Probing the mode of action of amphotericin B

• Main Author: James, Phillip Richard
• Published: University of Leicester 1996
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.674431

The antifungal agent amphotericin B is the treatment of choice for many topical and leep-seated systemic fungal infections, despite its potent human toxicity. The mode of action is believed to involve the formation of ion channels across the plasma membrane of the fungal cells, leading to the loss of essential electrolytes such as potassium ions. Evidence suggests that the channels arise as a result of complexation of the antifungal agent with membrane steroids, both in fungal cells (ergosterol) and human cells (cholesterol), the latter being responsible for the toxicity of the drug. In order to span the plasma membrane, to create a continuous channel, such channels would be expected to involve pairs of steroid molecules laying end to end along the radial axis of the cell. To examine this proposed mode of action, a series of tail-to-tail linked dimeric steroid analogues, capable of spanning the entire membrane bilayer, have been prepared, in a seven step synthesis from stigmasterol. These have been incorporated into artificial membranes (liposomes), and their ability to participate in transmembrane pore assembly, in the presence of amphotericin B, have been compared with that of cholesterol, using an assay based on sodium NMR, which measures efflux of sodium ions from the volume encapsulated by the membrane. The dimers did not support significant ion efflux. Furthermore, the proposed transmembrane channel containing cholesterol has been visualised at the molecular level by means of a computerised molecular modelling package, and compared with models of channels containing the dimers. The inability of the steroid dimers to promote efflux from liposomes, coupled with the modeling studies has led to a revised proposal for the mode of action.