Determination of incorporated amounts of poly(ethylene glycol)-derivatized lipids in liposomes for the physicochemical characterization of stealth liposomes.

We describe a method for determining incorporated amounts of poly(ethylene glycol) (PEG)-derivatized lipids in liposomes for the physicochemical characterization of PEG-coated liposomes. This method is based on the spectrophotometric determination of complexes of polyethers with sodium ions after their extraction as picrates into 1,2-dichloroethane, developed by Favretto for measuring levels of polyoxyethylene alkylphenyl-ether non-ionic surfactants in waste water. The same assay was applied to the estimation of PEG-derivatized lipids in liposomes and percent incorporation of PEG-derivatized lipids into liposomes was successfully determined. To prevent the interference from liposomal lipids other than PEG-derivatized lipids in this assay, liposomal samples were diluted at least to a concentration of less than 0.2 mM. The percent incorporation of PEG-lipids varied, depending on the molecular weight of PEG and anchor acyl chain length in PEG-lipids and it was suggested that the percent incorporation of PEG-lipids into liposomes would be a good parameter of quality control of PEG-liposomes in manufacturing facility and the picrate method used in the present study allows for the determination of this parameter without the need for hazardous radioisotopes.