Cholesterol phosphate derivatives: synthesis and incorporation into a phosphatase and calcium-sensitive triggered release liposome.

A series of cholesterol derivatives that position a phosphate monoester at increasing distance from the sterol ring system was synthesized, and their utility as a triggered release liposome tested. Stable anionic liposomes consisting of the novel cholesterol phosphate derivatives and dioleoylphosphatidylethanolamine (DOPE) can be induced to collapse upon phosphatase-catalyzed removal of the phosphate group. Control liposomes containing DOPE and cholesterol phosphate or phosphatidic acid, which are not phosphatase substrates, do not undergo phosphatase-mediated collapse. The phosphatase-sensitive liposomes also collapse in the presence of calcium. The precise concentration of calcium that induces the collapse is controlled by the structure of the cholesterol phosphate derivative. Plasmid DNA encoding luciferase, encapsulated in the cholesterol derivative/DOPE liposomes, transfected cells in vitro. The level of transfection is dependent upon the cholesterol derivative and is mediated by both a calcium-independent and a calcium-dependent pathway; however, the involvement of phosphatase in the latter mechanism is not yet resolved. The transfection efficiency is between 10(6) and 10(7) of luciferase activity in relative light units per milligram of protein, which is similar to transfection values reported using other triggered release liposomes.