Activity of amphipathic poly(ethylene glycol) 5000 to prolong the circulation time of liposomes depends on the liposome size and is unfavorable for immunoliposome binding to target

Dioleoyl-N-(monomethoxy polyethyleneglycol succinyl)-phosphatidylethanolamine (PEG-PE) (mol. wt. of PEG = 5000), an amphipathic polymer, can be incorporated into the liposome membrane and significantly prolong the blood circulation time of the liposome. As little as 3.7 mol% of PEG-PE in liposome resulted in maximal enhancement of liposome circulation time. However, this activity of PEG-PE was only seen with relatively small liposomes (d ≤ 200 nm); larger liposomes containing PEG-PE showed an unusually high level (approx. 35% injected dose) of accumulation in the spleen. We have tested whether the small, PEG-PE containing liposomes are suitable for immuno targeting by incorporating a lung-specific monoclonal antibody on the liposome surface. While another amphiphile, ganglioside GM₁, which is well known for its activity to prolong the liposome circulation time, significantly enhanced the lung binding of the immunoliposomes, PEG-PE incorporation of immunoliposomes resulted in a low level of target binding. To test if the reduced target binding is due to a steric barrier effect of the surface PEG polymer, we have incorporated a small amount of N-biotinaminocaproylphosphatidylethanolamine into the PEG-PE containing liposomes and examined the liposome agglutination induced by the addition of streptavidin. As little as 0.72 mol% PEG-PE in these liposomes completely abolished agglutination. In contrast, incorporation of GM₁ in liposomes only reduced the rate, but not the extent, of liposome agglutination. These results strongly support the hypothesis that PEG-PE prolongs liposome circulation time by providing a strong steric barrier which prevents close contact with another liposome or cell. Since GM₁ provides only a weak steric barrier effect, its activity to prolong the liposome circulation time must involve another yet unknown mechanism.