## 4 Combinations Including ETC Inhibitors  
TB treatment relies on the combination of several antibacterial agents. New regimens for TB need to be (1) effective against DS- and DR-TB, (2) contain drugs with new mechanisms of action, (3) are suitable for oral administration, and (4) do not interfere with drugs used to treat chronic conditions or chronic infections [194]. An effective regimen should combine drugs that preserve or even potentiate their activity (additivity or synergism) when given as a regimen. Due to their ability to perturb the energy metabolism of replicating and non-replicating _M. tb_, including drugs that target the ETC may shorten treatments against DS- and DR-TB. The recent approval of the BPAL regimen (BDQ-PA824-Linezolid; Nix-TB trial) for DR-TB highlights that inhibiting components of the ETC is key in developing new regimens against _M. tb_, even though the relative contribution of each drug to the sterilizing potency of the BPAL regimen remains to be further investigated in humans. Several inhibitors in lead optimisation were tested in combination with other anti-TB drugs. PAB, a Cyt-bcc oxidase inhibitor, resulted in the synergistic killing of _M. tb_ under both replicating and non-replicating conditions when combined with CFZ [195]. PABs in combination with BDQ demonstrated antagonism at early time points, particularly under non-replicating conditions. However, this antagonistic effect disappeared within three weeks, with PAB-BDQ combinations becoming highly bactericidal [195]. The specificity of the PAB series needs to be further studied since it kills nutrient-starved _M. tb_ while remaining bacteriostatic against replicating mycobacteria, a property \begin{table}
\begin{tabular}{c c c} \hline \hline
**Chemical Class** & **Represented by** & **Structure** & **Ref** \\ \hline Ethylenediamine & SQ109 & \\ Pyrazinamide & & \\ \hline
2-Aminoimidazoles & 2B8 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Inhibitors of the mycobacterial proton motive force (PMF) discuss in this review. not shared with other specific Cyt-bcc inhibitors [130; 153]. A MenA inhibitor, NM-4, was synergistic even at low doses together with sub-bactericidal concentrations of BDQ, CFZ, and the QcrB inhibitor ND-10885 [196], causing enhanced and efficient killing of _M. tb_ in a time-kill curve assay [117]. These preliminary results underline the need to assess the efficacy of candidates in combination for TB therapy in an early stage of development. Several ETC inhibitors are currently in ongoing pre-clinical and clinical trials to assess their efficacy against TB as part of new regimens [11]. These include BDQ, TBAJ-587 and TBAJ-876 (ATP synthase inhibitors), the first-line anti-TB drug PZA and SQ109 (inhibitors of the PMF), CFZ and its analogue TBI-166, and Q203 (inhibitor of Cyt-bcc-aa3 oxidase) [11]. SQ109 was reported to enhance the activity of anti-tuberculosis drugs INH, RIF and BDQ, and to shorten the time required to cure _M. tb_-infected mice [197]. Additionally, several inhibitors of the ETC compounds have a multi-target activity against the bacilli. The uncoupling effects of SQ109, PZA, BDQ, and CFZ suggest that these inhibitors may have a critical role due to their multi-targeting activity and should be taken into consideration for the further development of regimens against _M. tb_, particularly against DR-TB [183]. Assessing the in vitro or in vivo combinatory effects with repurposed or approved drugs for the treatment of TB has been another fruitful avenue to find new potential regimens comprising drug candidates that target the ETC. An example is TBI-166, an analogue of clofazimine with excellent potency alone or in combination with BDQ, PZA and linezolid in vitro and in vivo [198]. Q203 and BDQ were the first-in-class, orally-available representatives of the Cyt-bcc-aa3 and ATP synthase inhibitors, respectively. Q203 is currently in Phase 2b clinical trials and demonstrated potency in a 14-day, proof-of-concept design study of early bactericidal activity.