2-Mercapto-Quinazolinones as Inhibitors of Type II NADH Dehydrogenase and Mycobacterium tuberculosis: Structure-Activity Relationships, Mechanism of Action and Absorption, Distribution, Metabolism, and Excretion Characterization

Targeting type II NADH dehydrogenase in tuberculosis treatment: A review

Tuberculosis (TB), a critical global health issue, continues to impose significant harm on human populations worldwide, and the increasing prevalence of drug-resistant TB strains has exacerbated the challenges in effective treatment, underscoring an urgent need for the development of new therapeutic agents with innovative mechanisms of action. The introduction of bedaquiline highlights targeting Mycobacterium tuberculosis (Mtb) energy metabolism as a novel anti-TB strategy. Among the various targets within Mtb's metabolic pathways, type II NADH dehydrogenase (NDH-2) is of particular significance due to its critical function in bacterial respiration and its absence from human cells, rendering it an appealing candidate for selective inhibition. Recent advances have led to the identification of numerous NDH-2 inhibitors, some of which exhibit potent antibacterial activity at nanomolar concentrations, demonstrating their potential as lead compounds for future drug development. This review explores the biochemical function and molecular structure of NDH-2, underscoring its potential as a target for anti-TB therapies. It also discusses the progress made in discovering and optimizing NDH-2 inhibitors, offering insights into their mechanisms of action, efficacy, and pharmacological properties, with the aim of providing an overview that could inform and guide future research efforts towards developing more effective treatments against TB.

Structure of Mycobacterial NDH-2 Bound to a 2-Mercapto-Quinazolinone Inhibitor

Mycobacterial type II NADH dehydrogenase (NDH-2) is a promising drug target because of its central role in energy metabolism in Mycobacterium tuberculosis and other pathogens, and because it lacks a known mammalian homologue. To facilitate optimization of lead compounds, we used electron cryomicroscopy (cryo-EM) to determine the structure of NDH-2 from Mycobacterium smegmatis, a fast-growing nonpathogenic model for respiration in M. tuberculosis. The structure shows that active mycobacterial NDH-2 is dimeric, with an arrangement of monomers in the dimer that differs from the arrangement described for other prokaryotic NDH-2 dimers, instead resembling dimers formed by NDH-2 in the eukaryotes Saccharomyces cerevisiae and Plasmodium falciparum. A structure of the enzyme bound to a 2-mercapto-quinazolinone inhibitor shows that the compound interacts directly with the flavin adenine dinucleotide cofactor, blocking the menaquinone-reducing site. These results reveal structural elements of NDH-2 that could be used to design specific inhibitors of the mycobacterial enzyme.

Identification of a Series Containing a Pentafluorophenyl Moiety That Targets Pks13 to Inhibit Growth of Mycobacterium tuberculosis

Although not currently in the infectious disease spotlight, there is still a pressing need for new agents to treat tuberculosis caused by Mycobacterium tuberculosis. As there is an ever-increasing amount of clinical resistance to the current drugs, ideally new drugs would be found against novel targets to circumvent pre-existing resistance. A phenotypic growth screen identified a novel singleton, 1, as an inhibitor of M. tuberculosis growth. Mechanism-of-action studies determined that 1 targeted Pks13, an essential enzyme in cell wall biosynthesis that, as of yet, has not been targeted by agents in the clinic. The reactive nature of the pentafluorophenyl warhead meant that the molecule was inherently metabolically unstable. A medicinal chemistry optimization program is described that resulted in the identification of a compound that was reactive enough to still inhibit Pks13 and M. tuberculosis growth while being metabolically stable enough to explore in vivo.

Potential of Mycobacterium tuberculosis Type II NADH-Dehydrogenase in Antitubercular Drug Discovery

The type II NADH-dehydrogenase enzyme in Mycobacterium tuberculosis plays a critical role in the efficient functioning of the oxidative phosphorylation pathway. It acts as the entry point for electrons in the electron transport chain, which is essential for fulfilling the energy requirements of both replicating and nonreplicating mycobacterial species. Due to the absence of the type II NADH-dehydrogenase enzyme in mammalian mitochondria, targeting the type II NADH-dehydrogenase enzyme for antitubercular drug discovery could be a vigilant approach. Utilizing type II NADH-dehydrogenase inhibitors in antitubercular therapy led to bactericidal response, even in monotherapy. However, the absence of the cryo-EM structure of Mycobacterium tuberculosis type II NADH-dehydrogenase has constrained drug discovery efforts to rely on high-throughput screening methods, limiting the use of structure-based drug discovery. Here, we have delineated the literature-reported Mycobacterium tuberculosis type II NADH-dehydrogenase inhibitors and the rationale behind selecting this specific enzyme for antitubercular drug discovery, along with shedding light on the architecture of the enzyme structure and functionality. The gap in the current research and future research direction for TB treatment have been addressed.

New quinazolone-sulfonate conjugates with an acetohydrazide linker as potential antimicrobial agents: design, synthesis and molecular docking simulations

A novel molecular design based on a quinazolinone scaffold was developed via the attachment of aryl alkanesulfonates to the quinazolinone core through a thioacetohydrazide azomethine linker, leading to a new series of quinazolinone-alkanesulfonates 5a-r. The antimicrobial properties of the newly synthesized quinazolinone derivatives 5a-r were investigated to examine their bactericidal and fungicidal activities against bacterial pathogens like Bacillus subtilis, Staphylococcus aureus (Gram-positive), Pseudomonas aeruginosa, Klebsiella pneumonia, Sallmonella Typhimurium (Gram-negative), in addition to Candida albicans (unicellular fungal). The tested compounds demonstrated reasonable bactericidal activities compared to standard drugs. Notably, derivatives 5g and 5k exhibited the greatest MIC values against Candida albicans, while 5g was the best against Staphylococcus aureus with MIC of 11.3 ± 2.38 μg mL-1, two-fold efficacy more than that was recorded with sulfadiazine. Furthermore, 5k significantly prevented biofilm formation for all bacterial pathogens, with a percentage ratio reaching 63.9%, surpassing the standard drug Ciprofloxacin. Additionally, 5k caused elevated lipid peroxidation (LPO) when added to the tested microbial pathogens. Confocal Laser Scanning Microscopy (CLSM) visualization revealed fewer live cells after treatment. Molecular docking studies showed that the quinazolinone derivatives bind strongly to the DNA gyrase enzyme, with the acid hydrazide core interacting effectively with key residues GLU50, ASN46, GLY77, and ASP136, consistent with their antimicrobial activity. Additionally, these compounds exhibited promising physicochemical properties, paving the way for discovering new antimicrobial drugs.

Breaking the energy chain: importance of ATP synthase in Mycobacterium tuberculosis and its potential as a drug target

Unveiling novel pathways for drug discovery forms the foundation of a new era in the combat against tuberculosis. The discovery of a novel drug, bedaquiline, targeting mycobacterial ATP synthase highlighted the targetability of the energy metabolism pathway. The significant potency of bedaquiline against heterogeneous population of Mycobacterium tuberculosis marks ATP synthase as an important complex of the electron transport chain. This review focuses on the importance and unique characteristics of mycobacterial ATP synthase. Understanding these distinctions enables the targeting of ATP synthase subunits for drug discovery, without aiming at the mammalian counterpart. Furthermore, a brief comparison of the structural differences between mycobacterial and mitochondrial ATP synthase is discussed. Being a complex multi-subunit protein, ATP synthase offers multiple sites for potential inhibitors, including the a, c, ε, γ, and δ subunits. Inhibitors targeting these subunits are critically reviewed, providing insight into the design of better and more potent chemical entities with the potential for effective treatment regimens.

Advancements and challenges in tuberculosis drug discovery: A comprehensive overview

Tuberculosis continues to pose a health challenge causing the loss of millions of lives despite the existence of multiple drugs, for treatment. The emergence of drug-resistant strains has made the situation more complex making it increasingly difficult to fight against this disease. This review outlines the challenges associated with TB drug discovery, the nature of Mycobacterium tuberculosis shedding light on the mechanisms that lead to treatment failure and antibiotic resistance. We explore promising drug targets, encompassing inhibition of mycolyarabinogalactan peptidoglycan (MAGP) assembly, mycolic acid biosynthesis, DNA replication, transcription, translation, protein synthesis, and bioenergetics/metabolism pathways. A comprehensive overview of the global pipeline of anti-tuberculosis drugs at various stages of clinical trials, the diverse strategies being pursued to tackle this complex disease. By gaining an understanding of the mechanisms that contribute to resistance development and identifying suitable targets, we can pave the way for more effective treatments and contribute to global efforts to combat drug-resistant tuberculosis.

Cryptosporidium lysyl-tRNA synthetase inhibitors define the interplay between solubility and permeability required to achieve efficacy

Cryptosporidiosis is a diarrheal disease caused by infection with Cryptosporidium spp. parasites and is a leading cause of death in malnourished children worldwide. The only approved treatment, nitazoxanide, has limited efficacy in this at-risk patient population. Additional safe therapeutics are urgently required to tackle this unmet medical need. However, the development of anti-cryptosporidial drugs is hindered by a lack of understanding of the optimal compound properties required to treat this gastrointestinal infection. To address this knowledge gap, a diverse set of potent lysyl-tRNA synthetase inhibitors was profiled to identify optimal physicochemical and pharmacokinetic properties required for efficacy in a chronic mouse model of infection. The results from this comprehensive study illustrated the importance of balancing solubility and permeability to achieve efficacy in vivo. Our results establish in vitro criteria for solubility and permeability that are predictive of compound efficacy in vivo to guide the optimization of anti-cryptosporidial drugs. Two compounds from chemically distinct series (DDD489 and DDD508) were identified as demonstrating superior efficacy and prioritized for further evaluation. Both compounds achieved marked parasite reduction in immunocompromised mouse models and a disease-relevant calf model of infection. On the basis of these promising data, these compounds have been selected for progression to preclinical safety studies, expanding the portfolio of potential treatments for this neglected infectious disease.

2-Aryl-Benzoimidazoles as Type II NADH Dehydrogenase Inhibitors of Mycobacterium tuberculosis

The nonproton pumping type II NADH dehydrogenase in Mycobacterium tuberculosis is essential for meeting the energy needs in terms of ATP under normal aerobic and stressful hypoxic environmental states. Type II NADH dehydrogenase conduits electrons into the electron transport chain in Mycobacterium tuberculosis, which results in ATP synthesis. Therefore, the inhibition of NDH-2 ensures the abolishment of the entire ATP synthesis machinery. Also, type II NADH dehydrogenase is absent in the mammalian genome, thus making it a potential target for antituberculosis drug discovery. Herein, we have screened a commercially available library of drug-like molecules and have identified a hit having a benzimidazole core moiety (6, H37Rv mc26230; minimum inhibitory concentration (MIC) = 16 μg/mL and ATP IC50 = 0.23 μg/mL) interfering with the oxidative phosphorylation pathway. Extensive medicinal chemistry optimization resulted in analogue 8, with MIC = 4 μg/mL and ATP IC50 = 0.05 μg/mL against the H37Rv mc26230 strain of Mycobacterium tuberculosis. Compounds 6 and 8 were found to be active against mono- and multidrug-resistant mycobacterium strains and demonstrated a bactericidal response. The Peredox-mCherry experiment and identification of single-nucleotide polymorphisms in mutants of CBR-5992 (a known type II NADH dehydrogenase inhibitor) were used to confirm the molecules as inhibitors of the type II NADH dehydrogenase enzyme. The safety index >10 for the test active molecules revealed the safety of test molecules.

Emerging and Re-emerging Warheads for Targeted Covalent Inhibitors: An Update

Covalent inhibitors and other types of covalent modalities have seen a revival in the past two decades, with a variety of new targeted covalent drugs having been approved in recent years. A key feature of such molecules is an intrinsically reactive group, typically a weak electrophile, which enables the irreversible or reversible formation of a covalent bond with a specific amino acid of the target protein. This reactive group, often called the "warhead", is a critical determinant of the ligand's activity, selectivity, and general biological properties. In 2019, we summarized emerging and re-emerging warhead chemistries to target cysteine and other amino acids (Gehringer, M.; Laufer, S. A. J. Med. Chem. 2019, 62, 5673-5724; DOI: 10.1021/acs.jmedchem.8b01153). Since then, the field has rapidly evolved. Here we discuss the progress on covalent warheads made since our last Perspective and their application in medicinal chemistry and chemical biology.

Antitubercular activity of 2-mercaptobenzothiazole derivatives targeting Mycobacterium tuberculosis type II NADH dehydrogenase

Mycobacterium tuberculosis (Mtb) type II NADH dehydrogenase (NDH-2) transports electrons into the mycobacterial respiratory pathway at the cost of reduction of NADH to NAD+ and is an attractive drug target. Herein, we have synthesised a series of 2-mercaptobenzothiazoles (C1-C14) and evaluated their anti-tubercular potential as Mtb NDH-2 inhibitors. The synthesised compounds C1-C14 were evaluated for MIC90 and ATP depletion against Mtb H37Ra, M. bovis, and Mtb H37Rv mc2 6230. Compounds C3, C4, and C11 were found to be the active molecules in the series and were further evaluated for their MIC90 against Mtb-resistant strains and for their bactericidal potential against Mtb H37Rv mc26230. The Peredox-mCherry-expressing Mtb strain was used to examine whether C3, C4, and C11 possess NDH-2 inhibitory potential. Furthermore, cytotoxicity analysis against HepG2 displayed a safety index (SI) of >10 for C3 and C4. To get an insight into the mode of interaction at NDH-2, we have performed computational analysis of our active compounds.

Synthetic lethality of Mycobacterium tuberculosis NADH dehydrogenases is due to impaired NADH oxidation

IMPORTANCE

In 2022, it was estimated that 10.6 million people fell ill, and 1.6 million people died from tuberculosis (TB). Available treatment is lengthy and requires a multi-drug regimen, which calls for new strategies to cure Mycobacterium tuberculosis (Mtb) infections more efficiently. We have previously shown that simultaneous inactivation of type 1 (Ndh-1) and type 2 (Ndh-2) NADH dehydrogenases kills Mtb. NADH dehydrogenases play two main physiological roles: NADH oxidation and electron entry into the respiratory chain. Here, we show that this bactericidal effect is a consequence of impaired NADH oxidation. Importantly, we demonstrate that Ndh-1/Ndh-2 synthetic lethality can be achieved through simultaneous chemical inhibition, which could be exploited by TB drug development programs.

Identification and Optimization of Novel Inhibitors of the Polyketide Synthase 13 Thioesterase Domain with Antitubercular Activity

There is an urgent need for new tuberculosis (TB) treatments, with novel modes of action, to reduce the incidence/mortality of TB and to combat resistance to current treatments. Through both chemical and genetic methodologies, polyketide synthase 13 (Pks13) has been validated as essential for mycobacterial survival and as an attractive target for Mycobacterium tuberculosis growth inhibitors. A benzofuran series of inhibitors that targeted the Pks13 thioesterase domain, failed to progress to preclinical development due to concerns over cardiotoxicity. Herein, we report the identification of a novel oxadiazole series of Pks13 inhibitors, derived from a high-throughput screening hit and structure-guided optimization. This new series binds in the Pks13 thioesterase domain, with a distinct binding mode compared to the benzofuran series. Through iterative rounds of design, assisted by structural information, lead compounds were identified with improved antitubercular potencies (MIC < 1 μM) and in vitro ADMET profiles.

Comprehensive essentiality analysis of the Mycobacterium kansasii genome by saturation transposon mutagenesis and deep sequencing

Mycobacterium kansasii (Mk) is an opportunistic pathogen that is frequently isolated from urban water systems, posing a health risk to susceptible individuals. Despite its ability to cause tuberculosis-like pulmonary disease, very few studies have probed the genetics of this opportunistic pathogen. Here, we report a comprehensive essentiality analysis of the Mk genome. Deep sequencing of a high-density library of Mk Himar1 transposon mutants revealed that 86.8% of the chromosomal thymine-adenine (TA) dinucleotide target sites were permissive to insertion, leaving 13.2% TA sites unoccupied. Our analysis identified 394 of the 5,350 annotated open reading frames (ORFs) as essential. The majority of these essential ORFs (84.8%) share essential mutual orthologs with Mycobacterium tuberculosis (Mtb). A comparative genomics analysis identified 139 Mk essential ORFs that share essential orthologs in four other species of mycobacteria. Thirteen Mk essential ORFs share orthologs in all four species that were identified as being not essential, while only two Mk essential ORFs are absent in all species compared. We used the essentiality data and a comparative genomics analysis reported here to highlight differences in essentiality between candidate Mtb drug targets and the corresponding Mk orthologs. Our findings suggest that the Mk genome encodes redundant or additional pathways that may confound validation of potential Mtb drugs and drug target candidates against the opportunistic pathogen. Additionally, we identified 57 intergenic regions containing four or more consecutive unoccupied TA sites. A disproportionally large number of these regions were located upstream of pe/ppe genes. Finally, we present an essentiality and orthology analysis of the Mk pRAW-like plasmid, pMK1248. IMPORTANCE Mk is one of the most common nontuberculous mycobacterial pathogens associated with tuberculosis-like pulmonary disease. Drug resistance emergence is a threat to the control of Mk infections, which already requires long-term, multidrug courses. A comprehensive understanding of Mk biology is critical to facilitate the development of new and more efficacious therapeutics against Mk. We combined transposon-based mutagenesis with analysis of insertion site identification data to uncover genes and other genomic regions required for Mk growth. We also compared the gene essentiality data set of Mk to those available for several other mycobacteria. This analysis highlighted key similarities and differences in the biology of Mk compared to these other species. Altogether, the genome-wide essentiality information generated and the results of the cross-species comparative genomics analysis represent valuable resources to assist the process of identifying and prioritizing potential Mk drug target candidates and to guide future studies on Mk biology.

Mercapto-pyrimidines are reversible covalent inhibitors of the papain-like protease (PLpro) and inhibit SARS-CoV-2 (SCoV-2) replication

The papain-like protease (PLpro) plays a critical role in SARS-CoV-2 (SCoV-2) pathogenesis and is essential for viral replication and for allowing the virus to evade the host immune response. Inhibitors of PLpro have great therapeutic potential, however, developing them has been challenging due to PLpro's restricted substrate binding pocket. In this report, we screened a 115 000-compound library for PLpro inhibitors and identified a new pharmacophore, based on a mercapto-pyrimidine fragment that is a reversible covalent inhibitor (RCI) of PLpro and inhibits viral replication in cells. Compound 5 had an IC50 of 5.1 μM for PLpro inhibition and hit optimization yielded a derivative with increased potency (IC50 0.85 μM, 6-fold higher). Activity based profiling of compound 5 demonstrated that it reacts with PLpro cysteines. We show here that compound 5 represents a new class of RCIs, which undergo an addition elimination reaction with cysteines in their target proteins. We further show that their reversibility is catalyzed by exogenous thiols and is dependent on the size of the incoming thiol. In contrast, traditional RCIs are all based upon the Michael addition reaction mechanism and their reversibility is base-catalyzed. We identify a new class of RCIs that introduces a more reactive warhead with a pronounced selectivity profile based on thiol ligand size. This could allow the expansion of RCI modality use towards a larger group of proteins important for human disease.

Inhibiting respiration as a novel antibiotic strategy

The approval of the first-in-class antibacterial bedaquiline for tuberculosis marks a breakthrough in antituberculosis drug development. The drug inhibits mycobacterial respiration and represents the validation of a wholly different metabolic process as a druggable target space. In this review, we discuss the advances in the development of mycobacterial respiratory inhibitors, as well as the potential of applying this strategy to other pathogens. The non-fermentative nature of mycobacteria explains their vulnerability to respiration inhibition, and we caution that this strategy may not be equally effective in other organisms. Conversely, we also showcase fundamental studies that reveal ancillary functions of the respiratory pathway, which are crucial to some pathogens' virulence, drug susceptibility and fitness, introducing another perspective of targeting bacterial respiration as an antibiotic strategy.

Synthetic lethality of Mycobacterium tuberculosis NADH dehydrogenases is due to impaired NADH oxidation

Type 2 NADH dehydrogenase (Ndh-2) is an oxidative phosphorylation enzyme discussed as a promising drug target in different pathogens, including Plasmodium falciparum and Mycobacterium tuberculosis (Mtb). To kill Mtb, Ndh-2 needs to be inactivated together with the alternative enzyme type 1 NADH dehydrogenase (Ndh-1), but the mechanism of this synthetic lethality remained unknown. Here, we provide insights into the biology of NADH dehydrogenases and a mechanistic explanation for Ndh-1 and Ndh-2 synthetic lethality in Mtb. NADH dehydrogenases have two main functions: maintaining an appropriate NADH/NAD+ ratio by converting NADH into NAD+ and providing electrons to the respiratory chain. Heterologous expression of a water forming NADH oxidase (Nox), which catalyzes the oxidation of NADH, allows to distinguish between these two functions and show that Nox rescues Mtb from Ndh-1/Ndh-2 synthetic lethality, indicating that NADH oxidation is the essential function of NADH dehydrogenases for Mtb viability. Quantification of intracellular levels of NADH, NAD, ATP, and oxygen consumption revealed that preventing NADH oxidation by Ndh-2 depletes NAD(H) and inhibits respiration. Finally, we show that Ndh-1/ Ndh-2 synthetic lethality can be achieved through chemical inhibition.

Quinazolinones, the Winning Horse in Drug Discovery

Quinazolines are nitrogen-containing heterocycles that consist of a benzene ring fused with a pyrimidine ring. Quinazolinones, oxidized quinazolines, are promising compounds with a wide range of biological activities. In the pharmaceutical field, quinazolinones are the building blocks of more than 150 naturally occurring alkaloids isolated from different plants, microorganisms, and animals. Scientists give a continuous interest in this moiety due to their stability and relatively easy methods for preparation. Their lipophilicity is another reason for this interest as it helps quinazolinones in penetration through the blood-brain barrier which makes them suitable for targeting different central nervous system diseases. Various modifications to the substitutions around the quinazolinone system changed their biological activity significantly due to changes in their physicochemical properties. Structure-activity relationship (SAR) studies of quinazolinone revealed that positions 2, 6, and 8 of the ring systems are significant for different pharmacological activities. In addition, it has been suggested that the addition of different heterocyclic moieties at position 3 could increase activity. In this review, we will highlight the chemical properties of quinazolinones, including their chemical reactions and different methods for their preparation. Moreover, we will try to modify some of the old SAR studies according to their updated biological activities in the last twelve years.

Tricyclic SpiroLactams Kill Mycobacteria In Vitro and In Vivo by Inhibiting Type II NADH Dehydrogenases

It is critical that novel classes of antituberculosis drugs are developed to combat the increasing burden of infections by multidrug-resistant strains. To identify such a novel class of antibiotics, a chemical library of unique 3-D bioinspired molecules was explored revealing a promising, mycobacterium specific Tricyclic SpiroLactam (TriSLa) hit. Chemical optimization of the TriSLa scaffold delivered potent analogues with nanomolar activity against replicating and nonreplicating Mycobacterium tuberculosis. Characterization of isolated TriSLa-resistant mutants, and biochemical studies, found TriSLas to act as allosteric inhibitors of type II NADH dehydrogenases (Ndh-2 of the electron transport chain), resulting in an increase in bacterial NADH/NAD⁺ ratios and decreased ATP levels. TriSLas are chemically distinct from other inhibitors of Ndh-2 but share a dependence for fatty acids for activity. Finally, in vivo proof-of-concept studies showed TriSLas to protect zebrafish larvae from Mycobacterium marinum infection, suggesting a vulnerability of Ndh-2 inhibition in mycobacterial infections.

Lysyl-tRNA synthetase, a target for urgently needed M. tuberculosis drugs

Tuberculosis is a major global cause of both mortality and financial burden mainly in low and middle-income countries. Given the significant and ongoing rise of drug-resistant strains of Mycobacterium tuberculosis within the clinical setting, there is an urgent need for the development of new, safe and effective treatments. Here the development of a drug-like series based on a fused dihydropyrrolidino-pyrimidine scaffold is described. The series has been developed against M. tuberculosis lysyl-tRNA synthetase (LysRS) and cellular studies support this mechanism of action. DDD02049209, the lead compound, is efficacious in mouse models of acute and chronic tuberculosis and has suitable physicochemical, pharmacokinetic properties and an in vitro safety profile that supports further development. Importantly, preliminary analysis using clinical resistant strains shows no pre-existing clinical resistance towards this scaffold.

Fragment-Based Drug Discovery against Mycobacteria: The Success and Challenges

The emergence of drug-resistant mycobacteria, including Mycobacterium tuberculosis (Mtb) and non-tuberculous mycobacteria (NTM), poses an increasing global threat that urgently demands the development of new potent anti-mycobacterial drugs. One of the approaches toward the identification of new drugs is fragment-based drug discovery (FBDD), which is the most ingenious among other drug discovery models, such as structure-based drug design (SBDD) and high-throughput screening. Specialized techniques, such as X-ray crystallography, nuclear magnetic resonance spectroscopy, and many others, are part of the drug discovery approach to combat the Mtb and NTM global menaces. Moreover, the primary drawbacks of traditional methods, such as the limited measurement of biomolecular toxicity and uncertain bioavailability evaluation, are successfully overcome by the FBDD approach. The current review focuses on the recognition of fragment-based drug discovery as a popular approach using virtual, computational, and biophysical methods to identify potent fragment molecules. FBDD focuses on designing optimal inhibitors against potential therapeutic targets of NTM and Mtb (PurC, ArgB, MmpL3, and TrmD). Additionally, we have elaborated on the challenges associated with the FBDD approach in the identification and development of novel compounds. Insights into the applications and overcoming the challenges of FBDD approaches will aid in the identification of potential therapeutic compounds to treat drug-sensitive and drug-resistant NTMs and Mtb infections.

The Orphan Response Regulator Rv3143 Modulates the Activity of the NADH Dehydrogenase Complex (Nuo) in Mycobacterium tuberculosis via Protein–Protein Interactions

Two-component signal transduction systems enable mycobacterial cells to quickly adapt and adequately respond to adverse environmental conditions encountered at various stages of host infection. We attempted to determine the role of the Rv3143 “orphan” response regulator in the physiology of Mycobacterium tuberculosis and its orthologue Msmeg_2064 in Mycobacterium smegmatis. We identified the Rv3143 protein as an interaction partner for NuoD, a member of the type I NADH dehydrogenase complex involved in oxidative phosphorylation. The mutants Δrv3143 and Δmsmeg_2064 were engineered in M. tuberculosis and M. smegmatis cells, respectively. The Δmsmeg_2064 strain exhibited a significant reduction in growth and viability in the presence of reactive nitrogen species. The Rv3143-deficient strain was sensitive to valinomycin, which is known to reduce the electrochemical potential of the cell and overexpressed genes required for nitrate respiration. An increased level of reduction of the 2,3,5-triphenyltetrazolium chloride (TTC) electron acceptor in Δrv3143 and Δmsmeg_2064 cells was also evident. The silencing of ndh expression using CRISPRi/dCas9 affected cell survival under limited oxygen conditions. Oxygen consumption during entry to hypoxia was most severely affected in the double-mutant Δmsmeg_2064 ndhCRISPRi/dCas9. We propose that the regulatory protein Rv3143 is a component of the Nuo complex and modulates its activity.

A review on enzyme complexes of electron transport chain from Mycobacterium tuberculosis as promising drug targets

Energy metabolism is a universal process occurring in all life forms. In Mycobacterium tuberculosis (Mtb), energy production is carried out in two possible ways, oxidative phosphorylation (OxPhos) and substrate-level phosphorylation. Mtb is an obligate aerobic bacterium, making it dependent on OxPhos for ATP synthesis and growth. Mtb inhabits varied micro-niches during the infection cycle, outside and within the host cells, which alters its primary metabolic pathways during the pathogenesis. In this review, we discuss cellular respiration in the context of the mechanism and structural importance of the proteins and enzyme complexes involved. These protein-protein complexes have been proven to be essential for Mtb virulence as they aid the bacteria's survival during aerobic and hypoxic conditions. ATP synthase, a crucial component of the electron transport chain, has been in the limelight, as a prominent drug target against tuberculosis. Likewise, in this review, we have explored other protein-protein complexes of the OxPhos pathway, their functional essentiality, and their mechanism in Mtb's diverse lifecycle. The review summarises crucial target proteins and reported inhibitors of the electron transport chain pathway of Mtb.

Advances in Key Drug Target Identification and New Drug Development for Tuberculosis

Tuberculosis (TB) is a severe infectious disease worldwide. The increasing emergence of drug-resistant Mycobacterium tuberculosis (Mtb) has markedly hampered TB control. Therefore, there is an urgent need to develop new anti-TB drugs to treat drug-resistant TB and shorten the standard therapy. The discovery of targets of drug action will lay a theoretical foundation for new drug development. With the development of molecular biology and the success of Mtb genome sequencing, great progress has been made in the discovery of new targets and their relevant inhibitors. In this review, we summarized 45 important drug targets and 15 new drugs that are currently being tested in clinical stages and several prospective molecules that are still at the level of preclinical studies. A comprehensive understanding of the drug targets of Mtb can provide extensive insights into the development of safer and more efficient drugs and may contribute new ideas for TB control and treatment.

Recent advancement in drug development of nitro(NO2 )-heterocyclic compounds as lead scaffolds for the treatment of Mycobacterium tuberculosis

Tuberculosis (TB) is an infectious disease caused predominantly by Mycobacterium tuberculosis (Mtb). It was responsible for approximately 1.4 million deaths worldwide in 2019. The lack of new drugs to treat drug-resistant strains is a principal factor for the slow rise in TB infections. Our aim is to aid the development of new TB treatments by describing improvements (last decade, 2011-2021) to nitro(NO₂ )-based compounds that have shown activity or pharmacological properties (e.g., anti-proliferative, anti-kinetoplastid) against Mtb. For all compounds, we have included final correlations of minimum inhibitory concentrations against Mtb (H₃₇ Rv).

Optimization of TAM16, a Benzofuran That Inhibits the Thioesterase Activity of Pks13; Evaluation toward a Preclinical Candidate for a Novel Antituberculosis Clinical Target

With increasing drug resistance in tuberculosis (TB) patient populations, there is an urgent need for new drugs. Ideally, new agents should work through novel targets so that they are unencumbered by preexisting clinical resistance to current treatments. Benzofuran 1 was identified as a potential lead for TB inhibiting a novel target, the thioesterase domain of Pks13. Although, having promising activity against Mycobacterium tuberculosis, its main liability was inhibition of the hERG cardiac ion channel. This article describes the optimization of the series toward a preclinical candidate. Despite improvements in the hERG liability in vitro, when new compounds were assessed in ex vivo cardiotoxicity models, they still induced cardiac irregularities. Further series development was stopped because of concerns around an insufficient safety window. However, the demonstration of in vivo activity for multiple series members further validates Pks13 as an attractive novel target for antitubercular drugs and supports development of alternative chemotypes.

Screening approaches and therapeutic targets: The two driving wheels of tuberculosis drug discovery

Tuberculosis (TB) is an infectious disease, infecting a quarter of world's population. Drug resistant TB further exacerbates the grim scenario of the drying TB drug discovery pipeline. The limited arsenal to fight TB presses the need for thorough efforts for identifying promising hits to combat the disease. The review highlights the efforts in the field of tuberculosis drug discovery, with an emphasis on massive drug screening campaigns for identifying novel hits against Mtb in both industry and academia. As an intracellular pathogen, mycobacteria reside in a complicated intracellular environment with multiple factors at play. Here, we outline various strategies employed in an effort to mimic the intracellular milieu for bringing the screening models closer to the actual settings. The review also focuses on the novel targets and pathways that could aid in target-based drug discovery in TB. The recent high throughput screening efforts resulting in the identification of potent hits against Mtb has been summarized in this article. There is a pressing need for effective screening strategies and approaches employing innovative tools and recent technologies; including nanotechnology, gene-editing tools such as CRISPR-cas system, host-directed bacterial killing and high content screening to augment the TB drug discovery pipeline with safer and shorter drug regimens.

Evaluating the effect of clofazimine against Mycobacterium tuberculosis when given alone or in combination with pretomanid, bedaquiline or linezolid

Clofazimine (CFZ) has been regaining prominence for treating tuberculosis in recent years. However, as a single drug, it shows limited efficacy and optimal combination partners have not been identified. Therefore, the objective of our analysis was to evaluate the efficacy of CFZ-containing two-drug regimen with pretomanid (PMD), bedaquiline (BDQ) or linezolid (LZD) by determining: i) their pharmacodynamic (PD) mode of interaction against Mycobacterium tuberculosis (Mtb) strain H37Rv in log- and acid-metabolic states, and Mtb strain 18b in a non-replicating persister metabolic state, ii) to predict bacterial cell kill of the drugs alone and in combination, and iii) to evaluate the relationship between the interaction mode and bacterial cell kill amount. The results of our Greco universal response surface analysis showed that CFZ was at least additive with a clear trend towards synergy when combined with PMD, BDQ, and LZD against Mtb in all explored metabolic states under in vitro checkerboard assay conditions. They further showed that all 2-drug combination regimens exerted more bacterial kill than any of the drugs alone. CFZ alone showed the least antimicrobial efficacy amongst the evaluated drugs and there was a lack of correlation between the mode of interaction and the amount of bacterial kill. However, we may underestimate the effect of CFZ in this screening approach due to limited in vitro study duration and neglect of target site accumulation.

The Transcription Factor Rv1453 Regulates the Expression of qor and Confers Resistant to Clofazimine in Mycobacterium tuberculosis

Objective

Clofazimine plays an important role in the treatment of drug-resistant tuberculosis. However, the mechanism of clofazimine resistance remains unclear. In order to slow down the occurrence of clofazimine resistance, it is necessary to study its resistance mechanism.

Methods

In this study, we constructed Rv1453 knockout, complementary and overexpressed strain. The minimum inhibitory concentration (MIC) of clofazimine against Mycobacterium tuberculosis was detected by microplate alamar blue assay (MABA). The transcription levels of Rv1453 and its adjacent genes were detected by quantitative reverse transcriptase PCR. The purified Rv1453 protein was used for electrophoretic mobility shift assay (EMSA) to identify the binding site of Rv1453 protein.

Results

The minimum inhibitory concentration (MIC) of clofazimine increased about 4-fold for the Rv1453 knockout strain and decreased about 4-fold for the Rv1453 overexpressed strain compared with Mycobacterium tuberculosis H37Rv. Further analysis showed that Rv1453 protein, as a regulatory protein, binds to the RNA polymerase binding site of qor and blocks the transcription process.

Conclusion

This study preliminarily revealed that Rv1453 protein of Mycobacterium tuberculosis affects its susceptibility to clofazimine by regulating the transcription level of qor, which is shedding a new light on the mechanism of clofazimine resistance.

Quinazoline-tethered hydrazone: A versatile scaffold toward dual anti-TB and EGFR inhibition activities in NSCLC

Globally, lung cancer and tuberculosis are considered to be very serious and complex diseases. Evidence suggests that chronic infection with tuberculosis (TB) can often lead to lung tumors; therefore, developing drugs that target both diseases is of great clinical significance. In our study, we designed and synthesized a suite of 14 new quinazolinones (5a-n) and performed biological investigations of these compounds in Mycobacterium tuberculosis (MTB) and cancer cell lines. In addition, we conducted a molecular modeling study to determine the mechanism of action of these compounds at the molecular level. Compounds that showed anticancer activity in the preliminary screening were further evaluated in three cancer cell lines (A549, Calu-3, and BT-474 cells) and characterized in an epidermal growth factor receptor (EGFR) binding assay. Cytotoxicity in noncancerous lung fibroblast cells was also evaluated to obtain safety data. Our theoretical and experimental studies indicated that our compounds showed a mechanism of action similar to that of erlotinib by inhibiting the EGFR tyrosine kinase. In turn, the antituberculosis activity of these compounds would be produced by the inhibition of enoyl-ACP-reductase. From our findings, we were able to identify two potential lead compounds (5i and 5l) with dual activity and elevated safety toward noncancerous lung fibroblast cells. In addition, our data identified three compounds with excellent anti-TB activities (compounds 5i, 5l, and 5n).

Novel Aryl-Substituted Pyrimidones as Inhibitors of 3-Mercaptopyruvate Sulfurtransferase with Antiproliferative Efficacy in Colon Cancer

The enzyme 3-mercaptopyruvate sulfurtransferase (3-MST) is one of the more recently identified mammalian sources of H₂S. A recent study identified several novel 3-MST inhibitors with micromolar potency. Among those, (2-[(4-hydroxy-6-methylpyrimidin-2-yl)sulfanyl]-1-(naphthalen-1-yl)ethan-1-one) or HMPSNE was found to be the most potent and selective. We now took the central core of this compound and modified the pyrimidone and the arylketone sides independently. A 63-compound library was synthesized; compounds were tested for H₂S generation from recombinant 3-MST in vitro. Active compounds were subsequently tested to elucidate their potency and selectivity. Computer modeling studies have delineated some of the key structural features necessary for binding to the 3-MST's active site. Six novel 3-MST inhibitors were tested in cell-based assays: they exerted inhibitory effects in murine MC38 and CT26 colon cancer cell proliferation; the antiproliferative effect of the compound with the highest potency and best cell-based activity (1b) was also confirmed on the growth of MC38 tumors in mice.

Tuberculosis Drug Discovery: A Decade of Hit Assessment for Defined Targets

More than two decades have elapsed since the publication of the first genome sequence of Mycobacterium tuberculosis (Mtb) which, shortly thereafter, enabled methods to determine gene essentiality in the pathogen. Despite this, target-based approaches have not yielded drugs that have progressed to clinical testing. Whole-cell screening followed by elucidation of mechanism of action has to date been the most fruitful approach to progressing inhibitors into the tuberculosis drug discovery pipeline although target-based approaches are gaining momentum. This review discusses scaffolds that have been identified over the last decade from screens of small molecule libraries against Mtb or defined targets where mechanism of action investigation has defined target-hit couples and structure-activity relationship studies have described the pharmacophore.

Unconventional Antibacterials and Adjuvants

The need for new classes of antibacterials is genuine in light of the dearth of clinical options for the treatment of bacterial infections. The prodigious discoveries of antibiotics during the 1940s to 1970s, a period wistfully referred to as the Golden Age of Antibiotics, have not kept up in the face of emergence of resistant bacteria in the past few decades. There has been a renewed interest in old drugs, the repurposing of the existing antibiotics and pairing of synergistic antibiotics or of an antibiotic with an adjuvant. Notwithstanding, discoveries of novel classes of these life-saving drugs have become increasingly difficult, calling for new paradigms. We describe, herein, three strategies from our laboratories toward discoveries of new antibacterials and adjuvants using computational and multidisciplinary experimental methods. One approach targets penicillin-binding proteins (PBPs), biosynthetic enzymes of cell-wall peptidoglycan, for discoveries of non-β-lactam inhibitors. Oxadiazoles and quinazolinones emerged as two structural classes out of these efforts. Several hundred analogs of these two classes of antibiotics have been synthesized and fully characterized in our laboratories. A second approach ventures into inhibition of allosteric regulation of cell-wall biosynthesis. The mechanistic details of allosteric regulation of PBP2a of Staphylococcus aureus, discovered in our laboratories, is outlined. The allosteric site in this protein is at 60 Å distance to the active site, whereby ligand binding at the former makes access to the latter by the substrate possible. We have documented that both quinazolinones and ceftaroline, a fifth-generation cephalosporin, bind to the allosteric site in manifestation of the antibacterial activity. Attempts at inhibition of the regulatory phosphorylation events identified three classes of antibacterial adjuvants and one class of antibacterials, the picolinamides. The chemical structures for these hits went through diversification by synthesis of hundreds of analogs. These analogs were characterized in various assays for identification of leads with adjuvant and antibacterial activities. Furthermore, we revisited the mechanism of bulgecins, a class of adjuvants discovered and abandoned in the 1980s. These compounds potentiate the activities of β-lactam antibiotics by the formation of bulges at the sites of septum formation during bacterial replication, which are points of structural weakness in the envelope. These bulges experience rupture, which leads to bacterial death. Bulgecin A inhibits the lytic transglycosylase Slt of Pseudomonas aeruginosa as a likely transition-state mimetic for its turnover of the cell-wall peptidoglycan. Once damage to cell wall is inflicted by a β-lactam antibiotic, the function of Slt is to repair the damage. When Slt is inhibited by bulgecin A, the organism cannot cope with it and would undergo rapid lysis. Bulgecin A is an effective adjuvant of β-lactam antibiotics. These discoveries of small-molecule classes of antibacterials or of adjuvants to antibacterials hold promise in strategies for treatment of bacterial infections.

Metabolic Versatility of Mycobacterium tuberculosis during Infection and Dormancy

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is a highly successful intracellular pathogen with the ability to withstand harsh conditions and reside long-term within its host. In the dormant and persistent states, the bacterium tunes its metabolism and is able to resist the actions of antibiotics. One of the main strategies Mtb adopts is through its metabolic versatility-it is able to cometabolize a variety of essential nutrients and direct these nutrients simultaneously to multiple metabolic pathways to facilitate the infection of the host. Mtb further undergo extensive remodeling of its metabolic pathways in response to stress and dormancy. In recent years, advancement in systems biology and its applications have contributed substantially to a more coherent view on the intricate metabolic networks of Mtb. With a more refined appreciation of the roles of metabolism in mycobacterial infection and drug resistance, and the success of drugs targeting metabolism, there is growing interest in further development of anti-TB therapies that target metabolism, including lipid metabolism and oxidative phosphorylation. Here, we will review current knowledge revolving around the versatility of Mtb in remodeling its metabolism during infection and dormancy, with a focus on central carbon metabolism and lipid metabolism.

2-((3,5-Dinitrobenzyl)thio)quinazolinones: Potent Antimycobacterial Agents Activated by Deazaflavin (F420)-Dependent Nitroreductase (Ddn)

Swapping the substituents in positions 2 and 4 of the previously synthesized but yet undisclosed 5-cyano-4-(methylthio)-2-arylpyrimidin-6-ones 4, ring closure, and further optimization led to the identification of the potent antitubercular 2-thio-substituted quinazolinone 26. Structure-activity relationship (SAR) studies indicated a crucial role for both meta-nitro substituents for antitubercular activity, while the introduction of polar substituents on the quinazolinone core allowed reduction of bovine serum albumin (BSA) binding (63c, 63d). While most of the tested quinazolinones exhibited no cytotoxicity against MRC-5, the most potent compound 26 was found to be mutagenic via the Ames test. This analogue exhibited moderate inhibitory potency against Mycobacterium tuberculosis thymidylate kinase, the target of the 3-cyanopyridones that lies at the basis of the current analogues, indicating that the whole-cell antimycobacterial activity of the present S-substituted thioquinazolinones is likely due to modulation of alternative or additional targets. Diminished antimycobacterial activity was observed against mutants affected in cofactor F420 biosynthesis (fbiC), cofactor reduction (fgd), or deazaflavin-dependent nitroreductase activity (rv3547), indicating that reductive activation of the 3,5-dinitrobenzyl analogues is key to antimycobacterial activity.

Bioenergetic Inhibitors: Antibiotic Efficacy and Mechanisms of Action in Mycobacterium tuberculosis

Development of novel anti-tuberculosis combination regimens that increase efficacy and reduce treatment timelines will improve patient compliance, limit side-effects, reduce costs, and enhance cure rates. Such advancements would significantly improve the global TB burden and reduce drug resistance acquisition. Bioenergetics has received considerable attention in recent years as a fertile area for anti-tuberculosis drug discovery. Targeting the electron transport chain (ETC) and oxidative phosphorylation machinery promises not only to kill growing cells but also metabolically dormant bacilli that are inherently more drug tolerant. Over the last two decades, a broad array of drugs targeting various ETC components have been developed. Here, we provide a focused review of the current state of art of bioenergetic inhibitors of Mtb with an in-depth analysis of the metabolic and bioenergetic disruptions caused by specific target inhibition as well as their synergistic and antagonistic interactions with other drugs. This foundation is then used to explore the reigning theories on the mechanisms of antibiotic-induced cell death and we discuss how bioenergetic inhibitors in particular fail to be adequately described by these models. These discussions lead us to develop a clear roadmap for new lines of investigation to better understand the mechanisms of action of these drugs with complex mechanisms as well as how to leverage that knowledge for the development of novel, rationally-designed combination therapies to cure TB.

Small organic molecules targeting the energy metabolism of Mycobacterium tuberculosis

Causing approximately 10 million incident cases and 1.3-1.5 million deaths every year, Mycobacterium tuberculosis remains a global health problem. The risk is further exacerbated with latent tuberculosis (TB) infection, the HIV pandemic, and increasing anti-TB drug resistance. Therefore, unexplored chemical scaffolds directed towards new molecular targets are increasingly desired. In this context, mycobacterial energy metabolism, particularly the oxidative phosphorylation (OP) pathway, is gaining importance. Mycobacteria possess primary dehydrogenases to fuel electron transport; aa₃-type cytochrome c oxidase and bd-type menaquinol oxidase to generate a protonmotive force; and ATP synthase, which is essential for both growing mycobacteria as well as dormant mycobacteria because ATP is produced under both aerobic and hypoxic conditions. Small organic molecules targeting OP are active against latent TB as well as resistant TB strains. FDA approval of the ATP synthase inhibitor bedaquiline and the discovery of clinical candidate Q203, which both interfere with the cytochrome bc₁ complex, have already confirmed mycobacterial energy metabolism to be a valuable anti-TB drug target. This review highlights both preferable molecular targets within mycobacterial OP and promising small organic molecules targeting OP. Progressive research in the area of mycobacterial OP revealed several highly potent anti-TB compounds with nanomolar-range MICs as low as 0.004 μM against Mtb H37Rv. Therefore, we are convinced that targeting the OP pathway can combat resistant TB and latent TB, leading to more efficient anti-TB chemotherapy.

SAR Analysis of Small Molecules Interfering with Energy-Metabolism in Mycobacterium tuberculosis

Tuberculosis remains the world's top infectious killer: it caused a total of 1.5 million deaths and 10 million people fell ill with TB in 2018. Thanks to TB diagnosis and treatment, mortality has been falling in recent years, with an estimated 58 million saved lives between 2000 and 2018. However, the emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Mtb strains is a major concern that might reverse this progress. Therefore, the development of new drugs acting upon novel mechanisms of action is a high priority in the global health agenda. With the approval of bedaquiline, which targets mycobacterial energy production, and delamanid, which targets cell wall synthesis and energy production, the energy-metabolism in Mtb has received much attention in the last decade as a potential target to investigate and develop new antimycobacterial drugs. In this review, we describe potent anti-mycobacterial agents targeting the energy-metabolism at different steps with a special focus on structure-activity relationship (SAR) studies of the most advanced compound classes.

New tuberculosis drug targets, their inhibitors, and potential therapeutic impact

The current tuberculosis (TB) predicament poses numerous challenges and therefore every incremental scientific work and all positive socio-political engagements, are steps taken in the right direction to eradicate TB. Progression of the late stage TB-drug pipeline into the clinics is an immediate deliverable of this global effort. At the same time, fueling basic research and pursuing early discovery work must be sustained to maintain a healthy TB-drug pipeline. This review encompasses a broad analysis of chemotherapeutic strategies that target the DNA replication, protein synthesis, cell wall biosynthesis, energy metabolism and proteolysis of Mycobacterium tuberculosis (Mtb). It includes a status check of the current TB-drug pipeline with a focus on the associated biology, emerging targets, and their promising chemical inhibitors. Potential synergies and/or gaps within or across different chemotherapeutic strategies are systematically reviewed as well.

Oxidative Phosphorylation—an Update on a New, Essential Target Space for Drug Discovery in Mycobacterium tuberculosis

New drugs with new mechanisms of action are urgently required to tackle the global tuberculosis epidemic. Following the FDA-approval of the ATP synthase inhibitor bedaquiline (Sirturo®), energy metabolism has become the subject of intense focus as a novel pathway to exploit for tuberculosis drug development. This enthusiasm stems from the fact that oxidative phosphorylation (OxPhos) and the maintenance of the transmembrane electrochemical gradient are essential for the viability of replicating and non-replicating Mycobacterium tuberculosis (M. tb), the etiological agent of human tuberculosis (TB). Therefore, new drugs targeting this pathway have the potential to shorten TB treatment, which is one of the major goals of TB drug discovery. This review summarises the latest and key findings regarding the OxPhos pathway in M. tb and provides an overview of the inhibitors targeting various components. We also discuss the potential of new regimens containing these inhibitors, the flexibility of this pathway and, consequently, the complexity in targeting it. Lastly, we discuss opportunities and future directions of this drug target space.

Collocating Novel Targets for Tuberculosis (TB) Drug Discovery

BACKGROUND

Mycobacterium tuberculosis, being a resistive species is an incessant threat to the world population for the treatment of Tuberculosis (TB). An advanced genetic or a molecular level approach is mandatory for both diagnosis and therapy as the prevalence of multi drug-resistant (MDR) and extensively drug- resistant (XDR) TB.

METHODS

A literature review was conducted, focusing essentially on the development of biomarkers and targets to extrapolate the Tuberculosis Drug Discovery process.

RESULTS AND DISCUSSION

In this article, we have discussed several substantial targets and genetic mutations occurring in a diseased or treatment condition of TB patients. It includes expressions in Bhlhe40, natural resistance associated macrophage protein 1 (NRAMP1) and vitamin D receptor (VDR) with its mechanistic actions that have made a significant impact on TB. Moreover, recently identified compounds; imidazopyridine amine derivative (Q203), biphenyl amide derivative (DG70), azetidine, thioquinazole, tetrahydroindazole and 2- mercapto- quinazoline scaffolds for several targets such as adenosine triphosphate (ATP), amino acid and fatty acid have been briefed for their confirmed hits and therapeutic activity.

New potential drug leads against MDR-MTB: A short review

Multidrug resistant Mycobacterium tuberculosis (MDR-MTB) infections have created a critical health problem globally. The appalling rise in drug resistance to all the current therapeutics has triggered the need for identifying new antimycobacterial agents effective against multidrug-resistant Mycobacterium tuberculosis. Structurally unique chemical entities with new mode of action will be required to combat this pressing issue. This review gives an overview of the structures and outlines on various aspects of in vitro pharmacological activities of new antimycobacterial agents, mechanism of action and brief structure activity relationships in the perspective of drug discovery and development. This review also summarizes on recent reports of new antimycobacterial agents.

FAD/NADH Dependent Oxidoreductases: From Different Amino Acid Sequences to Similar Protein Shapes for Playing an Ancient Function

Flavoprotein oxidoreductases are members of a large protein family of specialized dehydrogenases, which include type II NADH dehydrogenase, pyridine nucleotide-disulphide oxidoreductases, ferredoxin-NAD+ reductases, NADH oxidases, and NADH peroxidases, playing a crucial role in the metabolism of several prokaryotes and eukaryotes. Although several studies have been performed on single members or protein subgroups of flavoprotein oxidoreductases, a comprehensive analysis on structure-function relationships among the different members and subgroups of this great dehydrogenase family is still missing. Here, we present a structural comparative analysis showing that the investigated flavoprotein oxidoreductases have a highly similar overall structure, although the investigated dehydrogenases are quite different in functional annotations and global amino acid composition. The different functional annotation is ascribed to their participation in species-specific metabolic pathways based on the same biochemical reaction, i.e., the oxidation of specific cofactors, like NADH and FADH₂. Notably, the performed comparative analysis sheds light on conserved sequence features that reflect very similar oxidation mechanisms, conserved among flavoprotein oxidoreductases belonging to phylogenetically distant species, as the bacterial type II NADH dehydrogenases and the mammalian apoptosis-inducing factor protein, until now retained as unique protein entities in Bacteria/Fungi or Animals, respectively. Furthermore, the presented computational analyses will allow consideration of FAD/NADH oxidoreductases as a possible target of new small molecules to be used as modulators of mitochondrial respiration for patients affected by rare diseases or cancer showing mitochondrial dysfunction, or antibiotics for treating bacterial/fungal/protista infections.

Plasticity of the Mycobacterium tuberculosis respiratory chain and its impact on tuberculosis drug development

The viability of Mycobacterium tuberculosis (Mtb) depends on energy generated by its respiratory chain. Cytochrome bc1-aa3 oxidase and type-2 NADH dehydrogenase (NDH-2) are respiratory chain components predicted to be essential, and are currently targeted for drug development. Here we demonstrate that an Mtb cytochrome bc1-aa3 oxidase deletion mutant is viable and only partially attenuated in mice. Moreover, treatment of Mtb-infected marmosets with a cytochrome bc1-aa3 oxidase inhibitor controls disease progression and reduces lesion-associated inflammation, but most lesions become cavitary. Deletion of both NDH-2 encoding genes (Δndh-2 mutant) reveals that the essentiality of NDH-2 as shown in standard growth media is due to the presence of fatty acids. The Δndh-2 mutant is only mildly attenuated in mice and not differently susceptible to clofazimine, a drug in clinical use proposed to engage NDH-2. These results demonstrate the intrinsic plasticity of Mtb's respiratory chain, and highlight the challenges associated with targeting the pathogen's respiratory enzymes for tuberculosis drug development.

Discovery of arylamide-5-anilinoquinazoline-8-nitro derivatives as VEGFR-2 kinase inhibitors: Synthesis, in vitro biological evaluation and molecular docking

Herein, we embarked on a structural optimization campaign aiming at the discovery of novel anticancer agents with our previously reported XL-6f as a lead compound. A library of 23 compounds has been synthesized based on the highly conserved active site of VEGFR-2. Several title compounds exhibited selective inhibitory activities against VEGFR-2, which also displayed selective anti-proliferation potency against HepG2 cell. All synthesized compounds were evaluated for anti-angiogenesis capability. Compound 7o showed the most potent anti-angiogenesis ability, the efficient cytotoxic activities (in vitro against HUVEC and HepG2 cell lines with IC₅₀ values of 0.58 and 0.23 µM, respectively). The molecular docking analysis revealed 7o is a Type-II inhibitor of VEGFR-2 kinase. In general, these results indicated these arylamide-5-anilinoquinazoline-8-nitro derivatives are promising inhibitors of VEGFR-2 for the potential treatment of anti-angiogenesis.

Carbon metabolism modulates the efficacy of drugs targeting the cytochrome bc1:aa3 in Mycobacterium tuberculosis

The influence of carbon metabolism on oxidative phosphorylation is poorly understood in mycobacteria. M. tuberculosis expresses two respiratory terminal oxidases, the cytochrome bc₁:aa₃ and the cytochrome bd oxidase, which are jointly required for oxidative phosphorylation and mycobacterial viability. The essentiality of the cytochrome bc₁:aa₃ for optimum growth is illustrated by its vulnerability to chemical inhibition by the clinical drug candidate Q203 and several other chemical series. The cytochrome bd oxidase is not strictly essential for growth but is required to maintain bioenergetics when the function of the cytochrome bc₁:aa₃ is compromised. In this study, we observed that the potency of drugs targeting the cytochrome bc₁:aa₃ is influenced by carbon metabolism. The efficacy of Q203 and related derivatives was alleviated by glycerol supplementation. The negative effect of glycerol supplementation on Q203 potency correlated with an upregulation of the cytochrome bd oxidase-encoding cydABDC operon. Upon deletion of cydAB, the detrimental effect of glycerol on the potency of Q203 was abrogated. The same phenomenon was also observed in recent clinical isolates, but to a lesser extent compared to the laboratory-adapted strain H37Rv. This study reinforces the importance of optimizing in vitro culture conditions for drug evaluation in mycobacteria, a factor which appeared to be particularly essential for drugs targeting the cytochrome bc₁:aa₃ terminal oxidase.

Transition metal-free synthesis of quinazolinones using dimethyl sulfoxide as a synthon

Biologically important quinazolinones have been synthesized from 2-aminobenzamides and DMSO.