Influence of stabilizing components on the integrity of antitumor liposomes loaded with lipophilic prodrug in the bilayer.

Previously, we proposed a liposomal formulation of melphalan (Mlph)-a chemotherapeutic alkylating agent-incorporated in a fluid lipid bilayer in the form of dioleoylglyceride ester. In this work, we compared the stabilizing effect of different amphiphiles included in the Mlph-liposomes, such as phosphatidylinositol (PI), ganglioside GM, a conjugate of N-carboxymethyl-modified oligoglycine with dioleoylphosphatidylethanolamine (acidic lipopeptide), and polyethylene glycol (2000 Da) conjugated with dipalmitoylphosphatidylethanolamine (PEG-lipid), upon incubation in human serum. Mean hydrodynamic diameter values (86-90 nm) were similar among different liposome samples, while zeta potential values considerably varied. The formulations were incubated in human serum at 37 °C for different time intervals up to 24 h. Liposome integrity was evaluated by changes in fluorescence upon leakage of calcein or disruption of Förster resonance energy transfer between donor and acceptor fluorescent lipid probes in the bilayer. The best stabilization of liposomes was achieved upon the addition of ganglioside GM or the acidic lipopeptide. Inclusion of 10 mol% PI improved liposome stability only for the first 4 h of incubation. Pegylated liposomal formulations of melphalan lipophilic prodrug with fluid phase bilayer were the least stable, which is probably due to the propensity of the PEG-lipid to exit liposome membranes. Cholesterol-containing bilayers of liquid ordered phase, supplemented with sufficient amounts of the PEG-lipid, showed good stability in serum.