Biol. Pharm. Bull. 30(1) 208—211 (2007)

avoid from catch by the reticuloendothelial system (RES). It has been clarified that the prolongation of liposome circulation in the blood and passive targeting to the tumor was achieved by PEG modification of the liposomal surface. In terms of their physicochemical properties by PEG modification, many studies have examined the interaction between the prolonged circulation time of liposomes and the conditions of the PEG-molecule on the liposomal surface. The fixed aqueous layer thickness (FALT) around the liposome was measured using interfacial electrochemistry and it was reported that the FALT of PEG-modified liposomes increased in comparison with PEG unmodified liposomes. PEGalkyl lipid derivatives have different physicochemical properties depending on the molecular weight of the PEG-moiety, the fatty acid length as the anchor, and the conformation of the spacer unit. In our previous paper, we reported the mixed PEG-modified liposomes obtained with a mixture of PEG of short and long polyoxyethylene chains were useful. Namely, the mixed PEG modification with different PEGmolecular weights increased the FALT, compared to that of single PEG modification. Furthermore, we confirmed that mixed PEG modification of the doxorubicin (DOX)-containing liposomes improved the biodistribution and increased in the antitumor activity compared to that of the single PEG modifications, and are superior. On the other hand, there are different mixed modifications by two PEGs that have different anchor units with the same molecular weight. However, the characteristics (maximum FALT, most suitable ratio of PEG lipid and incorporated ratio of PEG lipid, etc.) of these liposomes are not clear. In this study, we used PEG-2,3-distearoylglycerol (PEG-DSG) with an alkyl anchor and cholesterol-PEG (PEG-CHO) with a cholesterol anchor, which has different characteristics as the surface material of the liposomes, and examined the physicochemical characteristics of mixed PEG-modified liposomes with different anchor units (with the same molecular weight). MATERIALS AND METHODS