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.

Advancing the Therapeutic Potential of Indoleamides for Tuberculosis.

Indole-2-carboxamide derivatives are inhibitors of MmpL3, the cell wall-associated mycolic acid transporter of Mycobacterium tuberculosis In the present study, we characterized indoleamide effects on bacterial cell morphology and reevaluated pharmacokinetics and in vivo efficacy using an optimized oral formulation. Morphologically, indoleamide-treated M. tuberculosis cells demonstrated significantly higher numbers of dimples near the poles or septum, which may serve as the mechanism of cell death for this bactericidal scaffold. Using the optimized formulation, an expanded-spectrum indoleamide, compound 2, showed significantly improved pharmacokinetic (PK) parameters and in vivo efficacy in mouse infection models. In a comparative study, compound 2 showed superior efficacy over compound 3 (NITD-304) in a high-dose aerosol mouse infection model. Since indoleamides are equally active on drug-resistant M. tuberculosis, these findings demonstrate the therapeutic potential of this novel scaffold for the treatment of both drug-susceptible and drug-resistant tuberculosis.

Identification of a small molecule with activity against drug-resistant and persistent tuberculosis.

A cell-based phenotypic screen for inhibitors of biofilm formation in mycobacteria identified the small molecule TCA1, which has bactericidal activity against both drug-susceptible and -resistant Mycobacterium tuberculosis (Mtb) and sterilizes Mtb in vitro combined with rifampicin or isoniazid. In addition, TCA1 has bactericidal activity against nonreplicating Mtb in vitro and is efficacious in acute and chronic Mtb infection mouse models both alone and combined with rifampicin or isoniazid. Transcriptional analysis revealed that TCA1 down-regulates genes known to be involved in Mtb persistence. Genetic and affinity-based methods identified decaprenyl-phosphoryl-ß-D-ribofuranose oxidoreductase DprE1 and MoeW, enzymes involved in cell wall and molybdenum cofactor biosynthesis, respectively, as targets responsible for the activity of TCA1. These in vitro and in vivo results indicate that this compound functions by a unique mechanism and suggest that TCA1 may lead to the development of a class of antituberculosis agents.

Evaluation of moxifloxacin-containing regimens in pathologically distinct murine tuberculosis models.

In the recently concluded REMox-TB trial, two 4-month moxifloxacin-containing regimens did not meet the criteria for noninferiority compared to the current 6-month first-line regimen to treat tuberculosis (TB). Despite the disappointing result, this phase 3 clinical trial provides a rare opportunity to gauge the predictive accuracy of the nonclinical models used to support regimen development. In parallel with the REMox-TB trial, we compared the efficacy of the same three regimens against chronic TB infection in the commonly used BALB/c mouse strain and in C3HeB/FeJ mice, which have attracted recent interest as a nonclinical efficacy model because they develop caseous lung lesions which may better resemble human TB. In long-term treatment experiments at two institutions, using low-dose aerosol infection models with 6- to 8-week incubation periods in both mouse strains, control mice received rifampin, isoniazid, pyrazinamide, and ethambutol (RHZE), and test mice received the same regimen with moxifloxacin replacing isoniazid (RMZE) or ethambutol (RHZM). Outcome measures were lung CFU counts during treatment and relapse after various durations of treatment. At both institutions and in both mouse strains, RMZE and RHZM reduced by approximately 1 month and 0 to 1 month, respectively, the treatment duration needed to produce the same relapse rate as RHZE. These results demonstrating generally similar treatment-shortening effects of the moxifloxacin-containing regimens in each mouse strain, with effect sizes consistent with the REMox-TB trial results, reinforce the predictive value of murine models for TB regimen development.

1,4-azaindole, a potential drug candidate for treatment of tuberculosis.

New therapeutic strategies against multidrug-resistant (MDR) and extensively drug-resistant (XDR) Mycobacterium tuberculosis are urgently required to combat the global tuberculosis (TB) threat. Toward this end, we previously reported the identification of 1,4-azaindoles, a promising class of compounds with potent antitubercular activity through noncovalent inhibition of decaprenylphosphoryl-ß-D-ribose 2'-epimerase (DprE1). Further, this series was optimized to improve its physicochemical properties and pharmacokinetics in mice. Here, we describe the short-listing of a potential clinical candidate, compound 2, that has potent cellular activity, drug-like properties, efficacy in mouse and rat chronic TB infection models, and minimal in vitro safety risks. We also demonstrate that the compounds, including compound 2, have no antagonistic activity with other anti-TB drugs. Moreover, compound 2 shows synergy with PA824 and TMC207 in vitro, and the synergy effect is translated in vivo with TMC207. The series is predicted to have a low clearance in humans, and the predicted human dose for compound 2 is ≤1 g/day. Altogether, our data suggest that a 1,4-azaindole (compound 2) is a promising candidate for the development of a novel anti-TB drug.

Scaffold Morphing To Identify Novel DprE1 Inhibitors with Antimycobacterial Activity.

We report a novel benzimidazole (BI) based DprE1 inhibitor that resulted from scaffold morphing of a 1,4-azaindole series. The clinical progression of the 1,4-azaindole series from our previous work validates the potential of exploring newer chemical entities with antimycobacterial activity driven via a noncovalent inhibition of the decaprenylphosphoryl-ß-d-ribose-2'-epimerase (DprE1). The representative compounds from the new scaffold reported in this study exhibited an improved solubility and higher free plasma fraction, while retaining potent DprE1 inhibition and antimycobacterial activity. A representative compound from the benzimidazole series demonstrated good efficacy in a murine model of tuberculosis. Furthermore, molecular modeling of the BI scaffold suggests plausible modes of binding in the active site of DprE1 enzyme from Mycobacterium tuberculosis that can be used for further exploration of the series.

Discovery of Imidazo[1,2-a]pyridine Ethers and Squaramides as Selective and Potent Inhibitors of Mycobacterial Adenosine Triphosphate (ATP) Synthesis.

The approval of bedaquiline to treat tuberculosis has validated adenosine triphosphate (ATP) synthase as an attractive target to kill Mycobacterium tuberculosis (Mtb). Herein, we report the discovery of two diverse lead series imidazo[1,2-a]pyridine ethers (IPE) and squaramides (SQA) as inhibitors of mycobacterial ATP synthesis. Through medicinal chemistry exploration, we established a robust structure-activity relationship of these two scaffolds, resulting in nanomolar potencies in an ATP synthesis inhibition assay. A biochemical deconvolution cascade suggested cytochrome c oxidase as the potential target of IPE class of molecules, whereas characterization of spontaneous resistant mutants of SQAs unambiguously identified ATP synthase as its molecular target. Absence of cross resistance against bedaquiline resistant mutants suggested a different binding site for SQAs on ATP synthase. Furthermore, SQAs were found to be noncytotoxic and demonstrated efficacy in a mouse model of tuberculosis infection.

Lead optimization of 1,4-azaindoles as antimycobacterial agents.

In a previous report, we described the discovery of 1,4-azaindoles, a chemical series with excellent in vitro and in vivo antimycobacterial potency through noncovalent inhibition of decaprenylphosphoryl-ß-d-ribose-2'-epimerase (DprE1). Nevertheless, high mouse metabolic turnover and phosphodiesterase 6 (PDE6) off-target activity limited its advancement. Herein, we report lead optimization of this series, culminating in potent, metabolically stable compounds that have a robust pharmacokinetic profile without any PDE6 liability. Furthermore, we demonstrate efficacy for 1,4-azaindoles in a rat chronic TB infection model. We believe that compounds from the 1,4-azaindole series are suitable for in vivo combination and safety studies.

Alignment of new tuberculosis drug regimens and drug susceptibility testing: a framework for action.

New tuberculosis drug regimens are creating new priorities for drug susceptibility testing (DST) and surveillance. To minimise turnaround time, rapid DST will need to be prioritised, but developers of these assays will need better data about the molecular mechanisms of resistance. Efforts are underway to link mutations with drug resistance and to develop strain collections to enable assessment of new diagnostic assays. In resource-limited settings, DST might not be appropriate for all patients with tuberculosis. Surveillance data and modelling will help country stakeholders to design appropriate DST algorithms and to decide whether to change drug regimens. Finally, development of practical DST assays is needed so that, in countries where surveillance and modelling show that DST is advisable, these assays can be used to guide clinical decisions for individual patients. If combined judiciously during both development and implementation, new tuberculosis regimens and new DST assays have enormous potential to improve patient outcomes and reduce the burden of disease.