# Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs
## 3 Old TB Drugs
### Ethionamide/Prothionamide--Target: Mycolic Acids
In this study, the authors also noted that transposon mutants in _Rv0565c_ as well as in two other genes encoding Baeyer-Villiger monooxygenases (_Rv1393c_ and _Rv3049c_) did not lead to ETH resistance. However, a recent study determined that overexpression or deletion of _Rv0565c_ led to ETH hypersusceptibility or low resistance, respectively, in _M. tuberculosis_, leading the authors to conclude that Rv0565c was an additional activator of ETH in _M. tuberculosis_[130]. In both studies on _mymA_ and _Rv0565c_, no biochemical evidence was provided to show that MymA or Rv0565c actually activates ETH and what would be the resulting activated molecule(s). _Resistance._ Mutations in ETH-resistant laboratory and clinical _M. tuberculosis_ strains are found in the activators of ETH (_ethA, mymA, Rv0565c_), the negative regulator of _ethA (ethR_) and ETH target (_inhA_ gene and promoter region) [129, 130, 131]. Mutations in the _inhA_ promoter region are more frequent than mutations in the activator(s) of ETH [131]. Mutations in _inhA_ or in its promoter region are found in up to 68% of ETH-resistant clinical isolates while mutations in ETH activator _ethA_ are usually found in no more than 55% of ETH-resistant clinical isolates, and some of these _ethA_ mutations can also be found in ETH-susceptible strains [131]. The mutations in _ethA_ cover 91% of the gene (from 2 to 1341 bp; _ethA_ has 1470 bp) [71]. Mutations in _ethR_ have been identified in ETH-resistant clinical strains carrying the c-15t mutation in the _inhA_ promoter region [131]. Recently, cyclic dimeric guanosine monophosphate, a bacterial second messenger, was shown to boost the binding of EthR to _ethA_ promoter, causing a decrease in _ethA_ transcription levels and ETH resistance [132]. Other genes have been implicated in ETH resistance. _M. tuberculosis_ strains deleted for _mshA_, a gene encoding the glycosyltransferase involved in the biosynthesis of mycothiol, a major low-molecular-weight thiol, are eight times more resistant to ETH than their parental strains [133]. Mycothiol is thought to play a role in ETH resistance by increasing the rate of ETH activation by EthA [133]. In _M. smegmatis_ and _M. bovis_ BCG, mutants co-resistant to INH and ETH were isolated carrying mutations in _ndh_, a gene encoding an NADH dehydrogenase whose function is to oxidize NADH into NAD\({}^{+}\). In this case, _ndh_ mutants were shown to accumulate NADH. Excess NADH could act as a competitive inhibitor for the binding of the ETH-NAD to InhA, triggering ETH resistance [134]. _Area of investigation._ Regulation of _ethR_ expression plays a role in ETH resistance and susceptibility. While overexpression of _ethR_ was shown to cause ETH resistance, inhibition of _ethR_ triggers higher levels of _ethA_ expression and increases ETH susceptibility [135]. [PERSON] and colleagues have taken advantage of this point and generated EthR inhibitors to boost ETH activity. Screening of chemical libraries and SAR on potential EthR inhibitors led to a first series of EthR inhibitors, which by themselves had no activity on _M. tuberculosis_, yet, when combined with ETH, significantly \"boosted\" ETH activity in vitro and in _M. tuberculosis_-infected mice [136]. The most active of this first generation of boosters, the oxadiazole BDM41906, decreased the MIC for ETH to the nM range [137]; however, BDM41906 had no \"boosting\" activity in _M. tuberculosis_ strains carrying _ethA_ mutations [138]. The second generation of ETH booster, the spiroisoxazoline SMARt (Small Molecule Aborting Resistance)-420, does not inhibit EthR but instead triggers a different activation mechanism for ETH. This new activation system uses the oxidoreductase Rv0077c as the activator,which is negatively regulated by Rv0078. SMART-420 binds to Rv0078, releasing the activity of Rv0077c. SMART-420 increases the susceptibility of _M. tuberculosis_ to ETH 40-fold. In addition, ETH-resistant _M.