Mechanism of the selective toxicity of amphotericin B incorporated into liposomes.

Previously, it has been shown that incorporation of the membrane channel-forming polyene antibiotic, amphotericin B (AMB), into liposomes composed of dimyristoyl phosphatidylcholine/dimyristoyl phosphatidylglycerol (7:3 ratio) results in reduced drug toxicity to animals with full retention of therapeutic activity against systemic fungal infections. In this report we explore the cellular and biochemical bases of the enhanced therapeutic index of liposomal amphotericin B (L-AMB). AMB and L-AMB are equally potent and both promptly induce rapid cation efflux from Candida albicans cells. By contrast, AMB, but not L-AMB, induces cation efflux and cell lysis in mammalian erythrocytes, demonstrating the selectivity of L-AMB at the cellular level. The characteristics of the lipid of the erythrocyte membrane seem to be the most important determinant of cellular sensitivity, since AMB, but not L-AMB, induces cation release from large unilamellar liposomes composed of red cell membrane lipids, thus paralleling the observations on intact cells. The ability of L-AMB to induce cation release and cause toxicity to erythrocytes, however, can be modulated by changing the lipid composition of the liposome carrier. Thus, AMB-containing liposomes composed of phospholipids with saturated acyl chains are nontoxic, whereas AMB liposomes composed of phospholipids containing unsaturated acyl chains are almost as toxic as AMB itself. The acyl chain composition rather than the head group composition seems most important, although substitution of anionic phosphatidylglycerols for phosphatidylcholines contributes somewhat to the protective effect. Analysis of several types of liposomes containing AMB at concentrations up to 5 mol %, using electron paramagnetic resonance and freeze fracture electron microscopy, shows that the drug is incorporated in the lipid bilayer but produces only modest disruptive effects on bilayer structure. Current results are interpreted in terms of a selective transfer of AMB from "donor" liposomes to "target" cell membranes. The transfer process probably occurs by diffusion of AMB through the solvent but is regulated by the physical properties of both donor and target membranes.