Preventing the evolution of transmissible nitroimidazole resistance in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis (TB), the most significant infectious disease worldwide in terms of both morbidity and mortality. Nitroimidazole prodrugs are one of the two new classes of antitubercular agents in advanced clinical development. Delamanid is clinically approved for multiple drug resistant TB treatment, while pretomanid is part of a regimen under phase III trials. Both compounds undergo reductive activation by the deazaflavin (F420H2)-dependent nitroreductase (Ddn; rv3547) within Mycobacterium tuberculosis. Acquired resistance to nitroimidazoles, involving disruption to F420 biosynthesis and reduction, has been documented, but results in loss of bacterial fitness that makes transmission of such variants unlikely. In contrast, transmissible resistance, due to fitness-neutral genetic changes, is vastly more dangerous and could rapidly make this new, and desperately needed, class of drugs ineffective. By screening mutations to the activating enzyme, Ddn, we can identify mutations that can abolish nitroimidazole activation and confer resistance, without significantly affecting its natural physiological role thereby maintaining organismal fitness. Our preliminary data reveals a Mtb clinical isolate from the hypervirulent Beijing family possesses one such mutation and is resistant to pretomanid. Thus, transmissible resistance to pretomanid is present in a clinical strain, prior to the large-scale introduction of these drugs. This is an urgent situation: we must identify any other mutations that can confer resistance, establish whether alternative nitroimidazoles are less susceptible to resistance-mutations, and elucidate the physiological role of Ddn and its contribution to the fitness and virulence of Ddn. By tracking these mutations we can develop improved treatment strategies to slow the spread of resistance, and pre-empt the emergence of resistance by screening these variants against new nitroimidazole prodrugs.