pH sensitivities of egg phosphatidylcholine liposomes and dioleoylphosphatidylethanolamine liposomes triggered by poly(N-isopropylacrylamide-co-methacrylic acid-co-octadecylacrylate)

Egg phosphatidylcholine (PC) liposomes bearing pH-sensitive polymers and dioleoylphosphatidylethanolamine (DOPE) liposomes including the same polymers were prepared by a sonication method. As pH-sensitive polymers, copolymers of N-isopropylacrylamide, methacrylic acid, and octadecylacrylate were used. The liposomes were stable in neutral pH ranges in terms of release. But the release became marked at pH 5.5, and it was accelerated as pH further decreased. For example, the degree of release from egg PC liposomes (polymer/lipid ratio is 3:10, w/w) for 120 s increased from 2% to 63% as pH decreased from 7.5 to 4.5. Under the same condition, the degree of release from DOPE liposomes increased from 4% to 80%. These results indicate that DOPE liposome is more pH-sensitive than egg PC liposome.