Crystal structure of the Mycobacterium tuberculosis VirS regulator reveals its interaction with the lead compound SMARt751.

Ethionamide (ETO) is a prodrug that is primarily used as a second-line agent in the treatment of tuberculosis. Among the bacterial ETO activators, the monooxygenase MymA has been recently identified, and its expression is regulated by the mycobacterial regulator VirS. The discovery of VirS ligands that can enhance mymA expression and thereby increase the antimycobacterial efficacy of ETO, has led to the development of a novel therapeutic strategy against tuberculosis. This strategy involves the selection of preclinical candidates, including SMARt751. We report the first crystal structure of the AraC-like regulator VirS, in complex with SMARt751, refined at 1.69 Å resolution. Crystals were obtained via an in situ proteolysis method in the requisite presence of SMARt751. The elucidated structure corresponds to the ligand-binding domain of VirS, adopting an α/ß fold with structural similarities to H-NOX domains. Within the VirS structure, SMARt751 is situated in a completely enclosed hydrophobic cavity, where it forms hydrogen bonds with Asn11 and Asn149 as well as van der Waals contacts with various hydrophobic amino acids. Comprehensive structural comparisons within the AraC family of transcriptional regulators are conducted and analyzed to figure out the effects of the SMARt751 binding on the regulatory activity of VirS.

Loss-of-function mutations in ndh do not confer delamanid, ethionamide, isoniazid, or pretomanid resistance in Mycobacterium tuberculosis.

Results from clinical strains and knockouts of the H37Rv and CDC1551 laboratory strains demonstrated that ndh (Rv1854c) is not a resistance-conferring gene for isoniazid, ethionamide, delamanid, or pretomanid in Mycobacterium tuberculosis. This difference in the susceptibility to NAD-adduct-forming drugs compared with other mycobacteria may be driven by differences in the absolute intrabacterial NADH concentration.

ArfGAP1 restricts Mycobacterium tuberculosis entry by controlling the actin cytoskeleton.

The interaction of Mycobacterium tuberculosis (Mtb) with pulmonary epithelial cells is critical for early stages of bacillus colonization and during the progression of tuberculosis. Entry of Mtb into epithelial cells has been shown to depend on F-actin polymerization, though the molecular mechanisms are still unclear. Here, we demonstrate that mycobacterial uptake into epithelial cells requires rearrangements of the actin cytoskeleton, which are regulated by ADP-ribosylation factor 1 (Arf1) and phospholipase D1 (PLD1), and is dependent on the M3 muscarinic receptor (M3R). We show that this pathway is controlled by Arf GTPase-activating protein 1 (ArfGAP1), as its silencing has an impact on actin cytoskeleton reorganization leading to uncontrolled uptake and replication of Mtb. Furthermore, we provide evidence that this pathway is critical for mycobacterial entry, while the cellular infection with other pathogens, such as Shigella flexneri and Yersinia pseudotuberculosis, is not affected. Altogether, these results reveal how cortical actin plays the role of a barrier to prevent mycobacterial entry into epithelial cells and indicate a novel role for ArfGAP1 as a restriction factor of host-pathogen interactions.

A novel codrug made of the combination of ethionamide and its potentiating booster: synthesis, self-assembly into nanoparticles and antimycobacterial evaluation.

Ethionamide (ETH) is one of the most widely used second-line chemotherapeutic drugs for the treatment of multi-drug-resistant tuberculosis. The bioactivation and activity of ETH is dramatically potentiated by a family of molecules called "boosters" among which BDM43266 is one of the most potent. However, the co-administration of these active molecules is hampered by their low solubility in biological media and by the strong tendency of ETH to crystallize. A novel strategy that involves synthesizing a codrug able to self-associate into nanoparticles prone to be taken up by infected macrophages is proposed here. This codrug is designed by tethering N-hydroxymethyl derivatives of both ETH and its booster through a glutaric linker. This codrug self-assembles into nanoparticles of around 200 nm, stable upon extreme dilution without disaggregating as well as upon concentration. The nanoparticles of the codrug can be intranasally administered overcoming the unfavorable physico-chemical profiles of the parent drugs. Intrapulmonary delivery of the codrug nanoparticles to Mtb infected mice via the intranasal route at days 7, 9, 11, 14, 16 and 18 post-infection reduces the bacterial load in the lungs by a factor of 6.

Structural analysis of the interaction between spiroisoxazoline SMARt-420 and the Mycobacterium tuberculosis repressor EthR2.

Inhibition of transcriptional regulators of bacterial pathogens with the aim of reprogramming their metabolism to modify their antibiotic susceptibility constitutes a promising therapeutic strategy. One example is the bio-activation of the anti-tubercular pro-drug ethionamide, which activity could be enhanced by inhibiting the transcriptional repressor EthR. Recently, we discovered that inhibition of a second transcriptional repressor, EthR2, leads to the awakening of a new ethionamide bio-activation pathway. The x-ray structure of EthR2 was solved at 2.3 Å resolution in complex with a compound called SMARt-420 (Small Molecule Aborting Resistance). Detailed comparison and structural analysis revealed interesting insights for the upcoming structure-based design of EthR2 inhibitors as an alternative to revert ethionamide resistance in Mycobacterium tuberculosis.

Phenotypic assays for Mycobacterium tuberculosis infection.

Tuberculosis (TB) is still a major global threat, killing more than one million persons each year. With the constant increase of Mycobacterium tuberculosis strains resistant to first- and second-line drugs, there is an urgent need for the development of new drugs to control the propagation of TB. Although screenings of small molecules on axenic M. tuberculosis cultures were successful for the identification of novel putative anti-TB drugs, new drugs in the development pipeline remains scarce. Host-directed therapy may represent an alternative for drug development against TB. Indeed, M. tuberculosis has multiple specific interactions within host phagocytes, which may be targeted by small molecules. In order to enable drug discovery strategies against microbes residing within host macrophages, we developed multiple fluorescence-based HT/CS phenotypic assays monitoring the intracellular replication of M. tuberculosis as well as its intracellular trafficking. What we propose here is a population-based, multi-parametric analysis pipeline that can be used to monitor the intracellular fate of M. tuberculosis and the dynamics of cellular events such as phagosomal maturation (acidification and permeabilization), zinc poisoning system or lipid body accumulation. Such analysis allows the quantification of biological events considering the host-pathogen interplay and may thus be derived to other intracellular pathogens. © 2017 International Society for Advancement of Cytometry.

New active leads for tuberculosis booster drugs by structure-based drug discovery.

The transcriptional repressor EthR from Mycobacterium tuberculosis, a member of the TetR family of prokaryotic homo-dimeric transcription factors, controls the expression of the mycobacterial mono-oxygenase EthA. EthA is responsible for the bio-activation of the second-line tuberculosis pro-drug ethionamide, and consequently EthR inhibitors boost drug efficacy. Here, we present a comprehensive in silico structure-based screening protocol that led to the identification of a number of novel scaffolds of EthR inhibitors in subsequent biophysical screening by thermal shift assay. Growth inhibition assays demonstrated that five of the twenty biophysical hits were capable of boosting ethionamide activity in vitro, with the best novel scaffold displaying an EC50 of 34 µM. In addition, the co-crystal structures of EthR with four new ligands at resolution ranging from 2.1 to 1.4 Å confirm the binding and inactivation mode, and will enable future lead development.

Effects of Lipid-Lowering Drugs on Vancomycin Susceptibility of Mycobacteria.

Tuberculosis is still a cause of major concern, partly due to the emergence of multidrug-resistant strains. New drugs are therefore needed. Vancomycin can target mycobacteria with cell envelope deficiency. In this study, we used a vancomycin susceptibility assay to detect drugs hampering lipid synthesis in Mycobacterium bovis BCG and in Mycobacterium tuberculosis We tested three drugs already used to treat human obesity: tetrahydrolipstatin (THL), simvastatin, and fenofibrate. Only vancomycin and THL were able to synergize on M. bovis BCG and on M. tuberculosis, although mycobacteria could also be inhibited by simvastatin alone. Lipid analysis allowed us to identify several lipid modifications in M. tuberculosis H37Rv treated with those drugs. THL treatment mainly reduced the phthiocerol dimycocerosate (PDIM) content in the mycobacterial cell wall, providing an explanation for the synergy, since PDIM deficiency has been related to vancomycin susceptibility. Proteomic analysis suggested that bacteria treated with THL, in contrast to bacteria treated with simvastatin, tried to recover, inducing, among other reactions, lipid synthesis. The combination of THL and vancomycin should be considered a promising solution in developing new strategies to treat multidrug-resistant tuberculosis.

A fragment merging approach towards the development of small molecule inhibitors of Mycobacterium tuberculosis EthR for use as ethionamide boosters.

With the ever-increasing instances of resistance to frontline TB drugs there is the need to develop novel strategies to fight the worldwide TB epidemic. Boosting the effect of the existing second-line antibiotic ethionamide by inhibiting the mycobacterial transcriptional repressor protein EthR is an attractive therapeutic strategy. Herein we report the use of a fragment based drug discovery approach for the structure-guided systematic merging of two fragment molecules, each binding twice to the hydrophobic cavity of EthR from M. tuberculosis. These together fill the entire binding pocket of EthR. We elaborated these fragment hits and developed small molecule inhibitors which have a 100-fold improvement of potency in vitro over the initial fragments.

Mechanism of competence activation by the ComRS signalling system in streptococci.

In many streptococci, competence for natural DNA transformation is regulated by the Rgg-type regulator ComR and the pheromone ComS, which is sensed intracellularly. We compared the ComRS systems of four model streptococcal species using in vitro and in silico approaches, to determine the mechanism of the ComRS-dependent regulation of competence. In all systems investigated, ComR was shown to be the proximal transcriptional activator of the expression of key competence genes. Efficient binding of ComR to DNA is strictly dependent on the presence of the pheromone (C-terminal ComS octapeptide), in contrast with other streptococcal Rgg-type regulators. The 20 bp palindromic ComR-box is the minimal genetic requirement for binding of ComR, and its sequence directly determines the expression level of genes under its control. Despite the apparent species-specific specialization of the ComR-ComS interaction, mutagenesis of ComS residues from Streptococcus thermophilus highlighted an unexpected permissiveness with respect to its biological activity. In agreement, heterologous ComS, and even primary sequence-unrelated, casein-derived octapeptides, were able to induce competence development in S. thermophilus. The lack of stringency of ComS sequence suggests that competence of a specific Streptococcus species may be modulated by other streptococci or by non-specific nutritive oligopeptides present in its environment.

The Mycobacterium tuberculosis transcriptional repressor EthR is negatively regulated by Serine/Threonine phosphorylation.

Recent efforts have underlined the role of Serine/Threonine Protein Kinases (STPKs) in growth, pathogenesis and cell wall metabolism in mycobacteria. Herein, we demonstrated that the Mycobacterium tuberculosis EthR, a transcriptional repressor that regulates the activation process of the antitubercular drug ethionamide (ETH) is a specific substrate of the mycobacterial kinase PknF. ETH is a prodrug that must undergo bioactivation by the monooxygenease EthA to exert its antimycobacterial activity and previous studies reported that EthR represses transcription of ethA by binding to the ethA-ethR intergenic region. Mass spectrometry analyses and site-directed mutagenesis identified a set of four phosphoacceptors, namely Thr2, Thr3, Ser4 and Ser7. This was further supported by the complete loss of PknF-dependent phosphorylation of a phosphoablative EthR mutant protein. Importantly, a phosphomimetic version of EthR, in which all phosphosites were replaced by Asp residues, exhibited markedly decreased DNA-binding activity compared with the wild-type protein. Together, these findings are the first demonstration of EthR phosphorylation and indicate that phosphorylation negatively affects its DNA-binding activity, which may impact ETH resistance levels in M. tb.

Unconventional surface plasmon resonance signals reveal quantitative inhibition of transcriptional repressor EthR by synthetic ligands.

EthR is a mycobacterial repressor that limits the bioactivation of ethionamide, a commonly used anti-tuberculosis second-line drug. Several efforts have been deployed to identify EthR inhibitors abolishing the DNA-binding activity of the repressor. This led to the demonstration that stimulating the bioactivation of Eth through EthR inhibition could be an alternative way to fight Mycobacterium tuberculosis. We propose a new surface plasmon resonance (SPR) methodology to study the affinity between inhibitors and EthR. Interestingly, the binding between inhibitors and immobilized EthR produced a dose-dependent negative SPR signal. We demonstrate that this signal reveals the affinity of small molecules for the repressor. The affinity constants (K(D)) correlate with their capacity to inhibit the binding of EthR to DNA. We hypothesize that conformational changes in EthR during ligand interaction could be responsible for this SPR signal. Practically, this unconventional result opens perspectives onto the development of an SPR assay that would at the same time reveal structural changes in the target upon binding with an inhibitor and the binding constant of this interaction.

Structural activation of the transcriptional repressor EthR from Mycobacterium tuberculosis by single amino acid change mimicking natural and synthetic ligands.

Ethionamide is an antituberculous drug for the treatment of multidrug-resistant Mycobacterium tuberculosis. This antibiotic requires activation by the monooxygenase EthA to exert its activity. Production of EthA is controlled by the transcriptional repressor EthR, a member of the TetR family. The sensitivity of M. tuberculosis to ethionamide can be artificially enhanced using synthetic ligands of EthR that allosterically inactivate its DNA-binding activity. Comparison of several structures of EthR co-crystallized with various ligands suggested that the structural reorganization of EthR resulting in its inactivation is controlled by a limited portion of the ligand-binding-pocket. In silico simulation predicted that mutation G106W may mimic ligands. X-ray crystallography of variant G106W indeed revealed a protein structurally similar to ligand-bound EthR. Surface plasmon resonance experiments established that this variant is unable to bind DNA, while thermal shift studies demonstrated that mutation G106W stabilizes EthR as strongly as ligands. Proton NMR of the methyl regions showed a lesser contribution of exchange broadening upon ligand binding, and the same quenched dynamics was observed in apo-variant G106W. Altogether, we here show that the area surrounding Gly106 constitutes the molecular switch involved in the conformational reorganization of EthR. These results also shed light on the mechanistic of ligand-induced allosterism controlling the DNA binding properties of TetR family repressors.

Optimizing the use of current antituberculosis drugs to overcome drug resistance in Mycobacterium tuberculosis.

Antibiotic-resistant tuberculosis continues to be one of the major threats to global tuberculosis control. After a hiatus of over 40 years in antituberculosis drug development, the last decade has seen a resurgence of research, yielding a number of promising compounds in the tuberculosis drug pipeline, with some that are now game changers in the treatment of MDRTB. Despite this progress, there are still obstacles restricting the use of these molecules as first-line drugs. The quick appearance of bacteria resistant to these new treatments highlights a continuing need to fuel the discovery and development of new molecules. With this in mind, alternative strategies aimed at optimizing the utilization of existing antituberculosis agents are currently under evaluation. They are focused on enhancing the efficacy of antibiotics against their bacterial targets, primarily by augmenting the quantity of antibiotic that engages with these targets. This objective can be achieved through two primary approaches: (1) Provided that toxicity concerns are not a limiting factor, increased dosing is a viable avenue, as demonstrated by rifampicin, isoniazid, and fluoroquinolones, for which escalated dosing has been effective; and (2) Employing enhancers such as drug activator boosters (ethionamide), efflux pump inhibitors, or hydrolytic enzyme inhibitors (kanamycin) can elevate the concentration of antibiotics in bacterial cells. These strategies offer the potential to mitigate antibiotic obsolescence and complement the discovery of new antibiotics.

Both Nitro Groups Are Essential for High Antitubercular Activity of 3,5-Dinitrobenzylsulfanyl Tetrazoles and 1,3,4-Oxadiazoles through the Deazaflavin-Dependent Nitroreductase Activation Pathway.

3,5-Dinitrobenzylsulfanyl tetrazoles and 1,3,4-oxadiazoles, previously identified as having high in vitro activities against both replicating and nonreplicating mycobacteria and favorable cytotoxicity and genotoxicity profiles were investigated. First we demonstrated that these compounds act in a deazaflavin-dependent nitroreduction pathway and thus require a nitro group for their activity. Second, we confirmed the necessity of both nitro groups for antimycobacterial activity through extensive structure-activity relationship studies using 32 structural types of analogues, each in a five-membered series. Only the analogues with shifted nitro groups, namely, 2,5-dinitrobenzylsulfanyl oxadiazoles and tetrazoles, maintained high antimycobacterial activity but in this case mainly as a result of DprE1 inhibition. However, these analogues also showed increased toxicity to the mammalian cell line. Thus, both nitro groups in 3,5-dinitrobenzylsulfanyl-containing antimycobacterial agents remain essential for their high efficacy, and further efforts should be directed at finding ways to address the possible toxicity and solubility issues, for example, by targeted delivery.

Discovery of dual-active ethionamide boosters inhibiting the Mycobacterium tuberculosis ESX-1 secretion system.

Drug-resistant Mycobacterium tuberculosis (Mtb) remains a major public health concern requiring complementary approaches to standard anti-tuberculous regimens. Anti-virulence molecules or compounds that enhance the activity of antimicrobial prodrugs are promising alternatives to conventional antibiotics. Exploiting host cell-based drug discovery, we identified an oxadiazole compound (S3) that blocks the ESX-1 secretion system, a major virulence factor of Mtb. S3-treated mycobacteria showed impaired intracellular growth and a reduced ability to lyse macrophages. RNA sequencing experiments of drug-exposed bacteria revealed strong upregulation of a distinct set of genes including ethA, encoding a monooxygenase activating the anti-tuberculous prodrug ethionamide. Accordingly, we found a strong ethionamide boosting effect in S3-treated Mtb. Extensive structure-activity relationship experiments revealed that anti-virulence and ethionamide-boosting activity can be uncoupled by chemical modification of the primary hit molecule. To conclude, this series of dual-active oxadiazole compounds targets Mtb via two distinct mechanisms of action.

Structural and docking studies of potent ethionamide boosters.

Tuberculosis remains the second only to HIV as the leading cause of death by infectious disease worldwide, and was responsible for 1.4 million deaths globally in 2011. One of the essential drugs of the second-line antitubercular regimen is the prodrug ethionamide, introduced in the 1960s. Ethionamide is primarily used in cases of multi-drug resistant (MDR) and extensively drug resistant (XDR) TB due to severe adverse side effects. As a prodrug, ethionamide is bioactivated by EthA, a mono-oxygenase whose activity is repressed by EthR, a member of the TetR family of regulators. Previous studies have established that inhibition of EthR improves ethionamide potency. We report here the crystal structures of three EthR inhibitors at 0.8 Šresolution (3-oxo-3-{4-[3-(thiophen-2-yl)-1,2,4-oxadiazol-5-yl]piperidin-1-yl}propanenitrile (BDM31343), 4,4,4-trifluoro-1-{4-[3-(6-methoxy-1,3-benzothiazol-2-yl)-1,2,4-oxadiazol-5-yl]piperidin-1-yl}butanone (BDM41325) and 5,5,5-trifluoro-1-{4-[3-(4-methanesulfonylphenyl)-1,2,4-oxadiazol-5-yl]piperidin-1-yl}pentanone (BDM41907)), and the docking studies undertaken to investigate possible binding modes. The results revealed two distinct orientations of the three compounds in the binding channel, a direct consequence of the promiscuous nature of the largely lipophilic binding site.

Exploring the Antitubercular Activity of Anthranilic Acid Derivatives: From MabA (FabG1) Inhibition to Intrabacterial Acidification.

Mycobacterium tuberculosis, the pathogen that causes tuberculosis, is responsible for the death of 1.5 million people each year and the number of bacteria resistant to the standard regimen is constantly increasing. This highlights the need to discover molecules that act on new M. tuberculosis targets. Mycolic acids, which are very long-chain fatty acids essential for M. tuberculosis viability, are synthesized by two types of fatty acid synthase (FAS) systems. MabA (FabG1) is an essential enzyme belonging to the FAS-II cycle. We have recently reported the discovery of anthranilic acids as MabA inhibitors. Here, the structure-activity relationships around the anthranilic acid core, the binding of a fluorinated analog to MabA by NMR experiments, the physico-chemical properties and the antimycobacterial activity of these inhibitors were explored. Further investigation of the mechanism of action in bacterio showed that these compounds affect other targets than MabA in mycobacterial cells and that their antituberculous activity is due to the carboxylic acid moiety which induces intrabacterial acidification.

On the Hunt for Next-Generation Antimicrobial Agents: An Online Symposium Organized Jointly by the French Society for Medicinal Chemistry (Société de Chimie Thérapeutique) and the French Microbiology Society (Société Française de Microbiologie) on 9-10 December 2021.

The restrictions posed by the COVID-19 pandemic obliged the French Society for Medicinal Chemistry (Société de chimie thérapeutique) and the French Microbiology Society (Société Française de Microbiologie) to organize their joint autumn symposium (entitled "On the hunt for next-generation antimicrobial agents") online on 9-10 December 2021. The meeting attracted more than 200 researchers from France and abroad with interests in drug discovery, antimicrobial resistance, medicinal chemistry, and related disciplines. This review summarizes the 13 invited keynote lectures. The symposium generated high-level scientific dialogue on the most recent advances in combating antimicrobial resistance. The University of Lille, the Institut Pasteur de Lille, the journal Pharmaceuticals, Oxeltis, and INCATE, sponsored the event.

The small-molecule SMARt751 reverses Mycobacterium tuberculosis resistance to ethionamide in acute and chronic mouse models of tuberculosis.

The sensitivity of Mycobacterium tuberculosis, the pathogen that causes tuberculosis (TB), to antibiotic prodrugs is dependent on the efficacy of the activation process that transforms the prodrugs into their active antibacterial moieties. Various oxidases of M. tuberculosis have the potential to activate the prodrug ethionamide. Here, we used medicinal chemistry coupled with a phenotypic assay to select the N-acylated 4-phenylpiperidine compound series. The lead compound, SMARt751, interacted with the transcriptional regulator VirS of M. tuberculosis, which regulates the mymA operon encoding a monooxygenase that activates ethionamide. SMARt751 boosted the efficacy of ethionamide in vitro and in mouse models of acute and chronic TB. SMARt751 also restored full efficacy of ethionamide in mice infected with M. tuberculosis strains carrying mutations in the ethA gene, which cause ethionamide resistance in the clinic. SMARt751 was shown to be safe in tests conducted in vitro and in vivo. A model extrapolating animal pharmacokinetic and pharmacodynamic parameters to humans predicted that as little as 25 mg of SMARt751 daily would allow a fourfold reduction in the dose of ethionamide administered while retaining the same efficacy and reducing side effects.