# Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs
## 3 Old TB Drugs
### Ethionamide/Prothionamide--Target: Mycolic Acids  
_Discovery._ In 1954, [PERSON] and colleagues published the synthesis of an INH analog, where a thioamide group replaced the acyl hydrazide group in INH [118]. This thioisonicotinamide compound was active in a mouse model of _M. tuberculosis_ infection but was found to be less potent than INH. This publication was noticed by a French team who found that the thioamide derivative was more active than streptomycin and more importantly, potent against INH-, streptomycin- and PAS-resistant _M. tuberculosis_ strains. They synthesized a series of \(\alpha\)-alkyl derivatives of the thioisonicotinamide and found that adding an ethyl or propyl group at the \(\alpha\) position generated two compounds with activity in vitro and in mice greater than streptomycin but not as potent as INH [119]. They had synthesized ethionamide (ETH) and prothionamide (PTH). _Activity and toxicity._ ETH is a bactericidal drug, with a MIC ranging from 6 to 15 \(\upmu\)m against drug-susceptible _M. tuberculosis._ ETH and PTH are poorly soluble in water (0.84 and 0.28 g/L, respectively). ETH and PTH are in the group C (other core second-line agents) of second-line TB drugs used interchangeably for the treatment of MDR and XDR TB cases. ETH and PTH are oral drugs with some severe adverse effects (hepatoxicity, gastrointestinal disorders, neurotoxicity) [120]. _Mode of action._ [PERSON] and colleagues were the first to demonstrate that ETH inhibited mycolic acid biosynthesis [121]. They noticed that ETH affected _Mycobacterium bovis_ BCG similarly to INH although they observed no cross resistance between INH and ETH leading to the conclusion that the mode of action of INH and ETH were not identical [121]. Actually, cross resistance between INH and ETH had been observed in TB patients a few years earlier. Several studies noted that TB patients treated with INH developed resistance to both INH and ETH although the patients had never received ETH [122; 123; 124]. This suggested that ETH and INH shared a common mechanism of action. [PERSON] and colleagues had postulated that INH and ETH \"might differ in the means by which they reach the sensitive site\". That was prescient since INH and ETH were eventually revealed as prodrugs activated by different enzymes. While INH is activated by a catalase peroxidase, ETH is activated by the NADPH-specific flavin adenine dinucleotide-containing Baeyer-Villiger monooxygenase EthA (Rv3854c also called EtaA) [125; 126]. In vitro, activation of ETH by EthA leads first to the formation of ethionamide S-oxide, which is further metabolized by EthA to form either 2-ethyl-4-amidopyridine [126] or (2-ethyl-pyridin-4-yl) methanol [125] via radical intermediate(s). Once activated, the mechanism of action of ETH is very similar to INH. The activated form of ETHreacts with NAD\({}^{+}\) to form an ETH*-NAD adduct [127]. The structures of this adduct as well as the PTH*-NAD adduct were determined by X-ray crystallography, revealing an ethyl-isonicotinoyl or a propyl-isonicotinoyl covalently attached to the nicotinamide portion of reduced NAD\({}^{+}\) and bound to InhA. Inhibition of InhA by these adducts results in mycolic acid biosynthesis inhibition. [PERSON] and colleagues postulated that the activated form of ETH was an iminoyl radical but expressed doubt that this activation reaction was caused by EthA alone and hypothesized that additional enzymes might be involved [127]. Another study also concluded that another mechanism of activation might exist for ETH when demonstrating that deletion of _ethA_ and its regulator _ethR_ (_Rv3855_) in _M. tuberculosis_ caused only a modest increase (3-fold) in ETH resistance [128]. EthA has two close homologs: the monooxygenases Rv3083 (MymA) and Rv0565c [125]. [PERSON] and colleagues determined that MymA was indeed an activator of ETH by showing that loss of function of MymA or overexpression of _mymA_ conferred resistance or hypersusceptibility to ETH, respectively [129].