Amphotericin B is an antibiotic that forms ion channels in the membrane of a host cell. The change in permeability produced by these channels is greatly improved by sterols; nevertheless, the single channel conductivity remains invariant. Hence, it is proposed that sterols do not act directly, but rather through the modulation of the membrane phase. We look at the formation of these channels in the bacterial membrane to determine the mechanism of its known antibiotic resistance. We found that channels can indeed be formed in this membrane, but a substantial amount of amphotericin B is required. We also study the effects of the antibiotic concentration needed for channel expression as well as the dynamics of channels affected by both sterol and temperature in phosphatidylcholine membranes. The results support the idea that membrane structure is a determining factor in the action of the antibiotic.

Amphotericin B is the most potent antimycotic known to date. However due to its large collateral toxicity, its use, although long standing, had been limited. Many attempts have been made to produce derivatives with reduced collateral damage. The molecular mechanism of polyene has also been closely studied for this purpose and understanding it would contribute to the development of safe derivatives. Our study examined polyene action, including chemical synthesis, electrophysiology, pharmacology, toxicology and molecular dynamics. The results were used to support a novel Amphotericin B derivative with increased selectivity: L-histidine methyl ester of Amphotericin B. We found that this derivative has the same form of action as Amphotericin B, i.e. pore formation in the cell membrane. Its reduced dimerization in solution, when compared to Amphotericin B, is at least partially responsible for its increased selectivity. Here we also present the results of preclinical tests, which show that the derivative is just as potent as Amphotericin B and has increased safety.

A detailed and thorough characterization of nystatin-induced permeability on lipid bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)-containing ergosterol or cholesterol is presented. The results show that the same collection of transmembrane pores appears in membranes containing either sterol. The concentration range for the appearance of these pores is sterol-dependent. Another mechanism of action, membrane disruption, is also observed in ergosterol-POPC membranes. The greater potency of nystatin present in ergosterol-containing membranes cannot be explained simply by the longer opening times of its pores, as has been suggested; it is also due to an increased number of events in these membranes. The present results and those of a companion paper lead us to propose that membrane structure is the determining factor for drug selectivity in membranes with different sterols.

Pores formed by the polyene antibiotic nystatin were studied in solvent-free lipid membranes. The membranes were formed by the tip-dip technique using 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with different mol fractions (0-50%) of cholesterol or ergosterol. The effects of the mol fraction of sterol and of temperature variation (15-35°C) on the activity of the pores, their unitary conductances, lifetimes and time average conductances were studied. The results were used to analyze the behavior of nystatin channels along the phase diagrams previously reported for these lipid mixtures and to propose that membrane structure is the determinant factor for the known ergosterol/cholesterol selectivity.