A computational model of the inhibition of Mycobacterium tuberculosis ATPase by a new drug candidate R207910

One-pot multicomponent synthesis of novel pyridine derivatives for antidiabetic and antiproliferative activities

Background: Due to the close relationship of diabetes with hypertension reported in various research, a set of pyridine derivatives with US FDA-approved drug cores were designed and integrated by artificial intelligence. Methods: Novel pyridines were designed and synthesized. Compounds MNS-1-MNS-4 were evaluated for their structure and were screened for their in vitro antidiabetic (α-amylase) activity and anticancer (HepG2) activity by methyl thiazolyl tetrazolium assay. Comparative 3D quantitative structure-activity relationship analysis and pharmacophore generation were carried out. Results: The study revealed MNS-1 and MNS-4 as good alternatives to acarbose as antidiabetic agents, and MNS-2 as a more viable, better alternative to doxorubicin in the methyl thiazolyl tetrazolium assay. Conclusion: This combination of studies identifies new and more active analogs of existing FDA-approved drugs for the treatment of diabetes.

Targeting mycobacterial membranes and membrane proteins: Progress and limitations

Among the various bacterial infections, tuberculosis continues to hold center stage. Its causative agent, Mycobacterium tuberculosis, possesses robust defense mechanisms against most front-line antibiotic drugs and host responses due to their complex cell membranes with unique lipid molecules. It is now well-established that bacteria change their membrane composition to optimize their environment to survive and elude drug action. Thus targeting membrane or membrane components is a promising avenue for exploiting the chemical space focussed on developing novel membrane-centric anti-bacterial small molecules. These approaches are more effective, non-toxic, and can attenuate resistance phenotype. We present the relevance of targeting the mycobacterial membrane as a practical therapeutic approach. The review highlights the direct and indirect targeting of membrane structure and function. Direct membrane targeting agents cause perturbation in the membrane potential and can cause leakage of the cytoplasmic contents. In contrast, indirect membrane targeting agents disrupt the function of membrane-associated proteins involved in cell wall biosynthesis or energy production. We discuss the chronological chemical improvements in various scaffolds targeting specific membrane-associated protein targets, their clinical evaluation, and up-to-date account of their ''mechanisms of action, potency, selectivity'' and limitations. The sources of anti-TB drugs/inhibitors discussed in this work have emerged from target-based identification, cell-based phenotypic screening, drug repurposing, and natural products. We believe this review will inspire the exploration of uncharted chemical space for informing the development of new scaffolds that can inhibit novel mycobacterial membrane targets.

The evolving biology of Mycobacterium tuberculosis drug resistance

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb) is an ancient disease that has remained a leading cause of infectious death. Mtb has evolved drug resistance to every antibiotic regimen ever introduced, greatly complicating treatment, lowering rates of cure and menacing TB control in parts of the world. As technology has advanced, our understanding of antimicrobial resistance has improved, and our models of the phenomenon have evolved. In this review, we focus on recent research progress that supports an updated model for the evolution of drug resistance in Mtb. We highlight the contribution of drug tolerance on the path to resistance, and the influence of heterogeneity on tolerance. Resistance is likely to remain an issue for as long as drugs are needed to treat TB. However, with technology driving new insights and careful management of newly developed resources, antimicrobial resistance need not continue to threaten global progress against TB, as it has done for decades.

Cosolvent Sites-Based Discovery of Mycobacterium Tuberculosis Protein Kinase G Inhibitors

Computer-aided drug discovery methods play a major role in the development of therapeutically important small molecules, but their performance needs to be improved. Molecular dynamics simulations in mixed solvents are useful in understanding protein–ligand recognition and improving molecular docking predictions. In this work, we used ethanol as a cosolvent to find relevant interactions for ligands toward protein kinase G, an essential protein of Mycobacterium tuberculosis (Mtb). We validated the hot spots by screening a database of fragment-like compounds and another one of known kinase inhibitors. Next, we performed a pharmacophore-guided docking simulation and found three low micromolar inhibitors, including one with a novel chemical scaffold that we expanded to four derivative compounds. Binding affinities were characterized by intrinsic fluorescence quenching assays, isothermal titration calorimetry, and the analysis of melting curves. The predicted binding mode was confirmed by X-ray crystallography. Finally, the compounds significantly inhibited the viability of Mtb in infected THP-1 macrophages.

Bacterial F-type ATP synthases follow a well-choreographed assembly pathway

F-type ATP synthases are multiprotein complexes composed of two separate coupled motors (F₁ and F_O) generating adenosine triphosphate (ATP) as the universal major energy source in a variety of relevant biological processes in mitochondria, bacteria and chloroplasts. While the structure of many ATPases is solved today, the precise assembly pathway of F₁F_O-ATP synthases is still largely unclear. Here, we probe the assembly of the F₁ complex from Acetobacterium woodii. Using laser induced liquid bead ion desorption (LILBID) mass spectrometry, we study the self-assembly of purified F₁ subunits in different environments under non-denaturing conditions. We report assembly requirements and identify important assembly intermediates in vitro and in cellula. Our data provide evidence that nucleotide binding is crucial for in vitro F₁ assembly, whereas ATP hydrolysis appears to be less critical. We correlate our results with activity measurements and propose a model for the assembly pathway of a functional F₁ complex.

ATPases are the macromolecular machines for cellular energy production. Here the authors investigate factors that govern the assembly of the F₁ complex from a bacterial F-type ATPase and relate differences in activity of complexes assembled in cells and in vitro to structural changes.

Bedaquiline: Current status and future perspectives

The development of drug-resistant tuberculosis (TB) is a major threat worldwide. Based on World Health Organization (WHO) reports, it is estimated that more than 500 000 new cases of drug-resistant TB occur annually. In addition, there are alarming reports of increasing multidrug-resistant TB (MDR-TB) and the emergence of extensively drug-resistant TB (XDR-TB) from different countries of the world. Therefore, new options for TB therapy are required. Bedaquiline (BDQ), a novel anti-TB drug, has significant minimum inhibitory concentrations (MICs) both against drug-susceptible and drug-resistant TB. Moreover, BDQ was recently approved for therapy of MDR-TB. The current narrative review summarises the available data on BDQ resistance, describes its antimicrobial properties, and provides new perspectives on clinical use of this novel anti-TB agent.

Bedaquiline and delamanid for drug-resistant tuberculosis: a clinician's perspective

Drug-resistant tuberculosis (TB) represents a substantial threat to the global efforts to control this disease. After decades of stagnation, the treatment of drug-resistant TB is undergoing major changes: two drugs with a new mechanism of action, bedaquiline and delamanid, have been approved by stringent regulatory authorities and are recommended by the WHO. This narrative review summarizes the evidence, originating from both observational studies and clinical trials, which is available to support the use of these drugs, with a focus on special populations. Areas of uncertainty, including the use of the two drugs together or for prolonged duration, are discussed. Ongoing clinical trials are aiming to optimize the use of bedaquiline and delamanid to shorten the treatment of drug-resistant TB.

Design, synthesis and biological evaluation of novel triaryldimethylaminobutan-2-ol derivatives against Mycobacterium tuberculosis

Bedaquiline (TMC207), a typical diarylquinoline anti-tuberculosis drug, has been approved by FDA to specifically treat MDR-TB. Herein we describe design, synthesis, and in vitro biological evaluation against Mycobacterium tuberculosis of a series of triaryldimethylaminobutan-2-ol derivatives obtaining from the structural modification of TMC207. Compounds 23, 25, 28, 32, 39 and 43 provided superior anti-mycobacterial activity than positive control PC01 which shows the same configuration and contains TMC207. Compounds 16, 20, 29, 34, 37, 45 and 47 exhibited the similar activity to positive control PC01. Most importantly, the series of compounds showed excellent activity against XDR-Mtb. The result of acute toxicity suggested that this class of triaryldimethylaminobutan-2-ol derivatives should be graded as low. Further SAR analysis indicates that a large steric bulk of triaryl and 7-Br, 3-OCH₃ on 1-naphthyl are critical.

Molecule Property Analyses of Active Compounds for Mycobacterium tuberculosis

Tuberculosis (TB) continues to claim the lives of around 1.7 million people per year. Most concerning are the reports of multidrug drug resistance. Paradoxically, this global health pandemic is demanding new therapies when resources and interest are waning. However, continued tuberculosis drug discovery is critical to address the global health need and burgeoning multidrug resistance. Many diverse classes of antitubercular compounds have been identified with activity in vitro and in vivo. Our analyses of over 100 active leads are representative of thousands of active compounds generated over the past decade, suggests that they come from few chemical classes or natural product sources. We are therefore repeatedly identifying compounds that are similar to those that preceded them. Our molecule-centered cheminformatics analyses point to the need to dramatically increase the diversity of chemical libraries tested and get outside of the historic Mtb property space if we are to generate novel improved antitubercular leads.

Synthetic approaches towards bedaquiline and its derivatives

Bedaquiline is a diarylquinoline drug that demonstrates potent and selective inhibition of mycobacterial ATP synthase, and is clinically administered for the treatment of multi-drug resistant tuberculosis. Due to its excellent activity and novel mechanism of action, bedaquiline has been the focus of a number of synthetic studies. This review will discuss these synthetic approaches, as well as the synthesis and bioactivity of the numerous derivatives and molecular probes inspired by bedaquiline.

Whole Genome Sequencing for the Analysis of Drug Resistant Strains of Mycobacterium tuberculosis: A Systematic Review for Bedaquiline and Delamanid

Tuberculosis (TB) remains the deadliest Infectious disease worldwide, partially due to the increasing dissemination of multidrug and extensively drug-resistant (MDR/XDR) strains. Drug regimens containing the new anti-TB drugs bedaquiline (BDQ) and delamanid (DLM) appear as a last resort for the treatment of MDR or XDR-TB. Unfortunately, resistant cases to these drugs emerged just one year after their introduction in clinical practice. Early detection of resistant strains to BDQ and DLM is crucial to preserving the effectiveness of these drugs. Here, we present a systematic review aiming to define all available genotypic variants linked to different levels of resistance to BDQ and DLM that have been described through whole genomic sequencing (WGS) and the available drug susceptibility testing methods. During the review, we performed a thorough analysis of 18 articles. BDQ resistance was associated with genetic variants in Rv0678 and atpE, while mutations in pepQ were linked to a low-level of resistance for BDQ. For DLM, mutations in the genes ddn, fgd1, fbiA, and fbiC were found in phenotypically resistant cases, while all the mutations in fbiB were reported only in DLM-susceptible strains. Additionally, WGS analysis allowed the detection of heteroresistance to both drugs. In conclusion, we present a comprehensive panel of gene mutations linked to different levels of drug resistance to BDQ and DLM.

Synthesis of 2-methoxy-3-(thiophen-2-ylmethyl)quinoline containing amino carbinols as antitubercular agents

We have designed and synthesized 2-methoxy-3-(thiophen-2-ylmethyl)quinoline containing amino carbinols as possible anti-tubercular agents to combat the disease. These molecules were synthesized by tethering amino ether linkage with hydroxyl group to diarylquinoline skeleton; hydroxyl and amine chains were engrafted on diaryl ring. They were evaluated against strain (H₃₇R_a) of Mycobacterium tuberculosis and most of compounds showed in vitro antitubercular activity. Two compounds having diaryl quinoline hydroxyl amino ether scaffold and three compounds having diaryl amino alkyl carbinol core showed activities at 6.25 μg/mL. This study explores diaryl carbinol prototype as inhibitor against Mycobacterium tuberculosis.

Reduced susceptibility and resistance to bedaquiline in clinical M. tuberculosis isolates

OBJECTIVES

Bedaquiline is an effective drug used to treat MDR and XDR tuberculosis, providing high cure rates in complex therapy. Mutations in the mmpR (rv0678) and atpE genes are associated with reduced susceptibility to bedaquiline and have been identified in both in vitro selected strains and clinical isolates. However, the phenotypic criteria used to detect bedaquiline resistance have yet to be established due to the collection of few clinical isolates from patients receiving bedaquiline-containing treatment regimens.

METHODS

One hundred eighty-two clinical isolates from 74 patients receiving bedaquiline and 163 isolates from 107 patients not exposed to bedaquiline were analysed. The bedaquiline MICs were tested using serial dilutions on 7H11 agar plates and the Bactec MGIT 960 system. The mmpR and atpE genes were sequenced by Sanger sequencing.

RESULTS

The 7H11 agar method allowed for rapid discrimination between mutated and wild-type isolates and between exposed and non-exposed isolates. Seventy-three percent of bedaquiline-exposed isolates, as well as 91% of isolates with mutations, had an elevated bedaquiline MIC (≥ 0.12 mg/L on 7H11 media) compared to the reference isolates (89% had an MIC ≤ 0.03 mg/L). Previously reported in vitro-selected mutants (E61D and A63P) and novel AtpE substitutions (G25S and D28G) were observed in the clinical isolates. Substitutions in codon 63 of AtpE were likely associated with a higher bedaquiline MIC. Five new cases of pre-existing reduced susceptibility to bedaquiline, accompanied by mmpR mutations in most isolates, without a history of bedaquiline treatment were identified.

CONCLUSIONS

Bedaquiline treatment leads to an elevated bedaquiline MIC and the acquisition of mmpR and atpE gene mutations in tuberculosis strains. The standardisation of bedaquiline phenotypic susceptibility testing is urgently needed based on observed discrepancies between our study and previous studies and differences in solid and liquid media MIC determinations.

Development and validation of an LC-MS/MS method for the simultaneous determination of bedaquiline and rifabutin in human plasma

This article describes the simultaneous determination of bedaquiline fumarate (TMC-207) and rifabutin in human plasma by stable isotope dilution tandem mass spectrometry. The methodology was developed for an investigation of potential drug-drug interactions of the two anti-tuberculosis drugs when given together to healthy human volunteers. Use of the two drugs in combination to treat disease caused by Mycobacterium tuberculosis is contemplated as the bacterium becomes resistant to many currently available drugs.

Screening of Antitubercular Compound Library Identifies Inhibitors of Mur Enzymes in Mycobacterium tuberculosis

The rapid rise in the emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains of Mycobacterium tuberculosis (Mtb) mandates the discovery of novel tuberculosis (TB) drugs. Mur enzymes, which are identified as essential proteins in Mtb and catalyze the cytoplasmic steps in the peptidoglycan biosynthetic pathway, are considered potential drug targets. However, none of the clinical drugs have yet been developed against these enzymes. Hence, the aim of this study was to identify novel inhibitors of Mur enzymes in Mycobacterium tuberculosis. We screened an antitubercular compound library of 684 compounds, using MurB and MurE enzymes of the Mtb Mur pathway as drug targets. For experimental validation, the top hits obtained on in silico screening were screened in vitro, using Mtb Mur enzyme-specific assays. In all, seven compounds were found to show greater than 50% inhibition, with the highest inhibition observed at 77%, and the IC₅₀ for these compounds was found to be in the range of 28-50 μM. Compound 5175112 showed the lowest IC₅₀ (28.69 ± 1.17 μM), and on the basis of (1) the binding affinity, (2) the stability of interaction noted on molecular dynamics simulation, and (3) an in vitro assay, MurE appeared to be its target enzyme. We believe that the overall strategy followed in this study and the results obtained are a good starting point for developing Mur enzyme-specific Mtb inhibitors.

Drug-resistance in Mycobacterium tuberculosis: where we stand

Tuberculosis (TB), an infectious disease caused by the bacterium Mycobacterium tuberculosis (Mtb), has burdened vulnerable populations in modern day societies for decades. Recently, this global health threat has been heightened by the emergence and propagation of multi drug-resistant (MDR) and extensively drug-resistant (XDR) strains of Mtb that are resistant to current treatment regimens. The End-TB strategy, launched by the World Health Organization (WHO), aims to reduce TB-related deaths by 90%. This program encourages universal access to drug susceptibility testing, which is not widely available owing to the lack of laboratory capacity or resources in certain under-resourced areas. Clinical assays are further complicated by the slow growth of Mtb, resulting in the long turn-around time of tests which severely limits their application in guiding a patient's treatment regimen. This review provides a comprehensive overview of current TB treatments, mechanisms of resistance to anti-tubercular drugs and their diagnosis and the current pipeline of drugs targeting drug-resistant TB (DR-TB) with particular attention paid to ways in which drug-resistance is combated.

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.

Synthesis and Study of New Quinolineaminoethanols as Anti-Bacterial Drugs

The lack of antibiotics with a novel mode of action associated with the spread of drug resistant bacteria make the fight against infectious diseases particularly challenging. A quinoline core is found in several anti-infectious drugs, such as mefloquine and bedaquiline. Two main objectives were set in this work. Firstly, we evaluated the anti-mycobacterial properties of the previous quinolines 3, which have been identified as good candidates against ESKAPEE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp. and Escherichia coli) bacteria. Secondly, a new series 4 was designed and assessed against the same bacteria strains, taking the pair of enantiomers 3m/3n as the lead. More than twenty compounds 4 were prepared through a five-step asymmetric synthesis with good enantiomeric excesses (>90%). Interestingly, all compounds of series 3 were efficient on M. avium with MIC = 2-16 µg/mL, while series 4 was less active. Both series 3 and 4 were generally more active than mefloquine against the ESKAPEE bacteria. The quinolines 4 were either active against Gram-positive bacteria (MIC ≤ 4 µg/mL for 4c-4h and 4k/4l) or E. coli (MIC = 32-64 µg/mL for 4q-4v) according to the global lipophilicity of these compounds.

Halting ionic shuttle to disrupt the synthetic machinery-Structural and molecular insights into the inhibitory roles of Bedaquiline towards Mycobacterium tuberculosis ATP synthase in the treatment of tuberculosis

Therapeutic targeting of the adenosine triphosphate (ATP) machinery of Mycobacterium tuberculosis (Mtb) has recently presented a potent and alternative measure to halt the pathogenesis of tuberculosis. This has been potentiated by the development of bedaquiline (BDQ), a novel small molecule inhibitor that selectively inhibits mycobacterial F₁ F_o -ATP synthase by targeting its rotor c-ring, resulting in the disruption of ATP synthesis and consequential cell death. Although the structural resolution of the mycobacterial C₉ ring in co`mplex with BDQ provided the first-hand detail of BDQ interaction at the c-ring region of the ATP synthase, there still remains a need to obtain essential and dynamic insights into the mechanistic activity of this drug molecule towards crucial survival machinery of Mtb. As such, for the first time, we report an atomistic model to describe the structural dynamics that explicate the experimentally reported antagonistic features of BDQ in halting ion shuttling by the mycobacterial c-ring, using molecular dynamics simulation and the Molecular Mechanics/Poisson-Boltzmann Surface Area methods. Results showed that BDQ exhibited a considerably high ΔG while it specifically maintained high-affinity interactions with Glu65_B and Asp32_B , blocking their crucial roles in proton binding and shuttling, which is required for ATP synthesis. Moreover, the bulky nature of BDQ induced a rigid and compact conformation of the rotor c-ring, which impedes the essential rotatory motion that drives ion exchange and shuttling. In addition, the binding affinity of a BDQ molecule was considerably increased by the complementary binding of another BDQ molecule, which indicates that an increase in BDQ molecule enhances inhibitory potency against Mtb ATP synthase. Taken together, findings provide atomistic perspectives into the inhibitory mechanisms of BDQ coupled with insights that could enhance the structure-based design of novel ATP synthase inhibitors towards the treatment of tuberculosis.

Challenging the Drug-Likeness Dogma for New Drug Discovery in Tuberculosis

The emergence of multi- and extensively drug resistant tuberculosis worldwide poses a great threat to human health and highlight the need to discover and develop new, effective and inexpensive antituberculosis agents. High-throughput screening assays against well-validated drug targets and structure based drug design have been employed to discover new lead compounds. However, the great majority fail to demonstrate any antimycobacterial activity when tested against Mycobacterium tuberculosis in whole-cell screening assays. This is mainly due to some of the intrinsic properties of the bacilli, such as the extremely low permeability of its cell wall, slow growth, drug resistance, drug tolerance, and persistence. In this sense, understanding the pathways involved in M. tuberculosis drug tolerance, persistence, and pathogenesis, may reveal new approaches for drug development. Moreover, the need for compounds presenting a novel mode of action is of utmost importance due to the emergence of resistance not only to the currently used antituberculosis agents, but also to those in the pipeline. Cheminformatics studies have shown that drugs endowed with antituberculosis activity have the peculiarity of being more lipophilic than many other antibacterials, likely because this leads to improved cell penetration through the extremely waxy mycobacterial cell wall. Moreover, the interaction of the lipophilic moiety with the membrane alters its stability and functional integrity due to the disruption of the proton motive force, resulting in cell death. When a ligand-based medicinal chemistry campaign is ongoing, it is always difficult to predict whether a chemical modification or a functional group would be suitable for improving the activity. Nevertheless, in the “instruction manual” of medicinal chemists, certain functional groups or certain physicochemical characteristics (i.e., high lipophilicity) are considered red flags to look out for in order to safeguard drug-likeness and avoid attritions in the drug discovery process. In this review, we describe how antituberculosis compounds challenge established rules such as the Lipinski's “rule of five” and how medicinal chemistry for antituberculosis compounds must be thought beyond such dogmatic schemes.

A Probabilistic Framework for Constructing Temporal Relations in Replica Exchange Molecular Trajectories

Knowledge of the structure and dynamics of biomolecules is essential for elucidating the underlying mechanisms of biological processes. Given the stochastic nature of many biological processes, like protein unfolding, it is almost impossible that two independent simulations will generate the exact same sequence of events, which makes direct analysis of simulations difficult. Statistical models like Markov chains, transition networks, etc. help in shedding some light on the mechanistic nature of such processes by predicting long-time dynamics of these systems from short simulations. However, such methods fall short in analyzing trajectories with partial or no temporal information, for example, replica exchange molecular dynamics or Monte Carlo simulations. In this work, we propose a probabilistic algorithm, borrowing concepts from graph theory and machine learning, to extract reactive pathways from molecular trajectories in the absence of temporal data. A suitable vector representation was chosen to represent each frame in the macromolecular trajectory (as a series of interaction and conformational energies), and dimensionality reduction was performed using principal component analysis (PCA). The trajectory was then clustered using a density-based clustering algorithm, where each cluster represents a metastable state on the potential energy surface (PES) of the biomolecule under study. A graph was created with these clusters as nodes with the edges learned using an iterative expectation maximization algorithm. The most reactive path is conceived as the widest path along this graph. We have tested our method on RNA hairpin unfolding trajectory in aqueous urea solution. Our method makes the understanding of the mechanism of unfolding in the RNA hairpin molecule more tractable. As this method does not rely on temporal data, it can be used to analyze trajectories from Monte Carlo sampling techniques and replica exchange molecular dynamics (REMD).

Tomatidine Is a Lead Antibiotic Molecule That Targets Staphylococcus aureus ATP Synthase Subunit C

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of deadly hospital-acquired infections. The discovery of anti-Staphylococcus antibiotics and new classes of drugs not susceptible to the mechanisms of resistance shared among bacteria is imperative. We recently showed that tomatidine (TO), a steroidal alkaloid from solanaceous plants, possesses potent antibacterial activity against S. aureus small-colony variants (SCVs), the notoriously persistent form of this bacterium that has been associated with recurrence of infections. Here, using genomic analysis of in vitro-generated TO-resistant S. aureus strains to identify mutations in genes involved in resistance, we identified the bacterial ATP synthase as the cellular target. Sequence alignments were performed to highlight the modified sequences, and the structural consequences of the mutations were evaluated in structural models. Overexpression of the atpE gene in S. aureus SCVs or introducing the mutation found in the atpE gene of one of the high-level TO-resistant S. aureus mutants into the Bacillus subtilis atpE gene provided resistance to TO and further validated the identity of the cellular target. FC04-100, a TO derivative which also possesses activity against non-SCV strains, prevents high-level resistance development in prototypic strains and limits the level of resistance observed in SCVs. An ATP synthesis assay allowed the observation of a correlation between antibiotic potency and ATP synthase inhibition. The selectivity index (inhibition of ATP production by mitochondria versus that of bacterial ATP synthase) is estimated to be >10⁵-fold for FC04-100.

Insights into the regulatory function of the ɛ subunit from bacterial F-type ATP synthases: a comparison of structural, biochemical and biophysical data

ATP synthases catalyse the formation of ATP, the most common chemical energy storage unit found in living cells. These enzymes are driven by an electrochemical ion gradient, which allows the catalytic evolution of ATP by a binding change mechanism. Most ATP synthases are capable of catalysing ATP hydrolysis to varying degrees, and to prevent wasteful ATP hydrolysis, bacteria and mitochondria have regulatory mechanisms such as ADP inhibition. Additionally, ɛ subunit inhibition has also been described in three bacterial systems, Escherichia coli, Bacillus PS3 and Caldalkalibacillus thermarum TA2.A1. Previous studies suggest that the ɛ subunit is capable of undergoing an ATP-dependent conformational change from the ATP hydrolytic inhibitory 'extended' conformation to the ATP-induced non-inhibitory 'hairpin' conformation. A recently published crystal structure of the F1 domain of the C. thermarum TA2.A1 F1Fo ATP synthase revealed a mutant ɛ subunit lacking the ability to bind ATP in a hairpin conformation. This is a surprising observation considering it is an organism that performs no ATP hydrolysis in vivo, and appears to challenge the current dogma on the regulatory role of the ɛ subunit. This has prompted a re-examination of present knowledge of the ɛ subunits role in different organisms. Here, we compare published biochemical, biophysical and structural data involving ɛ subunit-mediated ATP hydrolysis regulation in a variety of organisms, concluding that the ɛ subunit from the bacterial F-type ATP synthases is indeed capable of regulating ATP hydrolysis activity in a wide variety of bacteria, making it a potentially valuable drug target, but its exact role is still under debate.

A Therapeutic Connection between Dietary Phytochemicals and ATP Synthase

For centuries, phytochemicals have been used to prevent and cure multiple health ailments. Phytochemicals have been reported to have antioxidant, antidiabetic, antitussive, antiparasitic, anticancer, and antimicrobial properties. Generally, the therapeutic use of phy-tochemicals is based on tradition or word of mouth with few evidence-based studies. Moreo-ver, molecular level interactions or molecular targets for the majority of phytochemicals are unknown. In recent years, antibiotic resistance by microbes has become a major healthcare concern. As such, the use of phytochemicals with antimicrobial properties has become perti-nent. Natural compounds from plants, vegetables, herbs, and spices with strong antimicrobial properties present an excellent opportunity for preventing and combating antibiotic resistant microbial infections. ATP synthase is the fundamental means of cellular energy. Inhibition of ATP synthase may deprive cells of required energy leading to cell death, and a variety of die-tary phytochemicals are known to inhibit ATP synthase. Structural modifications of phyto-chemicals have been shown to increase the inhibitory potency and extent of inhibition. Site-directed mutagenic analysis has elucidated the binding site(s) for some phytochemicals on ATP synthase. Amino acid variations in and around the phytochemical binding sites can re-sult in selective binding and inhibition of microbial ATP synthase. In this review, the therapeu-tic connection between dietary phytochemicals and ATP synthase is summarized based on the inhibition of ATP synthase by dietary phytochemicals. Research suggests selective target-ing of ATP synthase is a valuable alternative molecular level approach to combat antibiotic resistant microbial infections.

Bedaquiline, an FDA-approved antibiotic, inhibits mitochondrial function and potently blocks the proliferative expansion of stem-like cancer cells (CSCs)

Bedaquiline (a.k.a., Sirturo) is an anti-microbial agent, which is approved by the FDA for the treatment of multi-drug resistant pulmonary tuberculosis (TB). Bedaquiline is a first-in-class diaryl-quinoline compound, that mechanistically inhibits the bacterial ATP-synthase, and shows potent activity against both drug-sensitive and drug-resistant TB. Interestingly, eukaryotic mitochondria originally evolved from engulfed aerobic bacteria. Thus, we hypothesized that, in mammalian cells, bedaquiline might also target the mitochondrial ATP-synthase, leading to mitochondrial dysfunction and ATP depletion. Here, we show that bedaquiline has anti-cancer activity, directed against Cancer Stem-like Cells (CSCs). More specifically, we demonstrate that bedaquiline treatment of MCF7 breast cancer cells inhibits mitochondrial oxygen-consumption, as well as glycolysis, but induces oxidative stress. Importantly, bedaquiline significantly blocks the propagation and expansion of MCF7-derived CSCs, with an IC-50 of approx. 1-μM, as determined using the mammosphere assay. Similarly, bedaquiline also reduces both the CD44+/CD24low/− CSC and ALDH+ CSC populations, under anchorage-independent growth conditions. In striking contrast, bedaquiline significantly increases oxygen consumption in normal human fibroblasts, consistent with the fact that it is well-tolerated in patients treated for TB infections. As such, future pre-clinical studies and human clinical trials in cancer patients may be warranted. Interestingly, we also highlight that bedaquiline shares certain structural similarities with trans-piceatannol and trans-resveratrol, which are known natural flavonoid inhibitors of the mitochondrial ATP-synthase (complex V) and show anti-aging properties.

Type-II NADH Dehydrogenase (NDH-2): a promising therapeutic target for antitubercular and antibacterial drug discovery

INTRODUCTION

Tuberculosis (TB) is highly dangerous due to the development of resistance to first-line drugs. Moreover, Mycobacterium tuberculosis (Mtb) has also developed resistance to newly approved antitubercular drug bedaquiline. This necessitates the search for drugs acting on newer molecular targets. The energy metabolism of mycobacteria is the prime focus for the discovery of novel antitubercular drugs. Targeting type-2 NADH dehydrogenase (NDH-2) involved in the production of respiratory ATP could, therefore, be effective in treating the disease. Areas covered: This review describes the energetics of mycobacteria and the role of NDH-2 in ATP synthesis. Special attention has been given for genetic and chemical validations of NDH-2 as a molecular target. The reported kinetics and crystal structures of NDH-2 have been given in detail for better understanding of the enzyme. Expert opinion: NDH-2 is an essential enzyme for ATP synthesis and has a potential role in dormancy and persistence of Mtb. The human counterpart lacks this enzyme and hence NDH-2 inhibitors could have more clinical importance. Phenothiazines are potent inhibitor of NDH-2 and are effective against both drug-susceptible and drug-resistant Mtb. Thus, it is highly desirable to optimize phenothiazine class of compounds for the development of next generation anti-TB drugs.

Emerging drugs and drug targets against tuberculosis

Mycobacterium tuberculosis (M. tuberculosis), the causative agent of tuberculosis, uses various tactics to resist on antibiotics and evade host immunity. To control tuberculosis, antibiotics with novel mechanisms of action are urgently needed. Emerging new antibiotics and underlying novel drug targets are summarized in this paper.

Bedaquiline Targets the ε Subunit of Mycobacterial F-ATP Synthase

The tuberculosis drug bedaquiline inhibits mycobacterial F-ATP synthase by binding to its c subunit. Using the purified ε subunit of the synthase and spectroscopy, we previously demonstrated that the drug interacts with this protein near its unique tryptophan residue. Here, we show that replacement of ε's tryptophan with alanine resulted in bedaquiline hypersusceptibility of the bacteria. Overexpression of the wild-type ε subunit caused resistance. These results suggest that the drug also targets the ε subunit.

Mechanisms of action and therapeutic efficacies of the lipophilic antimycobacterial agents clofazimine and bedaquiline

Drug-resistant (DR)-TB is the major challenge confronting the global TB control programme, necessitating treatment with second-line anti-TB drugs, often with limited therapeutic efficacy. This scenario has resulted in the inclusion of Group 5 antibiotics in various therapeutic regimens, two of which promise to impact significantly on the outcome of the therapy of DR-TB. These are the 're-purposed' riminophenazine, clofazimine, and the recently approved diarylquinoline, bedaquiline. Although they differ structurally, both of these lipophilic agents possess cationic amphiphilic properties that enable them to target and inactivate essential ion transporters in the outer membrane of Mycobacterium tuberculosis. In the case of bedaquiline, the primary target is the key respiratory chain enzyme F₁/F₀-ATPase, whereas clofazimine is less selective, apparently inhibiting several targets, which may underpin the extremely low level of resistance to this agent. This review is focused on similarities and differences between clofazimine and bedaquiline, specifically in respect of molecular mechanisms of antimycobacterial action, targeting of quiescent and metabolically active organisms, therapeutic efficacy in the clinical setting of DR-TB, resistance mechanisms, pharmacodynamics, pharmacokinetics and adverse events.

Design and Stereochemical Research (DFT, ECD and Crystal Structure) of Novel Bedaquiline Analogs as Potent Antituberculosis Agents

A series of bedaquiline analogs containing H-bond donors were designed as anti-Mycobacterium tuberculosis drugs. A pair of diastereoisomers (R/S- and S/S-isomers) was selected from these designed compounds for synthetic and stereochemical research. The title compounds were synthesized from chiral precursors for the first time and the absolute configurations (ACs) were determined by electronic circular dichroism (ECD) with quantum chemical calculations. Moreover, a single crystal of the S/S compound was obtained for X-ray diffraction analysis, and the crystal structure showed high consistency with the geometry, confirming the reliability of ACs obtained by ECD analyses and theoretical simulation. Furthermore, the effect of stereochemistry on the anti-tuberculosis activity was investigated. The MICs of the R/S- and S/S-isomers against Mycobacterium phlei 1180 are 9.6 and 32.1 μg·mL⁻¹, respectively. Finally, molecular docking was carried out to evaluate the inhibitory nature and binding mode differences between diastereoisomers.

Insights into the pharmacokinetic properties of antitubercular drugs

INTRODUCTION

The furiously advancing cases of multidrug-resistant tuberculosis (TB) along with the recent emergence of total drug resistant TB and TB-AIDS comorbidity present an increased threat to global public health. Knowledge of pharmacokinetic properties helps in selecting an appropriate anti-TB dosage regimen to achieve optimal results in patients.

AREAS COVERED

This article provides a brief compilation of the information available regarding published pharmacokinetic data for anti-TB drugs and may act as a single window for investigators/medical practitioners in this field. The information regarding absorption, tissue distribution, elimination and pharmacokinetic interactions of the first- and second-line anti-TB drugs and candidate drugs under clinical trials is discussed.

EXPERT OPINION

Pharmacokinetic properties such as poor absorption, too short biological half-life, extensive first-pass metabolism, drug-food and drug-drug related interactions are not attractive for prospective anti-TB drugs and significantly contribute to treatment failure and further resistance. The long duration, monotonous and multidrug treatment plan leads to poor patient compliance and resulted in a greater occurrence of anti-TB drug resistance worldwide. Few new agents, which are in development phase, are considering the aspect of shortening duration of the treatment regimen and provide a boost in therapy that is sorely needed.

Energetics of Respiration and Oxidative Phosphorylation in Mycobacteria

Mycobacteria inhabit a wide range of intracellular and extracellular environments. Many of these environments are highly dynamic and therefore mycobacteria are faced with the constant challenge of redirecting their metabolic activity to be commensurate with either replicative growth or a non-replicative quiescence. A fundamental feature in this adaptation is the ability of mycobacteria to respire, regenerate reducing equivalents and generate ATP via oxidative phosphorylation. Mycobacteria harbor multiple primary dehydrogenases to fuel the electron transport chain and two terminal respiratory oxidases, an aa3 -type cytochrome c oxidase and cytochrome bd-type menaquinol oxidase, are present for dioxygen reduction coupled to the generation of a protonmotive force. Hypoxia leads to the downregulation of key respiratory complexes, but the molecular mechanisms regulating this expression are unknown. Despite being obligate aerobes, mycobacteria have the ability to metabolize in the absence of oxygen and a number of reductases are present to facilitate the turnover of reducing equivalents under these conditions (e.g. nitrate reductase, succinate dehydrogenase/fumarate reductase). Hydrogenases and ferredoxins are also present in the genomes of mycobacteria suggesting the ability of these bacteria to adapt to an anaerobic-type of metabolism in the absence of oxygen. ATP synthesis by the membrane-bound F1FO-ATP synthase is essential for growing and non-growing mycobacteria and the enzyme is able to function over a wide range of protonmotive force values (aerobic to hypoxic). The discovery of lead compounds that target respiration and oxidative phosphorylation in Mycobacterium tuberculosis highlights the importance of this area for the generation of new front line drugs to combat tuberculosis.

Scaffold morphing leading to evolution of 2,4-diaminoquinolines and aminopyrazolopyrimidines as inhibitors of the ATP synthesis pathway

2,4-Diaminoquinazolines, 2,4-diaminoquinolines and aminopyrazolopyrimidines, inhibitors of mycobacterial ATP synthesis, are novel lead molecules towards discovery and development of new anti-tubercular agents.

Mycobacterium tuberculosis histidinol dehydrogenase: biochemical characterization and inhibition studies

We describe a series of biochemical studies on recombinantMycobacterium tuberculosisHisD (MtHisD) and the synthesis of a series of compounds which are, to the best of our knowledge, the first inhibitors ofMtHisD activity reported in the literature.

Asymmetric Synthesis and Absolute Configuration Assignment of a New Type of Bedaquiline Analogue

Bedaquiline is the first FDA-approved new chemical entity to fight multidrug-resistant tuberculosis in the last forty years. Our group replaced the quinoline ring with a naphthalene ring, leading to a new type of triarylbutanol skeleton. An asymmetric synthetic route was established for our bedaquiline analogues, and the goal of assigning their absolute configurations was achieved by comparison of experimental and calculated electronic circular dichroism spectra, and was confirmed by the combined use of circular dichroism and NMR spectroscopy.

Nanoparticle Uptake: The Phagocyte Problem

Phagocytes are key cellular participants determining important aspects of host exposure to nanomaterials, initiating clearance, biodistribution and the tenuous balance between host tolerance and adverse nanotoxicity. Macrophages in particular are believed to be among the first and primary cell types that process nanoparticles, mediating host inflammatory and immunological biological responses. These processes occur ubiquitously throughout tissues where nanomaterials are present, including the host mononuclear phagocytic system (MPS) residents in dedicated host filtration organs (i.e., liver, kidney spleen, and lung). Thus, to understand nanomaterials exposure risks it is critical to understand how nanomaterials are recognized, internalized, trafficked and distributed within diverse types of host macrophages and how possible cell-based reactions resulting from nanomaterial exposures further inflammatory host responses in vivo. This review focuses on describing macrophage-based initiation of downstream hallmark immunological and inflammatory processes resulting from phagocyte exposure to and internalization of nanomaterials.

Bedaquiline for the treatment of multidrug-resistant tuberculosis: great promise or disappointment?

Acquired drug resistance by Mycobacterium tuberculosis (MTB) may result in treatment failure and death. Bedaquiline was recently approved for the treatment of multidrug-resistant tuberculosis (MDR-TB). This report examines the available data on this novel drug for the treatment of MDR-TB. PubMed searches, last updated 18 February 2015, using the terms bedaquiline, TMC 207 and R207910 identified pertinent English citations. Citation reference lists were reviewed to identify other relevant reports. Pertinent MDR-TB treatment reports on the US Food and Drug Administration, Centers for Disease Control and Prevention (CDC), World Health Organization (WHO) and Cochrane websites were also evaluated. Bedaquiline is an adenosine triphosphate (ATP) synthase inhibitor specific for MTB and some nontuberculous mycobacteria. The early bactericidal activity (EBA) of bedaquiline is delayed until ATP stores are depleted but subsequently it is similar to the EBA of isoniazid and rifampin. Bedaquiline demonstrated excellent minimum inhibitory concentrations (MICs) against both drug-sensitive and MDR-TB. Adding it to the WHO-recommended MDR-TB regimen reduced the time for sputum culture conversion in pulmonary MDR-TB. Rifampin, other cytochrome oxidase 3A4 inducers or inhibitors alter its metabolism. Adverse effects are common with MDR-TB treatment regimens with or without bedaquiline. Nausea is more common with bedaquiline and it increases the QTcF interval. It is not recommended for children, pregnant or lactating women. More patients died in the bedaquiline-treatment arms despite better microbiological outcomes in two recent trials. The WHO and CDC published interim guidelines that recommend restricting its use to patients with MDR-TB or more complex drug resistance who cannot otherwise be treated with a minimum of three effective drugs. It should never be added to a regimen as a single drug nor should it be added to a failing regimen to prevent the emergence of bedaquiline-resistant strains.

A comparative effectiveness analysis of treatment for latent tuberculosis infection using multilevel selection models

AIM

Nine months of isoniazid (9INH) is the gold standard for treatment of latent tuberculosis infection (LTBI). This paper compares the effectiveness of 9 months of isoniazid with 4 months of transitional rifampin (9H4R) to alternative therapies, including 9INH, 6 months of isoniazid (6INH) and 6 months of isoniazid with 4 months of transitional rifampin (6H4R), for treatment of LTBI.

MATERIALS & METHODS

Using an ethnically diverse clinic sample of 552 patients given treatment for LTBI with 9H4R, we use multilevel selection models to examine the adjusted comparative effectiveness of the regimens among ethnic groups that feature distinct genetic predispositions to side effects on INH. For unadjusted/absolute effectiveness, we simulated cost-effectiveness ratios for 4 months of rifampin (4RIF) and compared with bootstrapped confidence intervals for the alternative therapies.

RESULTS

There are variations in the comparative effectiveness across ethnic groups, with the most notable differences for 9H4R. For unadjusted/absolute effectiveness, 4RIF presents the greatest net benefit for US born black and African patients. For all other ethnic groups, 6H4R was the most effective.

CONCLUSION

Patient ethnicity affects tolerance to INH. 9H4R was the most effective LTBI treatment for all ethnicities. However, this result heavily depends on whether adjustments are made for self-selection.

Structure of the mycobacterial ATP synthase Fo rotor ring in complex with the anti-TB drug bedaquiline

Multidrug-resistant tuberculosis (MDR-TB) is more prevalent today than at any other time in human history. Bedaquiline (BDQ), a novel Mycobacterium-specific adenosine triphosphate (ATP) synthase inhibitor, is the first drug in the last 40 years to be approved for the treatment of MDR-TB. This bactericidal compound targets the membrane-embedded rotor (c-ring) of the mycobacterial ATP synthase, a key metabolic enzyme required for ATP generation. We report the x-ray crystal structures of a mycobacterial c9 ring without and with BDQ bound at 1.55- and 1.7-Å resolution, respectively. The structures and supporting functional assays reveal how BDQ specifically interacts with the rotor ring via numerous interactions and thereby completely covers the c-ring's ion-binding sites. This prevents the rotor ring from acting as an ion shuttle and stalls ATP synthase operation. The structures explain how diarylquinoline chemicals specifically inhibit the mycobacterial ATP synthase and thus enable structure-based drug design of next-generation ATP synthase inhibitors against Mycobacterium tuberculosis and other bacterial pathogens.

Bactericidal mode of action of bedaquiline

OBJECTIVES

It is not fully understood why inhibiting ATP synthesis in Mycobacterium species leads to death in non-replicating cells. We investigated the bactericidal mode of action of the anti-tubercular F1Fo-ATP synthase inhibitor bedaquiline (Sirturo™) in order to further understand the lethality of ATP synthase inhibition.

METHODS

Mycobacterium smegmatis strains were used for all the experiments. Growth and survival during a bedaquiline challenge were performed in multiple media types. A time-course microarray was performed during initial bedaquiline challenge in minimal medium. Oxygen consumption and proton-motive force measurements were performed on whole cells and inverted membrane vesicles, respectively.

RESULTS

A killing of 3 log10 cfu/mL was achieved 4-fold more quickly in minimal medium (a glycerol carbon source) versus rich medium (LB with Tween 80) during bedaquiline challenge. Assessing the accelerated killing condition, we identified a transcriptional remodelling of metabolism that was consistent with respiratory dysfunction but inconsistent with ATP depletion. In glycerol-energized cell suspensions, bedaquiline caused an immediate 2.3-fold increase in oxygen consumption. Bedaquiline collapsed the transmembrane pH gradient, but not the membrane potential, in a dose-dependent manner. Both these effects were dependent on binding to the F1Fo-ATP synthase.

CONCLUSIONS

Challenge with bedaquiline results in an electroneutral uncoupling of respiration-driven ATP synthesis. This may be a determinant of the bactericidal effects of bedaquiline, while ATP depletion may be a determinant of its delayed onset of killing. We propose that bedaquiline binds to and perturbs the a-c subunit interface of the Fo, leading to futile proton cycling, which is known to be lethal to mycobacteria.

Novel, potent, orally bioavailable and selective mycobacterial ATP synthase inhibitors that demonstrated activity against both replicating and non-replicating M. tuberculosis

The mycobacterial F0F1-ATP synthase (ATPase) is a validated target for the development of tuberculosis (TB) therapeutics. Therefore, a series of eighteen novel compounds has been designed, synthesized and evaluated against Mycobacterium smegmatis ATPase. The observed ATPase inhibitory activities (IC50) of these compounds range between 0.36 and 5.45μM. The lead compound 9d [N-(7-chloro-2-methylquinolin-4-yl)-N-(3-((diethylamino)methyl)-4-hydroxyphenyl)-2,3-dichlorobenzenesulfonamide] with null cytotoxicity (CC50>300μg/mL) and excellent anti-mycobacterial activity and selectivity (mycobacterium ATPase IC50=0.51μM, mammalian ATPase IC50>100μM, and selectivity >200) exhibited a complete growth inhibition of replicating Mycobacterium tuberculosis H37Rv at 3.12μg/mL. In addition, it also exhibited bactericidal effect (approximately 2.4log10 reductions in CFU) in the hypoxic culture of non-replicating M. tuberculosis at 100μg/mL (32-fold of its MIC) as compared to positive control isoniazid [approximately 0.2log10 reduction in CFU at 5μg/mL (50-fold of its MIC)]. The pharmacokinetics of 9d after p.o. and IV administration in male Sprague-Dawley rats indicated its quick absorption, distribution and slow elimination. It exhibited a high volume of distribution (Vss, 0.41L/kg), moderate clearance (0.06L/h/kg), long half-life (4.2h) and low absolute bioavailability (1.72%). In the murine model system of chronic TB, 9d showed 2.12log10 reductions in CFU in both lung and spleen at 173μmol/kg dose as compared to the growth of untreated control group of Balb/C male mice infected with replicating M. tuberculosis H37Rv. The in vivo efficacy of 9d is at least double of the control drug ethambutol. These results suggest 9d as a promising candidate molecule for further preclinical evaluation against resistant TB strains.

S. AK, Meena G, Sethi KP, Sharma R, Trivedi P, Khan SR, Verma AS, Singh S, Sharma S, Roy KK, Kant R, Krishnan MY, Singh BN, Sinha S, Chaturvedi V, Saxena AK, Dikshit DK. Synthesis and anti-tubercular activity of conformationally-constrained and bisquinoline analogs of TMC207

Conformationally-constrained and bisquinoline analogs of TMC207 as antitubercular agents.

Energetics of pathogenic bacteria and opportunities for drug development

The emergence and spread of drug-resistant pathogens and our inability to develop new antimicrobials to overcome resistance has inspired scientists to consider new targets for drug development. Cellular bioenergetics is an area showing promise for the development of new antimicrobials, particularly in the discovery of new anti-tuberculosis drugs where several new compounds have entered clinical trials. In this review, we have examined the bioenergetics of various bacterial pathogens, highlighting the versatility of electron donor and acceptor utilisation and the modularity of electron transport chain components in bacteria. In addition to re-examining classical concepts, we explore new literature that reveals the intricacies of pathogen energetics, for example, how Salmonella enterica and Campylobacter jejuni exploit host and microbiota to derive powerful electron donors and sinks; the strategies Mycobacterium tuberculosis and Pseudomonas aeruginosa use to persist in lung tissues; and the importance of sodium energetics and electron bifurcation in the chemiosmotic anaerobe Fusobacterium nucleatum. A combination of physiological, biochemical, and pharmacological data suggests that, in addition to the clinically-approved target F1Fo-ATP synthase, NADH dehydrogenase type II, succinate dehydrogenase, hydrogenase, cytochrome bd oxidase, and menaquinone biosynthesis pathways are particularly promising next-generation drug targets. The realisation of cellular energetics as a rich target space for the development of new antimicrobials will be dependent upon gaining increased understanding of the energetic processes utilised by pathogens in host environments and the ability to design bacterial-specific inhibitors of these processes.

SAR analysis of new anti-TB drugs currently in pre-clinical and clinical development

Despite enormous efforts have been made in the hunt for new drugs, tuberculosis (TB) still remains the first bacterial cause of mortality worldwide, causing an estimated 8.6 million new cases and 1.3 million deaths in 2012. Multi-drug resistant-TB strains no longer respond to first-line drugs and are inexorably spreading with an estimated 650,000 cases as well as extensively-drug resistant-TB strains, which are resistant to any fluoroquinolone and at least one of the second-line drugs, with 60,000 cases. Thus the discovery and development of new medicines is a major keystone for tuberculosis treatment and control. After decades of dormancy in the field of TB drug development, recent efforts from various groups have generated a promising TB drug pipeline. Several new therapeutic agents are concurrently studied in clinical trials together with much activity in the hittolead and lead optimization stages. In this article we will review the recent advances in TB drug discovery with a special focus on structure activity relationship studies of the most advanced compound classes.