Elastase activated liposomal delivery to nucleated cells.

The specific activation of liposomes for delivery has been explored by enzyme mediated cleavage of a peptide substrate covalently conjugated to a fusogenic lipid. We have previously shown an elastase sensitive peptide conjugated to 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine [corrected] (DOPE) could be activated by enzymatic cleavage, triggering liposome-liposome lipid mixing and fusion with erythrocyte ghosts (Pak et al., Biochim. Biophys. Acta, 1372 (1998) 13-27). Further optimization of this system has been aimed at obtaining substrate cleavage at or below physiological elastase levels and to demonstrate triggered delivery to living cells. Therefore a new peptide-lipid, MeO-suc-AAPV-DOPE (N-methoxy-succinyl-Ala-Ala-Pro-Val-DOPE), has been developed that exhibits greater sensitivity and selectivity for elastase cleavage and subsequent conversion to DOPE. This peptide-lipid was used with DODAP (dioleoyl dimethylammonium propane, a pH dependent cationic lipid) in a 1:1 mol ratio with the expectation that endocytosis would lead to a liposome with an overall positive charge if enzymatic cleavage had occurred. Elastase treated liposomes displayed pH dependent enhancement of binding, lipid mixing, and delivery of 10000 MW dextrans, relative to untreated liposomes, when incubated with HL60 human leukemic cells. Heat denatured elastase did not activate DODAP/MeO-suc-AAPV-DOPE liposomes, indicating enzymatic activity of elastase is necessary. Liposomes bound to ECV304 endothelial cells at physiological pH could be activated by elastase to deliver an encapsulated fluorescent probe, calcein, into the cell cytoplasm. These results suggest enzyme substrate peptides linked to a fusogenic lipid may be used to elicit specific delivery from liposomes to cells.