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Factors influencing the biodistribution of liposomal systems Sommerman, Eric Frank


Liposomes have important potential as drug delivery vehicles. However, in order to realize this potential, much basic research is required to elucidate the interactions experienced by liposomes in vivo. In this thesis two aspects of these interactions are investigated: the influence of vesicle size and lipid composition on the biodistribution observed in vivo; and the interaction of liposomes with plasma proteins. In order to determine the in vivo behavior of liposomal systems, a new vesicle marker is synthesized (¹²⁵I-tyraminyl-inulin, ¹²⁵ITI) and tested in vivo. It is shown that this probe satisfies the necessary criteria for an accurate marker of liposome behavior, and is superior to probes used by other workers in terms of accuracy, convenience, high specific activity, low tissue quenching and cost. The use of ¹²⁵ITI as a vesicle marker allows accurate measurements to be made with lower doses of liposomes than previously employed. The influence of vesicle size, composition, and dose on the blood residency times, leakage and tissue distributions of vesicles was therefore investigated at these lower doses, employing a cannulation procedure to monitor vesicles. It is demonstrated that the clearance of vesicles from the circulation exhibits biphasic kinetics. The relative number of vesicles cleared during the early phase (halflife <20 min) is decreased by increasing the vesicle dose or decreasing the size. The behavior of small vesicles produced by extrusion is also investigated, and the in vivo behavior of these systems is shown to be equivalent to conventional sonicated systems. The second part of this thesis investigates the binding of plasma proteins to vesicles in vitro. It is shown that vesicles bind a large number of plasma components and that the binding is strongly dependent on the surface charge of the vesicle. Some of the proteins have been tentatively identified with 2-D electrophoresis and several were positively identified via immuno- autoradiography. A hypothesis is advanced regarding the role of plasma proteins in the fate of liposomes in vivo.

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