Some negatively charged phospholipid derivatives prolong the liposome circulation in vivo.

A series of negatively charged phospholipid derivatives has been synthesized by coupling aliphatic dicarboxylic acids, HOOC(CH2)nCOOH, to dioleoylphosphatidylethanolamine (DOPE). The individual derivatives were incorporated into egg phosphatidylcholine/cholesterol liposomes (2:1, molar ratio) and injected into mice to test its effect on liposome circulation in vivo. The effectiveness of DOPE derivatives was dependent on the hydrocarbon chain length between the terminal carboxyl group and the amide bond. N-Glutaryl DOPE and N-adipyl DOPE were effective in prolonging the circulation time of liposomes. On the other hand, liposome uptake by the liver and spleen was increased by the addition of N-malonyl DOPE or N-succinyl DOPE, while it was not changed by the addition of N-pimelyl DOPE and N-suberyl DOPE. Our observation suggested that not all negatively charged phospholipids enhance liposome uptake by RES, some even reduce the uptake.