Determination of the Thickness of the Fixed Aqueous Layer Around Polyethyleneglycol-coated Liposomes

The zeta potentials of adriamycin-encapsulating liposomes containing 1-(monomethoxy polyethyleneglycol)-2,3-dimyristoylglycerol (PEG-DMG) were measured in an isotonic solution of 10mM lactate buffer (pH 4.0) with sodium chloride and sucrose. The negativity of the zeta potentials of adriamycin-encapsulating liposomes containing PEG-DMG decreased with increases in NaCl concentration more steeply than that of adriamycin-encapsulating liposomes without PEG coating. From this observation, the electrical potential distributions near the membrane surfaces were shown to be different between adriamycin-encapsulating liposomes with and without PEG coating. Based on these zeta potential data, the thickness of the fixed aqueous layer around PEG-DMG-containing adriamycin liposomes was determined from the slope of 1n zeta potential versus Debye-Hückel parameter plot. As a result, a correlation was indicated to exist between the circulation time of liposomes and the thickness of the aqueous layer around the liposomes.