Ethionamide activation and sensitivity in multidrug-resistant Mycobacterium tuberculosis

Ethionamide (ETA) is an important component of second-line therapy for the treatment of multidrug-resistant tuberculosis. Synthesis of radiolabeled ET A and an examination of drug metabolites formed by whole cells of Mycobacte rium tuberculosis (MTb) have allowed us to demonstrate that ETA is activate d by S-oxidation before interacting with its cellular target. ETA is metabo lized by MTb to a 4-pyridylmethanol product remarkably similar in structure to that formed by the activation of isoniazid by the catalase-peroxidase K atG, We have demonstrated that overproduction of Rv3855 (EtaR), a putative regulatory protein from MTb, confers ETA resistance whereas overproduction of an adjacent, clustered monooxygenase (Rv3854c, EtaA) confers ETA hyperse nsitivity, Production of EtaA appears to be negatively regulated by EtaR an d correlates directly with [C-14]ETA metabolism, suggesting that EtaA is th e activating enzyme responsible for thioamide oxidation and subsequent toxi city. Coding sequence mutations in EtaA were found in 11 of 11 multidrug-re sistant MTb patient isolates from Cape Town, South Africa. These isolates s howed broad cross-resistance to thiocarbonyl containing drugs including ETA , thiacetazone, and thiocarbide.