**1. Introduction**

Tuberculosis (TB) continues to have a significant global burden by remaining one of the top ten killers worldwide and the leading cause of death due to a single infectious agent [1]. The causative agent, *Mycobacterium tuberculosis*(*M. tb*), has evolved to adapt to a range of environmental stresses encountered during infection, giving rise to sub-populations of bacteria in heterogenous metabolic states, from actively replicating, to slow-growing, and dormant, non-replicating bacteria [2]. Conventional anti-TB drugs primarily target processes required for cell growth and replication, and are less efficacious against non-replicating bacteria, resulting in lengthy treatments in both drug-susceptible (DS-TB) and drug-resistant TB (DR-TB) cases [1,3]. In addition, current treatments for DR-TB are associated with low cure rates and toxicity [4–6]. Therefore, the successful elimination of the disease requires the development of new anti-TB drugs ideally active against non-replicating bacterial subpopulations and drug-resistant strains.

A new target space that has garnered increasing interest is energy metabolism, and in particular, the oxidative phosphorylation (OxPhos) pathway. The first clinical validation of this pathway as a mycobacterial drug target occurred in 2012, with the FDA approval of bedaquiline (BDQ) for treating

DR-TB by targeting the mycobacterial F1F0 ATP synthase [7]. This has been followed by the regulatory approvals of nitroimidazoles delamanid (Deltyba; DEL) and pretomanid (PA-824) [8,9], with the demonstration of PA-824 killing non-replicating *M. tb* by inhibition of the respiratory cytochromes through the release of nitric oxide (NO) [10]. Additionally, Telacebec (Q203), which is currently in Phase 2 clinical trials, targets the primary terminal oxidase cytochrome bcc-aa3 complex (Cyt-bcc-aa3) of *M. tb* [11,12]. Altogether, this highlights the vulnerability of energy metabolism in this pathogen and the sensitivity of components of the OxPhos pathway to specific chemical inhibition. Despite the high degree of conservation of this pathway between prokaryotes and eukaryotes, the ability to specifically target mycobacterial components without affecting the human mitochondrial counterparts has alleviated safety concerns, opening up a new and attractive target space for drug discovery.

From a biological viewpoint, this pathway is promising as a drug target space for several reasons. Whereas substrate-level phosphorylation can provide a sufficient amount of energy for replication in many bacteria, *M. tb* is dependent on the more energetically-efficient OxPhos to sustain growth [13–15]. This is probably linked to the absence of a NADH-dependent lactate dehydrogenase in mycobacteria, which would limit efficient fermentation [16,17]. During the OxPhos process, electrons are transferred from electron donors of central metabolism to oxygen through the electron transport chain (ETC). The energy released in this transfer is conserved by the proton-pumping components of the ETC, generating an electrochemical gradient in the form of a proton motive force (PMF). This stored energy is then mined through the ATP synthase as protons flow back down the gradient to yield ATP. Since a sustained PMF and ATP replenishment are essential even for the viability of non-replicating *M. tb* [18–21], it is believed that inhibition of the OxPhos pathway is an effective strategy to eradicate non-replicating subpopulations, thereby shortening future treatment durations. Moreover, since drug efflux pumps are energy-dependent in actively transporting drugs out of the bacterial cell, perturbation of the PMF and ATP levels may interfere with their function as well [13]. It has been demonstrated that efflux pump activities play a significant role in drug susceptibility of *M. tb* [22–27] and, therefore, perturbation of the OxPhos pathway may indirectly aid in overcoming issues of efflux-pump mediated drug resistance.

In light of these significant interests in targeting energy metabolism in mycobacteria, this review aims to (1) summarise key findings of the main components comprising the OxPhos pathway of *M. tb*, (2) provide an overview of validated inhibitors of this pathway which are either in the discovery stage, in clinical development, or have been approved, (3) discuss the potential of new regimens containing these inhibitors, (4) highlight possible concerns in targeting this pathway, and (5) discuss future opportunities and directions for drug development in this space.
