Evaluation of gyrase B as a drug target in Mycobacterium tuberculosis

Novobiocin primarily targets ParE in Neisseria gonorrhoeae

Multidrug-resistant Neisseria gonorrhoeae is a pathogenic bacterium that poses a public health concern. In this study, we aimed to elucidate the mode of action of the conventional antibiotic novobiocin, which has been selected as a leading compound for novel antigonococcal drugs. Unlike other previously studied bacteria strains, novobiocin-resistant N. gonorrhoeae strains have a mutation in the parE gene encoding DNA topoisomerase IV, strongly implying that the primary target of novobiocin is DNA topoisomerase IV and not DNA gyrase. The construction of genetically modified strains and structural biology analysis in silico suggest that this target discrepancy is from variations in the amino acid sequences in GyrB (Ile 78 in Escherichia coli, Met82 in N. gonorrhoeae) and ParE (Met 74 in E. coli, Ile76 in N. gonorrhoeae). This finding contributes to the development of drugs that target both GyrB and ParE enzymes to a similar extent.

Microbial-based natural products as potential inhibitors targeting DNA gyrase B of Mycobacterium tuberculosis: an in silico study

Introduction

Since the emergence of Mycobacterium tuberculosis (MBT) strains resistant to most currently used anti-tubercular drugs, there has been an urgent need to develop efficient drugs capable of modulating new therapeutic targets. Mycobacterial DNA gyrase is an enzyme that plays a crucial role in the replication and transcription of DNA in MBT. Consequently, targeting this enzyme is of particular interest in developing new drugs for the treatment of drug-resistant tuberculosis, including multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB).

Methods

In the present study, multiple computational tools were adopted to screen a microbial-based natural products database (NPAtlas) for potential inhibitors of the ATPase activity of MBT DNA gyrase.

Results and discussion

Twelve hits were initially identified as the top candidates based on their docking scores (ranging from -9.491 to -10.77 kcal/mol) and binding free energies (-60.37 to -73.21 kcal/mol). Following this, computational filters, including ADME-T profiling and pharmacophore modeling, were applied to further refine the selection. As a result, three compounds 1-Hydroxy-D-788-7, Erythrin, and Pyrindolol K2 emerged as the most promising, exhibiting favorable drug-like properties. Notably, 1-Hydroxy-D-788-7, an anthracycline derivative, demonstrated superior binding affinity in molecular dynamics simulations. The RMSD values, ranging from 1.7 to 2.5 Å, alongside RMSF analysis and a detailed evaluation of the established interaction forces, revealed that 1-Hydroxy-D-788-7 was the strongest binder to Mycobacterial DNA Gyrase B. The stable binding and favorable interaction profile highlighted 1-Hydroxy-D-788-7 as a top hit. These comprehensive computational findings strongly support the potential of 1-Hydroxy-D-788-7 as an effective anti-TB lead compound, warranting further experimental validation to confirm its therapeutic efficacy.

Mycobacterium tuberculosis inhibitors: an updated patent review (2021-present)

INTRODUCTION

Tuberculosis (TB) remains a major global health issue, causing around 10 million new cases and 1.3 million deaths in 2022. The challenge is compounded by multidrug-resistant (MDR) and extensively drug-resistant (XDR) TB strains, and co-infection with HIV.

AREAS COVERED

The present review examines significant patent literature on TB chemotherapeutics from September 2021 to the present using the following databases, reaxys, google patent and espacenet. Only patents reporting compounds with a minimum inhibitory concentration (MIC) on whole Mycobacterium tuberculosis cells of ≤5 µM were selected for review.

EXPERT OPINION

The fight against TB is advancing with the development of promising new compounds due to the challenge of drug-resistant strains. Notable among those reviewed in this paper are the benzothiazinones, showing high efficacy against both drug-sensitive and resistant TB strains. Additionally, Q203 analogues, demonstrate strong antitubercular activity, good microsomal stability, and favorable safety profiles. Finally, LysRS inhibitors also show significant promise in vivo models. These advancements underscore the importance of novel targets and innovative strategies in developing effective, resistance-resistant TB treatments.

Development of narrow-spectrum topoisomerase-targeting antibacterials against mycobacteria

New 2-pyrrolamidobenzothiazole-based inhibitors of mycobacterial DNA gyrase were discovered. Among these, compounds 49 and 51, show excellent antibacterial activity against Mycobacterium tuberculosis and Mycobacterium abscessus with a notable preference for mycobacteria. Both compounds can penetrate infected macrophages and reduce intracellular M. tuberculosis load. Compound 51 is a potent inhibitor of DNA gyrase (M. tuberculosis DNA gyrase IC50 = 4.1 nM, Escherichia coli DNA gyrase IC50 of <10 nM), selective for bacterial topoisomerases. It displays low MIC90 values (M. tuberculosis: 0.63 μM; M. abscessus: 2.5 μM), showing specificity for mycobacteria, and no apparent toxicity. Compound 49 not only displays potent antimycobacterial activity (MIC90 values of 2.5 μM for M. tuberculosis and 0.63 μM for M. abscessus) and selectivity for mycobacteria but also exhibits favorable solubility (kinetic solubility = 55 μM) and plasma protein binding (with a fraction unbound of 2.9 % for human and 4.7 % for mouse). These findings underscore the potential of fine-tuning molecular properties to develop DNA gyrase B inhibitors that specifically target the mycobacterial chemical space, mitigating the risk of resistance development in non-target pathogens and minimizing harm to the microbiome.

Mycobacterial biofilms: Understanding the genetic factors playing significant role in pathogenesis, resistance and diagnosis

Even though the genus Mycobacterium is a diverse group consisting of a majority of environmental bacteria known as non-tuberculous mycobacteria (NTM), it also contains some of the deadliest pathogens (Mycobacterium tuberculosis) in history associated with chronic disease called tuberculosis (TB). Formation of biofilm is one of the unique strategies employed by mycobacteria to enhance their ability to survive in hostile conditions. Biofilm formation by Mycobacterium species is an emerging area of research with significant implications for understanding its pathogenesis and treatment of related infections, specifically TB. This review provides an overview of the biofilm-forming abilities of different species of Mycobacterium and the genetic factors influencing biofilm formation with a detailed focus on M. tuberculosis. Biofilm-mediated resistance is a significant challenge as it can limit antibiotic penetration and promote the survival of dormant mycobacterial cells. Key genetic factors promoting biofilm formation have been explored such as the mmpL genes involved in lipid transport and cell wall integrity as well as the groEL gene essential for mature biofilm formation. Additionally, biofilm-mediated antibiotic resistance and pathogenesis highlighting the specific niches, sites of infection along with the possible mechanisms of biofilm dissemination have been discussed. Furthermore, drug targets within mycobacterial biofilm and their role as potential biomarkers in the development of rapid diagnostic tools have been highlighted. The review summarises the current understanding of the complex nature of Mycobacterium biofilm and its clinical implications, paving the way for advancements in the field of disease diagnosis, management and treatment against its multi-drug resistant species.

Metagenomics Analysis Reveals the Composition and Functional Differences of Fecal Microbiota in Wild, Farm, and Released Chinese Three-Keeled Pond Turtles (Mauremys reevesii)

The intestine of living organisms harbors different microbiota associated with the biological functioning and health of the host and influences the process of ecological adaptation. Here, we studied the intestinal microbiota's composition and functional differences using 16S rRNA and metagenomic analysis in the wild, farm, and released Chinese three-keeled pond turtle (Mauremys reevesii). At the phylum level, Bacteroidota dominated, followed by Firmicutes, Fusobacteriota, and Actinobacteriota in the wild group, but Chloroflexi was more abundant in the farm and released groups. Moreover, Chryseobacterium, Acinetobacter, Comamonas, Sphingobacterium, and Rhodobacter were abundant in the released and farm cohorts, respectively. Cetobacterium, Paraclostridium, Lysobacter, and Leucobacter showed an abundance in the wild group. The Kyoto Encyclopedia of Genes and Genomes (KEGG) database revealed that the relative abundance of most pathways was significantly higher in the wild turtles (carbohydrate metabolism, lipid metabolism, metabolism of cofactors, and vitamins). The comprehensive antibiotic resistance database (CARD) showed that the antibiotic resistance gene (ARG) subtype macB was the most abundant in the farm turtle group, while tetA was higher in the wild turtles, and srpYmcr was higher in the released group. Our findings shed light on the association between the intestinal microbiota of M. reevesii and its habitats and could be useful for tracking habitats to protect and conserve this endangered species.

Revealing the Interaction Mechanism between Mycobacterium tuberculosis GyrB and Novobiocin, SPR719 through Binding Thermodynamics and Dissociation Kinetics Analysis

With the rapid emergence of drug-resistant strains of Mycobacterium tuberculosis (Mtb), various levels of resistance against existing anti-tuberculosis (TB) drugs have developed. Consequently, the identification of new anti-TB targets and drugs is critically urgent. DNA gyrase subunit B (GyrB) has been identified as a potential anti-TB target, with novobiocin and SPR719 proposed as inhibitors targeting GyrB. Therefore, elucidating the molecular interactions between GyrB and its inhibitors is crucial for the discovery and design of efficient GyrB inhibitors for combating multidrug-resistant TB. In this study, we revealed the detailed binding mechanisms and dissociation processes of the representative inhibitors, novobiocin and SPR719, with GyrB using classical molecular dynamics (MD) simulations, tau-random acceleration molecular dynamics (τ-RAMD) simulations, and steered molecular dynamics (SMD) simulations. Our simulation results demonstrate that both electrostatic and van der Waals interactions contribute favorably to the inhibitors' binding to GyrB, with Asn52, Asp79, Arg82, Lys108, Tyr114, and Arg141 being key residues for the inhibitors' attachment to GyrB. The τ-RAMD simulations indicate that the inhibitors primarily dissociate from the ATP channel. The SMD simulation results reveal that both inhibitors follow a similar dissociation mechanism, requiring the overcoming of hydrophobic interactions and hydrogen bonding interactions formed with the ATP active site. The binding and dissociation mechanisms of GyrB with inhibitors novobiocin and SPR719 obtained in our work will provide new insights for the development of promising GyrB inhibitors.

An Arm-to-Disarm Strategy to Overcome Phenotypic AMR in Mycobacterium tuberculosis

Most front-line tuberculosis drugs are ineffective against hypoxic non-replicating drug-tolerant Mycobacterium tuberculosis (Mtb) contributing to phenotypic antimicrobial resistance (AMR). This is largely due to the poor permeability in the thick and waxy cell wall of persister cells, leading to diminished drug accumulation and reduced drug-target engagement. Here, using an "arm-to-disarm" prodrug approach, we demonstrate that non-replicating Mtb persisters can be sensitized to Moxifloxacin (MXF), a front-line TB drug. We design and develop a series of nitroheteroaryl MXF prodrugs that are substrates for bacterial nitroreductases (NTR), a class of enzymes that are over-expressed in hypoxic Mtb. Enzymatic activation involves electron-transfer to the nitroheteroaryl compound followed by protonation via water that contributes to the rapid cleavage rate of the protective group by NTR to produce the active drug. Phenotypic and genotypic data are fully consistent with MXF-driven lethality of the prodrug in Mtb with the protective group being a relatively innocuous bystander. The prodrug increased intracellular concentrations of MXF than MXF alone and is more lethal than MXF in non-replicating persisters. Hence, arming drugs to improve permeability, accumulation and drug-target engagement is a new therapeutic paradigm to disarm phenotypic AMR.

Mycobacterium tuberculosis Inhibitors Based on Arylated Quinoline Carboxylic Acid Backbones with Anti-Mtb Gyrase Activity

New antitubercular agents with either a novel mode of action or novel mode of inhibition are urgently needed to overcome the threat of drug-resistant tuberculosis (TB). The present study profiles new arylated quinoline carboxylic acids (QCAs) having activity against replicating and non-replicating Mycobacterium tuberculosis (Mtb), the causative agent of TB. Thus, the synthesis, characterization, and in vitro screening (MABA and LORA) of 48 QCAs modified with alkyl, aryl, alkoxy, halogens, and nitro groups in the quinoline ring led to the discovery of two QCA derivatives, 7i and 7m, adorned with C-2 2-(naphthalen-2-yl)/C-6 1-butyl and C-2 22-(phenanthren-3-yl)/C-6 isopropyl, respectively, as the best Mtb inhibitors. DNA gyrase inhibition was shown to be exhibited by both, with QCA 7m illustrating better activity up to a 1 μM test concentration. Finally, a docking model for both compounds with Mtb DNA gyrase was developed, and it showed a good correlation with in vitro results.

Bioisosteric Design Identifies Inhibitors of Mycobacterium tuberculosis DNA Gyrase ATPase Activity

Mutations in DNA gyrase confer resistance to fluoroquinolones, second-line antibiotics for Mycobacterium tuberculosis infections. Identification of new agents that inhibit M. tuberculosis DNA gyrase ATPase activity is one strategy to overcome this. Here, bioisosteric designs using known inhibitors as templates were employed to define novel inhibitors of M. tuberculosis DNA gyrase ATPase activity. This yielded the modified compound R3-13 with improved drug-likeness compared to the template inhibitor that acted as a promising ATPase inhibitor against M. tuberculosis DNA gyrase. Utilization of compound R3-13 as a virtual screening template, supported by subsequent biological assays, identified seven further M. tuberculosis DNA gyrase ATPase inhibitors with IC50 values in the range of 0.42-3.59 μM. The most active compound 1 showed an IC50 value of 0.42 μM, 3-fold better than the comparator ATPase inhibitor novobiocin (1.27 μM). Compound 1 showed noncytotoxicity to Caco-2 cells at concentrations up to 76-fold higher than its IC50 value. Molecular dynamics simulations followed by decomposition energy calculations identified that compound 1 occupies the binding pocket utilized by the adenosine group of the ATP analogue AMPPNP in the M. tuberculosis DNA gyrase GyrB subunit. The most prominent contribution to the binding of compound 1 to M. tuberculosis GyrB subunit is made by residue Asp79, which forms two hydrogen bonds with the OH group of this compound and also participates in the binding of AMPPNP. Compound 1 represents a potential new scaffold for further exploration and optimization as a M. tuberculosis DNA gyrase ATPase inhibitor and candidate anti-tuberculosis agent.

The pathogenic mechanism of Mycobacterium tuberculosis: implication for new drug development

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is a tenacious pathogen that has latently infected one third of the world's population. However, conventional TB treatment regimens are no longer sufficient to tackle the growing threat of drug resistance, stimulating the development of innovative anti-tuberculosis agents, with special emphasis on new protein targets. The Mtb genome encodes ~4000 predicted proteins, among which many enzymes participate in various cellular metabolisms. For example, more than 200 proteins are involved in fatty acid biosynthesis, which assists in the construction of the cell envelope, and is closely related to the pathogenesis and resistance of mycobacteria. Here we review several essential enzymes responsible for fatty acid and nucleotide biosynthesis, cellular metabolism of lipids or amino acids, energy utilization, and metal uptake. These include InhA, MmpL3, MmaA4, PcaA, CmaA1, CmaA2, isocitrate lyases (ICLs), pantothenate synthase (PS), Lysine-ε amino transferase (LAT), LeuD, IdeR, KatG, Rv1098c, and PyrG. In addition, we summarize the role of the transcriptional regulator PhoP which may regulate the expression of more than 110 genes, and the essential biosynthesis enzyme glutamine synthetase (GlnA1). All these enzymes are either validated drug targets or promising target candidates, with drugs targeting ICLs and LAT expected to solve the problem of persistent TB infection. To better understand how anti-tuberculosis drugs act on these proteins, their structures and the structure-based drug/inhibitor designs are discussed. Overall, this investigation should provide guidance and support for current and future pharmaceutical development efforts against mycobacterial pathogenesis.

Acyldepsipeptide Analogues: A Future Generation Antibiotics for Tuberculosis Treatment

Acyldepsipeptides (ADEPs) are a new class of emerging antimicrobial peptides (AMPs), which are currently explored for treatment of pathogenic infections, including tuberculosis (TB). These cyclic hydrophobic peptides have a unique bacterial target to the conventional anti-TB drugs, and present a therapeutic window to overcome Mycobacterium Tuberculosis (M. tb) drug resistance. ADEPs exerts their antibacterial activity on M. tb strains through activation of the protein homeostatic regulatory protease, the caseinolytic protease (ClpP1P2). ClpP1P2 is normally regulated and activated by the ClpP-ATPases to degrade misfolded and toxic peptides and/or short proteins. ADEPs bind and dysregulate all the homeostatic capabilities of ClpP1P2 while inducing non-selective proteolysis. The uncontrolled proteolysis leads to M. tb cell death within the host. ADEPs analogues that have been tested possess cytotoxicity and poor pharmacokinetic and pharmacodynamic properties. However, these can be improved by drug design techniques. Moreover, the use of nanomaterial in conjunction with ADEPs would yield effective synergistic effect. This new mode of action has potential to combat and eradicate the extensive multi-drug resistance (MDR) problem that is currently faced by the public health pertaining bacterial infections, especially TB.

Design, dynamic docking, synthesis, and in vitro validation of a novel DNA gyrase B inhibitor

Methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-intermediate-resistant Staphylococcus aureus (VRSA) are among the WHO's high priority pathogens. Among these two, MRSA is the most globally documented pathogen that necessitates the pressing demand for new classes of anti-MRSA drugs. Bacterial gyrase targeted therapeutics are unique strategies to overcome cross-resistance as they are present only in bacteria and absent in higher eukaryotes. The GyrB subunit is essential for the catalytic functions of the bacterial enzyme DNA Gyrase, thereby constituting a promising druggable target. The current study performed a structure-based virtual screening to designing GyrB target-specific candidate molecules. The de novo ligand design of novel hit molecules was performed using a rhodanine scaffold. Through a systematic in silico screening process, the hit molecules were screened for their synthetic accessibility, drug-likeness and pharmacokinetics properties in addition to its target specific interactions. Of the 374 hit molecules obtained through de novo ligand design, qsl-304 emerged as the most promising ligand. The molecular dynamic simulation studies confirmed the stable interaction between the key residues and qsl-304. qsl-304 was synthesized through a one-step chemical synthesis procedure, and the in vitro activity was proven, with an IC₅₀ of 31.23 µg/mL against the novobiocin resistant clinical isolate, Staphylococcus aureus sa-P2003. Further studies on time-kill kinetics showed the bacteriostatic nature with the diminished recurrence of resistance. The on-target gyrB inhibition further proved the efficacy of qsl-304.Communicated by Ramaswamy H. Sarma.

Identification of Potent DNA Gyrase Inhibitors Active against Mycobacterium tuberculosis

Mycobacterium tuberculosis DNA gyrase manipulates the DNA topology using controlled breakage and religation of DNA driven by ATP hydrolysis. DNA gyrase has been validated as the enzyme target of fluoroquinolones (FQs), second-line antibiotics used for the treatment of multidrug-resistant tuberculosis. Mutations around the DNA gyrase DNA-binding site result in the emergence of FQ resistance in M. tuberculosis; inhibition of DNA gyrase ATPase activity is one strategy to overcome this. Here, virtual screening, subsequently validated by biological assays, was applied to select candidate inhibitors of the M. tuberculosis DNA gyrase ATPase activity from the Specs compound library (www.specs.net). Thirty compounds were identified and selected as hits for in vitro biological assays, of which two compounds, G24 and G26, inhibited the growth of M. tuberculosis H37Rv with a minimal inhibitory concentration of 12.5 μg/mL. The two compounds inhibited DNA gyrase ATPase activity with IC₅₀ values of 2.69 and 2.46 μM, respectively, suggesting this to be the likely basis of their antitubercular activity. Models of complexes of compounds G24 and G26 bound to the M. tuberculosis DNA gyrase ATP-binding site, generated by molecular dynamics simulations followed by pharmacophore mapping analysis, showed hydrophobic interactions of inhibitor hydrophobic headgroups and electrostatic and hydrogen bond interactions of the polar tails, which are likely to be important for their inhibition. Decreasing compound lipophilicity by increasing the polarity of these tails then presents a likely route to improving the solubility and activity. Thus, compounds G24 and G26 provide attractive starting templates for the optimization of antitubercular agents that act by targeting DNA gyrase.

Spiropyrimidinetriones: a Class of DNA Gyrase Inhibitors with Activity against Mycobacterium tuberculosis and without Cross-Resistance to Fluoroquinolones

Described here is a series of spiropyrimidinetrione (SPT) compounds with activity against Mycobacterium tuberculosis through inhibition of DNA gyrase. The SPT class operates via a novel mode of inhibition, which involves Mg²⁺-independent stabilization of the DNA cleavage complex with DNA gyrase and is thereby not cross-resistant with other DNA gyrase-inhibiting antibacterials, including fluoroquinolones. Compound 22 from the series was profiled broadly and showed in vitro cidality as well as intracellular activity against M. tuberculosis in macrophages. Evidence for the DNA gyrase mode of action was supported by inhibition of the target in a DNA supercoiling assay and elicitation of an SOS response seen in a recA reporter strain of M. tuberculosis. Pharmacokinetic properties of 22 supported evaluation of efficacy in an acute model of M. tuberculosis infection, where modest reduction in CFU numbers was seen. This work offers promise for deriving a novel drug class of tuberculosis agent without preexisting clinical resistance.

In Vitro Resistance against DNA Gyrase Inhibitor SPR719 in Mycobacterium avium and Mycobacterium abscessus

The aminobenzimidazole SPR719 targets DNA gyrase in Mycobacterium tuberculosis. The molecule acts as inhibitor of the enzyme’s ATPase located on the Gyrase B subunit of the tetrameric Gyrase A₂B₂ protein. SPR719 is also active against non-tuberculous mycobacteria (NTM) and recently entered clinical development for lung disease caused by these bacteria. Resistance against SPR719 in NTM has not been characterized. Here, we determined spontaneous in vitro resistance frequencies in single step resistance development studies, MICs of resistant strains, and resistance associated DNA sequence polymorphisms in two major NTM pathogens Mycobacterium avium and Mycobacterium abscessus. A low-frequency resistance (10^−8/CFU) was associated with missense mutations in the ATPase domain of the Gyrase B subunit in both bacteria, consistent with inhibition of DNA gyrase as the mechanism of action of SPR719 against NTM. For M. abscessus, but not for M. avium, a second, high-frequency (10^−6/CFU) resistance mechanism was observed. High-frequency SPR719 resistance was associated with frameshift mutations in the transcriptional repressor MAB_4384 previously shown to regulate expression of the drug efflux pump system MmpS5/MmpL5. Our results confirm DNA gyrase as target of SPR719 in NTM and reveal differential resistance development in the two NTM species, with M. abscessus displaying high-frequency indirect resistance possibly involving drug efflux.

IMPORTANCE Clinical emergence of resistance to new antibiotics affects their utility. Characterization of in vitro resistance is a first step in the profiling of resistance properties of novel drug candidates. Here, we characterized in vitro resistance against SPR719, a drug candidate for the treatment of lung disease caused by non-tuberculous mycobacteria (NTM). The identified resistance associated mutations and the observed differential resistance behavior of the two characterized NTM species provide a basis for follow-up studies of resistance in vivo to further inform clinical development of SPR719.

In silico repurposing of a Novobiocin derivative for activity against latency associated Mycobacterium tuberculosis drug target nicotinate-nucleotide adenylyl transferase (Rv2421c)

Nicotinamide-nucleotide adenylyl transferase (Rv2421c) was selected as a potential drug target, because it has been shown, in vitro, to be essential for Mycobacterium tuberculosis growth. It is conserved between mycobacterium species, is up-regulated during dormancy, has a known 3D crystal structure and has no known human homologs. A model of Rv2421c in complex with nicotinic acid adenine dinucleotide and magnesium ion was constructed and subject tovirtual ligand screening against the Prestwick Chemical Library and the ZINC database, which yielded 155 potential hit molecules. Of the 155 compounds identified five were pursued further using an IC50 based 3D-QSAR study. The 3D-QSAR model validated the inhibition properties of the five compounds based on R2 value of 0.895 and Q2 value of 0.944 compared to known inhibitors of Rv2421c. Higher binding affinities was observed for the novel ZINC13544129 and two FDA approved compounds (Novobiocin sodium salt, Sulfasalazine). Similarly, the total interaction energy was found to be the highest for Cromolyn disodium system (-418.88 kJ/mol) followed by Novobiocin (-379.19 kJ/mol) and Sulfasalazine with (-330.13 kJ/mol) compared to substrate DND having (-185.52 kJ/mol). Subsequent in vitro testing of the five compounds identified Novobiocin sodium salt with activity against Mycobacterium tuberculosis at 50 μM, 25μM and weakly at 10μM concentrations. Novobiocin salt interacts with a MG ion and active site residues His20, Thr86, Gly107 and Leu164 similar to substrate DND of Mycobacterium tuberculosis Rv2421c. Additional in silico structural analysis of known Novobiocin sodium salt derivatives against Rv2421c suggest Coumermycin as a promising alternative for the treatment of Mycobacterium tuberculosis based on large number of hydrogen bond interactions with Rv2421c similar in comparison to Novobiocin salt and substrate DND.

In vitro activity of SPR719 against Mycobacterium ulcerans, Mycobacterium marinum and Mycobacterium chimaera

Nontuberculosis mycobacterial (NTM) infections are increasing in prevalence across the world. In many cases, treatment options for these infections are limited. However, there has been progress in recent years in the development of new antimycobacterial drugs. Here, we investigate the in vitro activity of SPR719, a novel aminobenzimidazole antibiotic and the active form of the clinical-stage compound, SPR720, against several isolates of Mycobacterium ulcerans, Mycobacterium marinum and Mycobacterium chimaera. We show that SPR719 is active against these NTM species with a MIC range of 0.125–4 μg/ml and that this compares favorably with the commonly utilized antimycobacterial antibiotics, rifampicin and clarithromycin. Our findings suggest that SPR720 should be further evaluated for the treatment of NTM infections.

Nontuberculosis mycobacteria represent a large group of diverse bacteria that live across a range of environments. Human contact with the habitats in which these organisms live can result in opportunistic infections. Among the NTM, Mycobacterium ulcerans causes necrotic skin ulcers that can lead to significant long term physical impairment; Mycobacterium marinum causes granulomatous skin lesions; and Mycobacterium chimaera has been linked to contaminated heater-cooler units used during cardiac surgery, resulting in prosthetic heart valve infections that are particularly difficult to treat. We performed laboratory experiments to test the susceptibility of these NTM species (M. ulcerans, M. marinum and M. chimaera) to a recently developed antibiotic, SPR719. We found that SPR719 inhibits the growth of these mycobacteria at concentration ranges similar to or better than commonly used anti-mycobacterial antibiotics. As SPR720, the oral prodrug of SPR719, has recently completed a Phase I safety, tolerability and PK study in healthy human volunteers, the potential exists for this product to be explored for the treatment of NTM infections, where new treatment options are urgently needed.

Current Insights into the Chemistry and Antitubercular Potential of Benzimidazole and Imidazole Derivatives

Tuberculosis is a disease caused by Mycobacterium tuberculosis (Mtb), affecting millions of people worldwide. The emergence of drug resistance is a major problem in the successful treatment of tuberculosis. Due to the commencement of MDR-TB (multi-drug resistance) and XDR-TB (extensively drug resistance), there is a crucial need for the development of novel anti-tubercular agents with improved characteristics such as low toxicity, enhanced inhibitory activity and short duration of treatment. In this direction, various heterocyclic compounds have been synthesized and screened against Mycobacterium tuberculosis. Among them, benzimidazole and imidazole containing derivatives have been found to have potential anti-tubercular activity. The present review focuses on various imidazole and benzimidazole derivatives (from 2015-2019) with their structure-activity relationships in the treatment of tuberculosis.

Anti-mycobacterial natural products and mechanisms of action

Covering: up to June, 2020Tuberculosis (TB) continues to be a major disease with high mortality and morbidity globally. Drug resistance and long duration of treatment make antituberculosis drug discovery more challenging. In this review, we summarize recent advances on anti-TB natural products (NPs) and their potential molecular targets in cell wall synthesis, protein production, energy generation, nucleic acid synthesis and other emerging areas. We highlight compounds with activity against drug-resistant TB, and reveal several novel targets including Mtb biotin synthase, ATP synthase, 1,4-dihydroxy-2-naphthoate prenyltransferase and biofilms. These anti-TB NPs and their targets could facilitate target-based screening and accelerate TB drug discovery.

Novel thiomorpholine tethered isatin hydrazones as potential inhibitors of resistant Mycobacterium tuberculosis

Novel chemotherapeutic agents against multidrug resistant-tuberculosis (MDR-TB) are urgently needed at this juncture to save the life of TB-infected patients. In this work, we have synthesized and characterized novel isatin hydrazones 4(a-o) and their thiomorpholine tethered analogues 5(a-o). All the synthesized compounds were initially screened for their anti-mycobacterial activity against the H₃₇Rv strain of Mycobacterium tuberculosis (MTB) under level-I testing. Remarkably, five compounds 4f, 4h, 4n, 5f and 5m (IC₅₀ = 1.9 µM to 9.8 µM) were found to be most active, with 4f (IC₅₀ = 1.9 µM) indicating highest inhibition of H₃₇Rv. These compounds were further evaluated at level-II testing against the five drug-resistant strains such as isoniazid-resistant strains (INH-R1 and INH-R2), rifampicin-resistant strains (RIF-R1 and RIF-R2) and fluoroquinolone-resistant strain (FQ-R1) of MTB. Interestingly, 4f and 5f emerged as the most potent compounds with IC₅₀ of 3.6 µM and 1.9 µM against RIF-R1 MTB strain, followed by INH-R1 MTB strain with IC₅₀ of 3.5 µM and 3.4 µM, respectively. Against FQ-R1 MTB strain, the lead compounds 4f and 5f displayed excellent inhibition at IC₅₀ 5.9 µM and 4.9 µM, respectively indicating broad-spectrum of activity. Further, molecular docking, ADME pharmacokinetic and molecular dynamics simulations of the compounds were performed against the DNA gyrase B and obtained encouraging results.

Structure-based drug repurposing to inhibit the DNA gyrase of Mycobacterium tuberculosis

Drug repurposing is an alternative avenue for identifying new drugs to treat tuberculosis (TB). Despite the broad-range of anti-tubercular drugs, the emergence of multi-drug-resistant and extensively drug-resistant strains of Mycobacterium tuberculosis (Mtb) H37Rv, as well as the significant death toll globally, necessitates the development of new and effective drugs to treat TB. In this study, we have employed a drug repurposing approach to address this drug resistance problem by screening the drugbank database to identify novel inhibitors of the Mtb target enzyme, DNA gyrase. The compounds were screened against the ATPase domain of the gyrase B subunit (MtbGyrB47), and the docking results showed that echinacoside, doxorubicin, epirubicin, and idarubicin possess high binding affinities against MtbGyrB47. Comprehensive assessment using fluorescence spectroscopy, surface plasmon resonance spectroscopy (SPR), and circular dichroism (CD) titration studies revealed echinacoside as a potent binder of MtbGyrB47. Furthermore, ATPase, and DNA supercoiling assays exhibited an IC50 values of 2.1-4.7 µM for echinacoside, doxorubicin, epirubicin, and idarubicin. Among these compounds, the least MIC90 of 6.3 and 12 μM were observed for epirubicin and echinacoside, respectively, against Mtb. Our findings indicate that echinacoside and epirubicin targets mycobacterial DNA gyrase, inhibit its catalytic cycle, and retard mycobacterium growth. Further, these compounds exhibit potential scaffolds for optimizing novel anti-mycobacterial agents that can act on drug-resistant strains.

An appraisal of anti-mycobacterial activity with structure-activity relationship of piperazine and its analogues: A review

Piperazine, is privileged six membered nitrogen containing heterocyclic ring also known as 1,4-Diazacyclohexane. Consequently, piperazine is a versatile medicinally important scaffold and is an essential core in numerous marketed drugs with diverse pharmacological activities. In recent years several potent molecules containing piperazine as an essential subunit of the structural frame have been reported, especially against Mycobacterium tuberculosis (MTB). Remarkably, a good number of these reported molecules also displayed potential activity against multidrug-resistant (MDR), and extremely drug-resistant (XDR) strains of MTB. In this review, we have made a concerted effort to retrace anti-mycobacterial compounds for the past five decades (1971-2019) specifically where piperazine has been used as a vital building block. This review will benefit medicinal chemists as it elaborates on the design, rationale and structure-activity relationship (SAR) of the reported potent piperazine based anti-TB molecules, which in turn will assist them in addressing the gaps, exploiting the reported strategies and developing safer, selective, and cost-effective anti-mycobacterial agents.

Structure-Based Screening of DNA GyraseB Inhibitors for Therapeutic Applications in Tuberculosis: a Pharmacoinformatics Study

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis (MTB) and considered as serious public health concern worldwide which kills approximately five thousand people every day. Therefore, TB drug development efforts are in gigantic need for identification of new potential chemical agents to eradicate TB from the society. The bacterial DNA gyrase B (GyrB) protein as an experimentally widely accepted effective drug target for the development of TB chemotherapeutics. In the present study, advanced pharmacoinformatics approaches were used to screen the Mcule database against the GyrB protein. Based on a number of chemometric parameters, five molecules were found to be crucial to inhibit the GyrB. A number of molecular binding interactions between the proposed inhibitors and important active site residues of GyrB were observed. The predicted drug-likeness properties of all molecules were indicated that compounds possess characteristics to be the drug-like candidates. The dynamic nature of each molecule was explored through the molecular dynamics (MD) simulation study. Various analyzing parameters from MD simulation trajectory have suggested rationality of the molecules to be potential GyrB inhibitor. Moreover, the binding free energy was calculated from the entire MD simulation trajectories highlighted greater binding free energy values for all newly identified compounds also substantiated the strong binding affection towards the GyrB in comparison to the novobiocin. Therefore, the proposed molecules might be considered as potential anti-TB chemical agents for future drug discovery purposes subjected to experimental validation. Graphical Abstract.

Mesenchymal stem cells offer a drug-tolerant and immune-privileged niche to Mycobacterium tuberculosis

Anti-tuberculosis (TB) drugs, while being highly potent in vitro, require prolonged treatment to control Mycobacterium tuberculosis (Mtb) infections in vivo. We report here that mesenchymal stem cells (MSCs) shelter Mtb to help tolerate anti-TB drugs. MSCs readily take up Mtb and allow unabated mycobacterial growth despite having a functional innate pathway of phagosome maturation. Unlike macrophage-resident ones, MSC-resident Mtb tolerates anti-TB drugs remarkably well, a phenomenon requiring proteins ABCC1, ABCG2 and vacuolar-type H⁺ATPases. Additionally, the classic pro-inflammatory cytokines IFNγ and TNFα aid mycobacterial growth within MSCs. Mechanistically, evading drugs and inflammatory cytokines by MSC-resident Mtb is dependent on elevated PGE2 signaling, which we verify in vivo analyzing sorted CD45^-Sca1⁺CD73⁺-MSCs from lungs of infected mice. Moreover, MSCs are observed in and around human tuberculosis granulomas, harboring Mtb bacilli. We therefore propose, targeting the unique immune-privileged niche, provided by MSCs to Mtb, can have a major impact on tuberculosis prevention and cure.

Advancement of GyrB Inhibitors for Treatment of Infections Caused by Mycobacterium tuberculosis and Non-tuberculous Mycobacteria

The prospect of ever increasing antibiotic resistance eroding currently available treatment options for bacterial infections underscores the need to continue to identify new antibiotics, preferably those that act on novel targets or with novel mechanisms of action. Bacterial gyrase B subunit (GyrB), an essential component of bacterial gyrase required for successful DNA replication, represents such a target. We describe recent examples of GyrB inhibitors and point out their potential utility for treatment of mycobacterial diseases caused by Mycobacterium tuberculosis (TB) and non-tuberculous mycobacteria (NTM). Current therapeutic options for these diseases are often suboptimal due to resistance to current standard of care antibiotics. A future GyrB inhibitor-based antibiotic could offer a new and effective addition to the armamentarium for treatment of mycobacterial diseases and possibly for infections caused by other bacterial pathogens. One GyrB inhibitor, SPR720, has recently completed a first-in-human clinical trial and is in clinical development for the treatment of NTM and TB infections.

Targeting Genome Integrity in Mycobacterium Tuberculosis: From Nucleotide Synthesis to DNA Replication and Repair

Mycobacterium tuberculosis (MTB) is the causative agent of tuberculosis (TB), an ancient disease which still today causes 1.4 million deaths worldwide per year. Long-term, multi-agent anti-tubercular regimens can lead to the anticipated non-compliance of the patient and increased drug toxicity, which in turn can contribute to the emergence of drug-resistant MTB strains that are not susceptible to first- and second-line available drugs. Hence, there is an urgent need for innovative antitubercular drugs and vaccines. A number of biochemical processes are required to maintain the correct homeostasis of DNA metabolism in all organisms. Here we focused on reviewing our current knowledge and understanding of biochemical and structural aspects of relevance for drug discovery, for some such processes in MTB, and particularly DNA synthesis, synthesis of its nucleotide precursors, and processes that guarantee DNA integrity and genome stability. Overall, the area of drug discovery in DNA metabolism appears very much alive, rich of investigations and promising with respect to new antitubercular drug candidates. However, the complexity of molecular events that occur in DNA metabolic processes requires an accurate characterization of mechanistic details in order to avoid major flaws, and therefore the failure, of drug discovery approaches targeting genome integrity.

Second-generation 4,5,6,7-tetrahydrobenzo[d]thiazoles as novel DNA gyrase inhibitors

Aim: DNA gyrase and topoisomerase IV are essential bacterial enzymes, and in the fight against bacterial resistance, they are important targets for the development of novel antibacterial drugs. Results: Building from our first generation of 4,5,6,7-tetrahydrobenzo[d]thiazole-based DNA gyrase inhibitors, we designed and prepared an optimized series of analogs that show improved inhibition of DNA gyrase and topoisomerase IV from Staphylococcus aureus and Escherichia coli, with IC₅₀ values in the nanomolar range. Importantly, these inhibitors also show improved antibacterial activity against Gram-positive strains. Conclusion: The most promising inhibitor, 29, is active against Enterococcus faecalis, Enterococcus faecium and S. aureus wild-type and resistant strains, with minimum inhibitory concentrations between 4 and 8 μg/ml, which represents good starting point for development of novel antibacterials.

Targeting DNA Gyrase to Combat Mycobacterium tuberculosis: An Update

DNA gyrase is a clinically validated drug target, currently targeted only by fluoroquinolone class of antibacterials. However, owing to increasing drug resistance as well as a concomitant reduction in the availability of newer classes of antibiotics, fluoroquinolones are increasingly being over-utilized in order to treat serious infections, including multi-drug resistant tuberculosis. This, in turn, increases the probability of resistance to fluoroquinolones, which is mediated by a single amino acid change in gyrA, leading to class-wide resistance. In this review, we provide an overview of the recent progress in identifying novel scaffolds which target DNA gyrase and provide an update on their discovery and development status.

An overview of new antitubercular drugs, drug candidates, and their targets

The causative agent of tuberculosis (TB), Mycobacterium tuberculosis and more recently totally drug-resistant strains of M. tuberculosis, display unique mechanisms to survive in the host. A four-drug treatment regimen was introduced 40 years ago but the emergence of multidrug-resistance and more recently TDR necessitates the identification of new targets and drugs for the cure of M. tuberculosis infection. The current efforts in the drug development process are insufficient to completely eradicate the TB epidemic. For almost five decades the TB drug development process remained stagnant. The last 10 years have made sudden progress giving some new and highly promising drugs including bedaquiline, delamanid, and pretomanid. Many of the candidates are repurposed compounds, which were developed to treat other infections but later, exhibited anti-TB properties also. Each class of drug has a specific target and a definite mode of action. These targets are either involved in cell wall biosynthesis, protein synthesis, DNA/RNA synthesis, or metabolism. This review discusses recent progress in the discovery of newly developed and Food and Drug Administration approved drugs as well as repurposed drugs, their targets, mode of action, drug-target interactions, and their structure-activity relationship.

Unpacking the Pathogen Box-An Open Source Tool for Fighting Neglected Tropical Disease

The Pathogen Box is a 400-strong collection of drug-like compounds, selected for their potential against several of the world's most important neglected tropical diseases, including trypanosomiasis, leishmaniasis, cryptosporidiosis, toxoplasmosis, filariasis, schistosomiasis, dengue virus and trichuriasis, in addition to malaria and tuberculosis. This library represents an ensemble of numerous successful drug discovery programmes from around the globe, aimed at providing a powerful resource to stimulate open source drug discovery for diseases threatening the most vulnerable communities in the world. This review seeks to provide an in-depth analysis of the literature pertaining to the compounds in the Pathogen Box, including structure-activity relationship highlights, mechanisms of action, related compounds with reported activity against different diseases, and, where appropriate, discussion on the known and putative targets of compounds, thereby providing context and increasing the accessibility of the Pathogen Box to the drug discovery community.

Design, synthesis, and in vitro biological evaluation of novel benzimidazole tethered allylidenehydrazinylmethylthiazole derivatives as potent inhibitors of Mycobacterium tuberculosis

Tuberculosis (TB) has become one of the most significant public health problems in recent years. Antibiotic therapy remains the mainstay of TB control strategies, but the increasing resistance of mycobacterial species has heightened alarm, requiring the development of novel drugs in order to improve treatment outcomes. Here, as an effort to identify novel and effective antitubercular agents, we designed and synthesized a series of novel substituted benzimidazolallylidenehydrazinylmethylthiazole derivatives via a multi-component molecular hybridization approach with single molecular architecture. Our design strategy involved assembling the antitubercular pharmacophoric fragments benzimidazole, 2-aminothiazole and substituted α,β-unsaturated ketones via condensation reactions. All the newly synthesized compounds were fully characterized via NMR and mass spectral data and evaluated for in vitro biological activity against the H37Ra strain of Mycobacterium tuberculosis. From the biological evaluation data, we identified some effective compounds, of which 8g and 7e were the most active ones (both having MIC values of 2.5 μg mL-1). In addition, compound 8g exhibited a lower cytotoxicity profile. We conceive that compound 8g may serve as a chemical probe of interest for further lead optimization studies with the general aim of developing novel and effective antitubercular agents.

Mechanistic investigation of resistance via drug-inactivating enzymes in Mycobacterium tuberculosis

Tuberculosis (TB) is a serious major health concern that has existed from millennia. According to annual WHO report 2016, it is considered as world's ninth highest killer disease by single infectious agent, ranking above HIV/AIDS. To worsen the scenario the development of multi-drug resistant tuberculosis (MDR-TB) and extremely drug-resistant tuberculosis (XDR-TB) have significantly reduced the success rate of TB treatment. Several efforts are being made to handle pharmacodynamic resistance (MDR and XDR-TB) involving designing of new inhibitors, targeting mutated target or by multi-targeting agents. However, the issue of pharmacokinetic resistance in TB is not being addressed appropriately till date. Pharmacokinetic mode of resistance involves an intrinsic mechanism of bacterial drug resistance via expression of various enzymes and efflux pumps that are responsible for the loss of activity of the therapeutic agents. Mycobacterium tuberculosis is also intrinsically resistant to various approved agents via pharmacokinetic mechanism of resistance. Several bacterial enzymes are encoded that either degrade or modifies the drugs and renders them ineffective. Targeting such inactivating bacterial enzymes provides a novel approach to make the current therapy effective and combat the problem of resistance. This review provides an insight into different bacterial enzymes which are responsible for pharmacokinetic drug resistance in TB. The structure attributes and mechanism of catalysis employed by these enzymes to inactivate drug have also been discussed which may provide basis for developing novel therapeutic agents for resistant TB.

Chemical classes targeting energy supplying GyrB domain of Mycobacterium tuberculosis

Tuberculosis (TB) is contagious in nature and immunocompromised patients have a higher probability of developing TB. The occurrence of drug resistance, has led to serious health concerns in the management of TB. In order to combat resistant tuberculosis there is an urgent need of identifying new drug targets and new drug combinations for the effective management and reduction in the duration of TB treatment. Targeting DNA gyrase that is involved in bacterial replication cycle, provides one rationale approach. Various fluoroquinolone based drugs have shown promising effect against DNA gyrase enzyme and in turn were successful in combat against MDR TB. However, GyrA domain mutations based resistance towards fluoroquinolones has put a question mark over current therapies for tuberculosis. Fluoroquinolones target GyrA domain of bacterial DNA gyrase therefore targeting DNA GyrB domain may overcome this resistance issue, establishing it as an attractive target. This review is a compilation of current research efforts on energy supplying domain of Mycobacterium tuberculosis that could provide breakthrough in development of more potent Mtb DNA GyrB inhibitors.

Design, synthesis and antibacterial properties of pyrimido[4,5-b]indol-8-amine inhibitors of DNA gyrase

According to the World Health Organization (WHO), approximately 1.7 million deaths per year are caused by tuberculosis infections. Furthermore, it has been predicted that, by 2050, antibacterial resistance will be the cause of approximately 10 million deaths annually if the issue is not tackled. As a result, novel approaches to treating broad-spectrum bacterial infections are of vital importance. During the course of our wider efforts to discover unique methods of targeting multidrug-resistant (MDR) pathogens, we identified a novel series of amide-linked pyrimido[4,5-b]indol-8-amine inhibitors of bacterial type II topoisomerases. Compounds from the series were highly potent against gram-positive bacteria and mycobacteria, with excellent potency being retained against a panel of relevant Mycobacterium tuberculosis drug-resistant clinical isolates.

Ribonucleotide reductase as a drug target against drug resistance Mycobacterium leprae: A molecular docking study

Leprosy is a chronic infection of skin and nerve caused by Mycobacterium leprae. The treatment is based on standard multi drug therapy consisting of dapsone, rifampicin and clofazamine. The use of rifampicin alone or with dapsone led to the emergence of rifampicin-resistant Mycobacterium leprae strains. The emergence of drug-resistant leprosy put a hurdle in the leprosy eradication programme. The present study aimed to predict the molecular model of ribonucleotide reductase (RNR), the enzyme responsible for biosynthesis of nucleotides, to screen new drugs for treatment of drug-resistant leprosy. The study was conducted by retrieving RNR of M. leprae from GenBank. A molecular 3D model of M. leprae was predicted using homology modelling and validated. A total of 325 characters were included in the analysis. The predicted 3D model of RNR showed that the ϕ and φ angles of 251 (96.9%) residues were positioned in the most favoured regions. It was also conferred that 18 α-helices, 6 β turns, 2 γ turns and 48 helix-helix interactions contributed to the predicted 3D structure. Virtual screening of Food and Drug Administration approved drug molecules recovered 1829 drugs of which three molecules, viz., lincomycin, novobiocin and telithromycin, were taken for the docking study. It was observed that the selected drug molecules had a strong affinity towards the modelled protein RNR. This was evident from the binding energy of the drug molecules towards the modelled protein RNR (-6.10, -6.25 and -7.10). Three FDA-approved drugs, viz., lincomycin, novobiocin and telithromycin, could be taken for further clinical studies to find their efficacy against drug resistant leprosy.

C-terminal lysine repeats in Streptomyces topoisomerase I stabilize the enzyme-DNA complex and confer high enzyme processivity

Streptomyces topoisomerase I (TopA) exhibits exceptionally high processivity. The enzyme, as other actinobacterial topoisomerases I, differs from its bacterial homologs in its C-terminal domain (CTD). Here, bioinformatics analyses established that the presence of lysine repeats is a characteristic feature of actinobacterial TopA CTDs. Streptomyces TopA contains the longest stretch of lysine repeats, which terminate with acidic amino acids. DNA-binding studies revealed that the lysine repeats stabilized the TopA–DNA complex, while single-molecule experiments showed that their elimination impaired enzyme processivity. Streptomyces coelicolor TopA processivity could not be restored by fusion of its N-terminal domain (NTD) with the Escherichia coli TopA CTD. The hybrid protein could not re-establish the distribution of multiple chromosomal copies in Streptomyces hyphae impaired by TopA depletion. We expected that the highest TopA processivity would be required during the growth of multigenomic sporogenic hyphae, and indeed, the elimination of lysine repeats from TopA disturbed sporulation. We speculate that the interaction of the lysine repeats with DNA allows the stabilization of the enzyme–DNA complex, which is additionally enhanced by acidic C-terminal amino acids. The complex stabilization, which may be particularly important for GC-rich chromosomes, enables high enzyme processivity. The high processivity of TopA allows rapid topological changes in multiple chromosomal copies during Streptomyces sporulation.

Targeting DNA Replication and Repair for the Development of Novel Therapeutics against Tuberculosis

Mycobacterium tuberculosis is the etiological agent of tuberculosis (TB), an infectious disease which results in approximately 10 million incident cases and 1.4 million deaths globally each year, making it the leading cause of mortality from infection. An effective frontline combination chemotherapy exists for TB; however, this regimen requires the administration of four drugs in a 2 month long intensive phase followed by a continuation phase of a further 4 months with two of the original drugs, and is only effective for the treatment of drug-sensitive TB. The emergence and global spread of multidrug-resistant (MDR) as well as extensively drug-resistant (XDR) strains of M. tuberculosis, and the complications posed by co-infection with the human immunodeficiency virus (HIV) and other co-morbidities such as diabetes, have prompted urgent efforts to develop shorter regimens comprising new compounds with novel mechanisms of action. This demands that researchers re-visit cellular pathways and functions that are essential to M. tuberculosis survival and replication in the host but which are inadequately represented amongst the targets of current anti-mycobacterial agents. Here, we consider the DNA replication and repair machinery as a source of new targets for anti-TB drug development. Like most bacteria, M. tuberculosis encodes a complex array of proteins which ensure faithful and accurate replication and repair of the chromosomal DNA. Many of these are essential; so, too, are enzymes in the ancillary pathways of nucleotide biosynthesis, salvage, and re-cycling, suggesting the potential to inhibit replication and repair functions at multiple stages. To this end, we provide an update on the state of chemotherapeutic inhibition of DNA synthesis and related pathways in M. tuberculosis. Given the established links between genotoxicity and mutagenesis, we also consider the potential implications of targeting DNA metabolic pathways implicated in the development of drug resistance in M. tuberculosis, an organism which is unusual in relying exclusively on de novo mutations and chromosomal rearrangements for evolution, including the acquisition of drug resistance. In that context, we conclude by discussing the feasibility of targeting mutagenic pathways in an ancillary, “anti-evolution” strategy aimed at protecting existing and future TB drugs.

α-pyrones and their hydroxylated analogs as promising scaffolds against Mycobacterium tuberculosis

Tuberculosis ranks as the leading cause of global human mortality from a single infectious agent. To address the uprising issues of drug resistance, intense research efforts have been directed towards drug discovery. However, it is a long and economically challenging process that is often associated with high failure rates. Therefore, it seems prudent to take forward the core scaffolds that have already acclaimed clinical relevance. In this direction, hydroxylated α-pyrone scaffold has received US FDA approval for human use against HIV. Interestingly, literature review reveals the potential applicability of α-pyrones in TB drug discovery. On one hand, α-pyrones play a vital role in the cell wall of Mycobacterium tuberculosis and on the other hand natural α-pyrones display appreciable anti-TB activity. This review aims to rekindle the interest of researchers toward α-pyrone as a new anti-TB drug that may possibly tackle drug resistance and open a dual frontier in TB and HIV drug discovery.

Versatility of 7-Substituted Coumarin Molecules as Antimycobacterial Agents, Neuronal Enzyme Inhibitors and Neuroprotective Agents

A medium-throughput screen using Mycobacterium tuberculosis H37Rv was employed to screen an in-house library of structurally diverse compounds for antimycobacterial activity. In this initial screen, eleven 7-substituted coumarin derivatives with confirmed monoamine oxidase-B and cholinesterase inhibitory activities, demonstrated growth inhibition of more than 50% at 50 µM. This prompted further exploration of all the 7-substituted coumarins in our library. Four compounds showed promising MIC₉₉ values of 8.31–29.70 µM and 44.15–57.17 µM on M. tuberculosis H37Rv in independent assays using GAST-Fe and 7H9+OADC media, respectively. These compounds were found to bind to albumin, which may explain the variations in MIC between the two assays. Preliminary data showed that they were able to maintain their activity in fluoroquinolone resistant mycobacteria. Structure-activity relationships indicated that structural modification on position 4 and/or 7 of the coumarin scaffold could direct the selectivity towards either the inhibition of neuronal enzymes or the antimycobacterial effect. Moderate cytotoxicities were observed for these compounds and slight selectivity towards mycobacteria was indicated. Further neuroprotective assays showed significant neuroprotection for selected compounds irrespective of their neuronal enzyme inhibitory properties. These coumarin molecules are thus interesting lead compounds that may provide insight into the design of new antimicrobacterial and neuroprotective agents.

Machine learning and docking models for Mycobacterium tuberculosis topoisomerase I

There is a shortage of compounds that are directed towards new targets apart from those targeted by the FDA approved drugs used against Mycobacterium tuberculosis. Topoisomerase I (Mttopo I) is an essential mycobacterial enzyme and a promising target in this regard. However, it suffers from a shortage of known inhibitors. We have previously used computational approaches such as homology modeling and docking to propose 38 FDA approved drugs for testing and identified several active molecules. To follow on from this, we now describe the in vitro testing of a library of 639 compounds. These data were used to create machine learning models for Mttopo I which were further validated. The combined Mttopo I Bayesian model had a 5 fold cross validation receiver operator characteristic of 0.74 and sensitivity, specificity and concordance values above 0.76 and was used to select commercially available compounds for testing in vitro. The recently described crystal structure of Mttopo I was also compared with the previously described homology model and then used to dock the Mttopo I actives norclomipramine and imipramine. In summary, we describe our efforts to identify small molecule inhibitors of Mttopo I using a combination of machine learning modeling and docking studies in conjunction with screening of the selected molecules for enzyme inhibition. We demonstrate the experimental inhibition of Mttopo I by small molecule inhibitors and show that the enzyme can be readily targeted for lead molecule development.

Targeting Phenotypically Tolerant Mycobacterium tuberculosis

While the immune system is credited with averting tuberculosis in billions of individuals exposed to Mycobacterium tuberculosis, the immune system is also culpable for tempering the ability of antibiotics to deliver swift and durable cure of disease. In individuals afflicted with tuberculosis, host immunity produces diverse microenvironmental niches that support suboptimal growth, or complete growth arrest, of M. tuberculosis. The physiological state of nonreplication in bacteria is associated with phenotypic drug tolerance. Many of these host microenvironments, when modeled in vitro by carbon starvation, complete nutrient starvation, stationary phase, acidic pH, reactive nitrogen intermediates, hypoxia, biofilms, and withholding streptomycin from the streptomycin-addicted strain SS18b, render M. tuberculosis profoundly tolerant to many of the antibiotics that are given to tuberculosis patients in clinical settings. Targeting nonreplicating persisters is anticipated to reduce the duration of antibiotic treatment and rate of posttreatment relapse. Some promising drugs to treat tuberculosis, such as rifampin and bedaquiline, only kill nonreplicating M. tuberculosisin vitro at concentrations far greater than their minimal inhibitory concentrations against replicating bacilli. There is an urgent demand to identify which of the currently used antibiotics, and which of the molecules in academic and corporate screening collections, have potent bactericidal action on nonreplicating M. tuberculosis. With this goal, we review methods of high-throughput screening to target nonreplicating M. tuberculosis and methods to progress candidate molecules. A classification based on structures and putative targets of molecules that have been reported to kill nonreplicating M. tuberculosis revealed a rich diversity in pharmacophores.