Development of a Novel Lead that Targets M. tuberculosis Polyketide Synthase 13.

Widespread resistance to first-line TB drugs is a major problem that will likely only be resolved through the development of new drugs with novel mechanisms of action. We have used structure-guided methods to develop a lead molecule that targets the thioesterase activity of polyketide synthase Pks13, an essential enzyme that forms mycolic acids, required for the cell wall of Mycobacterium tuberculosis. Our lead, TAM16, is a benzofuran class inhibitor of Pks13 with highly potent in vitro bactericidal activity against drug-susceptible and drug-resistant clinical isolates of M. tuberculosis. In multiple mouse models of TB infection, TAM16 showed in vivo efficacy equal to the first-line TB drug isoniazid, both as a monotherapy and in combination therapy with rifampicin. TAM16 has excellent pharmacological and safety profiles, and the frequency of resistance for TAM16 is ∼100-fold lower than INH, suggesting that it can be developed as a new antitubercular aimed at the acute infection. PAPERCLIP.

Mycobacterium tuberculosis Malate Synthase Structures with Fragments Reveal a Portal for Substrate/Product Exchange.

Fragment screening and high throughput screening are complementary approaches that combine with structural biology to explore the binding capabilities of an active site. We have used a fragment-based approach on malate synthase (GlcB) from Mycobacterium tuberculosis and discovered several novel binding chemotypes. In addition, the crystal structures of GlcB in complex with these fragments indicated conformational changes in the active site that represent the enzyme conformations during catalysis. Additional structures of the complex with malate and of the apo form of GlcB supported that hypothesis. Comparative analysis of GlcB structures in complex with 18 fragments allowed us to characterize the preferred chemotypes and their binding modes. The fragment structures showed a hydrogen bond to the backbone carbonyl of Met-631. We successfully incorporated an indole group from a fragment into an existing phenyl-diketo acid series. The resulting indole-containing inhibitor was 100-fold more potent than the parent phenyl-diketo acid with an IC50 value of 20 nm.

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.

SAR and Lead Optimization of an HIV-1 Vif-APOBEC3G Axis Inhibitor.

We describe structure-activity relationship and optimization studies of RN-18, an HIV-1 Vif-APOBEC3G axis inhibitor. Targeted modifications of RN-18 ring-C, ring-B, ring-A, bridge A-B, and bridge B-C were performed to identify the crucial structural features, which generated new inhibitors with similar (4g and 4i) and improved (5, 8b, and 11) activities. Two potent water-soluble RN-18 analogues, 17 and 19, are also disclosed, and we describe the results of pharmacological studies with compound 19. The findings described here will be useful in the development of more potent Vif inhibitors and in the design of probes to identify the target protein of RN-18 and its analogues.