Discovery of 2,4,5-Substituted Benzoxazole Derivatives as Pks13 Inhibitors via the Scaffold Hopping Strategy

Pks13, an essential enzyme for <i>Mycobacterium tuberculosis</i> (<i>Mtb</i>) cell wall biosynthesis, represents a promising target for antimicrobial intervention. Previously, the benzofuran derivative <b>TAM16</b> was identified as a potent inhibitor of Pks13 through interaction with the thioesterase (TE) domain, but its development was halted due to cardiotoxicity. Therefore, we sought to identify an alternative scaffold that demonstrated good whole-cell activity that we demonstrate had a mode of action (MOA) similar to that of TAM16. To achieve this, we employed a scaffold hopping approach, leading to the discovery of a benzoxazole (BZX) scaffold that was determined to target the Pks13 TE domain. We then explored various structure-activity relationship (SAR) studies of the series, which resulted in the identification of a prototype BZX lead. Several of the novel BZX compounds showed potent minimum inhibitory concentrations (MICs) against <i>Mtb</i> and low to no toxicity in cytotoxicity assays. These compounds showed on-target activity, as evidenced by the induction of the BCG <i>iniBAC</i> cell wall damage reporter, inhibition of mycolic acid synthesis, and resistance mutations mapping to the TE domain of Pks13 in <i>Mycobacterium smegmatis</i> (<i>Msm</i>). Overall, we believe that the BZX scaffold represents a new and promising structural class with high potential to advance antitubercular drug discovery.