Electronic tuning of site selectivity and semisynthesis of C2'-deoxyamphotericin B
The small molecule Amphotericin B (AmB) is a natural product that is utilized in the treatment of systemic fungal infections. AmB is effective against a broad spectrum of fungi, and resistance to this potent antifungal agent is rare. However, the use of AmB is limited due to its high toxicity. All of the details of the mechanism of AmB have not yet been elucidated, and a molecular understanding of the mechanisms and interactions of AmB would be an asset in the improvement of the therapeutic index of this antimycotic and in the development of other antimicrobials that would be refractory to resistance development. AmB is known to self-assemble into ion channels in phospholipid membranes containing sterol. The resulting membrane permeabilization has long been the leading hypothesis for the primary mechanism of antifungal activity. Three possible interactions involving the carboxylic acid and mycosamine appendages of AmB have been predicted to play important roles in the mechanism. To investigate the mode of action of AmB, derivatives of AmB lacking one or both of these appendages were synthesized and submitted to biophysical assays to determine their roles in the possible mechanistic interactions. It was found that the mycosamine was essential for antifungal activity, forming ion channels and mediating a direct interaction with ergosterol. The discovery that AmB directly binds ergosterol pointed to two possible primary mechanisms, membrane permeabilization through channel formation or ergosterol sequestration. The C35 hydroxyl group on AmB has been predicted to be important in the stabilization of the ion channel complex via hydrogen-bonding. A C35-deoxyAmB derivative was synthesized and evaluated for its antifungal activity, channel forming ability, and sterol binding capacity. C35-deoxyAmB was unable to permeablize yeast cells but still retained the ability to bind sterol and displayed significant antifungal activity. These findings support the conclusion that the primary mechanism of AmB is ergosterol sequestration. Investigation of the mycosamine-sterol interaction is needed to further elucidate the mechanism of AmB. The mycosamine subunit contains multiple functional groups that could be involved in the AmB-sterol binding event. The C2’ hydroxyl group has been implicated in a potential hydrogen-bond interaction with the sterol. Electronic tuning of site-selective acylation reactions was developed towards the synthesis of a C2’-deoxyAmB derivative. More electron-rich acyl donors create a less exothermic reaction in which the transition state is more product-like according to the Hammond Postulate. This more product-like transition state magnifies the site-discriminating interactions between the acylating complex and the substrate thereby engendering greater selectivity. The site-selective acylation of the C2’ hydroxyl group allowed this hydroxyl to be isolated and deoxygenated. In addition, a hybrid route with a higher yielding and streamlined protection-deprotection sequence was also developed to produce the C2’-deoxyAmB derivative.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign