The Inosine Monophosphate Dehydrogenase, GuaB2, Is a Vulnerable New Bactericidal Drug Target for Tuberculosis

VCC234718, a molecule with growth inhibitory activity against Mycobacterium tuberculosis (<i>Mtb</i>), was identified by phenotypic screening of a 15344-compound library. Sequencing of a VCC234718-resistant mutant identified a Y487C substitution in the inosine monophosphate dehydrogenase, GuaB2, which was subsequently validated to be the primary molecular target of VCC234718 in <i>Mtb</i>. VCC234718 inhibits <i>Mtb</i> GuaB2 with a <i>K</i>_i of 100 nM and is uncompetitive with respect to IMP and NAD⁺. This compound binds at the NAD⁺ site, after IMP has bound, and makes direct interactions with IMP; therefore, the inhibitor is by definition uncompetitive. VCC234718 forms strong pi interactions with the Y487 residue side chain from the adjacent protomer in the tetramer, explaining the resistance-conferring mutation. In addition to sensitizing <i>Mtb</i> to VCC234718, depletion of GuaB2 was bactericidal in <i>Mtb</i> in vitro and in macrophages. When supplied at a high concentration (≥125 μM), guanine alleviated the toxicity of VCC234718 treatment or GuaB2 depletion via purine salvage. However, transcriptional silencing of <i>guaB2</i> prevented <i>Mtb</i> from establishing an infection in mice, confirming that <i>Mtb</i> has limited access to guanine in this animal model. Together, these data provide compelling validation of GuaB2 as a new tuberculosis drug target