Imidazopyridine Compounds Inhibit Mycobacterial Growth by Depleting ATP Levels.

The imidazopyridines are a promising new class of antitubercular agents with potent activity in vitro and in vivo We isolated mutants of Mycobacterium tuberculosis resistant to a representative imidazopyridine; the mutants had large shifts (>20-fold) in MIC. Whole-genome sequencing revealed mutations in Rv1339, a hypothetical protein of unknown function. We isolated mutants resistant to three further compounds from the series; resistant mutants isolated from two of the compounds had single nucleotide polymorphisms in Rv1339 and resistant mutants isolated from the third compound had single nucleotide polymorphisms in QcrB, the proposed target for the series. All the strains were resistant to two compounds, regardless of the mutation, and a strain carrying the QcrB T313I mutation was resistant to all of the imidazopyridine derivatives tested, confirming cross-resistance. By monitoring pH homeostasis and ATP generation, we confirmed that compounds from the series were targeting QcrB; imidazopyridines disrupted pH homeostasis and depleted ATP, providing further evidence of an effect on the electron transport chain. A representative compound was bacteriostatic against replicating bacteria, consistent with a mode of action against QcrB. The series had a narrow inhibitory spectrum, with no activity against other bacterial species. No synergy or antagonism was seen with other antituberculosis drugs under development. In conclusion, our data support the hypothesis that the imidazopyridine series functions by reducing ATP generation via inhibition of QcrB.

Imidazoles Induce Reactive Oxygen Species in Mycobacterium tuberculosis Which Is Not Associated with Cell Death.

Azoles are a class of antimicrobial drugs used clinically to treat yeast and fungal infections. Against pathogenic yeast and fungi, azoles act by inhibiting the activity of the cytochrome P450 Cyp51, which is involved in the synthesis of a critical component of the yeast and fungal cell membrane. Azoles have antibacterial activity, including against mycobacteria, but the basis for this activity is not well-understood. We demonstrated that imidazoles are bactericidal to Mycobacterium tuberculosis. A marked increase in reactive oxygen species (ROS) was observed within imidazole-treated M. tuberculosis. The generation of ROS did not appear to be related to the mechanism of killing of imidazoles, as the addition of antioxidants or altered expression of detoxifying enzymes had no effect on growth. We examined the metabolic changes induced by econazole treatment in both wild-type and econazole-resistant mutant strains of M. tuberculosis. Econazole treatment induced changes in carbohydrates, amino acids, and energy metabolism in both strains. Notably, the untreated mutant strain had a metabolic profile similar to the wild-type drug-treated cells, suggesting that adaptation to similar stresses may play a role in econazole resistance.