Studies on Factors influencing the elimination of liposomes from the circulation
Creators: name: Oja, Conrad D.
Publisher: name: University of British Columbia
Year: 2009

description: This thesis concerns the interactions experienced by liposomal drug delivery systems as they circulate in the blood and their influence on elimination behaviour. These studies are divided into three areas, which investigate three factors influencing liposome elimination behaviour - lipid dose, poly(ethylene glycol) (PEG)-polymer coating and the generation of an immune response to liposome systems. First, the influence of lipid dose on elimination has previously been attributed to 'saturation' of the free and fixed macrophages of the reticuloendothelial system (RES) at high lipid doses. Here the potential role of blood protein binding by liposomes is examined. Protein binding and elimination properties of two representative compositions of large unilamellar vesicles were examined in mice over a dose range of 10-1000 mg lipid / kg body weight. Although longer half-lives were observed for higher doses even the highest lipid doses of the most rapidly cleared liposome compositions failed to completely saturate RES uptake. However, these higher dose liposomes did bind significantly less protein as measured on a protein-to-lipid basis. These results suggest the existence of a specific pool of blood proteins that interacts with liposomes of a given composition, and is diluted over larger surface areas at higher lipid doses, resulting in less efficient protein-mediated RES uptake. A second method of influencing liposome elimination behaviour is through "steric stabilization" of liposomes by incorporation of PEG polymer coatings. This enhances circulation lifetimes and increases delivery of drugs to sites of disease. It has been conjectured that reduction of blood protein adsorption to liposomes is the primary mechanism leading to longer circulation lifetimes of these systems, however this remains to be confirmed in vivo. These studies employed three representative lipid compositions to demonstrate that incorporation of PEG induces a nonspecific reduction of blood protein bound to vesicles in the circulation of mice. This reduced blood protein adsorption correlated with the increased circulation lifetimes of the vesicles. Dosedependent changes in circulation lifetime, similar to those observed for conventional lipid compositions, were noted. A third factor which can dramatically alter liposome elimination behaviour is the generation of immune responses - particularly against the surface-coupled antibodies or ligands of targeted liposomes. This results in rapid elimination of subsequent administrations, therefore limiting potential applications. Liposomes are known to interact with cells responsible for antigen processing and presentation, antibody production and cell-mediated immunity. Therefore, it was investigated whether the toxic effects of encapsulated drugs on cells of the immune system could solve this problem. Liposome elimination and humoral immune response were monitored for repeated administrations of doxorubicin encapsulated in liposomes with ovalbumin covalently coupled to the surface. The results showed that, at high drug-to-lipid ratios low doses of encapsulated doxorubicin prevented humoral immunity against repeated administration of ovalbumin-proteoliposomes. Immunosuppression was specific for the ovalbumin bound to drugloaded vesicles at low drug doses. This suggests a selective suppression of immunity against the target ligand for low doses of doxorubicin-loaded targeted liposomes which could prove advantageous for safe repeated administration.