**5. Phenotypic Screening**

This is a biology-driven effort where compound libraries are screened for antimycobacterial activity against mycobacterial cells in culture, and the 'hits' are progressed based on the potency both in vitro and in vivo [69]. This cell-based approach has been successful from the early days of TB discovery, and most of the frontline drugs were progressed based on the structure–activity relationship that was evaluated directly on the whole cells. Current-day phenotypic-screening approaches include the ability to identify inhibitors that are active against MTB cells growing in a variety of microenvironments that represent the replicating, the non-replicating, or bacilli in different physiological states [70]. Genetic probes have also facilitated the unravelling of the biological targets of the inhibitors.

Some of the examples of phenotypic-screening-derived compounds that have become drugs, or those in late clinical development, are discussed here.

**a. Bedaquiline:** Approved in 2012 for the treatment of MDR TB, Bedaquiline (TMC-207, R207910, shown in Figure 3) has the reputation of being the first FDA approved TB drug in more than four decades. It was discovered by Johnson & Johnson by screening more than 70,000 compounds against *Mycobacterium. smegmatis* (M.sm). The compound was first described in 2004, at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC). The target of TMC-207 was later conclusively proven to be the MTB ATP synthase [68]. Subsequently, in 2012, Bedaquiline became an essential part of the MDR regimen [71,72]. Several combinations of Bedaquiline are also being investigated for their potential to reduce the duration of therapy [73]. Discovery of Bedaquiline and the target link has made MTB ATP synthase an attractive validated target.

**Figure 3.** Structure of Bedaquiline (TMC-207, R207910).

**b. Benzothiazinone (BTZ) and Macozinone (PBTZ):** BTZ was discovered at the Russian academy of science and developed with NM4TB, and it is presently being clinically evaluated under the Innovative Medicines for Tuberculosis (iM4TB). The target for BTZ has been shown as decaprenylphosphoryl-β-d-ribose-2- -oxidase (DprE1) by genetic and biochemical studies [74,75]. Based on SAR explorations, a piperazine derivative, PBTZ-169 (Macozinone), which has superior pharmacokinetics (PK), as compared to that of BTZ [76], was discovered and is currently in Phase 2 clinical trials, in addition to BTZ 043. Structures of BTZ and PBTZ are shown in Figure 4.

**Figure 4.** Structures of BTZ and PBTZ.

**c. Azaindoles:** These compounds emerged from a literature search for compounds similar to Q-203 [77,78]. The compound had an MIC but was not bactericidal. Scaffold morphing efforts on this compound at AZ Bangalore in a collaboration with TB alliance led to the identification of TBA 7371 [79–81], a potent DprE1 inhibitor with non-covalent binding. TBA 7371, jointly owned by Global Alliance for TB and the Foundation for Neglected Disease Research (FNDR), was developed by GATB through Phase 1 safety trials in humans. Recently, GATB has licensed TBA 7371 to Gates Medical Research Institute (GMRI); the news was disclosed in Union World conference of lung health at Hyderabad, India, in November 2019. GMRI is currently testing TBA7371 in Phase 2A clinical trials in TB patients [82]. The progression is depicted in Figure 5.

**Figure 5.** Evolution of TBA7371 which is in Phase 2A clinical trials in TB patients.

**d. Q203:** Telacebec (Q203) is a compound that was discovered by the researchers at Institute Pasteur, Korea, and is structurally very similar to the one discussed above [77,78]. This compound inhibits the cytochrome bc1 complex of MTB that is critical for the electron transport chain. The compound is in Phase 2 clinical trials [83]. Structure of Telacebec is shown in Figure 6.

**Figure 6.** Structure of Telacebec.

**e. OPC-167832:** First discussed in 2016 at the 47th Union World Conference on Lung Health in Liverpool, UK, this is a carbostyril derivative that inhibits DPRE1. The structure shown in Figure 7 represents a balance of hydrophobic and hydrophilic residues. The compound is currently in Phase 2 clinical trials [84]. Structure of OPC-167832 is shown in Figure 7.

**Figure 7.** Structure of OPC-167832.
