# Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs
## 3 Old TB Drugs
### Isoniaid--Target: Mycolic Acids
INH is activated by the mycobacterial catalase peroxidase KatG (Rv1908c) [52] into what is most likely an isonicotinoyl radical that reacts with nicotinamide adenine dinucleotide (NAD\({}^{+}\)) to form the INH-NAD adduct [53; 54; 55; 56]. This adduct binds to and inhibits InhA [54; 57; 58; 59; 60], the NADH-dependent enoyl-ACP reductase [61; 62] of the FAS-II system [63], leading to the inhibition of mycolic acid biosynthesis and mycobacterial cell death [64; 60; 65; 66]. Although the mechanism of INH action seems rather straightforward, elucidation of the molecular details took almost fifty years [67]. _Resistance._ Shortly after the antimycobacterial activity of INH was published in 1952, the first report of INH-resistant clinical isolates appeared less than one year later [68]. In 1954, [PERSON] demonstrated that INH-resistant mutants isolated in vitro were catalase-negative [69]. It will then take another 40 years to discover the genetic basis of this phenotype and decipher the main mechanism of resistance to INH: mutation in _katG,_ the gene encoding the INH activator. [PERSON] and colleagues showed that 1) complementation of an INH-resistant _Mycobacterium smegmatis_ mutant with a single copy of _katG_ restored INH susceptibility [52]; 2) two highly INH-resistant clinical isolates had _katG_ deletion [52]; and 3) _M. tuberculosis_ INH-resistant mutants regained INH susceptibility when transformed with _katG_[70]. Since then, more than 300 _katG_ mutations covering 99% of the gene's length (_katG_ has 2223 base pairs (bp)) have been identified in INH-resistant laboratory and clinical strains [71]. The most frequent mutation in clinical isolates is the Ser315 Thr. Actually, each of the three bases of the serine codon (AGC) can be mutated leading to Asn, Arg, Ile, Gly or Leu amino acid change. Mutations in KatG alter its catalase peroxidase and oxidase activities causing a defect in KatG's ability to activate INH [72; 73; 74]. Thus, most _M. tuberculosis_ clinical strains carrying _katG_ mutations are highly resistant to INH [75]. KatG enzymatic activities can also be disrupted by mutations in _furA_ (_Rv190 qc_), a gene encoding a ferric uptake regulation protein and a negative regulator of _katG_ transcription. Isogenic strains carrying the mutations a-10c and g-7a in the intergenic region between _katG_ and _furA_ had reduced _katG_ expression leading to a decrease in INH oxidase activity and a modest increase in INH resistance [76]. Resistance to a drug can occur through either target mutation (preventing the binding of the drug to its target) or target overexpression (titration of the drug). This is the case for the second most common mutations in INH-resistant clinical isolates: mutations in _inhA_ and its promoter region. The c-15t mutation in the _inhA_ promoter region increases _inhA_ mRNA levels by 20-fold resulting in higher InhA protein levels and an 8-fold MIC increase in _M. tuberculosis_[60]. This mutation is found in about one third of the INH-resistant clinical isolates but more often in XDR TB cases than MDR or INH-monoresistant TB cases, suggesting that the c-15t mutation could be a marker for XDR TB [77]. In contrast to _katG_ mutations, clinical strains with _inhA_ mutations (either in the gene or in the promoter region) have a low INH resistance phenotype [75]. There are another 20 different mutations identified in the _inhA_ promoter region [71]. Mutations in the target of INH _inhA_, an essential gene, are rare, with only 17 identified so far [71]. The first _inhA_ mutation (Ser94 Ala) was isolated in vitro in _M. singematis_ during a screening for mutants co-resistant to INH and ethionamide (ETH), a second-line TB drug [57]. This mutant led to the hypothesis that InhA was the primary target of both INH and ETH [57]. Introduction of the Ser94 Ala mutation into wild-type _M. tuberculosis_ H37 Rv was shown to be sufficient to confer INH and ETH resistance [60] and to decrease the binding of the INH-NAD adduct to InhA [60; 78].