The antibiotic Acidomycin (ACM) discovered in 1952 was shown to possess selective activity against Mycobacterium tuberculosis (Mtb). Based on its structural similarity to biotin it was hypothesized to be an antimetabolite and early experiments demonstrated addition of exogenous biotin could rescue Mtb from ACM. However, the specific molecular mechanism of action of ACM remains unresolved. Given the paucity of new drugs for tuberculosis, the continued increase of drug-resistant Mtb strains, coupled with the simple chemical structure and selective antitubercular activity, we initiated a program to investigate the detailed mechanism of action, discern the structure-activity relationships, and use contemporary medicinal approaches to improve upon the drug disposition properties to enhance oral bioavailability and tissue distribution. Using classic feeding experiments of biotin pathway intermediates, we isolated the penultimate step performed by BioB, a radical S-adenosylmethionine (SAM) enzyme that catalyzes the C-H activation and insertion of sulfur in dethiobiotin (DTB) to afford biotin as the likely target based on the ability of biotin to rescue activity in ACM-treated Mtb and failure of DTB and all upstream metabolites to restore growth. Mtb strains that allow differential expression of BioB showed a monotonic relationship between BioB protein levels and ACM susceptibility further supporting this molecular target. To biochemically validate these findings, we established an in vitro enzyme assay for this notoriously challenging radical SAM enzyme and confirmed ACM is a potent enzyme inhibitor of BioB. Structure-activity relationships have confirmed the activity resides in the (–)-antipode and demonstrated the C5 alkyl chain is optimal and thiazolidinone heterocycle is essential for activity. These results provide a foundation for future studies aimed to revitalize this old antibiotic that acts by a novel mechanism of action.

Revitalizing an old molecule: Investigating acidomycin as an inhibitor of Mycobacterium tuberculosis biotin synthase

254th National Meeting (2017)

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