Temperature-dependent transfer of amphotericin B from liposomal membrane of AmBisome to fungal cell membrane

Abstract

Liposomal amphotericin B (AMPH-B), also known as AmBisome, exhibits a potent antifungal effect through its binding to ergosterol contained within the fungal cell membrane. However, the mechanism responsible for the movement of AmBisome-derived AMPH-B to the fungal cell membrane through the cell wall is not yet clear. Therefore, in the present study we aimed at elucidating this mechanism operating in Saccharomyces cerevisiae. AmBisome showed its antifungal effect against S. cerevisiae at 35 °C but not at 4 °C, whereas free AMPH-B was effective at both temperatures. A significant difference in the amount of AMPH-B transferred to the fungal cells between incubation at 4 and 35 °C was also observed when AmBisome was used. Confocal microscopic study, however, indicated that NBD-labeled AmBisome was localized on the surface of the fungal cells at either temperature. To decrease the affinity of AMPH-B for the liposomal membrane, we entrapped AMPH-B in fluid liposomes containing egg yolk phosphatidylcholine (EPC) instead of hydrogenated soy PC (HSPC). These liposomes showed the antifungal effect even at 4 °C. On the contrary, AMPH-B in liposomes containing ergosterol (Erg-AmB) instead of cholesterol showed a significantly weaker antifungal effect at 35 °C with reduced transfer of AMPH-B to the fungal cells. These results suggest that not the binding of AmBisome to target cells but the transfer of AMPH-B from liposomal membrane of AmBisome to the cell membrane is critical for the antifungal activity of AmBisome. This transfer is dependent on the temperature, fluidity of the liposomal membrane, and the affinity of AMPH-B for the fungal cell membrane.