3.2.4. MenG

Benkovic et al. identified inhibitors of CcrM, an essential DNA methyltransferase in the α-proteobacteria *Caulobacter crescentus*, which were later demonstrated to inhibit MenH in *B. subtilis*an orthologue of *M. tb* MenG [118]. The derivative compound 12a, a diphenylborinic acid quinoline ester, was the most active MenH inhibitor in *Bacillus subtillis*. In *M. tb*, the MICs of the derivatives

range from 0.31 to 0.64 μg/mL, and to our knowledge, no further SAR and target validation studies have been performed.

Another chemical class of MenG inhibitors, the biphenyl amides, were also identified in a whole-cell screen targeting the mycobacterial respiratory pathway. 168 small molecules with anti-TB activity from GlaxoSmithKline were screened against a *M. bovis* BCG strain containing a mWasabi reporter fused to the putative promoter of cydAB, a reporter system used to identify drugs targeting respiration [51,125]. The compound GSK1733953A, also known as DG70, induced the expression of the reporter strain compared to untreated control. This bactericidal compound showed specific activity against drug-susceptible and drug-resistant *M. tb*. DG70 was bactericidal against nutrient-starved *M. tb*, which mimics a sub-population that is particularly difficult to kill [51]. MK-4 supplementation rescued the bactericidal potency of DG70, implying that the MK biosynthesis pathway is involved in the mechanism of action of this compound. Resistant mutants selected in *M. tb* H37Rv with DG70 harboured mutations V20A, F118L, W75A, and S188A in MenG, leading to high-level resistance to DG70. DG70 was also shown to block *de novo* biosynthesis of MK, highlighting its role as an inhibitor of MenG. Interestingly, this compound showed synergism with BDQ, isoniazid (INH) and rifampicin (RIF) in time-kill assays, suggesting that MenG inhibitors could be part of a potent drug combination [51].
