The side effects associated with tuberculosis therapy bring with them the risk of noncompliance and subsequent drug resistance. Increasing the therapeutic index of antituberculosis drugs should thus improve treatment effectiveness. Several antituberculosis compounds require in situ metabolic activation to become inhibitory. Various thiocarbamide-containing drugs, including ethionamide, are activated by the mycobacterial monooxygenase EthA, the production of which is controlled by the transcriptional repressor EthR. Here we identify drug-like inhibitors of EthR that boost the bioactivation of ethionamide. Compounds designed and screened for their capacity to inhibit EthR-DNA interaction were co-crystallized with EthR. We exploited the three-dimensional structures of the complexes for the synthesis of improved analogs that boosted the ethionamide potency in culture more than tenfold. In Mycobacterium tuberculosis-infected mice, one of these analogs, BDM31343, enabled a substantially reduced dose of ethionamide to lessen the mycobacterial load as efficiently as the conventional higher-dose treatment. This provides proof of concept that inhibiting EthR improves the therapeutic index of thiocarbamide derivatives, which should prompt reconsideration of their use as first-line drugs.

Mycobacterium tuberculosis EthR is a repressor of ethA, a gene encoding a mono-oxygenase required for the activation of the prodrug ethionamide. Here we describe the X-ray crystal structure of EthR, a homodimer with an entirely helical structure showing similarities to TetR family members. Each monomer contained a fortuitous ligand identified as hexadecyl octanoate. The crystal structure of EthR purified in M. smegmatis revealed the presence of a comparable ligand. The binding of hexadecyl octanoate to EthR induces a conformational state incompatible with repressor function, which should lead to ethA derepression and consequently to an increased sensitivity to ethionamide and other thioamides. A related, more hydrophilic ketone was found to exhibit synergistic antimycobacterial effects when tested together with ethionamide, indicating that this strategy may help reduce the dosage of potent antibacterial compounds that otherwise are too toxic to be used as first-line drugs.