Isolation of resistant mutants to AX compounds in Mycobacterium tuberculosis

New anti-tuberculosis (TB) drugs are needed to handle the current TB situation worldwide caused by M. tuberculosis. We report here lead optimisation efforts for molecule GW861072X, one of 177 leads published in a GSK-led phenotypic screening campaign by Balell et al. (2013), generating the AX series. Along with the parent compound AX-35, four other derivatives with mild to no cytotoxicity showed potent in vitro and ex vivo activity in infected THP-1 macrophages against M. tuberculosis. Interestingly, activity of the AX series was also observed against other pathogenic mycobacteria, including non-tuberculous mycobacteria (NTM) such as the Mycobacterium abscessus complex (MABSC). Isolation of resistant mutants to AX compounds in M. tuberculosis revealed mutations in the QcrB of the cytochrome bc1 oxidase, one of two terminal oxidases of the mycobacterial electron transport chain. Cross-resistance studies, transcriptomic analyses and bioenergetics flux assays provide further evidence of QcrB as the target of the AX compounds, and that AX compounds likely interact differently with the quinol binding pocket compared to other QcrB inhibitors.