ESTIMATION OF SURFACE-STATE OF POLY(ETHYLENE GLYCOL)-COATED LIPOSOMESUSING AN AQUEOUS 2-PHASE PARTITIONING TECHNIQUE

Poly(ethylene glycol)-coated liposomes (PEG-liposomes) were prepared f rom distearoylphosphatidylcholine (DSPC)/cholesterol (Ch) (1:1, molar ratio) with various amounts of distearoyl-N-(monomethoxy poly(ethylene glycol)succinyl)phosphatidylethanolamine (DSPE-PEG), Surface potentia ls of PEG-liposomes showed negative values, however, the zeta potentia ls were almost neutral under physiological conditions (150 mM NaCl), T aking these electrical surface properties into consideration, a non-ch arge-sensitive phase system consisting of 5% PEG8000 and 5% dextran T- 500, 0.01 M sodium phosphate, 0.15 M sodium chloride (pH 7.0) was used to estimate the alteration of surface state of PEG-liposomes after in teraction with plasma in vitro and in vivo, PEG-liposomes showed incre ased partitioning to the PEG phase with increasing amount of DSPE-PEG. One hundred percent partitioning to the PEG phase was obtained when 2 or 1 mol% of DSPE-PEG1K or 2K was incorporated into the liposomes, re spectively. This PEG/lipid ratio (mol/mol) thus afforded complete prot ection of the Liposomal surface by the PEG moiety, When these PEG-lipo somes were incubated with plasma protein (in vitro) or were recovered from liposome-injected mice (in vivo), they showed decreased partition ing to the PEG phase, However, when the in vivo-treated PEG-liposomes were purified by column chromatography and ultracentrifugation, their partitioning to the PEG phase was restored to that of PEG-liposomes in cubated in phosphate-buffered saline, Thus although PEG acts as a ster ic barrier against the attachment of plasma protein to the liposome su rface and slows down liposome clearance from the circulation in vivo, a weak interaction remains between PEG-liposome and plasma protein whe n the incorporated amount of DSPE-PEG is low.