New Amphipatic Polymer-Lipid Conjugates Forming Long-Circulating Reticuloendothelial System-Evading Liposomes

Lipid-conjugates of two amphipatic polymers, poly(2-methyl-2-oxazoline) (PMOZ) and poly(2-ethyl-2-oxazoline) (PEOZ) (degree of polymerization approximately 50) were synthesized by linking glutarate esters of the polymers to distearoylphosphatidylethanolamine (DSPE) or alternatively by termination of the polymerization process with DSPE. Surface-modified liposomes (90 +/- 5 nm) prepared from either conjugate (5 mol % of total lipid) were injected into rats and followed by blood level and tissue distribution measurements. Both polymers PEOZ and PMOZ were found to convey long circulation and low hepatosplenic uptake to liposomes to the same extent as polyethylene glycol (PEG), the best known material for this purpose. This is the first demonstration of protection from rapid recognition and clearance conveyed by alternative polymers, which is equal to the effect of PEG.