Synthesis and function of the conserved motif of mycosamine-containing polyene macrolides

Mycosamine-containing polyene macrolides represent a large family of natural products with antifungal activity, of which amphotericin B (AmB) is an important member. Several of these clinically vital but toxic agents have been proposed to exert their biological activity through ion channel formation and thus represent small molecules with the capacity to perform a protein-like function. This function and the dose-limiting toxicity of AmB are thought to be dependent upon the presence of sterols in the cellular membrane, but the precise role of sterols in this mechanism and even whether channel formation and antifungal activity are causatively linked has remained unclear. An advanced understanding of this mechanism thus stands to enable the development of new antifungals with improved therapeutic index, as well as small molecule prosthetics which can replicate the function of missing or dysfunctional proteins for the treatment of human disease. All members of this family contain a conserved structural motif bearing a carboxylic acid and the rare aminosugar mycosamine. These groups are proposed to form polar interactions which stabilize the putative ion channel complex, anchor these compounds to the lipid bilayer, and/or promote an interaction with sterol. We employed a series of AmB derivatives lacking each of these functional groups to test these contested and unresolved hypotheses. We found that mycosamine is necessary for promoting a direct binding interaction with sterol, and that deletion of mycosamine abolishes sterol binding, channel formation, and antifungal activity. The C35 hydroxyl group of AmB has also been proposed to be critical for ion channel formation. We synthetically removed this group, and the resulting derivative had no membrane permeabilizing activity but retained the ability to bind ergosterol. This compound exhibited fungicidal activity like that of natamycin, another mycosamine-containing polyene macrolide which binds ergosterol and does not permeabilize membranes. Removal of mycosamine from natamycin also abolished its antifungal activity. Collectively, these results led us to conclude that mycosamine-mediated sterol binding is the primary mechanism of action of these natural products, and that channel formation represents a complementary mechanism which further increases drug potency. Given the functional importance of the conserved substructure of mycosamine-containing polyene macrolides, we synthesized a building block containing this motif with the potential for application to a general platform for small molecule synthesis based on iterative cross-coupling. This synthesis was enabled by the development of a new type of diastereotopic group-selective lactonization and a new directing group for the challenging installation of the mycosamine sugar. This building block may help enable the modular synthesis of these natural products and their derivatives not readily accessible via degradation. Additionally, the discovery that mycosamine-mediated sterol binding is the primary mechanism of action of polyene macrolides may inform future efforts to improve the therapeutic index of these compounds and to develop small molecule prosthetics for the treatment of human disease