Contribution of direct InhA inhibitors to novel drug regimens in a mouse model of tuberculosis.

Isoniazid is an important first-line medicine to treat tuberculosis (TB). Isoniazid resistance increases the risk of poor treatment outcomes and development of multidrug resistance, and is driven primarily by mutations involving katG, encoding the prodrug-activating enzyme, rather than its validated target, InhA. The chemical tractability of InhA has fostered efforts to discover direct inhibitors of InhA (DIIs). In this study, we bridge the gap in understanding the potential contribution of DIIs to novel combination regimens and demonstrate a clear distinction of DIIs, like GSK693 and the newly described GSK138, from isoniazid, based on activity against clinical isolates and contribution to novel drug regimens. The results suggest that DIIs, specifically GSK138 and GSK693, could be promising partners in novel drug regimens, including those used against isoniazid-resistant TB, potentially enhancing their efficacy and/or preventing the selection of resistant mutants and supporting the continued exploration of InhA as a promising target for TB drug development.

Lysyl-tRNA synthetase, a target for urgently needed M. tuberculosis drugs.

Tuberculosis is a major global cause of both mortality and financial burden mainly in low and middle-income countries. Given the significant and ongoing rise of drug-resistant strains of Mycobacterium tuberculosis within the clinical setting, there is an urgent need for the development of new, safe and effective treatments. Here the development of a drug-like series based on a fused dihydropyrrolidino-pyrimidine scaffold is described. The series has been developed against M. tuberculosis lysyl-tRNA synthetase (LysRS) and cellular studies support this mechanism of action. DDD02049209, the lead compound, is efficacious in mouse models of acute and chronic tuberculosis and has suitable physicochemical, pharmacokinetic properties and an in vitro safety profile that supports further development. Importantly, preliminary analysis using clinical resistant strains shows no pre-existing clinical resistance towards this scaffold.

Optimization of TAM16, a Benzofuran That Inhibits the Thioesterase Activity of Pks13; Evaluation toward a Preclinical Candidate for a Novel Antituberculosis Clinical Target.

With increasing drug resistance in tuberculosis (TB) patient populations, there is an urgent need for new drugs. Ideally, new agents should work through novel targets so that they are unencumbered by preexisting clinical resistance to current treatments. Benzofuran 1 was identified as a potential lead for TB inhibiting a novel target, the thioesterase domain of Pks13. Although, having promising activity against Mycobacterium tuberculosis, its main liability was inhibition of the hERG cardiac ion channel. This article describes the optimization of the series toward a preclinical candidate. Despite improvements in the hERG liability in vitro, when new compounds were assessed in ex vivo cardiotoxicity models, they still induced cardiac irregularities. Further series development was stopped because of concerns around an insufficient safety window. However, the demonstration of in vivo activity for multiple series members further validates Pks13 as an attractive novel target for antitubercular drugs and supports development of alternative chemotypes.

Easy-To-Synthesize Spirocyclic Compounds Possess Remarkable in Vivo Activity against Mycobacterium tuberculosis.

Society urgently needs new, effective medicines for the treatment of tuberculosis. To kick-start the required hit-to-lead campaigns, the libraries of pharmaceutical companies have recently been evaluated for starting points. The GlaxoSmithKline (GSK) library yielded many high-quality hits, and the associated data were placed in the public domain to stimulate engagement by the wider community. One such series, the spiro compounds, are described here. The compounds were explored by a combination of traditional in-house research and open source methods. The series benefits from a particularly simple structure and a short associated synthetic chemistry route. Many members of the series displayed striking potency and low toxicity, and highly promising in vivo activity in a mouse model was confirmed with one of the analogues. Ultimately the series was discontinued due to concerns over safety, but the associated data remain public domain, empowering others to resume the series if the perceived deficiencies can be overcome.

Screening of a Novel Fragment Library with Functional Complexity against Mycobacterium tuberculosis InhA.

Our findings reported herein provide support for the benefits of including functional group complexity (FGC) within fragments when screening against protein targets such as Mycobacterium tuberculosis InhA. We show that InhA fragment actives with FGC maintained their binding pose during elaboration. Furthermore, weak fragment hits with functional group handles also allowed for facile fragment elaboration to afford novel and potent InhA inhibitors with good ligand efficiency metrics for optimization.

A new 'golden age' for the antitubercular target InhA.

The increasing prevalence of multidrug-resistant strains of Mycobacterium tuberculosis is the main contributing factor in unfavorable outcomes in the treatment of tuberculosis. Studies suggest that direct inhibitors of InhA, an enoyl-ACP-reductase, might yield promising clinical candidates that can be developed into new antitubercular drugs. In this review, we describe the application of different hit-identification strategies to InhA, which clearly illustrate the druggability of its active site through distinct binding mechanisms. We further characterize four classes of InhA inhibitors that show novel binding modes, and provide evidence of their successful target engagement as well as their in vivo activity.

THPP target assignment reveals EchA6 as an essential fatty acid shuttle in mycobacteria.

Phenotypic screens for bactericidal compounds against drug-resistant tuberculosis are beginning to yield novel inhibitors. However, reliable target identification remains challenging. Here, we show that tetrahydropyrazo[1,5-a]pyrimidine-3-carboxamide (THPP) selectively pulls down EchA6 in a stereospecific manner, instead of the previously assigned target Mycobacterium tuberculosis MmpL3. While homologous to mammalian enoyl-coenzyme A (CoA) hydratases, EchA6 is non-catalytic yet essential and binds long-chain acyl-CoAs. THPP inhibitors compete with CoA-binding, suppress mycolic acid synthesis, and are bactericidal in a mouse model of chronic tuberculosis infection. A point mutation, W133A, abrogated THPP-binding and increased both the in vitro minimum inhibitory concentration and the in vivo effective dose 99 in mice. Surprisingly, EchA6 interacts with selected enzymes of fatty acid synthase II (FAS-II) in bacterial two-hybrid assays, suggesting essentiality may be linked to feeding long-chain fatty acids to FAS-II. Finally, our data show that spontaneous resistance-conferring mutations can potentially obscure the actual target or alternative targets of small molecule inhibitors.

Antitubercular drugs for an old target: GSK693 as a promising InhA direct inhibitor.

Despite being one of the first antitubercular agents identified, isoniazid (INH) is still the most prescribed drug for prophylaxis and tuberculosis (TB) treatment and, together with rifampicin, the pillars of current chemotherapy. A high percentage of isoniazid resistance is linked to mutations in the pro-drug activating enzyme KatG, so the discovery of direct inhibitors (DI) of the enoyl-ACP reductase (InhA) has been pursued by many groups leading to the identification of different enzyme inhibitors, active against Mycobacterium tuberculosis (Mtb), but with poor physicochemical properties to be considered as preclinical candidates. Here, we present a series of InhA DI active against multidrug (MDR) and extensively (XDR) drug-resistant clinical isolates as well as in TB murine models when orally dosed that can be a promising foundation for a future treatment.

New direct inhibitors of InhA with antimycobacterial activity based on a tetrahydropyran scaffold.

Tetrahydropyran derivative 1 was discovered in a high-throughput screening campaign to find new inhibitors of mycobacterial InhA. Following initial in-vitro profiling, a structure-activity relationship study was initiated and a focused library of analogs was synthesized and evaluated. This yielded compound 42 with improved antimycobacterial activity and low cytotoxicity. Additionally, the crystal structure of InhA in complex with inhibitor 1 was resolved, to reveal the binding mode and provide hints for further optimization.

Design, synthesis, and evaluation of new thiadiazole-based direct inhibitors of enoyl acyl carrier protein reductase (InhA) for the treatment of tuberculosis.

Mycobacterial enoyl acyl carrier protein reductase (InhA) is a clinically validated target for the treatment of tuberculosis infections, a disease that still causes the death of at least a million people annually. A known class of potent, direct, and competitive InhA inhibitors based on a tetracyclic thiadiazole structure has been shown to have in vivo activity in murine models of tuberculosis infection. On the basis of this template, we have here explored the medicinal chemistry of truncated analogues that have only three aromatic rings. In particular, compounds 8b, 8d, 8f, 8l, and 8n show interesting features, including low nanomolar InhA IC50, submicromolar antimycobacterial potency, and improved physicochemical profiles in comparison with the tetracyclic analogues. From this series, 8d is identified as having the best balance of potency and properties, whereby the resolved 8d S-enatiomer shows encouraging in vivo efficacy.

Encoded library technology as a source of hits for the discovery and lead optimization of a potent and selective class of bactericidal direct inhibitors of Mycobacterium tuberculosis InhA.

Tuberculosis (TB) is one of the world's oldest and deadliest diseases, killing a person every 20 s. InhA, the enoyl-ACP reductase from Mycobacterium tuberculosis, is the target of the frontline antitubercular drug isoniazid (INH). Compounds that directly target InhA and do not require activation by mycobacterial catalase peroxidase KatG are promising candidates for treating infections caused by INH resistant strains. The application of the encoded library technology (ELT) to the discovery of direct InhA inhibitors yielded compound 7 endowed with good enzymatic potency but with low antitubercular potency. This work reports the hit identification, the selected strategy for potency optimization, the structure-activity relationships of a hundred analogues synthesized, and the results of the in vivo efficacy studies performed with the lead compound 65.