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Liposomal immunosuppressants for the management of organ transplantation

University of British Columbia

This thesis examines liposomal drug carriers for the delivery of immunosuppressive agents. A finite amount of cyclosporine was found to incorporate into liposomes. The amount of cyclosporine incorporated was inversely related to the amount of cholesterol present in the liposomal bilayer. Cyclosporine incorporated into liposomes remained associated with this carrier when subject to pore filtration, gel exclusion chromatography or density gradient ultracentrifugation. When donor liposomes carrying cyclosporine are mixed with acceptor liposomes devoid of cyclosporine however, a rapid redistribution of cyclosporine occurred between two liposome populations. This redistribution, proportional to the relative size of the lipid sink (acceptor) compared to the initial liposomal cyclosporine formulation, observed in vitro would predict a similar rapid but even larger loss of the cyclosporine in vivo. In mice, liposomal cyclosporine plasma elimination and tissue biodistribution of the drug confirmed the hypothesis that the cyclosporine does not stay with the liposome as predicted by the in vitro studies. Furthermore, the tissue biodistribution of cyclosporine differed from that of its liposome carrier. A comparison between liposomal versus cremophor cyclosporine resulted in insignificant differences in the plasma elimination rates and the extent of accumulation in tissues of the drug. When liposomal cyclosporine was tested in a rat heterotopic heart transplant model, the liposomal carrier accumulated in statistically significant higher levels in the cardiac grafts compared to that of hosts' hearts. Cyclosporine delivery to the cardiac grafts was, however, not improved. Therefore, liposomes are good candidates to deliver drugs to cardiac graft sites but cyclosporine is not readily deliverable in liposomes. Fast protein liquid chromatography (FPLC) was studied to address the challenge of characterizing drug loss. FPLC was determined to be an extremely powerful method for the separation of liposomes from plasma components in order to monitor component loss from liposomal drug formulations. This method for isolating liposomes from plasma components is regarded to have broad applicability to the study of liposomal component exchange from various liposomal drug carrier formulations. The approach taken to address the problem of rapid drug exchange was to utilize antibodies that could be covalently attached to the liposome exterior. Certain antibodies directed against lymphocytes have been identified to have immunosuppressive properties. For the purposes of establishing the feasibility of delivering immunosuppressive antibodies coupled to liposomes, however, a readily available antibody, human IgG, was utilized to study whether the antibody remains with its liposomal carrier in vivo. When the antibodycoupled liposomes are injected into severely combined immunodeficient (SCfD) mice, similar plasma elimination characteristics of human IgG and liposomes provided strong evidence for the stability of the covalent attachment between human IgG and the liposome. The FPLC separation of liposomes from plasma components developed in this thesis confirmed that the antibody, human IgG, remained attached to liposome in plasma. The biodistribution of the antibody-coupled liposome formulation varied in each type of tissue. Histochemical analysis revealed co-localization of the human IgG and the liposomes in the spleen and, to a lesser extent, in the liver. These results provide evidence for the feasibility of attaching other antibodies to liposomes in order to take advantage of the carrier's ability to accumulate at sites of ongoing rejection.

Thesis/Dissertation

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2009-06-02

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http://hdl.handle.net/2429/8552

Pharmacology

University of British Columbia

UBCV

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