pH-dependent release property of dioleoylphosphatidyl ethanolamine liposomes

pH-sensitive liposomes were prepared by a detergent removal method. Dioleoylphosphatidylethanolamine (DOPE) and cholesteryl hemisuccinate (CHEMS) were combined for the preparation of the liposomes so that the molar ratios of DOPE to CHEMS are 9/1, 8/2, 6/4 and 5/5. On transmission electron micrographs, hundreds of nm sized-multilamella vesicles were observed. The degrees of fluorescence quenching were approximately 70–80%, indicating that closed vesicles were formed. According to the results of the pH-dependent release experiment with the liposome composed of DOPE/CHEMS (5/5), no significant release was observed in the pH region ranging from 6 to 8. At pH of 5, an appreciable amount of calcein was released. The patterns of pH-dependent releases from liposomes composed of DOPE/CHEMS (6/4) and DOPE/CHEMS (8/2) were almost the same as those from liposomes composed of DOPE/ CHEMS (5/5). With the liposomes composed of DOPE/CHEMS (9/1), unlike the other liposomes described above, almost 90% release was observed at pH 6. In this case, since the amount of a complementary molecule, CHEMS, is relatively small, the liposomes would be subjective to destabilization even at a small change in the degree of deionization of the carboxylic group. This may explain why the liposome of DOPE/CHEMS (9/1) exhibits a significant release at a relatively high pH, pH 6.0.