Florasulam Is a Potent Inhibitor of <i>Mycobacterium tuberculosis</i> Acetohydroxyacid Synthase and Possesses In Vivo Antituberculosis Activity

Tuberculosis (TB) remains as a leading cause of morbidity and mortality, accounting for ∼1.3 million fatalities worldwide per year. There are two major concerns: (i) the rise in the number of multi- and extensively drug-resistant strains of TB and (ii) the significant side-effects related to the use of many of the current therapies to treat drug-resistant and drug-sensitive TB alike. Thus, there is an ongoing need to discover new drugs and drug targets to combat this disease. Here, acetohydroxyacid synthase (AHAS), the first enzyme in the branched-chain amino acids (BCAAs) biosynthesis pathway, is comprehensively investigated as such a drug target. All five chemical classes of plant AHAS inhibitors, established as commercial herbicides, were assessed as leads. Members of the triazolopyrimidine family (e.g., metosulam, penoxsulam, and florasulam) are the most potent inhibitors of <i>Mycobacterium tuberculosis</i> AHAS (<i>MtbAHAS</i>) with <i>K</i>_i values as low as 20 nM. These compounds also exhibit the property of accumulative time-dependent inhibition, a feature that appears to be crucial for herbicidal activity and more generally for biocidal activity. Of these, the anti-TB activity of florasulam was the most effective, with an MIC of 500 nM against virulent <i>Mtb</i> grown in culture. This compound is also effective in killing intramacrophage <i>Mtb</i> and reduces bacterial load, as compared to vehicle-only by 13-fold in the lungs of mice infected with <i>Mtb</i>. Thus, triazolopyrimidines as AHAS inhibitors, and in-particular florasulam, represents a promising new class of leads for anti-TB drug development.