New therapeutic strategies are needed to combat the tuberculosis pandemic and the spread of multidrug-resistant (MDR) and extensively drug-resistant (XDR) forms of the disease, which remain a serious public health challenge worldwide. The most urgent clinical need is to discover potent agents capable of reducing the duration of MDR and XDR tuberculosis therapy with a success rate comparable to that of current therapies for drug-susceptible tuberculosis. The last decade has seen the discovery of new agent classes for the management of tuberculosis, several of which are currently in clinical trials. However, given the high attrition rate of drug candidates during clinical development and the emergence of drug resistance, the discovery of additional clinical candidates is clearly needed. Here, we report on a promising class of imidazopyridine amide (IPA) compounds that block Mycobacterium tuberculosis growth by targeting the respiratory cytochrome bc1 complex. The optimized IPA compound Q203 inhibited the growth of MDR and XDR M. tuberculosis clinical isolates in culture broth medium in the low nanomolar range and was efficacious in a mouse model of tuberculosis at a dose less than 1 mg per kg body weight, which highlights the potency of this compound. In addition, Q203 displays pharmacokinetic and safety profiles compatible with once-daily dosing. Together, our data indicate that Q203 is a promising new clinical candidate for the treatment of tuberculosis.

A critical feature of Mycobacterium tuberculosis, the causative agent of human tuberculosis (TB), is its ability to survive and multiply within macrophages, making these host cells an ideal niche for persisting microbes. Killing the intracellular tubercle bacilli is a key requirement for efficient tuberculosis treatment, yet identifying potent inhibitors has been hampered by labor-intensive techniques and lack of validated targets. Here, we present the development of a phenotypic cell-based assay that uses automated confocal fluorescence microscopy for high throughput screening of chemicals that interfere with the replication of M. tuberculosis within macrophages. Screening a library of 57,000 small molecules led to the identification of 135 active compounds with potent intracellular anti-mycobacterial efficacy and no host cell toxicity. Among these, the dinitrobenzamide derivatives (DNB) showed high activity against M. tuberculosis, including extensively drug resistant (XDR) strains. More importantly, we demonstrate that incubation of M. tuberculosis with DNB inhibited the formation of both lipoarabinomannan and arabinogalactan, attributable to the inhibition of decaprenyl-phospho-arabinose synthesis catalyzed by the decaprenyl-phosphoribose 2′ epimerase DprE1/DprE2. Inhibition of this new target will likely contribute to new therapeutic solutions against emerging XDR-TB. Beyond validating the high throughput/content screening approach, our results open new avenues for finding the next generation of antimicrobials.

We report in this article an extensive structure-activity relationships (SAR) study with 58 thiophen-2-yl-1,2,4-oxadiazoles as inhibitors of EthR, a transcriptional regulator controling ethionamide bioactivation in Mycobacterium tuberculosis. We explored the replacement of two key fragments of the starting lead BDM31343. We investigated the potency of all analogues to boost subactive doses of ethionamide on a phenotypic assay involving M. tuberculosis infected macrophages and then ascertained the mode of action of the most active compounds using a functional target-based surface plasmon resonance assay. This process revealed that introduction of 4,4,4-trifluorobutyryl chain instead of cyanoacetyl group was crucial for intracellular activity. Replacement of 1,4-piperidyl by (R)-1,3-pyrrolidyl scaffold did not enhance activity but led to improved pharmacokinetic properties. Furthermore, the crystal structures of ligand-EthR complexes were consistent with the observed SAR. In conclusion, we identified EthR inhibitors that boost antibacterial activity of ethionamide with nanomolar potency while improving solubility and metabolic stability.

Mycobacterial transcriptional repressor EthR controls the expression of EthA, the bacterial monooxygenase activating ethionamide, and is thus largely responsible for the low sensitivity of the human pathogen Mycobacterium tuberculosis to this antibiotic. We recently reported structure-activity relationships of a series of 1,2,4-oxadiazole EthR inhibitors leading to the discovery of potent ethionamide boosters. Despite high metabolic stability, pharmacokinetic evaluation revealed poor mice exposure; therefore, a second phase of optimization was required. Herein a structure-property relationship study is reported according to the replacement of the two aromatic heterocycles: 2-thienyl and 1,2,4-oxadiazolyl moieties. This work was done using a combination of structure-based drug design and in vitro/ex vivo evaluations of ethionamide boosters on the targeted protein EthR and on the human pathogen Mycobacterium tuberculosis. Thanks to this process, we identified compound 42 (BDM41906), which displays improved efficacy in addition to high exposure to mice after oral administration.

Macrophages are reservoirs for replicating mycobacterium during tuberculosis (TB) infections. In this study, small molecules to be developed as anti-tubercular treatments were investigated for their ability to kill intracellular bacteria in in vitro macrophage models. High-content imaging technologies offer a high-throughput method to quantify a drug's ability to inhibit Mycobacterium tuberculosis intracellular invasion and multiplication in host cells. Dedicated image analysis enables the automated quantification of infected macrophages, and compounds that inhibit mycobacteria proliferation can be tested using this method. Furthermore, the implementation of the assay in 384-well microtiter plates combined with automated image acquisition and analysis allows large-scale screening of compound libraries in M. tuberculosis-infected macrophages. Here we describe a high-throughput and high-content workflow and detail its utility for the development of new TB drugs.

In this paper, we describe the screening of a 14640-compound library using a novel whole mycobacteria phenotypic assay to discover inhibitors of EthR, a transcriptional repressor implicated in the innate resistance of Mycobacterium tuberculosis to the second-line antituberculosis drug ethionamide. From this screening a new chemical family of EthR inhibitors bearing an N-phenylphenoxyacetamide motif was identified. The X-ray structure of the most potent compound crystallized with EthR inspired the synthesis of a 960-member focused library. These compounds were tested in vitro using a rapid thermal shift assay on EthR to accelerate the optimization. The best compounds were synthesized on a larger scale and confirmed as potent ethionamide boosters on M. tuberculosis -infected macrophages. Finally, the cocrystallization of the best optimized analogue with EthR revealed an unexpected reorientation of the ligand in the binding pocket.

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