## 3 Overview of the Inhibitors Targeting the OxPhos Pathway  
### NADH Dehydrogenase  
NADH dehydrogenases play a major role in maintaining mycobacterial respiratory chain energization by using MK as an electron carrier. NDH-1 is non-essential for mycobacterial growth and for persistence [19; 34]. Therefore, no drug development is currently in progress for inhibitors of mycobacterial NDH-1 [39]. NDH-2 is widespread in bacteria and the mitochondria of fungi, plants and some protists. In some organisms, more than one copy is present [91]. Notably, NDH-2 (Ndh and NdhA) are absent in mammalian genomes, suggesting a potential leading target for anti-mycobacterial drug development. Several inhibitors have been found to target NDH-2 in mycobacteria, including the phenothiazazines, quinolinyl pyrimidines and 2-mercapto-quinazolinones (Table 1). It is also thought that NDH-2 plays a role in the reduction of the anti-leprosy drug clofazine (CFZ), leading to the production of reactive oxygen species (ROS) and, consequently, its bactericidal activity against _M. tb_[92]. However, CFZ is still potent against a _M. tb_ Andh-2 mutant strain, indicating that CFZ does not require activation by NDH-2 to exert its anti-mycobacterial potency [35]. Further studies will be needed to understand the mechanism of action of CFZ against mycobacteria. The CFZ analogue TBI-166 is more potent than CFZ against _M. tb in vitro,_ and it is at least as potent as CFZ in acute and chronic murine models of TB while being potentially associated with less skin discoloration [93]. A study by [PERSON] et al. demonstrated that the inactivation of both NDH-2 (\(\Delta\)ndh-2) can be achieved in the absence of fatty acids in the growth media. Interestingly, rotenone, an inhibitor of NDH-1, has bactericidal activity against Andh-2, which suggests that NDH-1 and NDH-2 have a redundant role in _M. tb_[35]. The phenothiazazines, a class of antipsychotic drugs, including thioridazine, chlorpromazine and trifluoperazine, have been widely studied against _M. tb_. They have activity in vitro against _M. tb_ with minimum inhibitory concentration (MIC) of thioridazine, chlorpromazine and trifluoperazine ranging between 5 to 32 ug/mL [94; 95; 96; 97; 98]. However, their limited potency coupled with an unfavourable toxicological profile preclude clinical use for TB treatment [99]. However, phenothiazazines accumulate in macrophages and are particularly potent against intracellular _M. tb_[100; 101]. Current efforts aim at developing new phenothiazazine derivatives with improved activity against _M. tb_ and reduced toxicity [102; 103; 104; 105]. Another class of NDH-2 inhibitors is the quinolinyl pyrimidines [106]. This class of compounds was identified during an NDH-2 target-based high-throughput screening of more than 100 000 compounds. The series showed a good inhibitory range against _M. tb_ in vitro, with MIC\({}_{50}\) below 1 ug/mL [106]. In contrast to the phenothiazazines, the quinolinyl pyrimidines series is not hemolytic. CBR-1825 and CBR-4032 are two representatives compounds of the thioquinazoline (TQZ) and tetrahydroroindazole scaffolds, respectively, that were identified in a high-throughput screen of over 800 000 compounds in mycobacterial inverted membrane vesicles (IMVs) [29]. The assay was designed to identify small-molecules interfering with ATP production. CBR-1825 and CBR-4032 had an MIC\({}_{50}\) of 0.43 uM and 6.6 uM, respectively, when tested, and no apparent cytotoxicity [29]. Both compounds are bactericidal against _M. tb_ in vitro and seem to perturb NADH turnover. Sequencing of three escape mutants resistant to TQZ revealed a non-synonymous mutation in the promoter region of ndhA (rv0392c), suggesting that these compounds target NDH-2. A 2-mercaptoethanol-quinazolinones cluster of hits (1, 2, and 3) was identified in a screening of a commercial diversity library [30]. Compound 1 potentially inhibiting _M. tb_ growth in vitro in the low micromolar range without cytotoxicity against HepG2 cells [30].