Amphotericin B toxicity and lethality: a tale of two channels

In spite of its toxic side effects, Amphotericin B (AmB) is the most effective drug in the treatment of visceral Leishmaniasis and systemic fungus infections. However, the toxic side effects of this drug can be substantially reduced when delivered as a complex with liposomes and other lipidic systems. Nonetheless, the mechanisms of AmB antifungal activity and of toxicity to the host are not yet fully understood. The classical pore model of AmB action postulates that a single type of channel permeable to monovalent cations and anions is responsible for the AmB fungicidal and leishmanicidal action, as well as for its toxic side effects. Thus, the loss of membrane cation selectivity, caused by AmB aqueous pores has been proposed as the primary cause of fungal cell death. The hypothesis has been that fungal cell death results from intracellular acidification associated with K+ leakage. However, this mechanism of AmB action has been challenged by several investigators on the basis that low AmB concentrations cause K+ leakage from sensitive cells which is dissociated from the lethal effects produced by higher AmB concentrations. Our own studies indicate that the formation of aqueous pores by AmB in sterol-containing liposomes is always preceded by the formation of non-aqueous channels. At low AmB concentrations or in the absence of sterols, AmB non-aqueous channels do not evolve to form aqueous pores, nor is the K+ leakage they produce lethal to cells. It is only when a 'critical' concentration of AmB is reached at the membrane that non-aqueous channels interact with ergosterol or cholesterol to form transmembrane aqueous pores. The pore diameter of the channels formed by AmB is critical for AmB toxicity or lethality because non-aqueous channels are only permeable to urea and monovalent cations, whereas AmB aqueous pores are permeable to monovalent cations and anions (including H+ and OH-) and divalent cations such as Ca2+. In fact, leishmanias are killed rapidly by colloid osmotic lysis due to a net salt influx across the AmB aqueous pores. Fungal cells are protected from osmotic lysis by the presence of a cell wall. but an increased H+/OH- permeability across AmB aqueous pores leads to an elevation of intracellular pH which then results in membrane damage. In host mammalian cells, non-aqueous channels appear to be responsible for some of the toxic but reversible side effects produced by AmB. However, more acute and damaging effects such as those exerted by AmB in kidney tubular cells may be caused by increased salt, Ca2+ and/or H+ permeability across aqueous pores. A sustained collapse of pH and Ca2+ gradients is a mechanism which is also exhibited by molecular inducers of programmed cell death (apoptosis) in eucaryotic cells. (C) 1998 Elsevier Science B.V. All rights reserved.