New anti-tuberculosis drugs in clinical trials with novel mechanisms of action

Mycobacterium tuberculosis: immune response, biomarkers, and therapeutic intervention

Although tuberculosis (TB) is an infectious disease, the progression of the disease following Mycobacterium tuberculosis (MTB) infection is closely associated with the host's immune response. In this review, a comprehensive analysis of TB prevention, diagnosis, and treatment was conducted from an immunological perspective. First, we delved into the host's immune response mechanisms against MTB infection as well as the immune evasion mechanisms of the bacteria. Addressing the challenges currently faced in TB diagnosis and treatment, we also emphasized the importance of protein, genetic, and immunological biomarkers, aiming to provide new insights for early and personalized diagnosis and treatment of TB. Building upon this foundation, we further discussed intervention strategies involving chemical and immunological treatments for the increasingly critical issue of drug-resistant TB and other forms of TB. Finally, we summarized TB prevention, diagnosis, and treatment challenges and put forward future perspectives. Overall, these findings provide valuable insights into the immunological aspects of TB and offer new directions toward achieving the WHO's goal of eradicating TB by 2035.

Oxazoline/amide derivatives against M. tuberculosis: experimental, biological and computational investigations

Tuberculosis (TB) is a treatable contagious disease that continuously kills approximately 2 million people yearly. Different oxazoline/amide derivatives were synthesized, and their anti-tuberculosis activity was performed against different strains of Mtb. This study designed the anti-Mtb compounds based on amide and oxazoline, two different structural moieties. The compounds were further synthesized and characterized by spectral techniques. Their anti-Tb activity was evaluated against strain (M. tuberculosis: H37Rv). Selectivity and binding affinity of all synthesized compounds (2a-2e, 3a-3e) against PanK in Mtb were investigated through molecular docking. Molecular dynamics simulation studies for the promising compounds 2d and 3e were performed for 100 ns. The stability of these complexes was assessed by calculating the root mean square deviation, solvent-accessible surface area, and gyration radius relative to their parent structures. Additionally, free energy of binding calculations were performed. Among all synthesized compounds, 2d and 3e had comparable antitubercular activity against standard drug, validated by their computational and biological study.

Natural products and their analogues acting against Mycobacterium tuberculosis: A recent update

Tuberculosis (TB) remains one of the deadliest infectious diseases caused by Mycobacterium tuberculosis (M.tb). It is responsible for significant causes of mortality and morbidity worldwide. M.tb possesses robust defense mechanisms against most antibiotic drugs and host responses due to their complex cell membranes with unique lipid molecules. Thus, the efficacy of existing front-line drugs is diminishing, and new and recurring cases of TB arising from multidrug-resistant M.tb are increasing. TB begs the scientific community to explore novel therapeutic avenues. A precise knowledge of the compounds with their mode of action could aid in developing new anti-TB agents that can kill latent and actively multiplying M.tb. This can help in the shortening of the anti-TB regimen and can improve the outcome of treatment strategies. Natural products have contributed several antibiotics for TB treatment. The sources of anti-TB drugs/inhibitors discussed in this work are target-based identification/cell-based and phenotypic screening from natural products. Some of the recently identified natural products derived leads have reached clinical stages of TB drug development, which include rifapentine, CPZEN-45, spectinamide-1599 and 1810. We believe these anti-TB agents could emerge as superior therapeutic compounds to treat TB over known Food and Drug Administration drugs.

Structure-Based Design of Promysalin Analogues to Overcome Mechanisms of Bacterial Resistance

The search for antibiotics that function through novel mechanisms of action is ongoing, and recent progress in our lab identified the tricarboxylic acid cycle as a viable option. Promysalin is a secondary metabolite capable of species-specific inhibition of Pseudomonas aeruginosa, a common opportunistic pathogen. Promysalin disrupts primary metabolism in this bacterium by competitively inhibiting succinate dehydrogenase at the ubiquinone binding site. However, the activity of promysalin in cellulo is marred potentially by its chemical instability and/or propensity for efflux. To assess the success of these novel analogues, a novel strain of P. aeruginosa harboring gene deletions of eight efflux pumps and porins was developed and implemented. Herein, we disclose the synthesis and biological investigation of six promysalin analogues to overcome these liabilities and demonstrate that efflux likely plays a significant role in tolerating the effect of the inhibitor.

Indole: A promising scaffold for the discovery and development of potential anti-tubercular agents

Indole-containing small molecules have been reported to have diverse pharmacological activities. The aromatic heterocyclic scaffold, which resembles various protein structures, has received attention from organic and medicinal chemists. Exploration of indole derivatives in drug discovery has rapidly yielded a vast array of biologically active compounds with broad therapeutic potential. Nature is the major source of indole scaffolds, but various classical and advanced synthesis methods for indoles have also been reported. One-pot synthesis is widely considered an efficient approach in synthetic organic chemistry and has been used to synthesize some indole compounds. The rapid emergence of drug-resistant tuberculosis is a major challenge to be addressed. Identifying novel targets and drug candidates for tuberculosis is therefore crucial. Researchers have extensively explored indole derivatives as potential anti-tubercular agents or drugs. Indole scaffolds containing the novel non-covalent (decaprenylphosphoryl-β-D-ribose2'-epimerase) DprE1 inhibitor 1,4-azaindole is currently in clinical trials to treat Mycobacterium tuberculosis. In addition, DG167 indazole sulfonamide with potent anti-tubercular activity is undergoing early-stage development in preclinical studies. Indole bearing cationic amphiphiles with high chemical diversity have been reported to depolarize and disrupt the mycobacterial membrane. Some indole-based compounds have potential inhibitory activities against distinct anti-tubercular targets, including the inhibition of cell wall synthesis, replication, transcription, and translation, as summarized in the graphical abstract. The success of computer-aided drug design in the fields of cancer and anti-viral drugs has accelerated in silico studies in antibacterial drug development. This review describes the sources of indole scaffolds, the potential for novel indole derivatives to serve as anti-tubercular agents, in silico findings, and proposed actions to facilitate the design of novel compounds with anti-tubercular activity.

Oxadiazole: A highly versatile scaffold in drug discovery

As a pharmacologically important heterocycle, oxadiazole paved the way to combat the problem associated with the confluence of many commercially available drugs with different pharmacological profiles. The present review focuses on the potential applications of five-membered heterocyclic oxadiazole derivatives, especially 1,2,4-oxadiazole, 1,2,5-oxadiazole, and 1,3,4-oxadiazole, as therapeutic agents. Designing new hybrid molecules containing the oxadiazole moiety is a better solution for the development of new drug molecules. The designed molecules may accumulate a biological profile better than those of the drugs currently available on the market. The present review will guide the way for researchers in the field of medicinal chemistry to design new biologically active molecules based on the oxadiazole nucleus. Antitubercular, antimalarial, anti-inflammatory, anti-HIV, antibacterial, and anticancer activities of various oxadiazoles have been reviewed extensively here.

3-Aryl-substituted imidazo[1,2-a]pyridines as antituberculosis agents

Novel inhibitors are needed to tackle tuberculosis. Herein, we report the 3-aryl-substituted imidazo[1,2-a]pyridines as potent antituberculosis agents. A small library of 3-aryl-substituted imidazo[1,2-a]pyridines was synthesized using direct arylation, followed by nitro reduction and finally Pd-catalyzed C-N coupling reactions. The compounds thus obtained were evaluated against Mycobacterium tuberculosis H37Rv. Compound 26 was identified as an antituberculosis lead with a minimum inhibitory concentration of 2.3 μg/ml against M. tuberculosis H37Rv. This compound showed a selectivity index of 35. The docking of 26 in the active site of the M. tuberculosis cytochrome bc1 complex cytochrome b subunit (Mtb QcrB) revealed key π-π interactions of compound 26 with the Tyr389 and Trp312 residues of Mtb QcrB.

Revealing Antibiotic Tolerance of the Mycobacterium smegmatis Xanthine/Uracil Permease Mutant Using Microfluidics and Single-Cell Analysis

To reveal rare phenotypes in bacterial populations, conventional microbiology tools should be advanced to generate rapid, quantitative, accurate, and high-throughput data. The main drawbacks of widely used traditional methods for antibiotic studies include low sampling rate and averaging data for population measurements. To overcome these limitations, microfluidic-microscopy systems have great promise to produce quantitative single-cell data with high sampling rates. Using Mycobacterium smegmatis cells, we applied both conventional assays and a microfluidic-microscopy method to reveal the antibiotic tolerance mechanisms of wild-type and msm2570::Tn mutant cells. Our results revealed that the enhanced antibiotic tolerance mechanism of the msm2570::Tn mutant was due to the low number of lysed cells during the antibiotic exposure compared to wild-type cells. This is the first study to characterize the antibiotic tolerance phenotype of the msm2570::Tn mutant, which has a transposon insertion in the msm2570 gene—encoding a putative xanthine/uracil permease, which functions in the uptake of nitrogen compounds during nitrogen limitation. The experimental results indicate that the msm2570::Tn mutant can be further interrogated to reveal antibiotic killing mechanisms, in particular, antibiotics that target cell wall integrity.

The Mur Enzymes Chink in the Armour of Mycobacterium tuberculosis cell wall

TUBERCULOSIS: (TB) transmitted by Mycobacterium tuberculosis (Mtb) is one of the top 10 causes of death globally. Currently, the widespread occurrence of resistance toward Mtb strains is becoming a significant concern to public health. This scenario exaggerated the need for the discovery of novel targets and their inhibitors. Targeting the "Mtb cell wall peptidoglycan synthesis" is an attractive strategy to overcome drug resistance. Mur enzymes (MurA-MurF) play essential roles in the peptidoglycan synthesis by catalyzing the ligation of key amino acid residues to the stem peptide. These enzymes are unique and confined to the eubacteria and are absent in humans, representing potential targets for anti-tubercular drug discovery. Mtb Mur ligases with the same catalytic mechanism share conserved amino acid regions and structural features that can conceivably exploit for the designing of the inhibitors, which can simultaneously target more than one isoforms (MurC-MurF) of the enzyme. In light of these findings in the current review, we have discussed the recent advances in medicinal chemistry of Mtb Mur enzymes (MurA-MurF) and their inhibitors, offering attractive multi-targeted strategies to combat the problem of drug-resistant in M. tuberculosis.

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.

Oxidations of Benzhydrazide and Phenylacetic Hydrazide by Hexachloroiridate(IV): Reaction Mechanism and Structure-Reactivity Relationship

Benz(o)hydrazide (BH) is the basic aryl hydrazide; aryl hydrazides have been pursued in the course of drug discovery. Oxidations of BH and phenylacetic hydrazide (PAH) by hexachloroiridate(IV) ([IrCl₆]²⁻) were investigated by use of stopped-flow spectral, rapid spectral scan, RP-HPLC and NMR spectroscopic techniques. The oxidation reactions followed well-defined second-order kinetics and the observed second-order rate constant k′ versus pH profiles were established over a wide pH range. Product analysis revealed that BH and PAH were cleanly oxidized to benzoic acid and phenylacetic acid, respectively. A reaction mechanism was proposed, resembling those suggested previously for the oxidations of isoniazid (INH) and nicotinic hydrazide (NH) by [IrCl₆]²⁻. Rate constants of the rate-determining steps were evaluated, confirming a huge reactivity span of the protolysis species observed previously. The enolate species of BH is extremely reactive towards reduction of [IrCl₆]²⁻. The determined middle-ranged negative values of activation entropies together with rapid scan spectra manifest that an outer-sphere electron transfer is probably taking place in the rate-determining steps. The reactivity of neutral species of hydrazides is clearly not correlated to the corresponding pK_a values of the hydrazides. On the other hand, a linear correlation, logk_enolate = (0.16 ± 0.07)pK_enol + (6.1 ± 0.8), is found for the aryl hydrazides studied so far. The big intercept and the small slope of this correlation may pave a way for a rational design of new antioxidants based on aryl hydrazides. The present work also provides the pK_a values for BH and PAH at 25.0 °C and 1.0 M ionic strength which were not reported before.

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.

A review on anti-tuberculosis peptides: Impact of peptide structure on anti-tuberculosis activity

Antibiotic resistance is a major public health problem globally. Particularly concerning amongst drug-resistant human pathogens is Mycobacterium tuberculosis that causes the deadly infectious tuberculosis (TB) disease. Significant issues associated with current treatment options for drug-resistant TB and the high rate of mortality from the disease makes the development of novel treatment options against this pathogen an urgent need. Antimicrobial peptides are part of innate immunity in all forms of life and could provide a potential solution against drug-resistant TB. This review is a critical analysis of antimicrobial peptides that are reported to be active against the M tuberculosis complex exclusively. However, activity on non-TB strains such as Mycobacterium avium and Mycobacterium intracellulare, whenever available, have been included at appropriate sections for these anti-TB peptides. Natural and synthetic antimicrobial peptides of diverse sequences, along with their chemical structures, are presented, discussed, and correlated to their observed antimycobacterial activities. Critical analyses of the structure allied to the anti-mycobacterial activity have allowed us to draw important conclusions and ideas for research and development on these promising molecules to realise their full potential. Even though the review is focussed on peptides, we have briefly summarised the structures and potency of the various small molecule drugs that are available and under development, for TB treatment.

Discovery of napabucasin derivatives for the treatment of tuberculosis

Tuberculosis is the contagious disease responsible for the highest number of deaths worldwide. Here, we screened a commercially available compound library and found napabucasin to possess a moderate anti-tubercular activity against M. tuberculosis H37Ra (MIC 2.5 μg mL-1, 10.4 μM). Three series of napabucasin derivatives were further evaluated for their in vitro anti-tubercular activities against Mtb H37Ra. The activity of most derivatives was either retained or enhanced compared with that of napabucasin. Compound 3s was the most active compound showing a MIC value of 0.3125 μg mL-1 (0.9 μM). Furthermore, several compounds were selected and evaluated against the Mtb H37Rv standard strain and six Mtb clinical isolates. Importantly, these compounds were found to be effective against Mtb clinical isolates with multi-resistance to isoniazid, rifampicin, and ethambutol.

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.

Biophysical characterization and ligand-binding properties of the elongation factor Tu from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the key devastating bacterial pathogen responsible for tuberculosis. Increasing emergence of multi-drug-resistant, extensively drug-resistant, and rifampicin/isoniazid-resistant strains of Mtb makes the discovery of validated drug targets an urgent priority. As a vital translational component of the protein biosynthesis system, elongation factor Tu (EF-Tu) is an important molecular switch responsible for selection and binding of the cognate aminoacyl-tRNA to the acceptor site on the ribosome. In addition, EF-Tu from Mtb (MtbEF-Tu) is involved in the initial step of trans-translation which is an effective system for rescuing the stalled ribosomes from non-stop translation complexes under stress conditions. Given its crucial role in protein biosynthesis, EF-Tu is identified as an excellent molecular target for drug design. Here, we reported the recombinant expression, purification, biophysical characterization, and structural modeling of the MtbEF-Tu protein. Our results demonstrated that prokaryotic expression plasmids of pET28a-MtbEF-Tu could be expressed efficiently in Escherichia coli. We successfully purified the 6× His-tagged proteins with a yield of 16.8 mg from 1 l of Luria Bertani medium. Dynamic light scattering experiments showed that MtbEF-Tu existed in a monomeric form, and circular dichroism experiments indicated that MtbEF-Tu was well structured. Moreover, isothermal titration calorimetry experiments displayed that the purified MtbEF-Tu protein possessed intermediate binding affinities for guanosine-5'-triphosphate (GTP) and GDP. The GTP/GDP-binding sites were predicted by flexible molecular docking approach which reveals that GTP/GDP binds to MtbEF-Tu mainly through hydrogen bonds. Our work lays the essential basis for further structural and functional studies of MtbEF-Tu as well as MtbEF-Tu-related novel drug developments.

Microbes, not humans: exploring the molecular basis of Pseudouridimycin selectivity towards bacterial and not human RNA polymerase

OBJECTIVE

Bacterial RNA polymerase (bRNAP) represent a crucial target for curtailing microbial activity but its structural and sequence similarities with human RNA polymerase II (hRNAPII) makes it difficult to target. Recently, Pseudouridimycin (PUM), a novel nucleoside analogue was reported to selectively inhibit bRNAP and not hRNAP. Till date, underlying mechanisms of PUM selectivity remains unresolved, hence the aim of this study.

RESULTS

Using sequence alignment method, we observed that the β' of bRNAP and the RPB1 subunits of hRNAPII were highly conserved while the β and RPB2 subunits of both proteins were also characterized by high sequence variations. Furthermore, the impact of these variations on the differential binding of PUM was evaluated using MMPB/SA binding free energy and per-residue decomposition analysis. These revealed that PUM binds better to bRNAP than hRNAP with prominent bRNAP active site residues that contributed the most to PUM binding and stabilization lacking in hRNAPII active site due to positional substitution. Also, the binding of PUM to hRNAP was characterized by the formation of unfavorable interactions. In addition, PUM assumed favorable orientations that possibly enhanced its mobility towards the hydrophobic core region of bRNAP. On the contrary, unfavorable intramolecular interactions characterize PUM orientations at the binding site of hRNAPII, which could restrict its movement due to electrostatic repulsions.

CONCLUSION

These findings would enhance the design of potent and selective drugs for broad-spectrum antimicrobial activity.

Kinetics and mechanism of oxidation of the anti-tubercular prodrug isoniazid and its analog by iridium(iv) as models for biological redox systems

A complex reaction mechanism of oxidation of the anti-tubercular prodrug isoniazid (isonicotinic hydrazide, INH) by [IrCl₆]^2- as a model for redox processes of such drugs in biological systems has been studied in aqueous solution as a function of pH between 0 and 8.5. Similar experiments have been performed with its isomer nicotinic hydrazide (NH). All reactions are overall second-order, first-order in [IrCl₆]^2- and hydrazide, and the observed second-order rate constants k' have been determined as a function of pH. Spectrophotometric titrations indicate a stoichiometry of [Ir(iv)] : [hydrazide] = 4 : 1. HPLC analysis shows that the oxidation product of INH is isonicotinic acid. The derived reaction mechanism, based on rate law, time-resolved spectra and stoichiometry, involves parallel attacks by [IrCl₆]^2- on all four protolytic species of INH and NH as rate-determining steps, depending on pH. These steps are proposed to generate two types of hydrazyl free radicals. These radicals react further in three rapid consecutive processes, leading to the final oxidation products. Rate constants for the rate-determining steps have been determined for all protolytic species I-IV of INH and NH. They are used to calculate reactivity-pH diagrams. These diagrams demonstrate that for both systems, species IV is ca. 10⁵ times more reactive in the redox process than the predominant species III at the physiological pH of 7.4. Thus, species IV will be the main reactant, in spite of the fact that its concentration at this pH is extremely low, a fact that has not been considered in previous work. The results indicate that pH changes might be an important factor in the activation process of INH in biological systems also, and that in such systems this process most likely is more complicated than previously assumed.

Effect of the alkyl group in the piperazine N-substitution on the therapeutic action of rifamycins: A drug-membrane interaction study

In this work, we studied the effects of the N-alkyl group (methyl, cyclopentyl) in the piperazine ring of, respectively, rifampicin (RIF) and rifapentine (RPT) to correlate this substitution with their differential pharmacokinetic properties and overall clinical performance. Since this group is their only structural change, and given that they share the same pharmacological target, differences in their therapeutic behavior may respond to this asset, particularly in their interaction with lipid membranes across the organism. In this study, surface pressure-area isotherms, as well as spectroscopic and microscopic techniques of characterization of phospholipid monolayers at the air/water interface were used to gain insight into drug-membrane interactions. Differences in the affinity for lipid membranes for both drugs, given by the vibration frequency of characteristic chemical groups in the lipid, as well as by reflectivity and mean molecular area of the monolayer, seem to be due to the N-alkyl substituent and can contribute to provide a molecular explanation as why they pose different choices in the chemotherapy against the deadliest infectious disease, tuberculosis.

Crystal structure of benzoato-κO-bis(1,3,5-triaza-7-phosphaadamantane-κP)silver(I) monohydrate C19H31AgN6O3P2

C19H31AgN6O3P2, monoclinic, P21/c (no. 14), a = 16.5661(8) Å, b = 6.1644(3) Å, c = 23.8822(10) Å, β = 114.553(3)°, V = 2218.32(18) Å3, Z = 4, R gt(F) = 0.0188, wR ref(F 2) = 0.0525, T = 100(2) K.

The Drug Discovery and Development Industry in India-Two Decades of Proprietary Small-Molecule R&D

This review provides a comprehensive survey of proprietary drug discovery and development efforts performed by Indian companies between 1994 and mid-2016. It is based on the identification and detailed analysis of pharmaceutical, biotechnology, and contract research companies active in proprietary new chemical entity (NCE) research and development (R&D) in India. Information on preclinical and clinical development compounds was collected by company, therapeutic indication, mode of action, target class, and development status. The analysis focuses on the overall pipeline and its evolution over two decades, contributions by type of company, therapeutic focus, attrition rates, and contribution to Western pharmaceutical pipelines through licensing agreements. This comprehensive analysis is the first of its kind, and, in our view, represents a significant contribution to the understanding of the current state of the drug discovery and development industry in India.

New drugs for the treatment of Mycobacterium tuberculosis infection

Tuberculosis presents a grave challenge to health, globally instigating 1.5 million mortalities each year. Following the breakthrough of first-line anti-TB medication, the number of mortalities reduced greatly; nonetheless, the swift appearance of tuberculosis which was drug-resistant, as well as the capability of the bacterium to survive and stay dormant are a considerable problem for public health. In order to address this issue, several novel possible candidates for tuberculosis therapy have been subjected to clinical trials of late. The novel antimycobacterial agents are acquired from different categories of medications, operate through a range of action systems, and are at various phases of advancement. We therefore talk about the present methods of treating tuberculosis and novel anti-TB agents with their action method, in order to advance awareness of these new compounds and medications.

New Insights in to the Intrinsic and Acquired Drug Resistance Mechanisms in Mycobacteria

Infectious diseases caused by clinically important Mycobacteria continue to be an important public health problem worldwide primarily due to emergence of drug resistance crisis. In recent years, the control of tuberculosis (TB), the disease caused by Mycobacterium tuberculosis (MTB), is hampered by the emergence of multidrug resistance (MDR), defined as resistance to at least isoniazid (INH) and rifampicin (RIF), two key drugs in the treatment of the disease. Despite the availability of curative anti-TB therapy, inappropriate and inadequate treatment has allowed MTB to acquire resistance to the most important anti-TB drugs. Likewise, for most mycobacteria other than MTB, the outcome of drug treatment is poor and is likely related to the high levels of antibiotic resistance. Thus, a better knowledge of the underlying mechanisms of drug resistance in mycobacteria could aid not only to select the best therapeutic options but also to develop novel drugs that can overwhelm the existing resistance mechanisms. In this article, we review the distinctive mechanisms of antibiotic resistance in mycobacteria.

Recent advancements in the development of anti-tuberculosis drugs

Modern chemotherapy has significantly improved patient outcomes against drug-sensitive tuberculosis. However, the rapid emergence of drug-resistant tuberculosis, together with the bacterium's ability to persist and remain latent present a major public health challenge. To overcome this problem, research into novel anti-tuberculosis targets and drug candidates is thus of paramount importance. This review article provides an overview of tuberculosis highlighting the recent advances and tools that are employed in the field of anti-tuberculosis drug discovery. The predominant focus is on anti-tuberculosis agents that are currently in the pipeline, i.e. clinical trials.

New antimycobacterial agents in the pre-clinical phase or beyond: recent advances in patent literature (2001-2016)

INTRODUCTION

Tuberculosis, an infectious disease, has caused more deaths worldwide than any other single infectious disease, killing more than 1.5 million people each year; equating to 4,100 deaths a day. In the past 60 years, no new drugs have been added to the first line regimen, in spite of the fact that thousands of papers have been published on drugs against tuberculosis and hundreds of drugs have received patents as new potential products. Thus, there is undoubtedly an urgent need for the deployment of new effective drugs against tuberculosis. Areas covered: This review brings to the reader the opportunity to understand the chemical and biological characteristics of all patented anti-tuberculosis drugs in North America, Europe, Japan, and Russia. The 116 patents discussed here concern new molecules in the early or advanced phase of development in the last 16 years. Expert opinion: Of all 116 patents, only one developed drug, bedaquiline, is used, and then, only in specific cases. Another three drugs are in clinical studies. However, many other compounds, for which there are in vitro and in vivo studies, seem to fulfil the requisite criteria to be a new anti-tuberculosis agent. However, why are they not in use? Why were so many studies interrupted? Why is there no more news for many of these drugs?

Combating Tuberculosis Infection: A Forbidding Challenge

After 50 years drought, several drugs are looming from the pipeline to combat tuberculosis. They will serve as a boon to the field that has been burdened with primitive, inadequate treatments and drug-resistant bacterial strains. From the decades, due to lack of interest and resources, the field has suffered a lot. Learning from the flaws, scientists have renovated their approaches to the finding of new antitubercular drugs. The first line drugs take about six months or more for the entire treatment. The second line remedy for resistant-tuberculosis requires daily injections which carry severe side effects. Drug resistance remains a constant menace because patients stop the medication once they start feeling better. So new drugs are required to be explored which are effective against tuberculosis especially drug resistant tuberculosis. These drugs need to work well with other drugs as well as with antivirals used for the treatment of human immunodeficiency virus. It is also very important to be considered that the treatments need to be cheap, as tuberculosis primarily affects people more in the developing countries. Further, new drugs must cure the disease in short span of time than the current six to nine month regimen. Recently a few new and potent drugs such as bedaquiline, delamanid, teixobactin have been evolved which may serve as a nice step forward, with a better outcome. Teixobactin, a new antibiotic has been found to have promising action against resistant strains, is also under consideration.

Recombinant preparation and functional studies of EspI ATP binding domain from Mycobacterium tuberculosis

The ESX-1 secretion system of Mycobacterium tuberculosis is required for the virulence of tubercle bacillus. EspI, the ESX-1 secretion-associated protein in Mycobacterium tuberculosis (MtEspI), is involved in repressing the activity of ESX-1-mediated secretion when the cellular ATP level is low. The ATP binding domain of MtEspI plays a crucial role in this regulatory process. However, further structural and functional studies of MtEspI are hindered due to the bottleneck of obtaining stable and pure recombinant protein. In this study, we systematically analyzed the structure and function of MtEspI using bioinformatics tools and tried various expression constructs to recombinantly express full-length and truncated MtEspI ATP binding domain. Finally, we prepared pure and stable MtEspI ATP binding domain, MtEspI415-493, in Escherichia coli by fusion expression and purification with dual tag, Glutathione S-transferase (GST) tag and (His)6 tag. (31)P NMR titration assay indicated that MtEspI415-493 possessed a moderate affinity (∼μM) for ATP and the residue K425 was located at the binding site. The protocol described here may provide a train of thought for recombinant preparation of other ESX-1 secretion-associated proteins.

Composing compound libraries for hit discovery--rationality-driven preselection or random choice by structural diversity?

AIMS

In order to identify new scaffolds for drug discovery, surface plasmon resonance is frequently used to screen structurally diverse libraries. Usually, hit rates are low and identification processes are time consuming. Hence, approaches which improve hit rates and, thus, reduce the library size are required.

METHODS

In this work, we studied three often used strategies for their applicability to identify inhibitors of PqsD. In two of them, target-specific aspects like inhibition of a homologous protein or predicted binding determined by virtual screening were used for compound preselection. Finally, a fragment library, covering a large chemical space, was screened and served as comparison.

RESULTS & CONCLUSION

Indeed, higher hit rates were observed for methods employing preselected libraries indicating that target-oriented compound selection provides a time-effective alternative.

Inhibition of glutamine synthetase: a potential drug target in Mycobacterium tuberculosis

Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis. Globally, tuberculosis is second only to AIDS in mortality and the disease is responsible for over 1.3 million deaths each year. The impractically long treatment schedules (generally 6-9 months) and unpleasant side effects of the current drugs often lead to poor patient compliance, which in turn has resulted in the emergence of multi-, extensively- and totally-drug resistant strains. The development of new classes of anti-tuberculosis drugs and new drug targets is of global importance, since attacking the bacterium using multiple strategies provides the best means to prevent resistance. This review presents an overview of the various strategies and compounds utilized to inhibit glutamine synthetase, a promising target for the development of drugs for TB therapy.

Rifampicin-loaded ‘flower-like’ polymeric micelles for enhanced oral bioavailability in an extemporaneous liquid fixed-dose combination with isoniazid

Background: Coadministration of rifampicin (RIF)/isoniazid (INH) is clinically recommended to improve the treatment of tuberculosis. Under gastric conditions, RIF undergoes fast hydrolysis (a pathway hastened by INH) and oral bioavailability loss. Aim: We aimed to assess the chemical stabilization and the oral pharmacokinetics of RIF nanoencapsulated within poly(ε-caprolactone)-b-PEG-b-poly(ε-caprolactone) ‘flower-like’ polymeric micelles. Materials & methods: The chemical stability of RIF was evaluated in vitro under acid conditions with and without INH, and the oral pharmacokinetics of RIF-loaded micelles in rats was compared with those of a suspension coded by the US Pharmacopeia. Results: Nanoencapsulation decreased the degradation rate of RIF with respect to the free drug. Moreover, in vivo data showed a statistically significant increase of RIF oral bioavailability (up to 3.3-times) with respect to the free drug in the presence of INH. Conclusion: Overall results highlight the potential of this nanotechnology platform to develop an extemporaneous liquid RIF/INH fixed-dose combination suitable for pediatric administration.

Original submitted 6 April 2013; Revised submitted 7 August 2013

A novel indigoid anti-tuberculosis agent

The structure of a novel indigoid component was characterized by X-ray crystallography. This compound exhibited excellent anti-tuberculosis activity against Mycobacterium tuberculosis H37Rv in whole cell culture showing a submicromolar minimum inhibitory concentration (MIC). A synthesis of this molecule was designed and carried out to produce sufficient material for further testing. The in vitro profile, structure, and first synthesis of this indigoid component is reported.

Isolation and evaluation of the enantiospecific antitubercular activity of a novel triazole compound

Cyclohex-3-enyl(5-phenyl-4H-1,2,4-triazol-3-yl)methanol (MSDRT 12) is a novel triazole-based antitubercular compound with two chiral centers. To evaluate the enantiospecific antitubercular activity, the four stereoisomers were isolated using preparative chiral chromatography and the individual stereoisomers were evaluated using the resazurin microtiter assay method (REMA) and a microbroth dilution technique against the Mycobacterium tuberculosis H37Rv strain. Isomer III of MSDRT 12 was found to be the most potent with a minimum inhibitory concentration (MIC) of 0.78 μg/mL, Isomer II had a MIC of 12.5 μg/mL, and isomers I and IV showed no activity. The diastereomeric mixture of MSDRT 12 showed a MIC of 3.125 μg/mL and isoniazid, used as the standard drug, showed a MIC of 0.4 μg/mL. This confirms the necessity of screening individual enantiomers for their pharmacological activity early in the discovery phase to identify the most potent isomer for further development efforts.