Triggerable liposomal fusion by enzyme cleavage of a novel peptide-lipid conjugate

A novel peptide-lipid sensitive to enzyme cleavage was designed to generate liposomes that could be triggered to fuse by enzymatic activation. Covalent linkage of dioleoyl phosphatidylethanolamine (DOPE) to an elastase substrate, N-acetyl-ala-ala-, resulted in a cleavable peptide-lipid (N-Ac-AA-DOPE) with no intrinsic fusogenic activity. Cleavage of N-Ac-AA-DOPE and concomitant conversion to the fusogenic lipid DOPE could be detected after treatment with human leukocyte elastase or proteinase K, two proteases with similar substrate specificities. A strategy to utilize this cleavage to trigger fusogenicity was tested by modeling the fusion of liposomes containing the expected product of complete cleavage. Based on these results liposomes were designed to contain N-Ac-AA-DOPE, DOTAP, and PE in the ratio of 15/15/70. These liposomes exhibited lipid mixing with acceptor liposomes after elastase or proteinase K protease treatment. Activation of fusion, as monitored by a lipid mixing assay, appeared to be dependent on protease activity, as (1) heat inactivated enzyme did not activate liposomal fusion, and (2) the time and concentration dependence of proteinase K mediated cleavage of N-Ac-AA-DOPE correlated with membrane mixing. Liposomes could also be formulated that exhibited lipid mixing and transfer of aqueous fluorescent probe with erythrocyte ghosts. These observations demonstrate fusogenic lipids conjugated to enzyme substrates serve as triggerable fusion systems that may be useful for gene and drug delivery.