Improved Phenoxyalkylbenzimidazoles with Activity against Mycobacterium tuberculosis Appear to Target QcrB

[Image: see text] The phenoxy alkyl benzimidazoles (PABs) have good antitubercular activity. We expanded our structure–activity relationship studies to determine the core components of PABs required for activity. The most potent compounds had minimum inhibitory concentrations against Mycobacterium tuberculosis in the low nanomolar range with very little cytotoxicity against eukaryotic cells as well as activity against intracellular bacteria. We isolated resistant mutants against PAB compounds, which had mutations in either Rv1339, of unknown function, or qcrB, a component of the cytochrome bc(1) oxidase of the electron transport chain. QcrB mutant strains were resistant to all PAB compounds, whereas Rv1339 mutant strains were only resistant to a subset, suggesting that QcrB is the target. The discovery of the target for PAB compounds will allow for the improved design of novel compounds to target intracellular M. tuberculosis.