# Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs
## 3 Old TB Drugs
### Thiacetazone--Target: Mycolic Acids  
_Discovery._ [PERSON] was awarded the Nobel Prize in 1939 for the discovery of prontosil, a prodrug that releases _p_-aminobenzzenesulfonamide, the first sulfonamide drug active against _Staphylococcus_ and _Streptococcus_ infections, but not against _M. tuberculosis_ infections [163]. While prontosil was quickly replaced by penicillin to fight antibacterial infections, [PERSON] and his team continued working on the synthesis of related compounds, the thiosemicarbazides, and discovered that one in particular, thiacetazone (TAC, also called Tibione, Tb I, Conteben), had impressive activity against _M. tuberculosis_ in guinea pigs [164]. In the late 1940s, TAC was tested in Germany on patients with various forms of TB and found to have promising activity although severe adverse effects were recorded [37; 165]. _Activity and toxicity._ TAC is an oral, inexpensive, effective and bacteriostatic drug, with a MIC ranging from 0.3 to 5 \(\upmu\)m against drug-susceptible _M. tuberculosis._ TAC is poorly soluble in water (0.09 g/L). TAC is part of the group D3 (add-on agents) of second-line TB drugs. TAC has been given in combination with INH in resource-poor countries [166; 167]. Its utilization as a TB drug was mostly discontinued because of its high toxicity (skin disorder, gastrointestinal symptoms, conjunctivitis, vertigo, liver and kidney damage) especially in HIV-positive TB patients [168]. TAC is, therefore, not recommended in HIV-positive TB patients. _Mode of action._ TAC is a prodrug that is activated by the monooxygenase EthA [127; 169]. Once activated, TAC inhibits the dehydatase HadAB of the FAS-II system by forming a disulfide bound with a cysteine (Cys61) residue of HadA [170; 171]. During the elongation of fatty acyl-ACPs by FAS-II, HadA binds the acyl-ACP while HadB performs the dehydratase reaction. When covalently bound to the Cys61 residue of HadA, TAC obstructs the acyl-ACP channel preventing the binding of the fatty acyl-ACP [172]. The covalent bond between TAC and HadA Cys61 blocks the activity of HadAB leading to inhibition of mycolic acid biosynthesis [173]. _Resistance._ The mode of action of TAC was discovered through analysis of TAC resistance. Since TAC treatment of _M. tuberculosis_results in inhibition of mycolic acids, [PERSON] and [PERSON] overexpressed every gene involved in the FAS-II system. Only overexpression of the dehydatase operon _hadAB_or its dimer _hadBC_resulted in highly TAC-resistant _M. tuberculosis_strains (MIC \(>\) 0.2 mm) [170]. Curiously, the level of TAC resistance was much lower when the dimer _hadAB_ was overexpressed in _M. tuberculosis_ (MIC 10 \(\upmu\)m). This was puzzling since HadAB is the target of TAC. Moreover, sequencing of spontaneous _M. tuberculosis_ TAC-resistant mutants revealed mutations in HadA (Cys61 Ser, Cys61 Gly) but also in HadC (Val85 Phe, Thr123 Ala, Lys157 Arg, Ala151 Val) [170; 174]. [PERSON] and colleagues set up to unmask the role of _hadC_ in TAC resistance [175]. The authors found that mutations in _hadC_ protected _M. tuberculosis_ from TAC and compensated for HadAB inhibition by TAC. The authors proposed that _hadC_ mutations prevented TAC from reaching the active site of HadAB [175]. TAC resistance is also mediated by mutations in _mmaA4_ (_Rv0642c_), a gene encoding a methoxy mycolic acid synthase required for the synthesis of keto- and methoxy-mycolic acids [170, 176]. The methyltransferases involved in the modifications of the meromycolic acids such as MmaA4 interact with the proteins of the FAS-II system including the dehydatase heterodimers HadAB and HadBC [31]. Mutations in _mmaA4_ might, therefore, modify the conformation of HadAB preventing the binding of TAC to HadA and causing TAC resistance [175]. _Area of investigation._In the late 1940s, the discovery of the antiTB activity of the thiosemicarbazide TAC propelled pharmaceutical companies into an extensive search for TAC analogs with antiTB properties. This led to the discovery of the most important antimycobacterial drug INH (see above).