AMPHOTERICIN-B TOXICITY AND LETHALITY - A TALE OF 2 CHANNELS

In spite of its toxic side effects, Amphotericin B (AmB) is the most e ffective 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 a ctivity and of toxicity to the host are not yet fully understood. The classical pore model of AmB action postulates that a single type of ch annel permeable to monovalent cations and anions is responsible for th e AmB fungicidal and leishmanicidal action, as well as for its toxic s ide effects. Thus, the loss of membrane cation selectivity, caused by AmB aqueous pores has been proposed as the primary cause of fungal cel l death. The hypothesis has been that fungal cell death results from i ntracellular acidification associated with K+ leakage. However, this m echanism 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 aq ueous pores by AmB in sterol-containing liposomes is always preceded b y the formation of non-aqueous channels. At low AmB concentrations or in the absence of sterols, AmB non-aqueous channels do not evolve to f orm 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 m embrane that non-aqueous channels interact with ergosterol or choleste rol to form transmembrane aqueous pores. The pore diameter of the chan nels formed by AmB is critical for AmB toxicity or lethality because n on-aqueous channels are only permeable to urea and monovalent cations, whereas AmB aqueous pores are permeable to monovalent cations and ani ons (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 f rom osmotic lysis by the presence of a cell wall. but an increased H+/ OH- permeability across AmB aqueous pores leads to an elevation of int racellular 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 c ells may be caused by increased salt, Ca2+ and/or H+ permeability acro ss aqueous pores. A sustained collapse of pH and Ca2+ gradients is a m echanism which is also exhibited by molecular inducers of programmed c ell death (apoptosis) in eucaryotic cells. (C) 1998 Elsevier Science B .V. All rights reserved.