Role of glycolipids on liposome biodistribution, tumor uptake and immunotargeting

Several variables important in the construction of liposomes with reduced affinity to the reticuloendothelial system (RES) and thus prolonged circulation time were examined. Studies using a series of dioleoyl N-(monomethoxy polyethyleneglycol succinyl) phosphatidylethanolamine (PEG-PE) of different polymer chain lengths showed that both the activity of PEG-PE in prolonging the circulation time of liposomes and the relative steric barrier activity of the amphipathic polymer, measured by a liposome agglutination assay, were directly proportional to the polymer chain length (PEG5000-PE > PEG2000-PE > PEG750-PE). These results strongly support the hypothesis that PEG-PE exerts its activity in prolonging the circulation time of liposomes by presenting a steric barrier on liposome surfaces such that interactions of liposomes with the RES were reduced. In contrast, the activity of GM₁ in prolonging the circulation time of liposomes did not correlate with its relatively weak steric barrier activity, suggesting a different mechanism for this glycolipid. Systematic size-dependent biodistribution study with GM₁-containing liposomes revealed that there was a relatively restricted size requirement for the activity of GM₁ in prolonging the circulation time of liposomes. The optimal size appeared to be 70~200 nm in average diameter; larger and smaller liposomes accumulated efficiently in the spleen and the liver, respectively. The effectiveness of the long-circulating liposomes was demonstrated in both passive and active targeting studies of liposomes to non-RES tissues. Tumor-targeting studies of liposomes, using the transplantable solid tumor (EMT6) model, showed that the long-circulating liposomes were able to accumulate more efficiently than conventional liposomes in tumor. Furthermore, target binding studies of immunoliposomes using a lung endothelial model showed that, when GM₁ or PEG2000-PE was included in the lipid composition to reduce their affinity to the RES, the resulting immunoliposomes showed more efficient target binding than the ones with the conventional lipid composition. However, PEG5000-PE caused a significantly reduced target binding of immunoliposomes presumably due to its overly strong steric barrier activity which may interfere with the antibody-antigen interaction.