistance, and decreased risk of systemic off-target effects 33. In addition to site-specific delivery, drug delivery systems are also able to protect the drug from chemical and/or HQ]\PDWLF degradation as well as from the interaction with other molecules, to locally release a sufficient dose of drug in a controlled manner, and to improve drug transport across biological barriers 33,44; all contributing enhanced drug bioavailability. Thus, for efficient pulmonary delivery of antibiotics to treat lung infections it is important that the carrier of the drug delivery systems is biocompatible and biodegradable, so that toxicity from its accumulation is prevented and repeated dose administration is possible. High doses of antibiotics are usually required, thus, delivery systems with high drug loading capacity are preferred 44.      Background  21  2.3 Lung Targeting  Several drug delivery systems have been developed and investigated to specifically target WKHOXQJVDQGLQSDUWLFXODUWKHUHVSLUDWRU\]RQHSuch drug delivery systems can target the lungs directly via inhalation or indirectly via intravenous administration.  2.3.1 Direct Lung Targeting via Inhalation  Compared to the traditional approaches with oral and intravenous administration, inhalation has some advantages, including being non-invasive and needle-free, avoiding ‘first-pass’ metabolism and antibiotic exposure to the gastrointestinal microflora 33, and, most importantly, being able to deliver relatively high drug doses directly to the target site, e.g. the infected lung tissue 91. Thus, antibiotics encapsulated, entrapped, or adsorbed on the surface of drug delivery systems can be delivered directly to the pulmonary site of infection by inhalation. As shown in Figure 5, after inhalation the particulate drug delivery system enter the respiratory tract and deposit deep in the respiratory tree (e.g., in the alveoli) with subsequent release of the active component(s).  Figure 5. Lung targeting via inhalation of drug delivery systems. Figure created with BioRender.com  Antibiotic drug delivery to the lungs by inhalation, however, present some disadvantages, such as the possibility of systemic antibiotic absorption and the difficulty to control theantibiotic dose at the infection site 33,38. Other challenges are related to the complexity of the pulmonary anatomy and physiology, and to the mechanisms of deposition of particulates. To be effective for the treatment of pulmonary infections, the inhaled drug delivery systems have to first reach the respiUDWRU\ ]RQH then be able to avoid the macrophages, penetrate any bacterial biofilm, release the drug into the low volume of lung fluid so that it can exert its effect on the target bacteria 92. Achieving this will depend on the   Background  22  characteristics of the inhaled formulation (e.g., mass median aerodynamic SDUWLFOH VL]Hparticle VL]H distribution, surface and particle morphology, density, surface charge and hygroscopicity 12), physico-chemical properties of the antibiotic, inhalant device, patient (e.g., inspiratory flow velocity and flow regime, patient´s lung volume, respiratory tract geometry) and disease state 23,93,94. For example,  the particles or nebulised droplets carrying the antibiotic shRXOG KDYH D SDUWLFOH VL]H RI -5 μm to be deposited deep in the lower respiratory tract and in the alveoli by gravitational sedimentation or by Brownian diffusion, respectively, without impacting in the larger airways or being exhaled 33. 3DUWLFOHVRIDVL]H below 500 nm may also deposit, although to a limited extent as they have a high risk of being exhaled resulting in reduced deposition 33. If deposited in the respiratory tract and if they have a neutral surface charge, smaller nano-VL]HGSarticulate carriers may, to some extent, permeate the mucus in the upper respiratory tract and any associated bacterial biofilm by diffusing through the network formed by its components (cut-off of 500 nm in healthy people, 300-100 nm in patients with cystic fibrosis) 45.  The formulations applied for pulmonary delivery are either liquid (solutions or suspensions) or dry powders, and require inhalation devices for administration, QHEXOL]HUVRUdry powder inhalers, respectively 95. Many dry powders DQGQHEXOL]HG VXVSHQVLRQV of drug delivery systems are currently investigated at preclinical and clinical stages for local delivery of drug to treat different pulmonary diseases, including asthma, cystic fibrosis, COPD, lung infections, lung hypertension, and lung cancer 7. Among these, there are micelles, liposomes, polymeric nanoparticles, microparticles and nano-embedded microparticles, as shown in Table 5.  To date, only one inhaled formulation of an antibiotic in drug delivery systems has been approved and has entered the market. It is a liposomal suspension of amikacin designed for QHEXOL]DWLRQ DQG DSSURYHG E\ WKHUS FDA for the treatment of Mycobacterium avium complex lung disease. Two other formulations are undergoing clinical trials. Ciprofloxacin, formulated in liposomes (Lipoquin) and in a mixture of non-encapsulated ciprofloxacin with ciprofloxacin-encapsulated liposomes (Pulmaquin), has been tested in phase III clinical trials for the treatment of non-cystic fibrosis bronchiectasis patients with chronic P. aeruginosa pulmonary infections. However, US FDA and EMA recently denied the market DXWKRUL]DWLRQ 96. A liposomal formulation of tobramycin (Tobramycin Fluidosomes™) received the orphan drug designation for use in respiratory tract infections associated to cystic fibrosis, and entered the clinical trials. Interestingly, all these formulations have been designed for P. aeruginosa treatment in cystic fibrosis. This lack of success for inhaled nanomedicines with antibiotics compared to inhaled antibiotics that have been approved for the treatment of lung infections (i.e., tobramycin (as solution for inhalation, TOBI®, Bramitob® 97 or dry powder, TOBI® Podhaler® 98), levofloxacin (as solution for inhalation, Quinsair® 99D]WUHRQDPas solution for inhalation, Cayston® 100), and colistimethate sodium (as dry powder, ColoBeathe® 101), is related to the higher complexity of these novel drug delivery systems for inhalation compared to the   Background  23  current formulations. This means that efficient scale-up of production ensuring good manufacturing practice can be challenging and expensive. Other obstacles to develop such systems are associated with the unique lung microenvironment and its defence mechanisms and to the high doses usually required for anti-infective treatments, as mentioned above 33,44,102. Table 5. Examples of lipid and polymeric inhaled formulations for the treatment of lung infections approved and under development. CF: cystic fibrosis, MP: microparticles, NDA: new drug application, NP: nanoparticles, PLA: poly(lactic acid), PLGA: poly(lactic-co-glycolic) acid, TB: tuberculosis. Drug delivery system Therapeutic indication Status Ref Carrier Drug Liposomes Amikacin Lung disease caused by M. avium complex Approved, 2018 (ALIS/Arikayce®) 103,104 Liposomes Tobramycin Respiratory infections in CF Phase II (2016) (Tobramycin Fluidosomes™) 105 Liposomes Ciprofloxacin Non-CF bronchiectasis Phase III (2018),  NDA denied (2019) 96 (Lipoquin®, Pulmaquin®) 91 Liposomes Clarithromycin P. aeruginosa infections in CF Preclinical 106 PLA MP Rifampicin Pulmonary infections Preclinical 107 PLGA MP Curcumin CF Preclinical 108 PLGA MP Levofloxacin CF Preclinical 109 PLGA MP Rifampicin TB Preclinical 110 PLGA NP/MP Tobramycin CF Preclinical 111 PLGA NP Levofloxacin P. aeruginosa biofilm infections Preclinical 112 PLGA NP Ciprofloxacin CF Preclinical 113 PLA MP Rifampicin LVRQLD]LG TB Preclinical 114 PLA MP Rifampicin TB Preclinical 115 Chitosan Rifampicin TB Preclinical 115 Chitosan-coated PLGA MP Rifampicin TB Preclinical 115 Chitosan Moxifloxacin Respiratory infections Preclinical 116 Chitosan Levofloxacin P. aeruginosa infections in CF Preclinical 117   Background  24  2.3.2 Passive Lung Targeting via Intravenous Administration An alternative approach to target the lungs is from the vasculature around the thin alveolar epithelium taking advantage of the mechanical filter properties of the lung capillaries. After intravenous administration, antibiotic-loaded particulate carrier systems are pumped through the heart into the pulmonary blood circulation. The restricted capillary diameter of the pulmonary capillary bed (i.e., 7.5 ± 2.3 μm for healthy adults 8,9, 6.6 ± 1.6 μm and 7.5 ± 1.7 μm in rats and dogs, respectively 10) (Figure 6) will lead to entrapment of particles with a slightly higher diameter in the capillaries surrounding the alveolar epithelium. This is known as passive lung targeting. As the matrix of the drug delivery system degrade, the antibiotic is released and can freely diffuse through the thin alveolar-capillary barrier into the alveolar space, and exert the bacteriostatic or bactericidal effect.   Figure 6. Lung targeting via intravenous administration of drug delivery systems.  The passive lung targeting is particularly interesting and a valid approach in case of seriously ill patients with compromised lung function or respiratory tract obstruction (e.g., due to inflammation and mucus plugs), such as in patients with pneumonia or cystic fibrosis. Here, in fact, respiratory tract narrowing may result in compromised ventilation and inhomogeneous distribution of the inhaled drug delivery systems in the lungs, and thus in inefficient treatment 118–120. Moreover, in these cases, lung perfusion is rarely compromised to a high degree compared to ventilation 121. To date, the passive lung targeting strategy is approved and used only in a diagnostic nuclear medicine procedure to determine the lung perfusion in humans with 99mTc-labeled albumin macroaggregates, FRPPHUFLDOL]HG DVPulmolite® or DraxImage® albumin macroaggregates (Montreal, Canada). This approach is naturally challenged by the safety concern of HPEROL]ing lung capillaries. However, this may be tailored as it is directly related to the SDUWLFOHVL]HDQGGLVWULEXWLRQ of the particulate delivery systems, the administered dose, the compatibility, and the carrier biodegradation. It is thus highly relevant WKDWWKHSDUWLFXODWHFDUULHUV\VWHPKDVDQDUURZVL]H  Background  25  distribution around an average value within the 10-15 μm range. In this way, only capillaries and not larger vessels (e.g., arterioles with a diameter of ~20 μm 122DUHHPEROL]HGDQGthus, the microvascular hemodynamics of the pulmonary circulation is maintained normal and an acute massive embolism as a consequence of vascular occlusion is avoided. The administered dose of the particulate carrier system should be low enough to only HPEROL]H a small percentage (< 1%) of the 280 billion capillaries in humans122,123, so that the remaining non-HPEROL]HG capillaries allow the lungs to continue to work normally. As with inhaled drug delivery systems, the passively targeted carrier has to be biocompatible as well as biodegradable, so that HPEROL]DWLRQLVUHYHUVLEOHtoxicity from carrier accumulation is prevented and repeated dosing is possible. As for other routes of administration it is important that the drug-related properties of the formulations should be tuned and selected according to their use. The properties (e.g., solubility and hydrophobicity) of both the drug and the carrier are thus important. Further, the preparation method for the drug delivery system constitutes a key factor to develop high-content drug carriers. The carrier must release the loaded drug within the time range suitable for the specific application ensuring appropriate pharmacokinetics. In the case of pulmonary infections, it is optimal to have a sufficiently high and sustained drug release during a few (i.e., three-six) days so that it is SRVVLEOHWRILJKWWKHLQIHFWLRQDQGDWWKHVDPHWLPHPLQLPL]HWKHGRVLQJIUHTXHQF\)LQDOO\the drug has to be able to diffuse and permeate the alveolar-capillary barrier.  Due to the severity of many pulmonary diseases, e.g., cancer and infections, and the GHPRQVWUDWHGSRVVLELOLW\RI FRQWUROOHGHPEROL]DWLRQRI DOYHRODU FDSLOODULHV the design of drug-loaded particulate carrier systems to treat lung diseases after intravenous administration has attracted attention. Different biodegradable polymers (e.g., PLGA, poly (lactic acid) (PLA), albumin, gelatin) have been recently tested as carrier systems for cisplatin 124, rifampicin 125, erythromycin 126D]LWKURP\FLQ127, ofloxacin 128 and cefquinome 129 for the treatment of lung tumors and pulmonary infections (Table 6). Many of these previous works have tested in vitro performance as well as in vivo release and biodistribution of the loaded drugs. However, in the majority of the studies the drug did not accumulate exclusively in the lungs, but distributed to non-targeted organs, such as the liver, kidneys and spleen 126–129ZKLFKPD\EHDWWULEXWHGWRWKHLUODUJHSDUWLFOHVL]HGLVWULEXWLRQUDQJH-50 μm) and/or to the non-optimal release profile. Up to now, the drug-loaded delivery systems have been investigated at the preclinical stage without testing the real-time kinetics or efficacy of the delivery system in terms of treatment of in vivo models.    Background  26  Table 6. Examples of injectable diagnostic and therapeutic drug delivery systems for passive lung targeting approved or under development.  MP: microparticles, MS: microspheres, PLA: poly (lactic acid), PLGA: poly (lactic-co-glycolic acid) * % lung targeting intended as distribution of the carrier or the drug (released by the carrier) in the lungs compared to other organs at different times post injection (indicated in brackets).  Drug delivery system Particle size  and % lung targeting * Status Ref  Carrier Drug Diagnostic lung perfusion Albumin macro-aggregates (MAA) - - 10-90 um (90%), all < 150  - MAA in lungs ~99.4% (10 min) Approved (1976) 130 Polystyrene MP - - 10-12 μm - MP in lungs 97.7% (1 h)  ~95% (48 h) Preclinical 131 PLA MS - - 9.0  ± 0.4 μm, - MS in lungs 99.4% (15 min) Preclinical 132 PLGA-PEG MS - - 10-50 μm (~80%) - MS in lungs 25% (15 min) Preclinical 133 PLGA MS -  - 10-50 μm (~93%) - MS in lungs 30% (15 min) Preclinical 133 Respiratory infections Albumin MS $]LWKURP\FLQ(AZI) - 3.9-19.8 μm - AZI in lungs ~70% (30 min), ~90% (12 h) Preclinical 127 PLGA MS Cefquinome (CEQ) - 5-50 μm - CEQ in lungs ~55% (15 min), ~90% (12 h) Preclinical 134 Gelatin MS Cefquinome (CEQ) - 5-30 μm - CEQ in lungs ~50% (15 min), ~90% (12 h) Preclinical 129 Albumin MS Ofloxacin (OFX) - 5-25 μm - OFX in lungs ~75% (10 min), ~90% @ 12 h Preclinical 128 Cancer PLGA MS Cisplatin (CisPt) - 5-30 μm - CisPt in lungs ~95% (15 min), ~95% @ 24 h Preclinical 124 Gelatin MS Carboplatin (CPt) - 5.0-28.6 μm - CPt in lungs 47% (15 min),  64% (12 h) Preclinical 135 Gelatin MS 5-fluorouracil (5FU) - 5-15 μm - 5FU in lungs 60% (10 min),  85% (12 h) Preclinical 136   Scientific Outcome  27  3 Scientific Outcome 3.1 Research Manuscript Title 0RQRVL]HG 3RO\PHULF 0LFURVSKHUHV IRU 3DVVLYH /XQJ 7DUJHWLQJ %LRGLVWULEXWLRQ DQGPharmacokinetics after Intravenous Administration  (Appendix)  Aim  The aim of this study was to prepare DQGFKDUDFWHUL]Hbiodegradable microspheres suitable for targeted delivery of antibiotics to the lungs after intravenous administration.  Outcome  PLGA microspheres encapsulating levofloxacin were prepared with a flow-focusing microfluidic chip. They RSWLPL]HG PLFURVSKHUHV showed a suitable SDUWLFOH VL]H DQGphysico-chemical properties for passive lung targeting. The microspheres encapsulated levofloxacin and preserved its antibacterial activity against P. aeruginosa, E.coli and S. aureus. The PLGA microspheres degraded within three weeks in vitro and within one week in vivo, while slowly releasing ~85% of the encapsulated levofloxacin over five days. The PLGA microspheres showed low toxicity towards endothelial, alveolar epithelial and red blood cells. The microspheres were radiolabeled with the gamma emitter 111Indium and intravenously administered in mice to assess their pharmacokinetics and tissue distribution. Immediately after tail vein injection, the 111In-labeled PLGA microspheres distributed homogenously throughout the lungs, from where they cleared within one week.  Overall, monodisperse PLGA microspheres with a diameter of 12 μm demonstrated to be a promising targeted delivery system to transport and release antibiotics for the treatment of pulmonary infections.    Discussion  28  4 Discussion 4.1 Levofloxacin-Loaded PLGA Microspheres as Drug Delivery Systems Different drug delivery systems can be used to target the lungs. Among these, biodegradable polymeric micro- and nanoparticles are particularly interesting due to their controlled drug delivery properties. Of the biodegradable polymers (e.g., polyesters, hyaluronan, chitosan), PLGA is RQHRIWKHPRVWFKDUDFWHUL]HGDQGwidely used for the preparation of nano- and microparticles. PLGA is a copolymer of PLA and poly(glycolic acid) (PGA), and it degrades in vivo by hydrolysis producing the two non-toxic monomers (Figure 7), which are further PHWDEROL]HG DQG HOLPLQDWHG IURP WKH ERG\ WKRXJK QRUPDO PHWDEROLF SDWKZD\V i.e., tricarboxylic acid cycle) as water and carbon dioxide 137,138. The PLGA polymer is biodegradable and biocompatible and US FDA and EMA approved for use in different drug delivery systems in humans and for a range of administration routes (including intravenous administration) to treat various diseases (e.g., cancer, diabetes mellitus, bipolar disorders) 137,138. Drug delivery systems composed of PLGA and approved by US FDA have been well VXPPDUL]HGE\/Let al.139, Pandey et al. 140, and Zhong et al. 141. Additionally, they have shown to encapsulate different classes of drugs (e.g., small molecules, proteins, nucleic acids) with different solubilities (i.e., hydrophilic, hydrophobic) 138,142, and release them in a controlled manner. Particles composed of PLGA can also be easily surface-IXQFWLRQDOL]HGor modified (e.g., with polyethylene glycol (PEG) to increase their blood circulation half-life) 137,138. Moreover PLGA’s degradation and release kinetics can be tuned according to the application by choosing one of the commercially available PLGAs with specific copolymer composition (i.e., PLA:PGA molar ratio), molecular weight and end group 137,138,143.    Figure 7. Chemical structures of PLGA and levofloxacin.   Discussion  29  In this project, the drug of choice for the PLGA microspheres was levofloxacin (Figure 7), a third generation fluoroquinolone antibiotic that inhibits the bacterial DNA gyrase and the topoisomerase IV, two HQ]\PHV responsible for the DNA synthesis and replication 144. Levofloxacin is a broad spectrum antibiotic active against Gram-positive, Gram-negative and atypical bacteria 144. Due to its exceptional activity against pathogens responsible for respiratory infections (e.g., S. pneumoniae, H. influenza, S. aureus) 145, it is colloquially called the “respiratory quinolone” together with the other molecules of its class (e.g., moxifloxacin, gemifloxacin). Clinically, it is currently indicated for the treatment of respiratory infections (e.g., acute bacterial exacerbations of chronic bronchitis, community-acquired and nosocomial pneumonia) 144,146, skin and skin structure infections144,147, genitourinary infections 144,148. In solid form, levofloxacin is a light yellowish crystalline powder with a molecular weight of 370.38 g/mol. At physiological pH, it exists mainly (~75% of the total amount of levofloxacin) as a ]ZLWWHULRQ due to the LRQL]DWLRQRIERWKionisable groups (i.e., the 3-carboxyl proton with pKa1 = 5.7DQGWKHSLSHUD]LQ\OQLWURJHQwith pKa2 = 8.0 149). It is sparingly soluble in water at physiological pH and soluble in common solvents such as dichloromethane, chloroform, dimethyl sulphoxide. The distribution coefficient in octanol/buffer system (logDoct/buffer) at pH 7.4 is 0.51, showing its affinity for both the lipophilic phase and the hydrophilic phase 149. According to the Biopharmaceutics Classification System, it belongs to class I, i.e., drugs with high solubility, high permeability and rapid dissolution. Regarding the pharmacokinetics, the oral and intravenous formulations are considered bioequivalent, as the bioavailability after oral administration is 99%. It has a plasma half-life of 6-8 h after oral, intravenous and pulmonary administration 99,150,151. It is mainly eliminated (>85%) through the kidneys in the urine 99,150,151. After systemic administration, its intrapulmonary concentration (i.e., in the epithelial lining fluid and in the alveolar macrophages) is 1.5-6 times higher than in the plasma compartment 152–154, confirming the desired distribution for the treatment of pulmonary infections. Levofloxacin is generally considered safe and one of the most tolerable fluoroquinolones 155,156. Very few adverse events on the gastrointestinal tract and skin includes photosensitivity reactions, nausea or vomiting, diarrhoea, pseudomembranous colitis 155,156. However, levofloxacin and other fluoroquinolones have recently showed long-lasting and potentially permanent adverse reactions associated to the musculoskeletal and nervous systems (e.g., tendon inflammation and rupture, depression, problems with memory and sleeping), and their use has been restricted in patients with underlying conditions, that predispose them to side effects 157,158. Current delivery routes of levofloxacin include oral, intravenous and inhalation. At the preclinical phase, levofloxacin has been encapsulated in various drug delivery systems intended for intravenous and pulmonary administration, such as PLGA nanoparticles 90,112, chitosan nanoparticles 117,159,160, liposomes 90, hybrid lipid-polymer nanoparticles 161,162, niosomes (i.e., non-ionic surfactant vesicles) 163, conjugated gold nanoparticles 164, hybrid polymer-calcium carbonate microparticles 165,166, and polymeric microparticles 109.   Discussion  30  4.2 Preparation of Microspheres  PLGA microspheres can be prepared by various bulk methods, such as conventional single or double emulsification, membrane emulsification, spray drying, but also with microfluidic techniques 137,142,167–171. 7RDFKLHYHWKHKLJKHVWXQLIRUPLW\LQSDUWLFOHVL]H, the microspheres were in this project prepared and loaded with levofloxacin using a microfluidic emulsification technique in a glass chip with a flow-focusing geometry  (Figure 8). To make the microspheres, an aqueous solution (i.e., continuous phase (CP)) containing an emulsifier (i.e., polyvinyl alcohol) was introduced in the outer inlet channels. At the same time, an organic solution (i.e., dispersed phase (DP)) of dichloromethane containing both PLGA and levofloxacin was introduced in the central inlet at lower (~30 ×) flow rate. The DP was hydrodynamically focused into the orifice (i.e., the junction point of the inlet channels) and, because of the shear force exerted by the CP, broken into oil/water (O/W) emulsion droplets containing polymer and drug 172. Subsequently, each of the droplets shrunk and turned into solid polymeric microspheres as the result of extraction of the organic solvent in the external water phase.  Figure 8. Preparation of microspheres on a flow-focusing microfluidic chip. aLVX: anhydrous LVX, hLVX: hemihydrate LVX, PVA: polyvinyl alcohol, *F0i contains 50% (w/w) PLGA-PEG-NH2 for radioactive imaging. Inset of the microfluidic chip is the snapshot during the production of droplets taken with optical microscope. Scale bar: 150 μm.   The microfluidics technique was chosen over the conventional bulk emulsification for its high reproducibility and superior control of the emulsification process 173–176. Because each droplet was formed individually and sequentially under constant and identical forces, more homogenous emulsions were produced, making it a very good platform for the continuous synthesis of microparticles with high monodispersity and low batch-to-batch variation 174–177. Various parameters are important for the droplet formation, with channel geometry  Discussion  31  (particularly the orifice width), flow rates, and fluid viscosity showing the largest impact 175. By controlling the fluid dynamics through the experimental and formulation parameters (e.g., flow rates, viscosity of the DP and CP, and polymer concentration) in the microfluidic system, it is in fact possible to control the entire production process and tune the physico-chemical properties of the resulting microspheres 175.  A V\VWHPDWLF RSWLPL]DWLRQ was carried out to select the best conditions to obtain the microspheres with the desired characteristics for targeted delivery of antibiotics to the lungs after intravenous administration, i.e., monodisperse microspheres with VL]HUDQJHRI ~10-20 μm, with high antibiotic content and the ability to slowly release the loaded drug. The variables that were first tested were the flow rates of the DP (QDP) and CP (QCP), followed by the composition of the DP, and lastly the flow control system. Different flow rates and flow ratios were investigated for their effect on the droplet break-off DQG RQ WKH GURSOHW VL]H In general, there are four regimes of droplet formation: VTXHH]LQJ GULSSLQJ MHWWLQJ WKUHDG IRUPDWLRQ 178. Several combinations of QDP (0.5-5 μL/min) and QCP (30-150 μL/min) were screened. At low flow rate ratios (QCP /QDP  < 20), the DP fully blocked the orifice, GHIRUPHG DQG VTXHH]HG until break-off. This droplet generation regime is known as VTXHH]LQJ, and produces droplets larger than the orifice width. Here WKHVL]HRIWKHGURSOHWVDQGSDUWLFOHVLVPDLQO\FRQWUROOHG by the dimension of the orifice and the viscosity of the fluids in lieu of the flow rate ratio 178. By gradually increasing the QCP (30-150 μL/min), and thus the flow ratio (QDP /QCP = 20-120), the particle VL]HGHFUHDVHGDQG WKH UHJLPHPRYHGIURPVTXHH]LQJ WRdripping (in which the droplets form near the orifice) or to jetting (in which the droplets form further downstream from the orifice at a distance < 20× the orifice width). This happens because the shear force imposed by the CP to the DP increases with increasing QCP for a fixed QDP. Above a certain value of flow ratio (QDP /QCP > 150), the shear force on the thread of the DP by the CP caused instability in the system, transition of the break-up pattern to an elongated thread yielding a jet of small (few micron) and polydisperse droplets. The best conditions for the production of monodisperse microspheres of 10-15 μm were found to be QDP = 3 μL/min, QCP = 90 μL/min, associated to the dripping regime. By keeping the flow rates constant, other formulation-related parameters, specifically the organic solvent, the PLGA concentration in the DP, the PLGA lactide/glycolide ratio composition, and different PLGA molecular weights were tested and/or carefully chosen based on literature to PDLQWDLQWKHGHVLUHGSDUWLFOHVL]HDQG distribution, and at the same time allow for the highest levofloxacin loading possible.  For the droplet generation in a flow focusing chip, the organic solvent chosen as DP needs to be volatile and able to dissolve the polymer and the drug. Moreover, it has to be immiscible in the aqueous CP, yet still be able to diffuse to the external water phase resulting in the formation of solid particles. Three types of organic solvents (i.e., chloroform, dichloromethane and dimethyl carbonate) with different water solubility values and   Discussion  32  toxicities were tested as DP. By keeping the flow rates constant at QDP = 3 μL/min, QCP = 90 μL/min, monodisperse particles were obtained with both chloroform and dichloromethane as the DP, while higher polydispersity was observed with dimethyl carbonate. This may be related to the much higher water solubility of dimethyl carbonate (13.9% for dimethyl carbonate 179, compared to 0.8% for chloroform, 1.75% for dichloromethane) and lower interfacial tension between the DP and CP (due to its partial solubility in water), resulting in an unstable emulsion being formed 180. Between chloroform and dichloromethane, the latter was chosen as it shows lower toxicity 180, lower boiling point (dichloromethane 39.6 °C and chloroform 61.2 °C, meaning that it can more easily be removed in the washing step), and diffuses faster in the aqueous phase (~20 sec 181) leading to faster solidification of the particles.  Higher solidification rates are also desirable when amphiphilic or hydrophilic drugs are encapsulated in hydrophobic polymers 172. Amphiphilic drugs, such as levofloxacin, but also hydrophilic drugs have intermediate/low logD values, which means that in a biphasic organic/aqueous system they distribute in both the aqueous and organic phases or at the interfase between the two phases (if amphiphilic) or preferably in the water phase (if hydrophilic). In the single O/W emulsion system, the distribution of the drug into the excess of the external aqueous phase is associated with low encapsulation efficiency 172. Therefore to RSWLPL]HWKHGUXJFRQWHQW in the PLGA microspheres the drug partitioning (or diffusion) from the organic (i.e., DP) to the aqueous phase (i.e., CP) should be reduced as much as possible during the preparation. Experimentally, it can be achieved by accelerating the solvent extraction (and thus, droplet solidification), strengthening the drug-PLGA interaction, increasing the viscosity of the DP by using a higher PLGA concentration or a higher molecular weight PLGA, or by reducing the drug solubility in the aqueous phase 172. For all these reasons, the optimal DP composition was found to be a 5% (w/v) PLGA (7-17 kDa, 50:50 PLA:PGA ratio, carboxylic-terminated end group) solution in dichloromethane.  The choice of the flow control system plays an important role for the use of microfluidic tools, and here it has shown to be a critical aspect especially for the preparation of homogenous microspheres, where the flow rates used were very low (in the order of μL/min) and had to be stable during the entire collection of microspheres in order to form uniform and reproducible results. This resulted in a very sensitive setup. It was especially critical for the DP, since that flow is responsible for the formation of the uniform monodisperse droplets, while small fluctuation in the CP flow did not represent a high enough percentage of the total flow rate to matter. Three different systems were tested: basic syringe pumps, high-precision syringe pumps (neMESYS, Cetoni) and pressure-controlled pumps (Flow EZ pressure pumps, Fluigent). The syringe pumps are one of the most established flow control systems in microfluidics. However, in the case of the basic version with a rather simple drive motors built in, flow fluctuations occurred because of the increments of the stepper motor, resulting in rather polydisperse microspheres and low day-to-day reproducibility. Almost negligible oscillations and, thus, better performance were obtained with the high-precision syringe pumps. Pressure pumps showed, however, the best   Discussion  33  performance by precisely controlling the pulseless and stable flow of both the DP and CP with sub-second response time and fast equilibration time.  4.3 Physico-Chemical Characterization  %DVHGRQWKHEHVWFRQGLWLRQVIRXQGLQWKHRSWLPL]DWLRQSKDVHQRQ-loaded (i.e., formulation F0) and three levofloxacin-loaded (i.e., formulations F1, F2, F3) PLGA microspheres were prepared by varying the theoretical drug content in the DP and the composition of the CP, DQGIXUWKHUFKDUDFWHUL]HG All the resulting microspheres had an average diameter of 12 μm and a narroZ VL]Hdistribution with coefficient of variation < 5.2% (Table 7).  The formation of homogenous microspheres with a smooth surface was confirmed by scanning electron microscopy (SEM) (Appendix, Figure S1). These properties of the microspheres are highly desirable, as spherical and monodisperse VL]H-distributed particles are expected to limit the variation in the rate of microparticle degradation and reduce burst drug release, compared to particles prepared with conventional emulsification methods 170 $GGLWLRQDOO\ WKH VL]H UDQJHperfectly matched the optimal VL]HQHHded for passive lung targeting.  Table 7. 3DUWLFOH VL]H GUXJ FRQWHQW DQG HQFDSVXODWLRQ HIILFLHQF\ RI IUHVK DQG IUHH]H-dried non-loaded and levofloxacin-loaded PLGA microspheres.  Formulation      (#, form) Particle size (ȝP) CV (%) Theoretical LVX loading (% w/w) LVX loading  (% w/w) EE  (% w/w) F0, fresh 12.3 ± 0.4 ns 3.5 - - )IUHH]H-dried 12.5 ± 0.6 ns 4.6 - - F0i, fresh 11.4 ± 0.5 ns 4.5 - - DTPA-F0i, fresh 12.0 ± 0.6 ns 4.7 - - F1, fresh 12.2 ± 0.4 ns 3.5 15 2.2 ± 0.1 r)IUHH]H-dried 11.5 ± 0.4 ns 3.3 15 r rF2, fresh 12.7 ± 0.5 ns 3.9 15  r  r)IUHH]H-dried 12.1 ± 0.5 ns 4.7 15 r rF3, fresh 12.5 ± 0.5 ns 3.3 30 r r)IUHH]H-dried 12.1 ± 0.6 ns 5.2 30 r rCV: coefficient of variation (calculated based on 500 MS), EE: encapsulation efficiency, LVX: levofloxacin, *p FDOFXODWHGEDVHGRQLQGHSHQGHQWEDWFKHV1 QVQRWVLJQLILFDQWO\GLIIHUHQW)RUSDUWLFOHVL]Hstatistical multiple comparison between all formulations were done. For LVX loading and EE, multiple FRPSDULVRQEHWZHHQIUHVKDQGIUHH]H-dried formulations as well as the different loaded formulations were done. The statistical analysis of the EE followed the analysis of the loading **ns ns ns *  Discussion  34  The drug content was measured with ultraviolet-visible spectroscopy after microsphere disintegration. The levofloxacin-loaded PLGA microspheres had a levofloxacin content of ~2-5.5% (Table 7). The drug loading (i.e., drug-to-polymer ratio) and the encapsulation efficiency (i.e., encapsulated drug-to-initially included drug) doubled by saturating the CP with anhydrous levofloxacin (Table 7, F1 vs F2). This confirms that limiting the drug diffusion from the organic to the aqueous phase is a good strategy to increase the encapsulation of highly water soluble drugs in hydrophobic particles, as previously shown 109. Doubling the theoretical drug loading along with replacing the anhydrous form with the hemihydrate form of levofloxacin further improved the final drug loading but lowered the encapsulation efficiency (Table 7, F1 vs F2). These results indicated a saturation of the loading capacity under the used preparation conditions. The hemihydrate form of the drug was used because its solubility in dichloromethane was higher and more drug molecules were loaded as a result hereof.  The drug loading, although low, was satisfactory when considering the nature of the polymer and the drug together with the single O/W emulsification preparation method. Previous work showed that ~2 μm PLGA microspheres could be loaded with levofloxacin with up to 10.5% drug loading when prepared with a double emulsion water/oil/water (W/O/W) by saturating the external water phase, and that the further addition of lauric acid to the organic phase resulted in further increase of levofloxacin loading (18.4%) and encapsulation efficiency (44.4%), possibly due to complexation between the molecules through electrostatic interactions between the laXULFDFLGDQGWKHEDVLFSLSHUD]inyl group of levofloxacin 109,159. Other studies showed that gentamicin loading in PLGA microspheres increased from 2.7% to 6.2% when the PLGA concentration in the organic phase was doubled from 10% to 20% 182. Overall, these results show that the encapsulation of small hydrophilic and amphiphilic drugs in PLGA microspheres is challenging and requires RSWLPL]DWLRQ RI WKH SUHSDUDWLRQ PHWKRG and formulation-related parameters to obtain efficient loading 172. 4.4 In Vitro Characterization The performance of the prepared PLGA microspheres were assessed in vitro. Specifically, the levofloxacin release from the microspheres and the associated degradation of the polymeric matrix was assessed. The antimicrobial activity upon encapsulation in the microspheres was also determined. Finally, the compatibility on endothelial, epithelial and red blood cells was tested.    Discussion  35  4.4.1 Levofloxacin Release and Microspheres Degradation The release of levofloxacin from the PLGA microspheres is an important aspect of the drug delivery system, being the precondition for drug diffusion through the alveolar-capillary barrier and for its antibacterial effect. All the PLGA microspheres slowly released the encapsulated levofloxacin over a period of five days following a biphasic diffusion-driven mechanism 183. A fast release corresponding to ~10-20% of the encapsulated levofloxacin associated to the diffusion of the levofloxacin close to the particle surface was followed by a slow and gradual release of the loaded levofloxacin that diffused out through water-filled pores of the polymeric matrix. After five days, up to 85% of the loaded levofloxacin was released (Figure 9). No burst effect was observed, probably due to the fact that most of the levofloxacin was encapsulated in the polymeric matrix, instead of being bound or adsorbed on the surface.   Figure 9. In vitro cumulative release profiles of levofloxacin from PLGA microspheres. F1 fresh  ( ) and IUHH]H-dried ( ); F2, fresh (  DQG IUHH]H-dried ( ), F3, fresh  ( DQGIUHH]H-dried ( )) in phosphate buffered saline (PBS) and 37 °C, fitted by the Peppas-Sahlin model. Results are expressed as mean ± SD (N = 3, n = 1).   Five mathematical models (i.e., Zero order, First order, Higuchi, Korsmeyer-Peppas, Peppas-Sahlin 184) were tested to fit the experimental data and describe the drug release from PLGA microspheres. Of the 5, the Peppas-Sahlin model best fitted the drug release profiles, as it showed the highest correlation coefficient values (R2 = 0.9847 – 0.9964) and the lowest root mean squared error (RMSE = 2.04 – 4.83) (Appendix, Table S2, Figure S2). According to this model, the drug release happens as a consequence of two events, i.e., the drug diffusion (first term of Peppas-Sahlin Eq., Appendix, Table S2) and the relaxation of the polymeric chains with transition from a semi-rigid to a flexible state (second term of Peppas-Sahlin Eq., Appendix, Table S2) 184,185. However, in this case the diffusion of the drug was the main mechanism with a small, but not negligible, contribution of the chainrelaxation, as shown by the much higher Fickian diffusion constant compared to the   Discussion  36  relaxation constant in the Eq. (k1 = 65.1–76.8 >> k2 = -11.7 – -16.5, Appendix, Table S2) 184,185. As mentioned above, the levofloxacin release profile is important in relation to the antibacterial activity, since it controls the drug concentrations available at the target tissue. The PLGA microspheres showed a favourable slow and controlled release profile, which would ideally contribute to levofloxacin diffusing through the alveolar-capillary barrier and reaching the bacteria in the alveolar space over a longer time period, DQGPLQLPL]HWKHULVNof systemic side effects. Moreover, prolonged release profiles may improve patience compliance given the reduced frequency of administration needed to have therapeutic concentration at the infected site. However, a critical point is related to the fact that the released levofloxacin at the infected site has to be at concentrations the above-minimum inhibitory concentration (MIC) for the specific bacteria at all time points in order to have a pharmacological effect and reduce the risk of development of antibiotic resistance. Based on the release experiments performed, the concentration of levofloxacin in the epithelial lining fluid of mice upon release from the PLGA microspheres can be predicted to almost instantly be higher than the MIC for P. aeruginosa and S. aureus (4 μg/mL) and for E. coli (0.03 μg/mL, see section 4.4.2). For PLGA microspheres, drug release occurs together with and as a consequence of microsphere degradation. Therefore, by choosing a PLGA with a convenient degradation behaviour, the release profile (mono-, bi-, tri-phasic), rate and the duration could be tuned. Specifically, the degradation rate is faster for PLGA with a low molecular weight 183, amorphous nature, with more hydrophilic end groups (e.g., carboxylic acid 186), and with higher content of the glycolic acid, as shorter polymer chains require less time to degrade and as hydrophilicity of the matrix increases water absorption, and thus degradation and release rates 137,142. Based on these considerations, PLGA with PLA:PGA molar ratio of 50:50, molecular weight of 7-17 kDa and carboxylic end groups was chosen. Importantly, the polymer composition (i.e., molar ratio of PLA to PGA), molecular mass, end functionalities and polymer crystallinity are only some of the factors that control the release profile. Others are associated to the encapsulated drug (e.g., nature of the drug 187, the initial drug loading 188, the location of the drug in the microspheres), to the physico-chemical properties of the microspheres (e.g., VL]HSRO\GLVSHUVLW\porosity of the microspheres) and to in vitro conditions (e.g., temperature, stirring, pH of the release medium, composition of the release medium) 185. In an in vivo environment, the HIIHFWRIHQ]ymes, lipids and immune system should also be considered 185. PLGA degradation mechanism is well-known and occurs via hydrolysis: Water molecules first penetrate into the polymeric matrix causing some swelling, then break the ester bonds in the PLGA backbone. This results in the formation of oligomers (i.e., PLA, PGA) and monomers (i.e., LA, GA), which gradually dissolve in the release medium in a process known as erosion (i.e., mass loss 143,185) of the polymer matrix until complete polymer degradation. This creates internal water-filled pores available for drug diffusion out of the   Discussion  37  particles. Importantly, the degradation products contain acidic groups, which themselves catalyse the hydrolysis, and thus, increase the formation of pores 142,143,185. To confirm and validate the diffusion-based release and the associated microsphere degradation pattern, changes in the surface morphology and internal structure of the polymeric matrix were investigated over time by using a focused ion beam (FIB)-SEM. During the first days (Figure 10, day 1-2), the microspheres showed a partially collapsed spherical shape (appearing as “deflated balls”) with small pores on the surface and in the internal matrix. 7KHSRUHVJUHZLQVL]HDQGHYHQWXDOO\PHUJHGZLWKQHDUSRUHVWRIRUPODUJHUpores and opening channels towards the surface (Figure 10, day 5 vs day 1-2), while more prominent wrinkles appeared on the surface (Appendix, Figure S3, up to day 21). In agreement with previous work on PLGA films 189,190 and the above mentioned PLGA degradation mechanism, the polymer chains hydrolysis caused the formation of degradation products that contributed to the process via autocatalysis. Faster autocatalysis is expected in the matrix compared to the surface, as the molecules formed are kept locally in the bulk and were QRWQHXWUDOL]HGand removed by the buffer solution (as it may have happened for the monomers produced on the microspheres surface) 143.   Figure 10. SEM images of F2 PLGA microspheres and their cross section during degradation.  Scale bars: 5 μm.   Discussion  38  4.4.2 Antibacterial Activity The antibacterial activity of the levofloxacin-loaded microspheres was evaluated using the broth microdilution method, which is a standard assay for determination of MIC. The assay was performed on S. aureus, a Gram-positive, as well as E. coli, P. aeruginosa, which are Gram-negative bacteria. The chosen pathogens are relevant for the tested indications as they frequently are associated to nosocomial infections in e.g., the respiratory tract. The non-loaded PLGA microspheres did not display any antibacterial activity against the tested bacteria, in agreement with previous studies where no bacterial growth inhibition was observed at concentration below 1 mg/mL 191. The MICs of non-formulated levofloxacin were 0.03 μg/mL, 1 μg/mL, and 0.5 μg/mL for E. coli, P. aeruginosa, S. aureus, respectively (Appendix, Table 2), in good agreement with previous studies for E.coli 192 and P. aeruginosa 193, and in the middle of the quoted MICs (0.15 to 4 μg/mL) for S. aureus 193,194. The three PLGA microspheres containing levofloxacin showed MIC values of 0.25-1 μg/mL for E. coli, 4 μg/mL for P. aeruginosa, and 2-4 μg/mL for S. aureus. This shows that encapsulating LVX in PLGA microspheres reduces the antimicrobial activity of levofloxacin by a factor of two to five. This decrease may be due to the slow and controlled drug release profile for the PLGA microspheres, which results in progressively release of levofloxacin from the microspheres, as previously shown for PLGA nanoparticles loaded with netilmicin 195 and ciprofloxacin 196. Despite the decrease of antibacterial activity of levofloxacin upon encapsulation in the microspheres, the dose of levofloxacin delivered is expected to have a bactericidal effect in mice. Levofloxacin, in fact, has shown bactericidal effect against P. aeruginosa and S. pneumoniae at concentrations 1-4× the respective MICs 197–200. As alveolar lining volumes of mice are not available in literature, based on body weight-dependant calculation from a 70 kg human who has ~36 mL of alveolar lining fluid 15, a 25 g mouse may have a volume of ~13 μL of alveolar lining fluid. The levofloxacin delivered and released already after 1 h from the 3 mg dose of PLGA microspheres (i.e., ~22 μg) is ~400× higher than the amount needed to reach the MIC values in the alveolar lining fluid in mice (i.e., 0.051 μg). In humans, however, the amount of levofloxacin needed to have a concentration higher than MIC in the alveolar lining fluid (i.e., 145 μg) is theoretically only reached after four days, VKRZLQJ WKDW RSWLPL]DWLRQ RI WKH IRUPXODWLRQ or higher doses are needed to have a pharmacological effect.  4.4.3 Cytotoxicity and Hemocompatibility A desired characteristic of a drug delivery system is low toxicity of not only the carrier but also the drug-loaded delivery system against host eukaryotic cells. The PLGA microspheres were tested for cytotoxicity to endothelial (HUVEC), epithelial (alveolar lung epithelial A549 and human lung epithelial H1299), and red blood cells, as they were designed for intravenous administration and HPEROL]DWLRQ in the lung capillaries. Cell toxicity was   Discussion  39  evaluated by assessing the function of metabolic activity in mitochondria by measuring the potential of living cells to reduce the colorless 3-(4,5-GLPHWK\OWKLD]RO-2-yl)-2,5-GLSKHQ\OWHWUD]ROLXPEURPLGHMTT) into a EOXHIRUPD]an. The non-loaded and levofloxacin-loaded PLGA microspheres showed similar reduction in cell viability of the tested endothelial and epithelial cell lines (Figure 11A-C). This could be related to the relatively low levofloxacin loading shown for all the formulations, and thus no additional effect on the cytotoxicity, as previously shown for levofloxacin -loaded PLGA nanoparticles on bronchial Calu-3 cells 109. The PLGA microspheres showed IC50 values of 1.4–2 mg/mL for HUVEC, 2.5–3.5 mg/mL for A549 and 0.7–1.5 mg/mL for H1299. These values correspond to concentrations of levofloxacin of 0.03–0.08 mg/mL for HUVEC, 0.06–0.09 mg/mL for A549, and 0.01–0.04 mg/mL for H1299. The observed decrease in cell viability was concluded to be associated to the high concentration of PLGA microspheres used in the experiments and not to the initial fast release of levofloxacin, as the levofloxacin-loaded PLGA microspheres showed higher reduction in cell viability compared to non-encapsulated levofloxacin at the corresponding levofloxacin concentrations (Appendix, Figure S5).  Figure 11. Cytotoxicity and hemocompatibility of PLGA microspheres. (A,B,C) Cell viability (%) of A549, H1299 and HUVEC cells incubated for 24 h with F0 ( ),F0i ( ), DTPA-F0i ( ), F1 ( ), F2 ( ), F3 ( ) and hemihydrate ( ). The dotted line represents 50% viability. Data are presented as mean ± SD, N = 3, except for F0i and DTPA-F0i where N = 1.  (D) Representative phase contrast images of HUVEC cells taken using the IncuCyte® Live-Cell Analysis System at 0, 12, 24 h after treatment with media (control, top) or F0 at 0.63 mg/mL PLGA concentration (concentration indicated with the pink arrow in Figure C) (bottom). Scale bars: 100 μm. (E) Hemolysis (%) of human red blood cells caused by free LVX, non-loaded PLGA MS and LVX-loaded PLGA MS were assessed after 1 h incubation at different concentrations. Data are presented as mean ± SD (N = 1, n = 3).   Discussion  40  Despite the cytotoxicity shown at high PLGA concentrations, 3 mg was chosen as the dose for in vivo experiments to control if the minimum PLGA dose needed to reach a therapeutic level of levofloxacin in the alveolar lining fluid, i.e., above MIC in humans (i.e., > 2.8 mg of PLGA MS) could still be tolerated in an in vivo setting using mice. This might be expected as PLGA is a biocompatible polymer, US FDA approved for clinical administration at concentrations up to 30 mg/mL for intramuscular injection. Also, differences between in vitro and in vivo cytotoxicity results are in fact attributed to the higher susceptibility of cells in the well-controlled and confined, but also more simplistic in vitro environment.  Inverted phase microscopy images confirmed decreased cell viability upon exposure to PLGA microspheres and non-encapsulated levofloxacin. The representative images showed morphological changes of HUVEC cells exposed to PLGA microspheres for 24 h, when the cells appeared more round and smaller than the control. As expected, cells exposed to PLGA microspheres at a concentration of 0.6 mg/mL, showed normal cell morphology  (Figure 11D).  The hemocompatibility (i.e., blood compatibility), is another important evaluation that has to be done with drug delivery systems intended for intravenous administration, as interaction and destruction of blood components result in cellular and humoral reactions that can lead to unwanted inflammation, formation of thrombus 201, and/or clearance of the drug delivery system from the bloodstream and consequent reduction of drug delivery to the target site, and, thus, reduction of the efficacy of the treatment 202,203. Determination of hemocompatibility includes the assessment of hemolysis,  protein adsorption on the surface of the particulates, thrombogenicity and complement activation 201. The ideal intravenous carrier should be non-hemolytic, non-thrombogenic, non-complement activating and invisible to the immune system. The hemolysis (i.e., red blood cells lysis), and subsequent leakage of free haemoglobin was assessed to estimate the toxicity of the PLGA microspheres to human red blood cells. The non-loaded and LVX-loaded PLGA microspheres were non-hemolytic, meaning that all the PLGA microsphere concentrations lysed less than 2% of the red blood cells, the permitted level in hemolysis assessment 201. In fact, after 1 h incubation with erythrocytes, minimal hemolysis (i.e., under 0.4%) was induced for all formulations at concentrations up to 6 mg/mL corresponding to 3× the dose chosen for the in vivo experiments (Figure 11E). This is in agreement with previous studies on PLGA nanoparticles, where no hemolysis was observed at PLGA concentrations up to 3 mg/mL 204 or 10 mg/mL 205. Overall, the PLGA microspheres could be safely administered in vivo at a dose of 3 mg/mouse, corresponding to ~300,000 PLGA microspheres, without concerns on animal toxicity.    Discussion  41  4.5 In Vivo Characterization  4.5.1 Microspheres Degradation  The microsphere degradation kinetics was assessed in vivo by visual investigation of their presence in the lungs capillaries of mice at different time points, after imaging 30-μm thick coronal sections. The microspheres were entrapped in the lung capillaries immediately after intravenous injection (Figure 12) and stayed in the lungs for one more week (Figure 12C). Overall, the degradation kinetic was faster in in vivo than in vitro (where it took 3 weeks for the microspheres to completely disintegrate (Appendix, Figure S3)). This was in agreement with the literature, as PLGA microspheres were shown to degrade 1.7-2.6 times faster in vivo than in vitro, independently of the polymer end group or molecular weight, probably due to the SODVWLFL]HUHIIHFWof lipids and biological compounds in vivo, resulting in higher water uptake 206. However, it was not possible to show the microstructure of the microspheres (e.g., surface wrinkling) in the lung tissue. This may be due to the relatively low resolution of optical microscopy (compared to the electron microscopy) and probably also influenced by interference with surrounding tissue.   Figure 12. Representative histology images over time of lungs exposed to PLGA microspheres (F0) after IV injection. H&E stained histological 30-μm coronal lungs sections after different treatments (1st-3rd rows), and processed images after hematoxylin extraction (4th row, see Appendix, Figure S4 for full-VL]HGLPDJHV(A) Healthy and untreated lungs. (B,C,D) Healthy lungs after 1 h, 1 week and 2 weeks of injection of 3 mg/mouse PLGA MS (corresponding to ~300,000 PLGA MS/mouse). Scale bars: 1st row: 5 mm, 2nd row: 200 μm, 3rd and 4th row: 50 μm.   Discussion  42  4.5.2 Lung Targeting after Intravenous Administration The PLGA microspheres were further investigated for lung targeting, after being radiolabelled with 111Indium (111In).  Microspheres with a radioactive tag were prepared in order to track them after systemic administration using the nuclear imaging technique SPECT/CT. To radiolabel the microspheres, PEGylated PLGA microspheres were firstly prepared by adding PLGA-PEG-NH2 to the PLGA in the DP to VXUIDFH IXQFWLRQDOL]H the resulting particles with amino groups (1, i.e., F0i PLGA MS, Figure 13A). The PEGylated PLGA microspheres were then reacted with S-2-(4-LVRWKLRF\DQDWREHQ]\O-diethylenetriamine pentaacetic acid (p-SCN-Bn-DTPA), resulting in DTPA-F0i PLGA MS (2, Figure 13A), and finally radiolabelled with 111In, yielding the final product 111In-DTPA-F0i PLGA MS (3, Figure 13A). 111In was chosen amongst the gamma emitter radioisotopes due to its relatively long half-life of 2.8 days, which allows imaging for up to 10 days, and due to its ability of forming very thermodynamically stable bonds with DTPA 207.  Figure 13. Synthesis of radiolabeled PLGA microspheres. (A) Synthesis of 111In-DTPA-F0i PLGA MS. (B) Radiolabeling efficiency and stability over 24 h by using ITLC.   The final polymeric composition (i.e., PLGA-PEG-NH2:PLGA at 50:50 weight ratio) used to prepare the F0i PLGA microspheres was based on the RSWLPL]DWLRQ process, in which microspheres were prepared at different weight ratios between PLGA-PEG-NH2 and PLGA   Discussion  43  (i.e., 5:95, 10:90, 25:75, 50:50, 100:0 (i.e., 100% PLGA-PEG-NH2)) in the DP, and FKDUDFWHUL]HG in terms of radiolabelling efficiency and radiochemical purity. The radiolabelling efficiency (i.e., the ratio between the activity measured on the 111In-DTPA-F0i PLGA MS and the initial activity) gradually increased from ~2% to ~8% proportionally to the PEGylated content in the DP mixture. These relatively low values of labelling efficiency could be associated to the presence of only few amino groups on the microspheres surface decreasing their overall reactivity with DTPA, and thus the chelation of 111In. Since PLGA-PEG-NH2 was added to the reaction mixture during the formation of microspheres, and not DVVXUIDFHIXQFWLRQDOL]DWLRQRISUHIRUPHGPLFURSDUWLFOHV LWPD\KDYHGLVWULEXWHGbetween the bulk and the surface of the microspheres 208,209. This would align with the low radiolabeling efficiency shown by the microspheres made of 100% PLGA-PEG-NH2. However, due to the emulsion-based preparation method, the preferred orientation of the PEG moieties in theory remains towards the aqueous phase with the more hydrophobic part of the copolymer PLGA in the PLGA matrix, resulting in the formation of PEG enriched surfaces and less PEG inside the polymeric matrix 208,209. The resulting 111In labelling was found sufficient for the planned in vivo studies, but further investigations on this matter could be supplemented by studying the VXUIDFHIXQFWLRQDOL]DWLRQZLWK3(*DQGWKHQZLWKDTPA by FT-IR and NMR with peaks and absorbance bands of the introduced chemical groups (e.g., introduction of new ether functions from the PEG fragments PEG-NH2, DTPA, PLGA-COOH), as previously shown 210. The radiochemical purity was tested using instant thin-layer chromatography (ITLC) after incubation of the 111In-DTPA-PLGA MS with a strong chelator for 111In (i.e., ethylenediaminetetraacetic acid (EDTA)). From the integration of the ITLC peaks, all the F0i microspheres prepared showed high radiochemical purity, as only a small (<5%) percentage of 111In migrated to the solvent front as 111In-EDTA complex (retention factor, Rf = 1), while the 111In-DTPA-PLGA MS remained at the origin (Rf = 0). Despite the low radiolabelling efficiency, the high radiochemical purity proved that111In was strongly chelated by DTPA, making the formulations ready for the in vivo studies. The best in vitro performance was observed for 111In-DTPA-PLGA MS made of polymeric blend containing 50% PEGylated PLGA, as they showed the highest labelling efficiency (i.e, ~5%), and thus highest activity linked to the microspheres, as well as excellent radiochemical purity (i.e., 97%) and 24 h stability after EDTA challenge (Figure 13B). Moreover, the modification in terms of partly exchanging PLGA with PEGylated PLGA did not drastically change the composition of matter of the microspheres.  Importantly, the radiolabelling reaction did not alter the physicochemical properties of the )L3/*$PLFURVSKHUHVZLWKDQGZLWKRXW'73$DVWKHSDUWLFOHVL]HDQGVL]HGLVWULEXWLRQ(Table 7), the shape and surface morphology (Appendix, Figure S1) and the cytotoxicity (Figure 11C,E) did not differ significantly from F0 PLGA microspheres. This is critical for the in vivo performance as the microspheres biodistribution can be affected by their VL]HDQGshape. However, F0i PLGA microspheres showed a faster degradation (complete degradation within ten days, compared to the F0 PLGA ones (Appendix, Figure S3). This   Discussion  44  was not surprising as it may be explained by the increased hydrophilicity due to PEG chains and, thus, their faster hydrolysis.  With the intent of performing dual-radioisotope imaging to assess the passive lung targeting of the microspheres, their ability to deliver and release levofloxacin with resulting appropriate pharmacokinetics, a cold (i.e., not radioactive) copper (II) (Cu) complex with LVX and the N-donor heterocyclic ligand phenanthroline was synthesised (Figure 14A), as previously described 211. The mass spectrum showed the parent ion of [Cu(levofloxacin)(phenanthroline)]+ ternary complex (m/z = 605)  together with the mono(levofloxacin)Cu(II) complex ([Cu(levofloxacin)]+, m/z = 425) (Figure 14B). However, since the ternary complex had a very low solubility (< ng/mL) in organic solvents such as dichloromethane or chloroform, due to its positive charge, it was not possible to load it into the PLGA microspheres with the microfluidic O/W single emulsion method and use it in the in vivo evaluation.  Figure 14. The Cu(II) levofloxacin complex. (A) Chemical structure of the cationic ternary complex [Cu(levofloxacin)(phenanthroline)(H20)]+. (B) ESI+ MS spectrum. The pharmacokinetics and biodistribution of the non-loaded 111In-DTPA-F0i PLGA MSwere assessed by SPECT/CT imaging in healthy mice. Following intravenous administration, ~85% of the injected PLGA microspheres ended up in the lungs (Figure 15), as a consequence of PLFURHPEROL]LQJ the pulmonary capillaries with particles larger than the diameter of the lung capillaries (i.e., ~8 um). The microspheres distributed homogenously throughout the left and right lobes (Appendix, Figure 5B) confirming the   Discussion  45  hypothesis that a more homogenous lung distribution can be achieved by intravenous administration of microspheres via passive lung targeting, compared to that obtained by inhalation 119,212,213. TKLV ZDV KLJKO\ GHVLUDEOH DV UHJLRQDO ORFDOL]DWLRQ LQ WKH OXQJVdetermines the clinical efficacy of the treatment.  Figure 15. Pharmacokinetics and biodistribution of radiolabeled PLGA microspheres. Representative maximum intensity projections SPECT/CT overlay images (dorsal view) of healthy C57BL/6 mice showing the in vivo distribution of 111In-DTPA-F0i PLGA microspheres after intravenous tail injection over time. The radioactivity is shown in blue. ID: injected dose. However, ~ 4% of the injected radioactivity was immediately excreted via the urinary pathway (Figure 15, day 0). This could be due to the detachment and elimination of the 111In-DTPA complex alone or linked to the PEG chains in lieu of the release of non-complexed 111In. The high values of the in vitro radiochemical purity and stability of 111In-DTPA after EDTA challenge (Figure 13B) favour this hypothesis. The activity in the bladder at day 0 is not likely related to un-chelated 111In, as intravenously administered 111InCl3 distributed mainly in the kidneys (~ 20% of the injected activity) and, in less extent, in the bladder (~2%) (Appendix, Figure S7).  The pharmacokinetics of the PLGA microspheres showed that they were slowly cleared from the lungs with only ~ 5% of the injected dose three days after administration, and 0.1% of the initial dose after seven days (Appendix, Figure 5C). After ten days, 0.02% of the injected activity still present in the mice was distributed to the lungs and the excretory organs, such as liver and kidneys, and to the spleen (Appendix, Figure 5D,E). Over time, in fact, the microspheres degrade via hydrolysis DQGGHFUHDVHLQSDUWLFOHVL]H. When they UHDFKVL]HVVPDOOHUWKDQaXPWKH\can traverse the pulmonary arteriolar capillary bed, be transported away and accumulate in the liver and spleen, as shown for macroaggregate albumin particles and polystyrene microparticles 131,214. The detachment of groups present on the surface, including PLGA and/or PLGA-PEG-NH2 fragments conjugated to 111In-  Discussion  46  DTPA is also possible. At the same time, the 111In-DTPA complex undergoes strong competition in vivo by the serum proteins (e.g., transferrin), which frequently causes demetalation or transchelation of 111In  over prolonged exposure 215–218. This could explain some of the isotope activity found in the kidneys, liver, spleen, and bones 2-3 days post injection, as it follows the same distribution profile of intravenously administered 111InCl3 (Appendix, Figure S6A), as previously shown 219.  From these findings, the F0i PLGA MS seemed to be cleared to a larger extent from the lungs, i.e., ~three days post injection (Figure 15, and Appendix, Figure 5C) following a faster kinetic rate compared to the degradation rate shown in vitro (Appendix, Figure S3). This behaviour is in line with previous studies showing 1.7-2.6 times faster degradation kinetics of PLGA microspheres in vivo compared to the one in vitro,  which was K\SRWKHVL]HG WREHUHODWHG WRWKH in vivo SODVWLFL]LQJHIIHFW of lipids and other biological compounds, resulting in higher water uptake in the polymer 206. However, further investigation is needed to confirm that the decline of radioactivity in the lungs after three days is indeed related to complete degradation of the microspheres, and not to the detachment of surface radiolabelled functionalities, giving a false-positive result. Assuming microspheres clearance from the lungs happens within three days, prediction on the possibility of repeated administration can be made. The safety of repeated administration potentially necessary during serious lung infection therapy depends on several factors, including the total microspheres dose (i.e., the number of microspheres injected, which relates to the numbers of capillaries clogged) and the degradation time (based on the polymer used DQG SDUWLFOH VL]H (with longer degradation time for larger particles, as previously seen for albumin macroaggregates 214). Importantly, the tested dose of 3 mg PLGA microspheres did not cause measurable alteration of pulmonary function, as the mice respiratory rate monitored during the entire in vivo experiments showed normal values. As mentioned above, a healthy adult has 280 billion pulmonary capillaries 122,123, and occluding a few million of them (e.g., 3,000,000, corresponding to 10 repeated doses of 300,000 particles each) will thus only influence <0.1% of the capillaries. The repetition interval depends on the degradation rate and the dose, which in turn depends on its antibacterial efficacy. Ideally, the second and subsequent doses should be administered to maintain the antibiotic concentration at the site of infection above the MIC, i.e., administered at a time point when the previous injected microspheres have almost completely been cleared from the lungs. With the results shown in this work, we assume that 3 mg PLGA microspheres are enough to deliver a therapeutic dose of levofloxacin for the desired pharmacological effect for three days (see section 4.4.3). Thus, it is expected that several administrations in series would be possible, as it likely would be needed for the treatment of lung infections and chronic pathologies (e.g., cystic fibrosis) where periodic administrations of antibiotics are currently prescribed.     Conclusions and Future Perspectives  47  5 Conclusions and Future Perspectives   In this work, a wide range of methods was combined to developFKDUDFWHUL]HERWKin vitro and in vivo, and gain a comprehensive understanding of levofloxacin-loaded PLGA microspheres to selectively target the alveolar capillaries of infected lungs after intravenous administration.  The PLGA microspheres KDGDPRQRGLVSHUVHVL]HGLVWULEXWLRQ with a mean diameter of 12 μm. They degraded in vitro in three weeks while showing surface and internal morphological changes, and released the encapsulated levofloxacin in a controlled manner over five days. While the levofloxacin loading of the PLGA microspheres was relatively low, it still had antimicrobial activity against selected Gram-positive and Gram-negative bacteria. The microspheres showed compatibility with endothelial, alveolar epithelial and red blood cells. After tail vein injection in mice, the 111In-radiolabeled PLGA microspheres distributed preferentially and homogenously throughout the lung capillaries. The microspheres were retained in the lungs for up to one week while slowly degrading and the degradation products being eliminated through the urinary pathway.  Overall, the results support the safe use of monoGLVSHUVH VL]H-distributed polymeric microspheres to target the lungs from the vascular side, and potentially deliver drugs to treat lung diseases via passive lung targeting. It should be noted that the levofloxacin-loaded PLGA microspheres are not directly translatable to human, and, thus, have to be further RSWLPL]HGWRprovide WKHUDSHXWLFOHYHOVRIDQWLELRWLFVLQWKHKXPDQUHVSLUDWRU\]RQH The flow-focusing microfluidic chip was shown to be a very good platform to prepare PRQRGLVSHUVHPLFURVSKHUHVZKLOHFRQWUROOLQJWKHLUVL]HDQGVL]HGLVWULEXWLRQ+RZHYHUfor this strategy to be truly useful, some limitation and drawbacks should be solved/improved to have its application in larger scales, including clinical translation. The clogging of the PLFURVL]HGFKDQQHOVDQGRURULILFHGXHWRWKHSUHVHQFHRIFRQWDPLQDWLRQRUSUHFLSLWDWLRQRIthe materials in the fluid, as well as wettability and hydrophobicity of the channels, due to prolonged exposure and flow of organic solvent should be taken in consideration. To limit the number of interruptions in the continuous process, these problems should be solved, for example by using chips made of cheaper materials that could be discarded as soon as the issues arise.  Importantly, the material has to be compatible with the organic solvents used for the droplet formation. The glass chip has shown to be resistant to many harsh solvents, but is associated to high fabrication costs, and thus, ideally not replaceable in case of orifice clogging or reduction of channel hydrophilicity. Cheaper materials such as polymers (e.g., polydimthylsiloxane, thiol-ene) and milder solvents could be therefore considered to make the process more flexible. Another limitation of droplet-based microfluidics is the relatively limited/low productivity of a single chip (~10 mg/h, with the used conditions), which makes the production and collection of suitable amounts of microspheres tedious and lasts for many hours (in some cases). As a consequence, the DP and CP reservoirs (usually of ~2 mL and 10 mL, respectively) in this case had to be refilled halfway or during the experiment,   Conclusions and Future Perspectives  48  interrupting the continuous and stable flow, and requiring its re-equilibration, which may introduce variability in the sample. Several microfluidic chips can be connected in parallel to scale up the productivity, while larger syringe pumps (for the syringe pumps) or reservoirs (for the pressure-driven pumps) should be used in order to limit the refilling.  Drug content is one of the most important factors to confirm the success of a drug delivery system. Especially in the case of antibiotics treatments, the microspheres need to have sufficiently high drug content and preferably also high encapsulation efficiency. Here, the PLGA microspheres showed many good properties (e.g., biocompatibility, sustained release without a significant burst effect, and slow degradation within one week), but also poor loading with amphiphilic drugs. This disadvantage may indeed limit their use in clinical trials. )XUWKHURSWLPL]DWLRQRIWKHOHYRIOR[DFLQ-loaded PLGA microspheres is thus needed to increase the drug loading and obtain improved in vivo performance in the treatment of lung infections. The aim should be to have a high drug concentration at the site of infection and reduced dose of microspheres administered, as well as PLQLPL]H the drug waste during the preparation process. Several strategies should be considered, including the modification of the preparation method (from O/W emulsification to non-aqueous emulsification methods, such as W/O/O or solid (S)/O/O), modification of the polymer to a less hydrophobic material HJE\IXQFWLRQDOL]ing the entire surface with PEG molecules), or replacement of the amphiphilic drug with a more hydrophobic, preferably more potent molecule. However, these RSWLPL]DWLRQ steps should take the microfluidic approach into account as it is important to have microspheres with uniform and appropriate VL]Hdistribution for a safe passive lung targeting strategy. To confirm the sustained release of the drug observed in vitro and to prove the ability of the drug to diffuse through the alveolar-FDSLOODU\ PHPEUDQH DQG ORFDOL]H LQ WKH DOYHROLbiodistribution and pharmacokinetics studies for the drug should be performed with the PLGA microspheres encapsulating levofloxacin, preferably in both healthy and infected animals in which efficacy study could also be performed. Dual SPECT/CT imaging could for example be done after labeling the polymer and the drug with two different gamma-emitter radioisotopes (e.g., 111In, 67Ga, 67Cu) so that both the drug and the carrier could be followed over time in the same animal. To solve the solubility problems experienced with the ternary complex of levofloxacin during the radiolabeling procedure, monoanionic ligand (e.g., maltol, a natural FDA approval molecule) could be used.  Possible toxic effects should also be further evaluated. In vivo pulmonary toxicity studies should be performed in addition to the microscopic examination of lung tissue already performed to prove the absence of pathologic changes. Determining the minimum toxic dose and the minimum lethal dose not only in healthy, but also in infected animals is important to fully define the biocompatibility. Additionally, the effect of repeated administration should be investigated to define the possible treatment regime and maximum possible doses without toxicity. Repeated administrations can be for example tested with microspheres radiolabeled with different radioisotopes, which allows for observation of possible   Conclusions and Future Perspectives  49  differences in microspheres distribution in the lungs as well as in degradation rates between the different injections. Finally, to provide the evidence of the therapeutic benefit of the drug delivery system for the treatment of pulmonary infections, efficacy studies in established S. pneumoniae, P. aeruginosa and/or S. aureus infected in vivo models should be performed. Ideally, susceptible and resistant strains (e.g., penicillin-susceptible S. pneumoniae and penicillin-resistant S. pneumoniae) should be used to investigate its applications in case of antibiotic-resistance pathogens. Further proof of the drug delivery efficacy via passive lung targeting could be obtained by comparing it with current treatments (i.e., oral, intravenous and pulmonary administration of levofloxacin) as well as by combining it with inhaled microparticles encapsulating levofloxacin.        References  50  6 References (1)  Troeger, C.; Blacker, B.; Khalil, I. A.; Rao, P. C.; Cao, J.; Zimsen, S. R. M.; Albertson, S. B.; Deshpande, A.; Farag, T.; Abebe, Z.; et al. 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(196)  Dillen, K.; Vandervoort, J.; Van Den Mooter, G.; Verheyden, L.; Ludwig, A. Factorial Design, Physicochemical Characterisation and Activity of Ciprofloxacin-PLGA Nanoparticles. Int. J. Pharm. 2004, 275, 171–187. (197)  Nemeth, J.; Oesch, G.; Kuster, S. P. Bacteriostatic versus Bactericidal Antibiotics for Patients with Serious Bacterial Infections: Systematic Review and Meta-Analysis. J. Antimicrob. Chemother. 2015, 70, 382–395. (198)  .OHSVHU 0 ( (UQVW ( - 3HW]ROG & 5 5KRPEHUJ 3 'RHUQ * 9 &RPSDUDWLYHBactericidal Activities of Ciprofloxacin, Clinafloxacin, Grepafloxacin, Levofloxacin, Moxifloxacin, and Trovafloxacin against Streptococcus Pneumoniae in a Dynamic in Vitro Model. Antimicrob. Agents Chemother. 2001, 45, 673–678. (199)  Golini, G.; Favari, F.; Marchetti, F.; Fontana, R. Bacteriostatic and Bactericidal Activity of Levofloxacin against Clinical Isolates from Cystic Fibrosis Patients. Eur. J. Clin. Microbiol. Infect. Dis. 2004, 23, 798–800. (200)  Tasso, L.; De Andrade, C.; Dalla Costa, T. Pharmacokinetic/Pharmacodynamic Modelling of the Bactericidal Activity of Free Lung Concentrations of Levofloxacin and Gatifloxacin against Streptococcus Pneumoniae. Int. J. Antimicrob. Agents 2011, 38, 307–313. (201)  Weber, M.; Steinle, H.; Golombek, S.; Hann, L.; Schlensak, C.; Wendel, H. P.; Avci-Adali, M. Blood-Contacting Biomaterials: In Vitro Evaluation of the Hemocompatibility. Front.   References  63  Bioeng. Biotechnol. 2018, 6, 99. (202)  Dobrovolskaia, M. A.; McNeil, S. E. Immunological Properties of Engineered Nanomaterials: An Introduction. Handb. Immunol. Prop. Eng. Nanomater. Second Ed. 2016, 1, 1–24. (203)  Dobrovolskaia, M. A.; Aggarwal, P.; Hall, J. B.; McNeil, S. E. Preclinical Studies to Understand Nanoparticle Interaction with the Immune System and Its Potential Effects on Nanoparticle Biodistribution. Mol. Pharm. 2008, 5, 487–495. (204)  Fornaguera, C.; Calderó, G.; Mitjans, M.; Vinardell, M. P.; Solans, C.; Vauthier, C. Interactions of PLGA Nanoparticles with Blood Components: Protein Adsorption, Coagulation, Activation of the Complement System and Hemolysis Studies. Nanoscale 2015, 7, 6045–6058. (205)  7KDVQHHP<06DMHHVK66KDUPD&3(IIHFWRI7KLRO)XQFWLRQDOL]DWLRQRQWKH+HPR-Compatibility of PLGA Nanoparticles. J. Biomed. Mater. Res. - Part A 2011, 99 A, 607–617. (206)  7UDF\0$:DUG./)LURX]DEDGLDQ/:DQJ<'RQJ14LDn, R.; Zhang, Y. Factors Affecting the Degradation Rate of Poly(Lactide-Co-Glycolide) Microspheres In Vivo and In Vitro. Biomaterials 1999, 20, 1057–1062. (207)  Wadas, T. J.; Wong, E. H.; Weisman, G. R.; Anderson, C. J. Coordinating Radiometals of Copper, Gallium, Indium, Yttrium, and Zirconium for PET and SPECT Imaging of Disease. Chem. Rev. 2010, 110, 2858–2902. (208)  Wattendorf, U.; Merkle, H. P. PEGylation as a Tool for the Biomedical Engineering of Surface Modified Microparticles. J. Pharm. Sci. 2008, 97, 4655–4669. (209)  2ZHQV ' ( 3HSSDV 1 $ 2SVRQL]DWLRQ %LRGLVWULEXWLRQ DQG 3KDUPDFRNLQHWLFV RIPolymeric Nanoparticles. Int. J. Pharm. 2006, 307, 93–102. (210)  Madani, F.; Bessodes, M.; Lakrouf, A.; Vauthier, C.; Scherman, D.; Chaumeil, J. C. 3(*\ODWLRQ RI0LFURVSKHUHV IRU 7KHUDSHXWLF (PEROL]DWLRQ 3UHSDUDWLRQ &KDUDFWHUL]DWLRQand Biological Performance Evaluation. Biomaterials 2007, 28, 1198–1208. (211)  Sousa, I.; Claro, V.; Pereira, J. L.; Amaral, A. L.; Cunha-Silva, L.; De Castro, B.; Feio, M. - 3HUHLUD ( *DPHLUR 3 6\QWKHVLV &KDUDFWHUL]DWLRQ DQG $QWLEDFWHULDO 6WXGLHV RI DCopper(II) Levofloxacin Ternary Complex. J. Inorg. Biochem. 2012, 110, 64–71. (212)  7KDNXU $ 5RGUtJXH]-5RGUtJXH] & 6DDWFKL . 5RVH ) (VSRVLWR 7 1RVUati, Z.; Andersen, P.; Christensen, D.; Häfeli, U. O.; Foged, C. Dual-Isotope SPECT/CT Imaging of the Tuberculosis Subunit Vaccine H56/CAF01: Induction of Strong Systemic and Mucosal IgA and T-Cell Responses in Mice upon Subcutaneous Prime and Intrapulmonary Boost ,PPXQL]DWLRQFront. Immunol. 2018, 9, 2825. (213)  Yang, L.; Gradl, R.; Dierolf, M.; Möller, W.; Kutschke, D.; Feuchtinger, A.; Hehn, L.; Donnelley, M.; Günther, B.; Achterhold, K.; et al. Multimodal Precision Imaging of Pulmonary Nanoparticle Delivery in Mice: Dynamics of Application, Spatial Distribution, and Dosimetry. Small 2019, 15. (214)  Taplin, V. G.; MacDonald, N. S. Radiochemistry of Macroaggregated Albumin and Newer Lung Scanning Agents. Semin. Nucl. Med. 1971, 1, 132–152. (215)  Sun, H.; Li, H.; Sadler, P. J. Transferrin as a Metal Ion Mediator. Chem. Rev. 1999, 99, 2817–2842. (216)  Price, E. W.; Cawthray, J. F.; Bailey, G. A.; Ferreira, C. L.; Boros, E.; Adam, M. J.; Orvig,   References  64  C. H 4octapa: An Acyclic Chelator for 111In Radiopharmaceuticals. J. Am. Chem. Soc. 2012, 134, 8670–8683. (217)  Price, E. W.; Orvig, C. Matching Chelators to Radiometals for Radiopharmaceuticals. Chem. Soc. Rev. 2014, 43, 260–290. (218)  Harris, W. R.; Chen, Y.; Wein, K. Equilibrium Constants for the Binding of Indium(III) to Human Serum Transferrin. Inorg. Chem. 1994, 33, 4991–4998. (219)  Ando, A.; Ando, I.; Hiraki, T.; Hisada, K. Relation between the Location of Elements in the Periodic Table and Various Organ-Uptake Rates. Int. J. Radiat. Appl. Instrumentation. 1989, 16, 57–80.  Appendix  65  7 Appendix Research manuscript: Agnoletti M, 5RGUÕJXH]-5RGULJXH] C, .áRG]LQVND SN, Esposito TVF, Saatchi K, Mørck Nielsen H, Häfeli UO0RQRVL]HG3RO\PHULF0LFURVSKHUHV IRU3DVVLYH/XQJ7DUJHWLQJBiodistribution and Pharmacokinetics after Intravenous Administration. ACS Nano. 2020, 14, 6693-6706.         Appendix      Monosized Polymeric Microspheres for Passive Lung Targeting:  Biodistribution and Pharmacokinetics after Intravenous Administration Research manuscript   Agnoletti M5RGUÕJXH]-5RGULJXH]&.áRG]LQVND61Esposito TVF, Saatchi K,  Mørck Nielsen H, Häfeli UO ACS Nano, 2020, 14, 6693-6706   Monosized Polymeric Microspheres Designedfor Passive Lung Targeting: Biodistributionand Pharmacokinetics after IntravenousAdministrationMonica Agnoletti, Cristina Rodríguez-Rodríguez, Sylvia N. Kłodzinśka, Tullio V. F. Esposito,Katayoun Saatchi, Hanne Mørck Nielsen,* and Urs O. Haf̈eli*Cite This: ACS Nano 2020, 14, 6693−6706 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Local as well as systemic therapy is often used totreat bacterial lung infections. Delivery of antibiotics to thevascular side of infected lung tissue using lung-targetingmicrospheres (MS) is a good alternative to conventionaladministration routes, allowing for localized high levels ofantibiotics. This delivery route can also complement inhaledantibiotic therapy, especially in the case of compromised lungfunction. We prepared and characterized monodisperse poly-(lactic-co-glycolic acid) (PLGA) MS loaded with levofloxacinusing a flow-focusing glass microfluidic chip. In vitro character-ization showed that the encapsulated LVX displayed a biphasiccontrolled release during 5 days and preserved its antibacterialactivity. The MS degradation was investigated in vitro by cross-sectioning the MS using a focused ion beam scanning electron microscope and in vivo by histological examination of lungtissue from mice intravenously administered with the MS. The MS showed changes in the surface morphology and internalmatrix, whereas the degradation in vivo was 3 times faster than that in vitro. No effect on the viability of endothelial and lungepithelial cells or hemolytic activity was observed. To evaluate the pharmacokinetics and biodistribution of the MS, completequantitative imaging of the 111indium-labeled PLGA MS was performed in vivo with single-photon emission computedtomography imaging over 10 days. The PLGA MS distributed homogeneously in the lung capillaries. Overall, intravenousadministration of 12 μm PLGA MS is suitable for passive lung targeting and pulmonary therapy.KEYWORDS: microfluidics, levofloxacin, PLGA, microspheres, passive lung targeting, FIB-SEM, SPECT/CTLower respiratory tract infections, such as pneumonia orbronchiolitis, are some of the leading causes of mortalityand morbidity worldwide. In 2016, 2.4 million deathswere caused by such infections, making them the sixth leadingcause of mortality for all ages and the leading cause of deathamong children under 5 years of age.1 The recommendedtreatment for bacterial infections in the lungs includes oral orintravenous (IV) administration of amoxicillin, tetracyclines,macrolides, respiratory fluoroquinolones, such as levofloxacinand moxifloxacin, or, in the case of pseudomonal infections,administration of antipseudomonal fluoroquinolones, such aslevofloxacin or ciprofloxacin, in combination with an anti-pseudomonal β-lactam antibiotic.2,3 Although fluoroquinoloneshave been used extensively in the last decades, their use hasrecently been associated with long-lasting, disabling, andpotentially permanent side effects, such as tendon inflammationand rupture, depression, as well as problems with memory andsleeping.4,5 This brought the European Medicines Agency andthe United States Food andDrug Administration to restrict theiruse in patients with underlying conditions, which predisposethem to the occurrence of side effects.4,5 Additionally, theuncontrolled use of antibiotics, including fluoroquinolones, inthe last decades has dramatically increased the amount ofReceived: December 12, 2019Accepted: May 11, 2020Published: May 11, 2020Articlewww.acsnano.org© 2020 American Chemical Society6693https://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−6706Downloaded via COPENHAGEN UNIV LIBRARY on July 2, 2020 at 12:49:10 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.antibiotic-resistant pathogens.6 In the clinic, this translates to theneed for larger doses of antibiotics in order to obtain effectivedrug concentrations in the lungs. Such high doses are associatedwith an increase in undesirable side effects.The obstacles encountered during antibiotic treatment havedirected the attention of researchers toward development offormulations that allow targeted delivery of sufficiently highantibiotic doses directly to the infection site. This is particularlyinteresting when considering the treatment of pulmonaryinfections as delivering the antibiotic selectively to the lungswill not only reduce the dose needed to achieve a therapeuticeffect and reduce systemic side effects caused by the antibiotic,but also diminish the risk of development of multi-drug-resistantbacteria. Although guidelines for treating lung infectionscurrently recommend oral and IV administration routes, manylung-targeted inhalation drug delivery systems have beendeveloped and investigated during the last decades. Several drypowder and nebulized suspensions, emulsions, and solutions areapproved (e.g., tobramycin (TOBI Podhaler)7,8 and colistime-thate sodium (Colobreathe)9,10 for Pseudomonas aeruginosainfections) or investigated at preclinical and clinical stages.11,12However, for patients with compromised lung function orrespiratory tract obstruction (e.g., due to inflammation andmucus plugs), the delivery of antibiotics by inhalation is difficultand often not sufficiently effective to eliminate all of thepathogens. For this reason, targeted delivery of drugs to the lungsfrom the vasculature around the thin alveolar epithelium isconsidered to be the most relevant, and using biocompatibledrug carriers with sustained drug release willminimize the dosingfrequency. To entrap the IV-injected particulate carrier system inthe alveolar capillaries, the particles must be sized slightly largerthan the capillary diameter (i.e., 7.5± 2.3 μm in healthy adults,136.6 ± 1.6 and 7.5 ± 1.7 μm in rats and dogs14), leading to thecomplete entrapment in the lungs after being pumped throughthe heart and into the pulmonary blood circulation. This passivelung targeting strategy is currently approved in nuclear medicinefor determining lung perfusion in humans with 99mTc-labeledmacroaggregated albumin (MAA) (Pulmolite, Pharmalucence,MA,USA;DraxImage, Draxis Health,Montreal, Canada), whereonly 0.5−0.7% of healthy lung capillaries are embolized after IVinjection of a standard dose of MAA (i.e., 200,000−700,000MAA particles).15−17 Passive lung targeting was also recentlyinvestigated in preclinical work for the treatment of lung tumors(with cisplatin),18 tuberculosis (with rifampicin),19 andpulmonary infections (with erythromycin,20 azithromycin,21ofloxacin,22 and cefquinome23,24). However, previous works thatevaluated particles for IV treatment of pulmonary infectiondisplayed a particle size distribution in the range of 3−50 μm.Such large particle size distributions are associated with a highrisk of local vascular resistance alterations as not only capillariesbut also arterioles could be clogged. This may result inincomplete dose delivery to the lungs and distribution innontargeted organs, such as liver and kidneys.20−22,24With the hypothesis that a narrow particle size distribution isessential for optimal passive lung targeting, we preparedmicrospheres (MS) loaded with the antibiotic levofloxacin(LVX) using a flow-focusing microfluidic chip. We combined awide range of methods to optimize, characterize, and gain acomprehensive understanding of the physicochemical propertiesof the delivery system. The monodisperse, LVX-loaded poly-(lactic-co-glycolic acid) (PLGA) MS were investigated for invitro drug release, and the associated MS degradation wasexamined by assessing changes in the surface morphology and inthe particle internal structure using the focused ion beamscanning electron microscopy (FIB-SEM) technique. Theantibacterial activity, cytotoxicity, and hemocompatibility wereevaluated, and the PLGA MS were radiolabeled with 111indium(111In) to quantitatively determine lung uptake in mice byimaging the MS in vivo with single-photon emission computedtomography (SPECT/CT) over 10 days and to determine ifsystemically administered drug delivery systems can target thelungs and potentially treat pulmonary infections.RESULTS AND DISCUSSIONPLGA Microspheres Displayed a Mean Diameter of 12μm and a Uniform Size Distribution. Monodisperse PLGAMS with a mean diameter of 12 μm and a coefficient of variation<5.2% were successfully prepared (Table 1 and Figure S1A−D).No significant differences in physicochemical properties wereobserved between the fresh and freeze-dried formulations(Table 1). The high uniformity of particle size was achievedusing flow focusing in the microfluidic chip, producing uniformdroplets (Movie S1), which each turned into a solid and alsouniform microsphere after solvent extraction and evaporation.This exquisite control was made possible by controlling manyexperimental parameters, including flow rates, polymer concen-tration, and composition of the dispersed phase (DP) and thecontinuous phase (CP). More specifically, only few combina-tions of the DP and CP flow rates resulted in the production ofmonodisperse MS, whereas others resulted in polydisperse MS.In general, in the range of tested flow rates Q (QDP = 0.5−5 μL/min and QCP = 30−150 μL/min), we found that particle sizedecreased by increasing the flow rate of the CP and thus byincreasing the flow rate ratio, as previously shown.25 Moreover,polydispersity decreased by increasing the concentration ofPLGA in theDP (0.5−5%w/w), due to the increased viscosity ofthe DP. From the systematic optimization, the optimalTable 1. Particle Size, Drug Content and EncapsulationEfficiency of Fresh and Freeze-Dried Nonloaded andLevofloxacin-Loaded PLGA MicrospheresaaCV, coefficient of variation (calculated based on 500 microspheres);EE, encapsulation efficiency; LVX, levofloxacin, *p < 0.05, calculatedbased on three independent batches (N = 3); ns, not significantlydifferent. For particle size, statistical multiple comparisons between allformulations were done. For LVX loading and EE, multiplecomparisons between fresh and freeze-dried formulations as well asthe different loaded formulations were done. The statistical analysis ofthe EE followed the analysis of the loading.ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066694Figure 1.Degradation of PLGAmicrospheres and associated release of the drug levofloxacin. (A) In vitro cumulative release profiles of LVX fromPLGA MS (F1, fresh and freeze-dried (dark green); F2, fresh and freeze-dried (light green); F3, fresh and freeze-dried (teal)) in phosphatebuffered saline (PBS) and 37 °C, fitted by the Peppas−Sahlin model. Results are expressed as mean± SD (N = 3, n = 1). (B) SEM images of F2PLGAMS and their cross section during degradation. Red arrowheads point to some of the surface pores. Orange arrowheads point to some ofthe internal cavities. Scale bars: 5 μm.ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066695conditions were found to beQDP = 3 μL/min,QCP = 90 μL/min,and 5% (w/v) PLGA concentration in the DP.Controlled particle size and uniform size distribution arecritical for safe passive lung targeting. According to the UnitedStates Pharmacopeia, “>90% of albumin aggregated particlesmust have a size between 10 and 90 μm and no microparticlesmay be larger than 150 μm”.26 With a more monodisperseparticle size distribution, higher selectivity in lung targeting isachieved as measured by the amount of particles reaching thecapillaries, without embolizing larger arterioles or accumulatingin other organs (e.g., liver, kidneys). According to theliterature,18−24 all previous preclinical work was done withparticles with size distributions between 3 and 50 μm thatresulted in high lung uptake and relatively high levels of particlebiodistribution to other organs.All of the LVX-loaded PLGA MS had a spherical shape andsmooth surface, with evidence of small surface cavities (Figure1B, day 0). The nonloaded PLGA MS presented a smooth anduniform surface morphology, as shown in Figure S1E−H. Adifference in surfacemorphology between nonloaded and loadedPLGA particles was also reported by Bragagni et al.,27 whereprilocaine-loaded PLGA MS showed large surface cavities (or apartially collapsed spherical shape). This effect was concluded tobe attributed to the plasticizing effect of the loaded prilocaine.The LVX-loaded PLGAMS formulations F1, F2, and F3 had aLVX content of approximately 2, 4, and 5.5%, respectively(Table 1). Saturating the CP with LVX doubled both the LVXcontent and the encapsulation efficiency (EE) (F1 vs F2),indicating that by reducing the diffusion of the drug from theorganic (i.e., DP) to the water phase (i.e., CP) the drug content ofhighly water-soluble drugs in hydrophobic polymeric particlescan be significantly increased, in agreement with previouswork.28 The LVX content (drug-to-polymer ratio), but not theEE, further significantly increased when the anhydrous form ofLVXwas replaced with the hemihydrate form (F2 vs F3), and thetheoretical drug loading was increased from 15 to 30%. Thehigher drug loading together with an unchanged EEmay indicatesuperior loading of the hemihydrate form but also saturation ofthe loading capacity. The LVX loading in F3 PLGA MS wassatisfactory considering the high water solubility of LVX and thehydrophobic nature of PLGA combined with the oil/water (O/W) single emulsion method of preparation.These findings support previous reports on the importance ofthe method of preparation of polymeric MS and formulationparameters (e.g., polymerMw, composition and concentration ofthe polymer, type and concentration of the emulsifier, watermiscibility of the organic solvent or cosolvents used) for anefficient encapsulation of small hydrophilic and amphiphilicdrugs.29 In line with these reports, increasing the PLGAconcentration from 10 to 20% resulted in an increase ingentamicin loading from 2.7 to 6.2% in PLGAMS prepared withthe water/oil/water (W/O/W) double emulsion method.30Previous work on 2−3 μmPLGA particles loaded with LVX andprepared using W/O/W double emulsion showed a maximumLVX content of 10%.28,31The residual amount of poly(vinyl alcohol) (PVA) present inthe suspension of PLGA MS after three washing stepscorresponded to 3.3% of the total amount of PVA added perbatch (Table S1).Encapsulated LevofloxacinWas Slowly Releasedwith-in 5 Days during PLGADegradation.All LVX-loaded PLGAMS formulations showed similar release profiles, with a fastrelease of 10−20% of the encapsulated dose within the first 5 h,followed by a gradual slow release up to maximum 85% within 5days (Figure 1A). This illustrates a typical biphasic diffusion-driven release curve32 of the LVX-loaded PLGA MS, where theinitial fast release is likely due to surface-bound LVX detachingfrom the MS or to LVX molecules encapsulated close to theperiphery of the particles that are easily accessible by hydrationand can thus diffuse out through the polymeric matrix. The slowrelease phase is due to the diffusion of LVX molecules throughthe pores formed by polymer hydration and degradation. Thekinetic profiles of LVX release fromall of the PLGAMSwere bestfitted by the Peppas−Sahlin model, having the highestcorrelation coefficient values (R2 = 0.9847−0.9964) and thelowest root mean squared error (RMSE = 2.04−4.83) (Table S2and Figure S2). According to the Peppas−Sahlinmodel, the drugrelease from the polymeric matrix occurs as a result of thecombination of two independent mechanisms: the diffusion ofthe drug (first term of Peppas−Sahlin equation, Table S2) andthe transition of the polymer from a semirigid to a flexible state,generating a relaxation of the polymer chains (second term ofPeppas−Sahlin equation, Table S2).33,34 The contribution of theFickian diffusion was much greater than the polymer relaxation(k1 = 65.1−76.8 vs k2 =−11.7 to−16.5), indicating that the drugrelease of LVX from all PLGA MS is mainly controlled by theFickian diffusion. However, the relaxation mechanism is notnegligible as the purely Fickian diffusion exponent m (Peppas−Sahlin equation, Table S2) is not equal to n in the Korsmeyers−Peppas equation (Table S2).33,34Similar release profiles have previously been reported for 3−5μmLVX-loaded PLGAMS with a higher and faster burst releaseof 40%within the first 30 min followed by a slow release phase of72 h.28 Larger PLGA MS of 17 μm prepared using the doubleemulsion W/O/W method also showed to have a biphasicrelease curve with a dominant burst phase corresponding to 70%of the encapsulated LVX within 30 min.28 PLGA (Resomer RG502H, lactide/glycolide ratio 50:50, Mw 7−17 kDa, acidterminated) MS loaded with rifampicin showed to have similarbiphasic release profile with 50−80% of rifampicin released after12 h and release rates directly proportional to the polymerconcentrations (3−30%).35 On the other hand, a completelydifferent release profile was observed for gentamicin-loadedPLGA (lactide/glycolide ratio 60:40) MS where the drug wasreleased over 60 days with a triphasic profile with rates indirectlyproportional to the PLGA concentration used.30 A shift from abi- to a triphasic release profile was shown byMylonaki et al. over30 days for atorvastatin-loaded PLGA MS of 16−18 μmprepared with PLGA of increasing Mw.32 Different releaseprofiles of LVX and ofloxacin were observed in the case of morehydrophilic delivery systems: an almost complete and immediaterelease of LVX (90% within 30 min) from cross-linked chitosanMSwas presented by Gaspar et al., confirming that water-solublepolymers are more suitable for fast release systems of LVX.36 Adifferent release profile was observed for albumin MSencapsulating ofloxacin, which was completely released within6 h, showing a fast release of 42% in the first hour.22 Overall, thisshows how release kinetics depend on different factors, includingthe polymer type (copolymer ratio, crystallinity, Mw), particlediameter, polymer−drug interaction, and diffusion coefficient ofthe drug.30,32,37To gain a better understanding of particle degradation and itsrelation to the release mechanism, the MS surface morphologyand internal polymeric structure were investigated. Polymerdegradation was evident upon exposure to the release medium asboth LVX-loaded and nonloaded MS assumed a partiallyACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066696collapsed spherical shape, whereas the surface became rough andporous (Figures 1B and S3). Small pores were visible on thesurface of the LVX-loaded PLGAMS and cavities in the internalmatrix already after 1 day (cross section images in Figure 1B),whereas no pores were observed on the surface of the nonloadedPLGA MS (Figure S3). The pores and cavities increased in sizeover time (cross section images in Figure 1B, days 2−5 vs day 1).On day 5, substantial degradation of the MS was observed, withthe appearance of large cavities close to the surface of the MS.The presence of wrinkles on the surface of MS could not beattributed to the vacuum used during the scanning electronmicroscopy (SEM) investigation, as optical microscopy imagesof theMS showed the same behavior (Figure S3). This formationof wrinkles and partially collapsed surface morphology haspreviously been described38,39 as the initial event of heteroge-neous degradation of PLGA films. The wrinkles occur on the softsurface layer as a consequence of water diffusion in the bulkmatrix and the consequent decrease in the polymer glasstransition temperature. The hydrolysis of PLGA results in theformation of glycolic acid and lactic acid, which locally decreasethe microenvironment pH and catalyze further degradation ofthe polymer. Although this process takes place in the entirepolymermatrix, neutralization of the degradation product on theparticle surface by the release media results in reducedautocatalytic activity at the surface.38,39 Further wrinkling ofthe particle surface was observed until complete dissolution ofthe polymer matrix on day 21, similarly to previous reports for16−18 μm PLGA MS.32 The nonloaded PLGA MS showed thesame degradation mechanism, though without visible surfacepores on day 0 (Figure S3).To evaluate whether the degradation of the MS occurred in asimilar manner in vivo after IV administration, the MS wereadministered tomice and their appearance in situ in the lungswasassessed. The PLGAMS seemed to degrade faster in vivo than invitro, with all MS seemingly dissolved after 1 week (Figure 2C),whereas in vitro, the MS took 3 weeks to completely disintegrate(Figure S3). Similarly, Tracy et al. showed that the degradation ofPLGA MS in vivo was 1.7−2.6 times faster than in vitro,independently of the polymer end group or molecular weight.40They hypothesized that this could be attributed to the presenceFigure 2. Representative histology images over time of lungs exposed to PLGA microspheres (F0) after intravenous injection. Representativeimages of H&E stained histological 30 μm coronal lungs sections at 0×, 10×, and 40× magnification after different treatments (first−thirdrows), and processed images after hematoxylin extraction (fourth row; see Figure S4 for full-sized images). For more details, see the Materialsand Methods section. (A) Healthy and untreated lungs. (B−D) Healthy lungs after 1 h, 1 week, and 2 weeks of injection of 3 mg/mouse PLGAMS (corresponding to approximately 300,000 PLGAMS/mouse). Orange arrows point to some of theMS. Scale bars: (first row) 5mm, (secondrow) 200 μm, (third and fourth row) 50 μm.ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066697of lipids and biological compounds in vivo acting as plasticizersand resulting in increasing the uptake of water into thepolymer.40 This results in faster degradation and may explainwhy particles are only visible in the lungs during the first week.No major surface wrinkling was observed for the MS in the lungtissue. This may be due to relatively low resolution of opticalmicroscopy and the visual interference from the surroundingtissue.Levofloxacin Antimicrobial Activity Decreased uponEncapsulation in PLGA Microspheres. Both anhydrous andhemihydrate LVX presented the same minimum inhibitoryconcentration (MIC) against the tested pathogens. However,upon encapsulation of LVX in PLGA particles, a decrease inantimicrobial activity was observed. All three formulationsshowed an increase in MIC to 4 μg/mL, from 0.5 and 1 μg/mLactive compound against Staphylococcus aureus and P. aeruginosa,respectively. A larger increase in MIC as compared to thenonformulated drug was observed for the formulations whentested against Escherichia coli, with an increase from 0.03 to 0.5−1 μg/mL (Table 2). Similarly, a small increase in MIC toward S.aureus and P. aeruginosa for ciprofloxacin after formulation intoPLGA nanoparticles has previously been reported by Dillen etal.41 A 3-fold increase in MIC toward P. aeruginosa was alsoreported for netilmicin after encapsulation in PLGA nano-particles, and this increase in MIC was attributed to theprogressive release of netilmicin from the particles.42 A smalldifference inMIC was observed for some of the fresh and freeze-dried formulations, in particular, formulation F1 against E. coliand formulation F3 against E. coli and S. aureus. However, thedifference between theMICs for these formulations correspondsTable 2. Minimum Inhibitory Concentration Values of Levofloxacin-Loaded PLGA Microspheres against E. coli, P. aeruginosa,and S. aureusaMIC rangeb (μg/mL)F0 F1 F2 F3bacteria strain aLVX hLVX freshc freshc freeze-dried freshc freeze-dried freshc freeze-driedE. coli ATCC 25922 0.03 0.03 >30 0.5 1 1 1 0.25 0.5P. aeruginosa PA01 1 1 >30 4 4 4 4 4 4S. aureus 15981 0.5 0.5 >30 4 4 4 4 2 4aMIC, minimum inhibitory concentration; aLVX, anhydrous levofloxacin; hLVX, hemihydrate levofloxacin; N = 1, n = 3. bMIC values for F1, F2,and F3 were corrected for the LVX amount released from the PLGA MS during the experimental time (20 h). cThe fresh MS were stored at 4 °Cfor 48 h prior to MIC experiments, whereas the freeze-dried MS were dried immediately after preparation. Drug loading of those batches was notsignificantly different from fresh or freeze-dried MS.Figure 3. Cytotoxicity and hemocompatibility of PLGAmicrospheres. (A−C) Cell viability (%) of A549, H1299m and HUVEC cells incubatedfor 24 h with F0 (light purple), F0i (light blue), DTPA-F0i (blue), F1 (dark green), F2 (light green), F3 (teal) microspheres (MS). The dottedline represents 50% viability. Data are presented as mean ± SD, N = 3, except for F0i and DTPA-F0i, where N = 1. (D) Representative phasecontrast images of HUVEC cells taken using the IncuCyte live-cell analysis system at 0, 12, and 24 h after treatment withmedia (control, top) orF0 at 0.63mg/mL PLGA concentration (concentration indicated with the pink arrow in C) (bottom). Scale bars: 100 μm. (E) Hemolysis (%) ofhuman red blood cells caused by free LVX, nonloaded PLGA MS, and LVX-loaded PLGA MS were assessed after 1 h incubation at differentconcentrations. Data are presented as mean ± SD (N = 1, n = 3).ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066698to a 2-fold dilution and is not considered significant, as theprecision of the broth dilutionmethod is generally regarded to bewithin a 2-fold concentration range.43 Moreover, the dose ofLVX delivered and released from the 3 mg dose of PLGA MS isexpected to have a bactericidal effect in mice already at 1 h afterdelivery. In fact, its concentration in the murine alveolar liningfluid is calculated to be much greater (∼400×) than theconcentration needed for LVX to display a bacteriostatic effectagainst P. aeruginosa and S. pneumoniae. No growth inhibitionwas observed upon exposure to the nonloaded F0 PLGA MS inthe tested concentration range (30−0.234 μg/mL, correspond-ing to the amount of PLGA in LVX-loaded MS) (Table 2). Thisis in accordance with literature, as PLGA nanoparticles show noinhibition of bacterial growth at concentrations below 1 mg/mL.44PLGA Microspheres Showed Low Toxicity for Endo-thelial, Lung Epithelial, andRed BloodCells at the In VivoDose. As the particles are intended for IV administration andpassive targeting to the lungs, the cytotoxicity of PLGA MS wasevaluated on endothelial (HUVEC), alveolar lung epithelial(A549), and human lung epithelial (H1299) cell lines.Nonloaded PLGA MS presented IC50 values of 1.5, 5, and 1.4mg/mL for HUVEC, A549, and H1299, respectively (Figure3A−C), in line with previous reports.45,46 The LVX-loadedPLGAMS presented similar toxicity for the three tested cell lineswith IC50 values of 1.4−2 mg/mL for HUVEC (correspondingto 0.03−0.08mg/mL of loaded LVX), 2.5−3.5 mg/mL for A549(corresponding to 0.06−0.09 mg/mL of loaded LVX), and 0.7−1.5 mg/mL for H1299 (i.e., 0.01−0.04 mg/mL of loaded LVX)(Figure 3A−C). The cytotoxicity of LVX-loaded PLGAMS (i.e.,F1, F2, F3) was not related to the initial fast release of the loadedLVX, as no cytotoxicity was observed at the correspondingconcentrations of free LVX (Figure S5A−C). These resultsindicate that loading the PLGA MS with LVX does notsubstantially affect the cytotoxicity, similar to the report byGaspar et al. applying bronchial Calu-3 cells for testing.28 F0i andDTPA-F0i PLGA MS slightly reduced cell viability of HUVECcells at PLGAMS concentrations above 0.5mg/mL compared toF0 PLGA MS (Figure 3C). This may be due to the presence ofthe amino group added to the PLGA to allow chelatorconjugation, as nonloaded PLGA MS showed an IC50 of 1.4mg/mL when tested on HUVEC cells. The cytotoxicity of theF0i PLGA MS was not affected by conjugation of DTPA to theamino groups on the PLGA (Figure 3C). The integrity of all celllines exposed to formulations was confirmed by microscopicobservation using the IncuCyte live-cell imaging system.Representative images of HUVEC cells exposed to 0.63 mg/mL of F0 PLGA MS are presented in Figure 3D.The hemolytic effect of a drug delivery system is an importantfactor to consider before IV administration. Nonloaded (i.e., F0,F0i, DTPA-F0i) and LVX-loaded PLGAMS (i.e., F3, loadedMSwith the highest LVX content) as well as free LVX did not lysehuman erythrocytes in the tested concentration range. Addi-tionally, none of the tested formulations inducedmore than 0.4%hemolysis, even at concentrations corresponding to 3× the doseadministered in vivo for the components, namely, 6 and 0.3 mg/mL, respectively, for PLGAMS and free LVX, as shown in Figure3E. The results indicate that the PLGAMS are hemocompatibleand approved for safe administration in vivo. Previous reportsindicate no hemolysis caused by PLGAwas observed up to 3mg/mL of PLGA nanoparticles;47 however, to the authors’knowledge, no higher concentrations have been reported.111In-Labeled PLGA Microspheres Distributed Homo-geneously in the Lung Capillaries Immediately afterIntravenous Injection. The PLGAMS were radiolabeled firstby functionalizing the surface with amino groups by addingPLGA-block-poly(ethylene glycol)-amine (PLGA-PEG-NH2)to the DP (i.e., formulation F0i PLGA MS, 1, Figure 4A). TheFigure 4. Synthesis of radiolabeled PLGA microspheres. (A)Synthesis of 111In-DTPA-F0i PLGAMS. (B)Radiolabeling efficiencyand stability over 24 h using ITLC.ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066699amino functionality was then used to attach the p-SCN-Bn-DTPA (resulting in DTPA-F0i PLGA MS, 2, Figure 4A) andfinally radiolabeling with the gamma emitter 111In (3, Figure 4A).The labeling efficiency was approximately 5%, whereas theradiochemical purity measured using instant thin-layer chroma-tography (ITLC) was approximately 97% (Figure 4B). The111In-DTPA-PLGAMS remained at the origin (retention factor,Rf = 0) after incubation with 10 mM EDTA for 10 min at roomtemperature (RT), whereas the free 111In3+ migrated with thesolvent front (Rf = 1). The111In labeling presented high stabilityafter 24 h incubation in 10 mM EDTA, a strong chelator,indicating very strong binding between DTPA and 111In (Figure4B).To study the in vivo pharmacokinetics and biodistribution ofthe MS, healthy mice were IV injected with 111In-DTPA-F0iPLGA MS and SPECT/CT images taken at different timepoints. Following IV administration, the 111In-DTPA-F0i PLGAMS immediately accumulated in the lungs (Figure 5A, MovieS2). This is expected as particles present in IV fluids will passthrough the heart and into the pulmonary circulation. Thisrepresents the first stage of filtration in which any material with aparticle size larger than∼5 μm is trapped in the small pulmonarycapillaries. The pulmonary distribution of the MS within bothlobes is homogeneous (Figure 5B), which supports thehypothesis that passive lung targeting after IV administrationof drug-releasing delivery systems with a monodispersedistribution could lead to more homogeneous localization inlung tissue compared to conventional pulmonary inhalation ofaerosols of MS.SPECT/CT images of maximum intensity projections arerepresented in Figure 5A. Assessment of the organ-specific timeactivity curves for 111In-DTPA-F0i PLGA MS showed a rapiddeposition in the lungs within the first 15 min (Figure 5A, day 0)and a slow elimination over 3 days (Figure 5A,C,D). To be ableto visualize the uptake in other organs that presented less uptake,the intensity of each scan was adjusted individually to visualizesmall amounts of radioactivity in other organs (Figure S6A).This figure clearly showed that after 2−3 days postinjection,some radioactivity accumulated and stayed in the kidneys fordays, following the same trend as free 111In3+ (Figure S7A, 0−48h). This activity might come from the degradation of MS and ischaracteristic for free 111In3+ (Figures S7A−C). From ourdegradation studies (in vitro, Figures 1 and S3, and histology-based in vivo, Figure 2), we concluded that in vivo degradation ofPLGAMS seemed to happen at a faster kinetic rate. Here, we canconclude that the F0i PLGA MS seem to disappear to a largeFigure 5. Pharmacokinetics and biodistribution of radiolabeled PLGA microspheres. (A) Representative maximum intensity projectionSPECT/CT overlay images (dorsal view) of healthy C57BL/6 mice showing the in vivo distribution of 111In-DTPA-F0i PLGA MS after IV tailinjection over time. The radioactivity is shown in blue. (B) Left and right lung distribution of 111In-DTPA-F0i PLGAMS immediately after IVinjection, calculated from the SPECT images (N = 3), in standardized uptake value (SUV) (g/mL) (mean± SD). (C) Lung tissue retention ofPLGAMS. (D,E) Organ SUV in g/mL (mean± SD) of the 111In-DTPA-F0i PLGAMS over 10 days, calculated from the SPECT images (N = 3).(F) Biodistribution (mean± SD,N = 3) of 111In-DTPA-F0i PLGAMS on day 10 after injection. Tissue uptake is expressed as % of injected dose(ID)/g of tissue.ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066700extent from the lungs in approximately 3 days after injection(Figure 5A,C,D). After the last SPECT/CT scan on day 10, abiodistribution revealed that overall only 0.02% of the injectedactivity remained in the animals, with no radioactivity present inorgans other than the lungs and the excretory organs liver,kidneys, and spleen (Figures 5F and S6I).However, it should be noted that a small amount ofradioactivity (up to 4%) was excreted immediately uponinjection through the urinary tract into the bladder as seen bythe high activity in the bladder during the first imaging time point(Figures 5A and 5D, day 0, and Movie S2). This is potentiallydue to immediate release of primarily the 111In-DTPA complexassociated with the PEG linker and partially hydrolyzed PLGApolymer rather than free noncomplexed 111In3+. This issupported by the high labeling purity of 111In-DTPA-PLGAMS; by the absence of detection of 111In3+ in the MS batchobserved even after 10mMEDTA challenge (Figure 4B); and bythe distribution and elimination of radioactivity after IV injectionof free 111In3+, which showed 25-fold higher levels of activity inthe kidneys (∼20% of the injected radioactivity) compared tothat in the bladder (∼2% of the injected radioactivity) (FigureS7A)clearly something we did not see after the embolizationwith 111In-DTPA-F0i PLGA MS. Additionally, the 111In-DTPAlinked to short peptides was previously reported not to beexcreted through the urinary pathway but accumulated in thecortex and medulla of the kidneys,48,49 similarly to 99mTc-DTPAcommonly used in clinical routine for the evaluation ofglomerular filtration rate.50CONCLUSIONIn summary, we have successfully prepared antibiotic-loadedbiodegradable MS as a passive lung-targeting drug deliverysystem. The prepared PLGAMS displayed highly homogeneoussize distributions with a mean diameter of 12 μm suitable foralveolar capillary targeting. Almost exclusive distribution to thelungs after IV injection was seen, and the MS remained there forat least a week. Biodistribution and pharmacokinetics illustratedthat the particles immediately and homogeneously localized tothe lung capillaries with up to 85% of the dose and confirmedcontinuous degradation and elimination from the target site viathe urinary excretion pathway without any observed side effects.The particles were feasibly prepared by an optimized flow-focused microfluidics system, encapsulated a selected antibioticof which up to approximately 80% was released as an active drugin a sustained manner over a period of 5 days in vitro. Otherpotent drug molecules can be encapsulated alone or incombination for the preparation of therapeutically effectivedosage forms. Thus, the present work demonstrates an approachto explore lung-targeting MS further.MATERIALS AND METHODSMaterials. Resomer RG 502H (PLGA 50:50, Mw 7−17 kDa, acidterminated), levofloxacin hemihydrate (Pharmaceutical SecondaryStandard), levofloxacin anhydrous, and poly(vinyl alcohol) (Mw 30−70 kDa, 87−90% hydrolyzed) were all purchased from Sigma-Aldrich(St. Louis, MO, USA). PLGA-PEG-NH2 (Mw PLGA 10 kDa,Mw PEG:1 kDa) was bought from Nanosoft Polymers (Winston-Salem, NC,USA). (S)-2-(4-Isothiocyanatobenzyl)diethylenetriamine pentaaceticacid (p-SCN-Bn-DTPA) was purchased fromMacrocyclics (Plano, TX,USA). 111Indium (111InCl3) in 0.1 M HCl was supplied by BWXT(Vancouver, BC, Canada). Dichloromethane (DCM) and PBS at pH7.4 were obtained from VWR (Radnor, PA, USA). 3-(4,5-Dimethylth-iazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES) were pur-chased from Sigma-Aldrich. Ultrapure water for sample preparation wasobtained from a PURELAB flex machine (ELGA LabWater, HighWycombe, UK).P. aeruginosa PA01 (reference strain, wild-type), S. aureus 15981(clinical isolate strain), and E. coli ATCC 25922 (reference strain) werekindly provided by the Institute of Immunology and Microbiology,University of Copenhagen. Mueller Hinton broth (MHB) waspurchased from Sigma-Aldrich. Human lung adenocarcinoma cells(A549) and human non-small cell lung cancer (H1299) cells werepurchased from American Type Cell Cultures (ATCC, Manassas, VA,USA). RPMI-1640 medium, Eagle’s minimum essential medium(EMEM), fetal bovine serum (FBS), penicillin−streptomycin (10,000U/mL), 0.25% (w/v) trypsin-ethylenediaminetetraacetic (EDTA)(1×), and UltraPure sucrose were purchased from Thermo Fisher(Waltham, MA, USA). Human umbilical vein endothelial cells(HUVEC), endothelial basal medium-2 (EBM-2), and endothelialgrowth medium-2 (EGM-2) single quots supplements were purchasedfrom Lonza (Walkersville, MD, USA). A 4% (w/v) paraformaldehydesolution in PBS was purchased from Boster Biological Technology(Pleasanton, CA, USA).Microfluidic Preparation of PLGA Microspheres. PLGA MSwere prepared using a previously described flow-focusing microfluidicglass chip design.51 The geometries were 150 and 240 μmwide channelsfor the DP and the CP channels, respectively, and with a 100 μm wideand 90 μm long orifice. All channels were 45 μm deep. The chip wasconnected through a 4 linear connector 4-way system (Mitos EdgeConnector, Dolomite, Roystone, UK) and Teflon microtubing to thereservoirs of the DP and the CP. They, in turn, were connected to FlowEZ pressure pumps and flow sensors (Fluigent, France) that injected thesolutions in the central and outer channels, ensuring superior control ofthe flow rates, compared to traditional syringe pumps (Figure 6).The MS preparations were optimized by varying the composition ofboth the DP and CP, as shown in Figure 6, and keeping the flow ratesconstant at QDP = 3 μL/min and QCP = 90 μL/min. The organic DPconsisted of a 5% (w/v) PLGA inDCMand for the loaded particles (i.e.,Figure 6. Preparation of microspheres on a flow-focusing microfluidic chip. aLVX, anhydrous LVX; hLVX, hemihydrate LVX; PVA, poly(vinylalcohol); *F0i contains 50% (w/w) PLGA-PEG-NH2 for radioactive imaging. Inset of the microfluidic chip is the snapshot during theproduction of droplets taken with optical microscope. Scale bar: 150 μm. Full video available in the Supporting Information (Movie S1).ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066701F1, F2, F3), 0.8−2.2% (w/v) of LVX, corresponding to 15% (for F1 andF2), and 30% (for F3) of theoretical drug loading. Anhydrous (F1, F2)as well as hemihydrate (F3) forms of the LVX were added to DP andalso to theCP for F2 and F3 to increase the loading. TheCPwas 2% (w/v) PVA in ultrapure water, 0.22 μm filtered (Q-max PES syringe filters,Frisenette, Knebel, Denmark) prior to use, saturated with LVX for F2(0.1% w/v) and F3 (2% w/v). To radiolabel MS (F0i) for the in vivoimaging studies, the DP was a 5% (w/v) solution of polymer in DCM.The polymer consisted of PLGA and PLGA-PEG-NH2 at a weight ratioof 50:50. All solutions were prepared immediately prior to injection inthe chip. After collection over 20−80 min depending on the desiredbatch size, the MS were washed three times with cycles of resuspensionin purified water and centrifugation at 500g for 5min at 4 °C and freeze-dried without any cryo- and lyo-protectants.Radiolabeling of PLGA Microspheres. The F0i PLGA MS (9mg) were prepared for radiolabeling. In order to avoid precipitation ofthe MS during radiolabeling, the whole amount was divided into sixbatches of 1.5 mg. Each batch was resuspended in 150 μL of NaHCO3(0.1 M, pH 8.0) and sonicated for 5 s. Then, 3.5 μL of 142 mg/mL p-SCN-Bn-DTPA in DMSO was added and shaken in an EppendorfThermomixer R (Brinkmann Instruments, Westbury, NY, USA) at 850rpm and 20 °C overnight. Unreacted DTPA was removed bycentrifugation (500g, 5 min, 4 °C), and the MS were washed withultrapure water and resuspended in 200 μL ofHEPES buffer (0.1M, pH7.0) for radiolabeling. 111InCl3 (49.95 MBq, 3 μL in 0.1 M HCl) wasadded to the MS, and the mixture was incubated for 1 h 45 min on anEppendorf shaker at 850 rpm at 20 °C. Noncomplexed 111In3+ wasremoved by centrifugation (500g, 5min, 4 °C), and theMSwerewashedwith ultrapure water. The activity of the MS was measured with a dosecalibrator (CRC-55tR; Capintec, Florham Park, NJ, USA). The sixbatches were pooled together to achieve samples with uniform MSlabeling for in vivo administration.The labeling efficiency was determined for the pooled radiolabeledMS and calculated from the ratio between the activity on the 111In-DTPA-F0i PLGA MS and the activity initially added. The radio-chemical purity and stability of 111In-DTPA-F0i PLGA MS wereassessed with ITLC using green Tec-Control strips as the stationaryphase (Biodex Medical Systems, Shirley, NY, USA) and 10 mM EDTAin ultrapure water (pH 4.0) as the mobile phase after incubating theMSwith 10mMEDTA for 10min and 24 h at RT. ITLCs were exposed to aphotosensitive phosphor sheet (Packard, Mississauga, ON, Canada) for5 min. The phosphor sheet was then developed using a cyclonephosphor imager (Packard) to determine the location of radioactivityon the ITLCs from which γ-rays were being emitted. The 111In-DTPA-F0i PLGA MS remain at the origin (Rf = 0), whereas the free111In3+migrated with the mobile phase to the solvent front (Rf = 1). Activitypeaks were then integrated using Optiquant software to determine theradiochemical purity and stability.Residual PVA Quantification. The residual amount of PVApresent on the MS in suspension was quantified indirectly, byquantifying the residual amount of PVA in the suspension of 3 mg ofPLGA MS using a colorimetric method based on the formation of acolored complex between the two adjacent hydroxyl groups of PVA andan iodine molecule.52 Briefly, the supernatants obtained during the MSwashing process were collected, and 0.4 mL of the solution was mixedwith 1.5mLof boric acid solution (0.65M) and 0.3mLof I2/KI solution(0.05/0.15 M). The samples were incubated for 20 min, and theabsorbance was measured at 630 nm with the SPECTROstar NanoUV−vis spectrophotometer (BMG LABTECH, Ortenberg, Germany).Particle Size Distribution and Morphology. The particle sizedistribution of fresh and freeze-dried MS was determined using lightmicroscopy. The MS were suspended in water and imaged with anOlympus IX71 inverted microscope (Olympus, Tokyo, Japan). Imageswere analyzed with ImageJ, and the diameters of at least 500 MS weremeasured in six fields from three different images (N = 3). Gaussian fitfor obtaining the histogram of distribution was performed usingGraphPad Prism 8 (GraphPad Software, La Jolla, CA, USA). Themorphology of the fresh MS was analyzed by SEM after sputter-coatingwith a 6 nm layer of gold using a Leica EM ACE200 coater (LeicaMikrosysteme, Vienna, Austria). The images were acquired with a FEIQuanta 3D FEG microscope (FEI, Hillsboro, OR, USA) at anacceleration voltage of 2.0 kV.Levofloxacin Content. LVX content in PLGA MS was assessedspectroscopically. The LVX-loaded PLGA MS were dissolved inacetonitrile for 4 h under magnetic stirring and analyzed with aSPECTROstar Nano UV−vis spectrophotometer (BMG LABTECH).Absorbance was detected at 298 nm where the interference with PLGAwas negligible. The LVX loading was determined from the mass ratio ofthe encapsulated LVX to the total MS produced (eq 1), whereas the EEwas defined as the mass ratio of the encapsulated LVX to the LVXinitially added (eq 2).=+×LVX loading (%) LVX encapsulated in the MSPLGA LVX encapsulated in the MS100(1)= ×EE (%) LVX encapsulated in the MSLVX initially added100(2)Studies were conducted in triplicates on independent batches of MS (N= 3).In Vitro Levofloxacin Release. The in vitro release studies werecarried out in PBS under sink conditions. The LVX-loaded PLGA-MS(3 mg) were dispersed in 2 mL of the release medium in an Eppendorfmicrocentrifuge tube (Sigma-Aldrich) and incubated at 37 °C underconstant rotation. At predetermined time points, the test tube wascentrifuged at 500g for 5 min at RT, and a 150 μL sample of thesupernatant was collected and analyzed by UV−vis spectroscopy at 288nm using a calibration curve (3−30 μg/mL LVX in PBS). Thewithdrawn sample was replaced with 150 μL of 37 °C release medium,the MS redispersed, and the test tube placed back in the incubator.The cumulative release of LVX (%) was calculated according to eq 3:∑==× ∑ + ×== −V C V CMcumulative LVX release (%)LVXreleasedLVXenctttttt t0s 0 1 totLVXenc (3)where LVXreleasedt is the amount of LVX released at time t, LVXenc isthe amount of LVX loaded in theMS, Vtot and Vs are the total volume ofthe release reservoir and the volume of the sample withdrawn at eachtime point, respectively, and Ct is the concentration of LVX released attime point t.The cumulative LVX release (%) versus time curves were fitted by fivedifferent mathematical models of drug release (i.e., zero order, firstorder, Higuchi, Korsmeyer−Peppas, Peppas−Sahlin)33 using MAT-LAB software (Mathworks, Natick, MA, USA, version R2019a). The R2and the RMSE were obtained and used to evaluate the goodness of thefitting and to choose the model that best described the experimentaldata.The release study was performed on three separate days(corresponding to biological triplicates, N = 3), each in one technicalreplicate (n = 1) from an independent batch of each type of MS.In Vitro PLGAMicrospheres Degradation: Imaging and CrossSection. The degradation of the MS matrix during the drug releasestudy was investigated by analyzing both the surface modification andthe internal matrix organization after cross sectioning of the MS. Atpredetermined time points (i.e., 0, 1, 2, 5, 10, 14, 21 days), the MS werecollected, resuspended in water, concentrated, and a drop was mountedon an aluminum stub and dried overnight at RT.Once completely dried,the MS were sputter coated with a 20 nm layer of iridium (Leica EMMED020, Leica Microsystems Canada, Concord, ON, Canada) on arotate-tilt stage for improved uniform coating. Cross sections of the MSfrom each time point were prepared using a FIB and imaged with anSEM (Helios NanoLab 650, FEI, Hillsboro, OR, USA). The crosssections were prepared by tilting the sample to 52° and then applying aplatinum protection layer on the surface of the particles of interest. Thisprotection layer was used to prevent any surface damage by the ionbeam and applied by gradually increasing the beam current from 7.7 pAto 2.5 nA in a stepwise manner until the ideal coating conditions wereACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066702achieved. Then, a regular cross section of approximately 8−12 MS wasselected for milling. The cross section face was gradually polished bydecreasing the ion beam current from 2.5 to 0.79 nA, and selectedparticle surfaces were then further polished using an 80 pA ion beamcurrent. SEM images were acquired at low keV for true surface imagingin the secondary electron mode (1 keV, 50 μA) and in backscatteredelectron mode (2 keV, 0.2 nA) for imaging cross sections. All imageswere corrected for tilt in the imaging setup (−38°) and acquired with1536 × 1024 pixels at 30 μs/pixel dwell time and saved as 16-bit TIFfiles. Figure S8 summarizes the steps of the FIB-SEM procedure.In Vivo PLGA Microsphere Degradation. Twelve female miceC57BL/6 were tail vein injected with 100 μL of 30 mg/mL F0 PLGAMS (corresponding to approximately 300,000 PLGAMS from the samepreparation per mouse). The exact number of MS was determined withflow cytometry (CytoFLEX Flow Cytometer, Beckman Coulter,Mississauga, ON, Canada). At each time point (i.e., 0, 1, 2 weeks),three mice were sacrificed by CO2 asphyxiation under isofluraneanesthesia. The lungs were harvested, rinsed in ice cold PBS, and fixedvia submersion overnight in 4% (w/v) paraformaldehyde in PBS. Thelungs were then moved to a 15% (w/v) sucrose solution in PBS for 10 hat 4 °C, after which they were placed in a 30% (w/v) sucrose solution inPBS at 4 °C until analysis, that is, for maximum of 3 weeks, with weeklyreplacements of fresh sucrose solution. The tissue samples were cut in30 μm thick coronal sections, stained with hematoxylin and eosin(H&E) using a standard protocol, film coverslipped using TissueTek-Prisma and Coverslipper (Sakura Finetek, Torrance, CA, USA), andmounted onto microscope slides for imaging (Histowiz, Brooklyn, NY,USA). Whole-slide images were taken at 40× magnification anddigitized with the Aperio AT2 slide scanner (Leica Biosystems Canada,Concord, ON, Canada). Each group contained three mice correspond-ing to three biological replicates (N = 3). To make the embolized MSimprint clearly visible from the 30 μm thick histology sections, imageanalysis was performed using Image Pro Plus (Media Cybernetics,Rockville, MD, USA, version 6.0), taking advantage of the fact that MSlodged in the capillaries or arterioles stretching the endothelial cells in acircular fashion around the MS. To make this clearly visible, the “blue”information from the histology pictures in RGB space, which highlightsthe nuclei, as they were stained blue from the hematoxylin dye in theH&E stain, was first extracted. To further highlight the cells, localequalization was applied, thus enhancing the differences between nucleiand the surrounding areas. MS could thus be shown very clearly ascircular endothelial cell rings.Antibacterial Activity of Levofloxacin-Loaded PLGA Micro-spheres. The MIC of LVX and PLGA MS containing LVX wasdetermined using the broth microdilution method according to theClinical and Laboratory Standards Institute guidelines.53 Bacterialgrowth inhibition of the LVX-loaded PLGAMS was assessed against E.coli ATCC 25922, P. aeruginosa PA01, and S. aureus 15981. The LVXsolutions (both anhydrous LVX and hemihydrate LVX) and the LVX-loaded PLGA MS suspensions (F1, F2, F3, both fresh and freeze-driedMS) were prepared in 2-fold dilutions in MHB in amountscorresponding to a concentration range of 8−0.007 μg/mL of LVX insolution and encapsulated LVX in a 96-well flat-bottomed plates(Costar Corning 3596 cell culture plates, Corning, NY, USA). Thenonloaded PLGAMSwere tested in PLGA concentrations correspond-ing to the amount of PLGA in the LVX-loadedMS (30−0.029 μg/mL).The samples were then inoculated with bacteria in log phase to achieve afinal concentration of (2−5) × 105 colony forming units/mL andincubated for 20 h at 37 °C in an INCUCELL incubator (MMMMedcenter Einrichtungen, Munich, Germany) before visual inspectionof growth. The MIC values were determined as the lowestconcentration showing no visible bacteria growth. The experimentwas performed once in triplicate (N = 1, n = 3).In Vitro Compatibility Assessment. Cell Culturing.TheHUVECcell culture was maintained in EBM-2 supplemented with (EGM-2)SingleQuots kit, whereas the H1299 and A549 cell cultures weremaintained in RPMI1640 or FK-12 media, respectively, supplementedwith 10% (v/v) FBS, penicillin (100 U/mL), and streptomycin (100μg/mL). All cells were cultured in T-75 flasks (Corning, Corning, NY,USA) at standard culture conditions of 37 °C, 5% CO2, and 95%humidity. Once the cells reached approximately 80% confluence, theywere detached from the culture flasks using 0.25% (w/v) trypsin−EDTA solution (isotonic solution containing 2.5 and 0.3 mg/mL oftrypsin and EDTA, respectively). During culturing growth, mediumwasreplaced every 2 days.Cytotoxicity. Cytotoxicity of nonloaded and LVX-loaded PLGAMSon HUVEC, H1299, and A549 cells was determined using the MTTassay. Briefly, the cells were seeded in flat-bottomed and transparent 96-well plates at densities of 10,000 cells/well (HUVEC) or 5000 cells/well(A549, H1299) and incubated at 37 °C for 24 h until approximately80% confluency was achieved. The cells were washed with 37 °C PBSand exposed to 200 μL of culture medium containing MS in nineconcentrations over a 2-fold serial dilution series (5−0.012 mg/mL).Cells incubated in culture medium (untreated cells) were used as thenegative control. After the cells were incubated with the differentformulations for 24 h, 5μLof 5mg/mLMTT reagentwas added and thecells were subsequently incubated for 3.5 h (HUVEC) or 3 h (H1299and A549) at 37 °C until the formation of formazan crystals. Themedium was then removed, and 150 μL of DMSO was added to eachwell to dissolve the formazan crystals and lyse the cells. After completedissolution, the absorbancewasmeasured at 540 and 650 nm to subtractthe background of the cells using a Synergy MX plate reader (BioTek,Winooski, VT, USA). Cell viability was calculated according to eq 4:=−−×A AA Acell viability (%) 100treated cells @ 540 nm treated cells @ 650 nmuntreated cells @ 540 nm untreated cells @ 650 nm(4)where Atreated cells @ 540 nm and Auntreated cells @ 540 nm are the absorbances at540 nm of the wells incubated with either the MS or LVX and culturemedia, respectively, whereas Atreated cells @ 650 nm and Auntreated cells @ 650 nmare the respective background absorbances at 650 nm.IC50 values representing 50% dehydrogenase inhibition in the cells asa result of the treatment were determined by fitting the data with anonlinear regression function with variable slope with GraphPad Prism8. The cell viability (%) versus concentration curves were fitted using eq5:=+cell viability (%)100X1ICHill slope50Hill slope (5)whereX is the concentration of the formulation,Hill slope is a factor thatdescribes the steepness of the linear part of the curve, and IC50 is theconcentration of the formulations that inhibits 50% of cell dehydrogen-ase activity. The cytotoxicity assay was performed in biological (N = 3)and technical (n = 3) triplicates.Cell Morphology Changes. The effects of the MS on HUVEC,H1299, and A549 cell morphology were investigated with a live cellimaging system. Cells were grown on a 96-well microplate tray andexposed to MS and LVX, as described in the previous section, andplaced in an IncuCyte ZOOM live-cell analysis system (EssenBiosciences, Ann Arbor, MI, USA), equipped with a 10× objective ina 5% CO2 incubator at 37 °C. Real-time images of each well were takenevery 3 h for a period of 48 h.Hemocompatibility.Hemocompatibility of theMS was investigatedwith the hemolysis assay, following a method described by Evans et al.54Briefly, blood from a healthy human donor was collected in K2-EDTA-coated Vacutainer and centrifuged at 700g for 5 min at RT to obtainhuman red blood cells (hRBC). After the plasma was discarded, thehRBC were washed twice with 150 mM NaCl in ultrapure water andtwice with PBS with cycles consisting of gently mixing, centrifuging at700g for 5 min at RT, and gently removing the supernatant. The hRBCswere diluted to 3% (v/v) in PBS, and 100 μL of this dispersion wasadded to 100 μL of F0, F0i, DTPA-F0i, F3, and LVX in PBS at 4 and 12mg/mL concentrations, corresponding to final 1× (2 mg/mL) and 3×(6 mg/mL) of the in vivo dose (used in the histology and imagingstudies) when related to blood volume of themouse (1.5mL). TritonX-100 at a concentration of 2% (v/v) and PBS were used as positive andnegative controls, respectively. The Eppendorf tubes were incubated for1 h at 37 °C on an orbital shaker, and the hRBCs were pelleted byACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066703centrifugation at 700g for 5 min at RT. The supernatant was transferredto a round-bottom 96-well plate (Falcon, Corning, NY, USA), and theabsorbance of hemoglobinwasmeasured at 450 nmusing a SynergyMXplate reader (BioTek). The hemolysis (%) was calculated according toeq 6:=−−×A AA Ahemolysis (%) 100sample negative controlpositive control negative control (6)whereAsample is the absorbance of supernatant from the hRBC incubatedwith the different concentrations of the formulations, whereasAnegative control and Apositive control are the absorbances of the supernatantsfrom the hRBC incubated with PBS and Triton X-100, respectively, thatcorrespond to 0 and 100% hemolysis. The hemolysis assay wasperformed once in technical triplicates (N = 1, n = 3).In Vivo Pharmacokinetics and Biodistribution of PLGAMicrospheres. SPECT/CT Imaging. The in vivo studies wereconducted in accordance with the Canadian Council on Animal Care(CCAC) and protocol A16-0150 approved by the Animal CareCommittee (ACC) of the University of British Columbia. Five-month-old healthy female C57BL/6 mice were purchased from Charles Riverand allowed free access to food and water. The mice were allocated intotwo groups of three individuals (corresponding to three biologicalreplicates, N = 3). Each animal was anesthetized using isoflurane on aprecision vaporizer (5% in oxygen for induction, and between 1.5 and2% in oxygen for maintenance) and received a subcutaneous dorsalinjection of lactated Ringer’s solution (0.5 mL) for hydration prior toeach imaging scan. After the induction of anesthesia, an injection of 100μL of 30 mg/mL 111In-DTPA F0i PLGAMS in saline was administeredvia the tail vein. The administered dose corresponded to approximately300,000 MS weighing 3 mg, resulting in 2 mg/mL target concentrationof PLGAMS in the 1.5 mL average blood volume of a mouse. Similarly,the control group was administered an injection of 100 μL 111InCl3 insaline via the tail vein. The average injected activities were 2.04 and 9.25MBq for the 111In-DTPA-F0i PLGA MS and the control group,respectively. Immediately after injection, whole-body images wereacquired at different time points: 0, 1, 2, 3, 7, and 10 days for PLGAMSand 0, 4, 24, and 48 h for 111InCl3. A multimodal SPECT/CT scanner(VECTor/CT, MILabs, The Netherlands) equipped with a XUHS-2mm mouse pinhole collimator was used for the study. The firstmeasurements (corresponding to day 0) were performed within 15 minafter injection. Throughout the entire scanning procedure, therespiratory rate and body temperature of the mice were monitoredand isoflurane dose and animal bed temperature adjusted accordingly.Following each SPECT acquisition, a whole-body CT scan (55 kV, 615μA)was obtained to gain anatomical information, and both images wereregistered. The 111In photopeak window was centered at 171 keV with a20% energywindowwidth (backgroundweight 2.5), 0.4mm3 voxel, 128subsets, and 5 iterations using the SROSEM image reconstructionalgorithm. The images were decay corrected and after CT registration,and attenuation correction was applied. For visual representation, thereconstructed volumes of SPECT scans were postfiltered with a 3DGaussian filter. Volumes of interest were manually defined usingAMIDE55 (UCLA, USA, version 1.0.4) to determine the time−activitypattern per target organ. The presented regions were heart, lungs, liver,kidneys, bladder, and bone. The average organ activity per volume wasobtained from the SPECT images, and the standardized uptake value(SUV) was extracted from each organ using eq 7:× = ××−−SUV (g mL )organ activity concentration (MBq mL )injected dose (MBq)body weight (g)11(7)To relate the scanner units (counts/voxel) to radioactivity concen-tration, a calibration factor was determined by scanning a source with aknown concentration of 111In3+.Biodistribution. Following the last scan at day 2 for 111InCl3 and atday 10 for 111In-DTPA-F0i PLGA MS, mice were euthanized by CO2asphyxiation under isoflurane anesthesia. Cardiac puncture waspromptly performed to recover blood, and the organs of interest wereharvested (brain, skull, muscle, femur, bladder, spleen, pancreas,stomach, kidneys, small intestine, large intestine, liver, heart, right andleft lungs, shoulder, and tail), rinsed with PBS, and blotted dry. Eachorgan was weighed, and the radioactivity was quantified using acalibrated gamma counter (Packard Cobra II Autogamma counter,PerkinElmer, Waltham, MA, USA) followed by calculating the amountrelative to the injected dose per gram of tissue (%ID/g) and per organ(%ID/organ), respectively.Statistical Analysis. All of the in vitro and in vivo experiments andmeasurements were conducted in triplicate, unless otherwise specified.Results are presented as mean ± SD. Statistical analyses of data from invitro and in vivo experiments were performed with GraphPad Prism 8(GraphPad Software). Probability values of p < 0.05 were consideredstatistically significant.ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsnano.9b09773.Morphology and size distribution of PLGA microspheres(Figure S1); poly(vinyl alcohol) quantification duringwashing (Table S1); mathematical models of the in vitrorelease profiles (Table S2); representative drug releasefittingmodels (Figure S2); degradation of F0 vs F0i PLGAmicrospheres (Figure S3); representative full-size pro-cessed histology images of lungs exposed to PLGAmicrospheres (F0) after intravenous injection over time(Figure S4); cell toxicity of PLGA microspheres,normalized for levofloxacin content (Figure S5);pharmacokinetics and biodistribution of radiolabeledPLGA microspheres (Figure S6); pharmacokinetics andbiodistribution of 111InCl3 after intravenous administra-tion (Figure S7); steps to obtain cross sections of PLGAmicrospheres with FIB-SEM (Figure S8) (PDF)Movie S1: Production of microspheres by flow focusing(AVI)Movie S2: SPECT/CT animated image 15 min afterintravenous administration of 111In-DTPA-PLGA micro-spheres (MPG)AUTHOR INFORMATIONCorresponding AuthorsHanne Mørck Nielsen − Department of Pharmacy, Faculty ofHealth and Medical Sciences, University of Copenhagen,Copenhagen DK-2100, Denmark; orcid.org/0000-0002-7285-9100; Email: hanne.morck@sund.ku.dkUrs O. Ha ̈feli − Department of Pharmacy, Faculty of Health andMedical Sciences, University of Copenhagen, Copenhagen DK-2100, Denmark; Faculty of Pharmaceutical Sciences, University ofBritish Columbia, Vancouver, BC V6T 1Z3, Canada;orcid.org/0000-0003-0671-4509; Email: urs.hafeli@ubc.caAuthorsMonica Agnoletti − Department of Pharmacy, Faculty of Healthand Medical Sciences, University of Copenhagen, CopenhagenDK-2100, Denmark; Faculty of Pharmaceutical Sciences,University of British Columbia, Vancouver, BC V6T 1Z3,Canada; orcid.org/0000-0002-5103-9568Cristina Rodríguez-Rodríguez − Faculty of PharmaceuticalSciences and Department of Physics and Astronomy, University ofBritish Columbia, Vancouver, BC V6T 1Z3, Canada;orcid.org/0000-0002-3313-4422Sylvia N. Kłodzińska − Department of Pharmacy, Faculty ofHealth and Medical Sciences, University of Copenhagen,ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−67066704Copenhagen DK-2100, Denmark; orcid.org/0000-0001-7443-2234Tullio V. F. Esposito − Department of Pharmacy, Faculty ofHealth and Medical Sciences, University of Copenhagen,Copenhagen DK-2100, Denmark; Faculty of PharmaceuticalSciences, University of British Columbia, Vancouver, BC V6T1Z3, Canada; orcid.org/0000-0003-4047-0405Katayoun Saatchi − Faculty of Pharmaceutical Sciences,University of British Columbia, Vancouver, BC V6T 1Z3,Canada; orcid.org/0000-0002-5372-6791Complete contact information is available at:https://pubs.acs.org/10.1021/acsnano.9b09773Author ContributionsM.A., H.M.N., and U.O.H. designed the experiments. M.A.analyzed the data. M.A., C.R.R., S.N.K., T.V.F.E., and K.S.performed the experiments:M.A. prepared and characterized themicrospheres; M.A. and S.N.K. performed the antimicrobialactivity study; M.A. and T.V.F.E. performed the cytotoxicitystudy; K.S. modified the microspheres with DTPA; M.A. andT.V.F.E. radiolabeled and characterized the radiotracer; M.A.,T.V.F.E., andC.R.R. performed the SPECT/CT, biodistributionand histology studies, and analyzed the data. M.A. and S.N.K.drafted the manuscript. All authors revised and approved thefinal manuscript. H.M.N. and U.O.H. oversaw the research.U.O.H. provided financial support.NotesThe authors declare no competing financial interest.ACKNOWLEDGMENTSThe authors would like to thank the Lundbeck Foundation,Denmark (Joint Professorship to UOH, Grant No. 2014-4176),for financial support of this project. Furthermore, the CoreFacility for Integrated Microscopy (University of Copenhagen,Denmark) is acknowledged for access to their SEM facilities andsupport, and G. Owen and the Centre for High-ThroughputPhenogenomics (University of British Columbia, Canada) areacknowledged for their help with the cross section images withthe FIB-SEM. The authors would also like to thank M. Osoolyfor her skillful assistance in the SPECT/CT imaging and ex vivobiodistribution, J. Hodasova for her support with histologyexperiments, and the Canada Foundation for Innovation(Project No. 25413) for its support of the imaging facility(http://invivoimaging.ca/). K.S. acknowledges the generoussupport of BWXT Isotope Technology Group for the supply ofthe radioisotope.REFERENCES(1) Troeger, C.; Blacker, B.; Khalil, I. A.; Rao, P. C.; Cao, S.; Zimsen, S.R. M.; Albertson, S. B.; Stanaway, J. D.; Deshpande, A.; Farag, T.;Forouzanfar, M. H.; Abebe, Z.; Adetifa, I. M. O.; Adhikari, T. B.; Akibu,M.; Al Lami, F. H.; Al-Eyadhy, A.; Alvis-Guzman, N.; Amare, A. T.;Amoako, Y. A.; et al. 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Imaging 2003, 2, 131−137.ACS Nano www.acsnano.org Articlehttps://dx.doi.org/10.1021/acsnano.9b09773ACS Nano 2020, 14, 6693−670667061  Supporting Information     Monosized Polymeric Microspheres Designed for Passive Lung Targeting: Biodistribution and Pharmacokinetics  after Intravenous Administration Monica Agnoletti, a,b Cristina Rodríguez-Rodríguez, b,c Sylvia N. Kłodzińska, a  Tullio V.F. Esposito, a,b Katayoun Saatchi, b Hanne Mørck Nielsen,a,* and Urs O. Häfeli a,b,*  a Department of Pharmacy, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, DK-2100, Denmark b Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, BC, V6T 1Z3,  Canada c Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada *Corresponding authors: urs.hafeli@ubc.ca (UOH), hanne.morck@sund.ku.dk (HMN)               2    Figure S1. Morphology and size distribution of PLGA microspheres. (A-D) Size distribution of F0, F0i, DTPA-F0i and F3 PLGA MS. (E-G) Representative SEM images of F3, F0i and DTPA-F0i LVX-PLGA MS. Scale bars: 10 µm (H) Representative optical image of F3 LVX-loaded PLGA MS. Scale bar: 10 µm.   3  Table S1. Polyvinyl alcohol quantification during washing. PVA concentration present in the supernatant of the microsphere (MS) suspension, amount of PVA in the supernatant of the MS suspension and cumulative amount removed by the washing during up to three cycles of washing. Results are in triplicates (N = 3). † indicated as % of the amount of PVA measured in the dispersing medium after preparation and before washing            PVA concentration (%, w/v) in the  supernatant of the MS suspension Amount PVA (mg) in the supernatant of the MS suspension Cumulative amount of PVA (%) removed during the different washing cycles † Before washing 2.015 ± 0.080 96.77 ± 3.81 - After the 1st washing 0.016 ± 0.003 8.29 ± 1.60 88.7 After the 2nd washing 0.0001 ± 0.0005 0.05 ± 0.06 96.7 After the 3rd washing 0.0001 ± 0.0001 0.05 ± 0.06 96.7 4  Table S2. Mathematical models of the in vitro release profiles. Estimated parameters, coefficient of determination, root mean squared error obtained from fitting experimental data to the main release kinetic mathematical models in MATLAB® software. FD: freeze-dried, k0: zero-order release constant; k1º: first-order release constant; kH: Higuchi release constant; kKP: release constant incorporating structural and geometric characteristics of the drug-dosage form; n: diffusional exponent indicating the drug-release mechanism; k1: constant related to the Fickian kinetics; k2: constant related to relaxation kinetics; m: diffusional exponent, R2: correlation coefficient values, RMSE: root mean squared error.  Mathematical model Fitting parameters Formulation F1, fresh F1, FD F2, fresh F2, FD F3, fresh F3, FD Zero order 𝐹 = 𝑘0 × 𝑡 R2 0.6328 0.6476 0.5522 0.6144 0.6485 0.4411 RMSE 20.69 20.12 26.10 23.88 21.15 24.90 Parameters k0: 23.6 k0: 23.6 k0: 25.2 k0: 24.8 k0: 24.4 k0: 24.7 First order 𝐹 = 100 × (1 − 𝑒−𝑘1º×𝑡) R2 0.9689 0.9685 0.9735 0.9762 0.9789 0.9328 RMSE 6.02 6.01 6.34 5.93 5.1 8.63 Parameters k1º: 0.69 k1º: 0.67 k1º: 0.94 k1º: 0.82 k1º: 0.74 k1º: 0.87 Higuchi 𝐹 = 𝑘𝐻 × 𝑡½ R2 0.9560 0.9620 0.8863 0.9066 0.9538 0.9131 RMSE 7.16 6.61 13.16 11.75 7.66 9.82 Parameters kH: 45.3 kH: 45.2 kH: 48.9 kH: 47.3 kH: 46.8 kH: 48.1 Korsmeyer-Peppas 𝐹 = 𝑘𝐾𝑃 × 𝑡𝑛 R2 0.9648 0.9699 0.8882 0.9025 0.9584 0.9660 RMSE 6.40 5.88 13.03 12.05 7.28 6.14 Parameters kKP: 49.2 n: 0.420 kKP: 48.8 n: 0.424 kKP: 53.4 n: 0.415 kKP: 50.7 n: 0.440 kKP: 50.3 n: 0.431 kKP: 55.8 n: 0.351 Peppas-Sahlin 𝐹 = 𝑘1 × 𝑡𝑚 + 𝑘2 × 𝑡2𝑚 R2 0.9964 0.9948 0.9847 0.9927 0.9936 0.9888 RMSE 2.04 2.44 4.83 3.29 2.86 3.52 Parameters k1: 65.1 m: 0.69 k2:-12.2 k1: 63.7 m: 0.68 k2:-11.7 k1: 73.9 m: 0.81 k2:-14.1 k1: 67.5 m: 0.86 k2:-11.9 k1: 65.9 m: 0.73 k2:-12.0 k1: 76.8 m: 0.58 k2:-16.5 5   Figure S2. Representative drug release fitting models. Different kinetic models (Higuchi (red), First order (turquoise), Zero order (pink), Korsmeyer-Peppas (light green), Peppas-Sahlin (blue)) were fitted to the experimental release data of fresh F1 PLGA microspheres (blue dots).  6   Figure S3. Degradation of F0 vs F0i PLGA microspheres. Optical microscope and SEM images of microspheres incubated in PBS at pH 7.4 and 37 °C. Scale bars: 5 µm.  7    Figure S4. Representative full-size histology images of lungs exposed to PLGA microspheres (F0) after intravenous injection over time. Representative full-size processed images of the H&E stained histological 30-µm coronal lungs sections at 40× magnification after different treatments. (A) Healthy and untreated lungs. (B,C,D) Healthy lungs after 1 h, 1 week and 2 weeks of injection of 3 mg/mouse PLGA microspheres (corresponding to approximately 300,000 PLGA MS/mouse). Red arrows point to some of the microspheres. Scale bars: 50 µm.  8    Figure S5. Cell toxicity of PLGA microspheres, normalized for levofloxacin content. Cell viability (%) of (A) A549, (B) H1299 and (C) HUVEC cells incubated for 24 h with F1 (dark green),  F2 (light green), F3 (teal) PLGA microspheres and hemihydrate levofloxacin (yellow). The dotted line represents 50% viability. Data are presented as mean ± SD, N = 3, n = 3.      9   Figure S6. Pharmacokinetics and biodistribution of radiolabeled PLGA microspheres.  (A) SPECT/CT images of 111In-DTPA-PLGA MS at time points of 1 to 10 days are shown with intensity individually adjusted to visualize the small activity in other organs. Radioactivities shown are thus not to scale. (B-F) Organ SUV in g/mL (mean ± SD) of the 111In-DTPA-F0i PLGA MS over 10 days, calculated from the SPECT images (N = 3) (G) Biodistribution (mean ± SD, N = 3) of 111In-DTPA-PLGA MS on day 10 after injection. Tissue uptake is expressed as % of injected dose/organ.  10 Figure S7. Pharmacokinetics and biodistribution of 111InCl3 after intravenous administration. (A) Representative maximum intensity projections (MIPs) SPECT/CT overlay images (dorsal view) of healthy C57BL/6 mice showing the in vivo distribution of 111InCl3 after intravenous tail injection over time. The radioactivity is shown in red. (B-C) Organ SUV in g/mL (mean ± SD) of the free 111InCl over 48 h, calculated from the SPECT images (N = 3). (D) Biodistribution (mean ± SD, N = 3) of 111InCl3 at 48 h after injection as % of injected dose (ID)/g of tissue and % of ID/organ.  11  Figure S8. Steps to obtain cross-sections of PLGA microspheres with FIB-SEM.  Scale bars: 10 µm.    Movie S1. Production of microspheres by flow focusing.  Movie S2. SPECT/CT animated image 15 min after intravenous administration of 111In-DTPA-PLGA microspheres.  

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