Science 355, 1206–1211 (2017)

Combating antibiotic resistance is crucial in preventing the spread of drug-resistant tuberculosis. Ethionamide (ETH) is a prodrug widely used to treat Mycobacterium tuberculosis (Mtb) that is converted to its active form by the bacterial monooxygenase EthA. The transcriptional repressor EthR blocks ethA transcription and ETH activation. Following up on their previous work identifying EthR inhibitors, Blondiaux et al. characterized a family of spiroisoxazoline-based drugs called Small Molecules Aborting Resistance (SMARt) that failed to bind EthR yet boosted ETH conversion in Mtb, suggesting an alternative pathway for ETH activation. SMARt-420, a representative member of these spiroisoxazolines, upregulated expression of EthA- and EthR-related genes in Mtb (EthA2 and EthR2). SMARt-420 bound to EthR2, preventing its binding to DNA, resulting in induction of EthA2 and increased ETH activation. Importantly, clinical isolates of Mtb with mutations in ethA that conferred resistance to ETH were rendered sensitive to the prodrug in the presence of SMARt-420 both in vitro and in vivo. The findings suggest that identifying alternative pathways of prodrug activation may extend the utility of antibiotics, even in the face of resistance-conferring mutations.