A microbiological assessment of novel nitrofuranylamides as anti-tuberculosis agents

Novel quinoline derivatives with broad-spectrum antiprotozoal activities

Several quinoline derivatives incorporating arylnitro and aminochalcone moieties were synthesized and evaluated in vitro against a broad panel of trypanosomatid protozoan parasites responsible for sleeping sickness (Trypanosoma brucei rhodesiense), nagana (Trypanosoma brucei brucei), Chagas disease (Trypanosoma cruzi), and leishmaniasis (Leishmania infantum). Several of the compounds demonstrated significant antiprotozoal activity. Specifically, compounds 2c, 2d, and 4i displayed submicromolar activity against T. b. rhodesiense with half-maximal effective concentration (EC50) values of 0.68, 0.8, and 0.19 µM, respectively, and with a high selectivity relative to human lung fibroblasts and mouse primary macrophages (∼100-fold). Compounds 2d and 4i also showed considerable activity against T. b. brucei with EC50 values of 1.4 and 0.4 µM, respectively.

Evaluation of novel compounds as anti-bacterial or anti-virulence agents

Antimicrobial resistance is a global threat, leading to an alarming increase in the prevalence of bacterial infections that can no longer be treated with available antibiotics. The World Health Organization estimates that by 2050 up to 10 million deaths per year could be associated with antimicrobial resistance, which would equal the annual number of cancer deaths worldwide. To overcome this emerging crisis, novel anti-bacterial compounds are urgently needed. There are two possible approaches in the fight against bacterial infections: a) targeting structures within bacterial cells, similar to existing antibiotics; and/or b) targeting virulence factors rather than bacterial growth. Here, for the first time, we provide a comprehensive overview of the key steps in the evaluation of potential new anti-bacterial and/or anti-virulence compounds. The methods described in this review include: a) in silico methods for the evaluation of novel compounds; b) anti-bacterial assays (MIC, MBC, Time-kill); b) anti-virulence assays (anti-biofilm, anti-quorum sensing, anti-adhesion); and c) evaluation of safety aspects (cytotoxicity assay and Ames test). Overall, we provide a detailed description of the methods that are an essential tool for chemists, computational chemists, microbiologists, and toxicologists in the evaluation of potential novel antimicrobial compounds. These methods are cost-effective and have high predictive value. They are widely used in preclinical studies to identify new molecular candidates, for further investigation in animal and human trials.

Designing molecular diagnostics for current tuberculosis drug regimens

Diagnostic development must occur in parallel with drug development to ensure the longevity of new treatment compounds. Despite an increasing number of novel and repurposed anti-tuberculosis compounds and regimens, there remains a large number of drugs for which no rapid and accurate molecular diagnostic option exists. The lack of rapid drug susceptibility testing for linezolid, bedaquiline, clofazimine, the nitroimidazoles (i.e pretomanid and delamanid) and pyrazinamide at any level of the healthcare system compromises the effectiveness of current tuberculosis and drug-resistant tuberculosis treatment regimens. In the context of current WHO tuberculosis treatment guidelines as well as promising new regimens, we identify the key diagnostic gaps for initial and follow-on tests to diagnose emerging drug resistance and aid in regimen selection. Additionally, we comment on potential gene targets for inclusion in rapid molecular drug susceptibility assays and sequencing assays for novel and repurposed drug compounds currently prioritized in current regimens, and evaluate the feasibility of mutation detection given the design of existing technologies. Based on current knowledge, we also propose design priorities for next generation molecular assays to support triage of tuberculosis patients to appropriate and effective treatment regimens. We encourage assay developers to prioritize development of these key molecular assays and support the continued evolution, uptake, and utility of sequencing to build knowledge of tuberculosis resistance mechanisms and further inform rapid treatment decisions in order to curb resistance to critical drugs in current regimens and achieve End TB targets.Trial registration: ClinicalTrials.gov identifier: NCT05117788..

Pretomanid resistance: An update on emergence, mechanisms and relevance for clinical practice

Pretomanid (PA-824), a novel anti-tuberculosis (TB) nitroimidazoxazine, has been approved for multi-drug-resistant TB treatment for a few years. Pretomanid has been demonstrated to be highly active against Mycobacterium tuberculosis when combined with other anti-TB drugs. This review provides an update of the current knowledge on the modes of action, resistance mechanisms, emergence of drug resistance, and status of antimicrobial susceptibility testing for pretomanid and its relevance for clinical practice. Pretomanid resistance has been reported in in-vitro and animal models but not yet in clinical trials. Pretomanid-resistance-associated mutations have been reported in the fbiA, fbiB, fbiC, fbiD, ddn and fgd1 genes. However, understanding of in-vivo molecular resistance mechanisms remains limited, and complicates the development of accurate antimicrobial susceptibility testing methods for pretomanid. As such, no reference method for antimicrobial susceptibility testing of pretomanid has been established to guide clinical use. Further studies linking specific mutations, in-vitro susceptibility, drug exposure and resistance mechanisms to treatment failure with pretomanid should be prioritized.

Discovery and characterization of antimycobacterial nitro-containing compounds with distinct mechanisms of action and in vivo efficacy

Nitro-containing compounds have emerged as important agents in the control of tuberculosis (TB). From a whole-cell high-throughput screen for Mycobacterium tuberculosis (Mtb) growth inhibitors, 10 nitro-containing compounds were prioritized for characterization and mechanism of action studies. HC2209, HC2210, and HC2211 are nitrofuran-based prodrugs that need the cofactor F420 machinery for activation. Unlike pretomanid which depends only on deazaflavin-dependent nitroreductase (Ddn), these nitrofurans depend on Ddn and possibly another F420-dependent reductase for activation. These nitrofurans also differ from pretomanid in their potent activity against Mycobacterium abscessus. Four dinitrobenzamides (HC2217, HC2226, HC2238, and HC2239) and a nitrofuran (HC2250) are proposed to be inhibitors of decaprenyl-phosphoryl-ribose 2'-epimerase 1 (DprE1), based on isolation of resistant mutations in dprE1. Unlike other DprE1 inhibitors, HC2250 was found to be potent against non-replicating persistent bacteria, suggesting additional targets. Two of the compounds, HC2233 and HC2234, were found to have potent, sterilizing activity against replicating and non-replicating Mtb in vitro, but a proposed mechanism of action could not be defined. In a pilot in vivo efficacy study, HC2210 was orally bioavailable and efficacious in reducing bacterial load by ~1 log in a chronic murine TB infection model.

Arylnitro monocarbonyl curcumin analogues: Synthesis and in vitro antitubercular evaluation

Curcumin is a natural product that has been reported to exhibit myriad pharmacological properties, one of which is antitubercular activity. It demonstrates antitubercular activity by directly inhibiting Mycobacterium tuberculosis (M.tb) and also enhances immune responses that ultimately lead to the elimination of M.tb by macrophages. This natural product is, however, unstable, and several analogues, noticeably monocarbonyl analogues, have been synthesized to overcome this challenge. Curcumin and its monocarbonyl analogues reported so far exhibit moderate antitubercular activity in the range of 7 to 16 μM. Herein, we report a straightforward synthesis of novel monocarbonyl curcumin analogues, their antitubercular activity, and the structure-activity relationship. The hit compound from this study, 3a, exhibits potent MIC₉₀ values in the range of 0.2 to 0.9 μM in both ADC and CAS media.

SigH stress response mediates killing of Mycobacterium tuberculosis by activating nitronaphthofuran prodrugs via induction of Mrx2 expression

The emergence of drug-resistant Mycobacterium tuberculosis strains highlights the need to discover anti-tuberculosis drugs with novel mechanisms of action. Here we discovered a mycobactericidal strategy based on the prodrug activation of selected chemical derivatives classified as nitronaphthofurans (nNFs) mediated by the coordinated action of the sigH and mrx2 genes. The transcription factor SigH is a key regulator of an extensive transcriptional network that responds to oxidative, nitrosative, and heat stresses in M. tuberculosis. The nNF action induced the SigH stress response which in turn induced the mrx2 overexpression. The nitroreductase Mrx2 was found to activate nNF prodrugs, killing replicating, non-replicating and intracellular forms of M. tuberculosis. Analysis of SigH DNA sequences obtained from spontaneous nNF-resistant M. tuberculosis mutants suggests disruption of SigH binding to the mrx2 promoter site and/or RNA polymerase core, likely promoting the observed loss of transcriptional control over Mrx2. Mutations found in mrx2 lead to structural defects in the thioredoxin fold of the Mrx2 protein, significantly impairing the activity of the Mrx2 enzyme against nNFs. Altogether, our work brings out the SigH/Mrx2 stress response pathway as a promising target for future drug discovery programs.

MDN-6, a Possible Therapeutic Candidate for Multidrug-Resistant and Extensively Drug-Resistant Mycobacterium tuberculosis

Background: The rise of antibiotic-resistant Mycobacterium tuberculosis strains has accelerated the hunt for novel drugs for tuberculosis (TB). Objectives: This study identified a novel compound with strong anti-TB efficacy against several resistant M. tuberculosis strains from a chemical library of naphthoquinone derivatives. Methods: The identified chemical was designated as MDN-6 (methyl-1,4-bis(2-(diethylamino)ethoxy)-2-naphthoate). Results: It significantly inhibited all the tested Mycobacterium strains, including 24 clinically isolated resistant strains. The minimum inhibitory concentrations of MDN-6 were between 0.02 and 25 g/mL. It also had partially synergistic activity against extensively drug-resistant M. tuberculosis when coupled with rifampicin and streptomycin. Additionally, MDN-6 demonstrated a superior post-antibiotic effect over isoniazid and exhibited comparable inhibitory efficacy against Mycobacterium marinum and Mycobacterium kansasii. Besides the antimicrobial effect, MDN-6 had a 50% lethal dosage (LD50) of 279.1 mg/kg in female BALB/c mice. Conclusions: MDN-6 is a promising anti-TB therapeutic candidate against drug-resistant M. tuberculosis. However, further investigation is necessary to elucidate the action mechanism and assess the drug’s in vivo therapeutic potential.

Discovery and preclinical evaluations of JBD0131, a novel nitrodihydro-imidazooxazole anti-tuberculosis agent

Multidrug-resistant pulmonary tuberculosis (MDR-TB) is a major health problem worldwide. The treatment for MDR-TB requires medications for a long duration (up to 20-24 months) with second-line drugs resulting in unfavorable outcomes. Nitroimidazoles are promising antimycobacterial agents known to inhibit both aerobic and anaerobic mycobacterial activity. Delamanid and pretomanid are two nitroimidazoles approved by the regulatory agencies for MDR-TB treatment. However, both agents possess unsatisfactory absorption and QTc prolongation. In our search for a safer nitroimidazole, we discovered JBD0131 (2). It exhibited excellent anti-mycobacterial activity against M. tuberculosis H37Rv in vitro and in vivo, improved PK and absorption, reduced QT prolongation potential of delamanid. JBD0131 is currently in clinical development in China for pulmonary tuberculosis (CTR20202308).

Tuberculosis Drug Discovery: Challenges and New Horizons

Over the past 2000 years, tuberculosis (TB) has claimed more lives than any other infectious disease. In 2020 alone, TB was responsible for 1.5 million deaths worldwide, comparable to the 1.8 million deaths caused by COVID-19. The World Health Organization has stated that new TB drugs must be developed to end this pandemic. After decades of neglect in this field, a renaissance era of TB drug discovery has arrived, in which many novel candidates have entered clinical trials. However, while hundreds of molecules are reported annually as promising anti-TB agents, very few successfully progress to clinical development. In this Perspective, we critically review those anti-TB compounds published in the last 6 years that demonstrate good in vivo efficacy against Mycobacterium tuberculosis. Additionally, we highlight the main challenges and strategies for developing new TB drugs and the current global pipeline of drug candidates in clinical studies to foment fresh research perspectives.

Application of Next Generation Sequencing for Diagnosis and Clinical Management of Drug-Resistant Tuberculosis: Updates on Recent Developments in the Field

The World Health Organization’s End TB Strategy prioritizes universal access to an early diagnosis and comprehensive drug susceptibility testing (DST) for all individuals with tuberculosis (TB) as a key component of integrated, patient-centered TB care. Next generation whole genome sequencing (WGS) and its associated technology has demonstrated exceptional potential for reliable and comprehensive resistance prediction for Mycobacterium tuberculosis isolates, allowing for accurate clinical decisions. This review presents a descriptive analysis of research describing the potential of WGS to accelerate delivery of individualized care, recent advances in sputum-based WGS technology and the role of targeted sequencing for resistance detection. We provide an update on recent research describing the mechanisms of resistance to new and repurposed drugs and the dynamics of mixed infections and its potential implication on TB diagnosis and treatment. Whilst the studies reviewed here have greatly improved our understanding of recent advances in this arena, it highlights significant challenges that remain. The wide-spread introduction of new drugs in the absence of standardized DST has led to rapid emergence of drug resistance. This review highlights apparent gaps in our knowledge of the mechanisms contributing to resistance for these new drugs and challenges that limit the clinical utility of next generation sequencing techniques. It is recommended that a combination of genotypic and phenotypic techniques is warranted to monitor treatment response, curb emerging resistance and further dissemination of drug resistance.

Profiling Pretomanid as a Therapeutic Option for TB Infection: Evidence to Date

Tuberculosis (TB) is the most deadly infectious disease globally. Although most individuals achieve a cure, a substantial portion develop multi-drug resistant TB which is exceedingly difficult to treat, and the number of effective agents is dwindling. Development of new anti-tubercular medications is imperative to combat existing drug resistance and accelerate global eradication of TB. Pretomanid (PA-824) represents one of the newest drug classes (ie, nitroimidazooxazines) approved in 2019 by the United States Food and Drug Administration as part of a multi-drug regimen (with bedaquiline and linezolid, BPaL) and recommended by the World Health Organization (WHO) to treat extensively-resistant (XR-TB) and multi-drug resistant tuberculosis (MDR-TB). Approval was granted through the FDA's Limited Population Pathway for Antibacterial and Antifungal Drugs, which accelerates approval for antimicrobial drugs used to treat life-threatening or serious infections in a limited population with unmet need. This review details the pharmacology, efficacy, and safety of this new agent and describes evidence to date for its role in the treatment of drug resistant TB including published, ongoing, and planned studies.

Systematic review of mutations associated with resistance to the new and repurposed Mycobacterium tuberculosis drugs bedaquiline, clofazimine, linezolid, delamanid and pretomanid

BACKGROUND

Improved genetic understanding of Mycobacterium tuberculosis (MTB) resistance to novel and repurposed anti-tubercular agents can aid the development of rapid molecular diagnostics.

METHODS

Adhering to PRISMA guidelines, in March 2018, we performed a systematic review of studies implicating mutations in resistance through sequencing and phenotyping before and/or after spontaneous resistance evolution, as well as allelic exchange experiments. We focused on the novel drugs bedaquiline, delamanid, pretomanid and the repurposed drugs clofazimine and linezolid. A database of 1373 diverse control MTB whole genomes, isolated from patients not exposed to these drugs, was used to further assess genotype-phenotype associations.

RESULTS

Of 2112 papers, 54 met the inclusion criteria. These studies characterized 277 mutations in the genes atpE, mmpR, pepQ, Rv1979c, fgd1, fbiABC and ddn and their association with resistance to one or more of the five drugs. The most frequent mutations for bedaquiline, clofazimine, linezolid, delamanid and pretomanid resistance were atpE A63P, mmpR frameshifts at nucleotides 192-198, rplC C154R, ddn W88* and ddn S11*, respectively. Frameshifts in the mmpR homopolymer region nucleotides 192-198 were identified in 52/1373 (4%) of the control isolates without prior exposure to bedaquiline or clofazimine. Of isolates resistant to one or more of the five drugs, 59/519 (11%) lacked a mutation explaining phenotypic resistance.

CONCLUSIONS

This systematic review supports the use of molecular methods for linezolid resistance detection. Resistance mechanisms involving non-essential genes show a diversity of mutations that will challenge molecular diagnosis of bedaquiline and nitroimidazole resistance. Combined phenotypic and genotypic surveillance is needed for these drugs in the short term.

A Combined Experimental and Theoretical Study of Nitrofuran Antibiotics: Crystal Structures, DFT Computations, Sublimation and Solution Thermodynamics

Single crystal of furazolidone (FZL) has been successfully obtained, and its crystal structure has been determined. Common and distinctive features of furazolidone and nitrofurantoin (NFT) crystal packing have been discussed. Combined use of QTAIMC and Hirshfeld surface analysis allowed characterizing the non-covalent interactions in both crystals. Thermophysical characteristics and decomposition of NFT and FZL have been studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and mass-spectrometry. The saturated vapor pressures of the compounds have been measured using the transpiration method, and the standard thermodynamic functions of sublimation were calculated. It was revealed that the sublimation enthalpy and Gibbs energy of NFT are both higher than those for FZL, but a gain in the crystal lattice energy of NFT is leveled by an entropy increase. The solubility processes of the studied compounds in buffer solutions with pH 2.0, 7.4 and in 1-octanol was investigated at four temperatures from 298.15 to 313.15 K by the saturation shake-flask method. The thermodynamic functions of the dissolution and solvation processes of the studied compounds have been calculated based on the experimental data. Due to the fact that NFT is unstable in buffer solutions and undergoes a solution-mediated transformation from an anhydrate form to monohydrate in the solid state, the thermophysical characteristics and dissolution thermodynamics of the monohydrate were also investigated. It was demonstrated that a combination of experimental and theoretical methods allows performing an in-depth study of the relationships between the molecular and crystal structure and pharmaceutically relevant properties of nitrofuran antibiotics.

Hybridization Approach to Drug Discovery Inhibiting Mycobacterium tuberculosis-An Overview

Tuberculosis is one of the top 10 causes of death worldwide and the leading cause of death from a single infectious agent, mainly due to Mycobacterium tuberculosis (MTB). Recently, clinical prognoses have worsened due to the emergence of multi-drug resistant (MDR) and extensive-drug resistant (XDR) tuberculosis, which lead to the need for new, efficient and safe drugs. Among the several strategies, polypharmacology could be considered one of the best solutions, in particular, the multitarget directed ligands strategy (MTDLs), based on the synthesis of hybrid ligands acting against two targets of the pathogen. The framework strategy comprises linking, fusing and merging approaches to develop new chemical entities. With these premises, this review aims to provide an overview of the recent hybridization approach, in medicinal chemistry, of the most recent and promising multitargeting antimycobacterial candidates.

Mutations in fbiD (Rv2983) as a Novel Determinant of Resistance to Pretomanid and Delamanid in Mycobacterium tuberculosis

The nitroimidazole prodrugs delamanid and pretomanid comprise one of only two new antimicrobial classes approved to treat tuberculosis (TB) in 50 years. Prior in vitro studies suggest a relatively low barrier to nitroimidazole resistance in Mycobacterium tuberculosis, but clinical evidence is limited to date. We selected pretomanid-resistant M. tuberculosis mutants in two mouse models of TB using a range of pretomanid doses. The frequency of spontaneous resistance was approximately 10^-5 CFU. Whole-genome sequencing of 161 resistant isolates from 47 mice revealed 99 unique mutations, of which 91% occurred in 1 of 5 genes previously associated with nitroimidazole activation and resistance, namely, fbiC (56%), fbiA (15%), ddn (12%), fgd (4%), and fbiB (4%). Nearly all mutations were unique to a single mouse and not previously identified. The remaining 9% of resistant mutants harbored mutations in Rv2983 (fbiD), a gene not previously associated with nitroimidazole resistance but recently shown to be a guanylyltransferase necessary for cofactor F₄₂₀ synthesis. Most mutants exhibited high-level resistance to pretomanid and delamanid, although Rv2983 and fbiB mutants exhibited high-level pretomanid resistance but relatively small changes in delamanid susceptibility. Complementing an Rv2983 mutant with wild-type Rv2983 restored susceptibility to pretomanid and delamanid. By quantifying intracellular F₄₂₀ and its precursor Fo in overexpressing and loss-of-function mutants, we provide further evidence that Rv2983 is necessary for F₄₂₀ biosynthesis. Finally, Rv2983 mutants and other F₄₂₀H₂-deficient mutants displayed hypersusceptibility to some antibiotics and to concentrations of malachite green found in solid media used to isolate and propagate mycobacteria from clinical samples.

Synthesis and activity of BNF15 against drug-resistant Mycobacterium tuberculosis

Aim: Tuberculosis is the leading cause of mortality among infectious diseases worldwide. Finding a new competent anti tubercular therapy is essential. Materials & methods: We screened thousands of compounds and evaluated their efficacy against Mycobacterium tuberculosis. Results: Initially, 2-nitronaphtho[2,3-b]benzofuran-6,11-dione was active against M. tuberculosis. Next, among 15 newly synthesized derivatives, BNF15 showed promising effect against all drug-sensitive and drug-resistant M. tuberculosis (MIC: 0.02-0.78 μg/ml). BNF15 effectively killed intracellular M. tuberculosis and nontuberculous mycobacteria. BNF15 exhibited a prolonged post antibiotic effect superior to isoniazid, streptomycin, and ethambutol and synergistic interaction with rifampicin. In acute oral toxicity test, BNF15 did not show toxic effect at a concentration up to 2000 mg/kg. Conclusion: These results highlight the perspective of BNF15 to treat drug-resistant M. tuberculosis.

Minimum Bactericidal Concentration Techniques in Mycobacterium tuberculosis: A Systematic Review

Minimum bactericidal concentration (MBC) assay is an accepted parameter for evaluating new antimicrobial agents, and it is frequently used as a research tool to provide a prediction of bacterial eradication. To the best of our knowledge, there is no standardization among researchers regarding the technique used to detect a drug's MBC in Mycobacterium tuberculosis. Thus, the aim of this systematic review is to discuss the available literature in determining a drug's MBC in M. tuberculosis, to find the most commonly used technique and standardize the process. A broad and rigorous literature search of three electronic databases (PubMed, Web of Knowledge, and LILACS) was performed according to the PRISMA statement. We considered studies that were published from January 1, 1990 to February 19, 2019. Google Scholar was also searched to increase the number of publications. We searched for articles using the MeSH terms "microbiological techniques," "Mycobacterium," "antibacterial agents." In addition, free terms were used in the search. The search yielded 6,674 publications. After filter application, 5,348 publications remained. Of these, we evaluated the full text of 187 publications. By applying the inclusion criteria, 69 studies were included in the present systematic review. In the literature analyzed, a great variety in the techniques used to determine a drug's MBC in M. tuberculosis was observed. The most common variability is related to the culture media used, culture incubation time, and the percentage of bacterial death for the drug to be considered as bactericidal. The most commonly used technique for drug's MBC determination was carried out using the drug's minimum inhibitory concentration (MIC) assay. Aliquots from prior MIC values were subcultured in Middlebrook agar and incubated for 4 weeks at 35°C for determining the colony forming unit (CFU) with relevance to detect 99.9% bacilli killed or reduction in 3 log₁₀ viable bacilli.

Current development of 5-nitrofuran-2-yl derivatives as antitubercular agents

Pulmonary tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis (MTB) and still remains one of the foremost fatal infectious diseases, infecting nearly a third of the worldwide population. The emergencies of multidrug-resistant and extensively drug-resistant tuberculosis (MDR and XDR-TB) prompt the efforts to deliver potent and novel anti-TB drugs. Research aimed at the development of new anti-TB drugs based on nitrofuran scaffold led to the identification of several candidates that were effective against actively growing as well as latent mycobacteria with unique modes of action. This review focuses on the recent advances in nitrofurans that could provide intriguing potential leads in the area of anti-TB drug discovery.

Attachment of a 5-nitrofuroyl moiety to spirocyclic piperidines produces non-toxic nitrofurans that are efficacious in vitro against multidrug-resistant Mycobacterium tuberculosis

A selectively antimycobacterial compound belonging to the nitrofuran class of antimicrobials has been developed via conjugation of the nitrofuran moiety to a series of spirocyclic piperidines through an amide linkage. It proved to have comparable activity against drug-sensitive (H37Rv) strain as well as multidrug-resistant, patient-derived strains of Mycobacterium tuberculosis. The compound is druglike, showed no appreciable cytotoxicity toward human retinal pigment epithelial cell line ARPE-19 in concentrations up to 100 μM and displayed low toxicity when evaluated in mice.

Optimization of N-benzyl-5-nitrofuran-2-carboxamide as an antitubercular agent

The optimization campaign for a nitrofuran antitubercular hit (N-benzyl-5-nitrofuran-2-carboxamide; JSF-3449) led to the design, synthesis, and biological profiling of a family of analogs. These compounds exhibited potent in vitro antitubercular activity (MIC = 0.019-0.20 μM) against the Mycobacterium tuberculosis H37Rv strain and low in vitro cytotoxicity (CC₅₀ = 40->120 μM) towards Vero cells. Significant improvements in mouse liver microsomal stability and mouse pharmacokinetic profile were realized by introduction of an α, α-dimethylbenzyl moiety. Among these compounds, JSF-4088 is highlighted due to its in vitro antitubercular potency (MIC = 0.019 μM) and Vero cell cytotoxicity (CC₅₀ > 120 μM). The findings suggest a rationale for the continued evolution of this promising series of antitubercular small molecules.

Dynamic time-kill curve characterization of spectinamide antibiotics 1445 and 1599 for the treatment of tuberculosis

Spectinamides are a novel class of antibiotics under development for the treatment of MDR- and XDR-tuberculosis, with 1599 and 1445 as early lead candidates within this group. In order to evaluate and differentiate the pharmacological properties of these compounds and assist in candidate selection and design of optimal dosing regimens in animal models of Mtb infection, time kill curve assessments were performed in a previously established in vitro PK/PD model system. The performed studies and subsequent pharmacometric analysis indicate that the anti-mycobacterial activity of 1599 exhibits concentration-dependent killing whereas 1445 shows time-dependent killing. These findings are supported by the fact that the PKPD index that best describes bacterial killing is T > MIC for 1445, but fC_max/AUC for 1599. The differential killing behavior among the lead candidates can be rationalized by the differences in post-antibiotic effect: 15.7 h for 1445 compared the 133 h for 1599. Overall, the PK/PD based analysis of the in vitro pharmacologic killing profile of spectinamides 1599 and 1445 on mycobacteria provided valuable insights that contributed to lead candidate selection and preclinical development of these compounds.

In vitro activity of alpha-viniferin isolated from the roots of Carex humilis against Mycobacterium tuberculosis

This study explores the antitubercular activity of α-viniferin, a bioactive phytochemical compound obtained from Carex humilis. α-Viniferin was active against both drug-susceptible and -resistant strains of Mycobacterium tuberculosis at MIC₅₀s of 4.6 μM in culture broth medium and MIC₅₀s of 2.3-4.6 μM inside macrophages and pneumocytes. In combination with streptomycin and ethambutol, α-viniferin exhibited an additive effect and partial synergy, respectively, against M. tuberculosis H37Rv. α-Viniferin also did not show cytotoxicity in any of the cell lines tested up to a concentration of 147 μM, which gives this compound a selectivity index of >32. Moreover, α-viniferin was active against 3 Staphylococcus species, including methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus aureus (MRSA), and methicillin-resistant Staphylococcus epidermidis (MRSE).

Structure-Activity Relationships of Spectinamide Antituberculosis Agents: A Dissection of Ribosomal Inhibition and Native Efflux Avoidance Contributions

Spectinamides are a novel class of antitubercular agents with the potential to treat drug-resistant tuberculosis infections. Their antitubercular activity is derived from both ribosomal affinity and their ability to overcome intrinsic efflux mediated by the Mycobacterium tuberculosis Rv1258c efflux pump. This study explores the structure-activity relationships through analysis of 50 targeted spectinamides. Compounds are evaluated for ribosomal translational inhibition, MIC activity in Rv1258c efflux pump deficient and wild type tuberculosis strains, and efficacy in an acute model of tuberculosis infection. The results of this study show a narrow structure-activity relationship, consistent with a tight ribosome-binding pocket and strict structural requirements to overcome native efflux. Rationalization of ribosomal inhibition data using molecular dynamics simulations showed stable complex formation for halogenated spectinamides consistent with the long post antibiotic effects observed. The lead spectinamides identified in this study demonstrated potent MIC activity against MDR and XDR tuberculosis and had desirable antitubercular class specific features including low protein binding, low microsomal metabolism, no cytotoxicity, and significant reductions in bacterial burdens in the lungs of mice infected with M. tuberculosis. The structure-activity relationships detailed here emphasize the need to examine efflux-mediated resistance in the design of antituberculosis drugs and demonstrate that it is possible to overcome intrinsic efflux with synthetic modification. The ability to understand the structure requirements for this class has produced a variety of new substituted spectinamides, which may provide useful alternative candidates and promote the further development of this class.

Current strategies to treat tuberculosis

Tuberculosis (TB) has been a leading cause of death for more than a century. While effective therapies exist, treatment is long and cumbersome. TB control is complicated by the overlapping problems created by global inadequacy of public health infrastructures, the interaction of the TB and human immunodeficiency virus (HIV) epidemics, and the emergence of drug-resistant TB. After a long period of neglect, there is now significant progress in the development of novel treatment regimens for TB. Focusing on treatment for active disease, we review pathways to TB regimen development and the new and repurposed anti-TB agents in clinical development.

Synthesis and evaluation of pretomanid (PA-824) oxazolidinone hybrids

Pretomanid (PA-824) is an important nitroimidazole antitubercular agent in late stage clinical trials. However, pretomanid is limited by poor solubility and high protein binding, which presents opportunities for improvement in its physiochemical properties. Conversely, the oxazolidinone linezolid has excellent physicochemical properties and has recently shown impressive activity for the treatment of drug resistant tuberculosis. In this study we explore if incorporation of the outer ring elements found in first and second generation oxazolidinones into the nitroimidazole core of pretomanid can be used to improve its physicochemical and antitubercular properties. The synthesis of pretomanid outer oxazolidinone ring hybrids was successfully performed producing hybrids that maintained antitubercular activity and had improved in vitro physicochemical properties. Three lead compounds were identified and evaluated in a chronic model of tuberculosis infection in mice. However, the compounds lacked efficacy suggesting that portions of PA-824 tail not found in the hybrid molecules are required for in vivo efficacy.

Delamanid: a review of its use in patients with multidrug-resistant tuberculosis

Delamanid (Deltyba(®)), a nitroimidazo-oxazole derivative, is a new anti-tuberculosis (TB) drug which exhibits potent in vitro and in vivo antitubercular activity against drug-susceptible and -resistant strains of Mycobacterium tuberculosis. It is approved in several countries, including Japan and those of the EU, for use as part of an appropriate combination regimen in adults with multidrug-resistant tuberculosis (MDR-TB) when an effective treatment regimen cannot otherwise be composed due to resistance or tolerability. In a robust phase II trial in adult patients with MDR-TB, oral delamanid 100 mg twice daily for 2 months plus an optimized background regimen improved sputum culture conversion rates to a significantly greater extent than placebo. In a 6-month extension study, long-term (≤8 months) treatment with delamanid was associated with a higher incidence of favourable outcomes (i.e. cured or completed all treatment) than short-term (≤2 months) treatment, with an accompanying reduction inunfavourable outcomes as defined by the WHO (i.e. pre-specified proportion of TB-positive sputum cultures, death or treatment discontinuation for ≥2 months without medical approval). Delamanid was not associated with clinically relevant drug-drug interactions, including with antiretroviral drugs and those commonly used in treating TB. Delamanid was generally well tolerated in patients with MDR-TB, with gastrointestinal adverse events and insomnia reported most commonly. Although the incidence of QT interval prolongation was higher with delamanid-based therapy, it was not associated with clinical symptoms such as syncope and arrhythmia. In conclusion, delamanid is a useful addition to the treatment options currently available for patients with MDR-TB.

Mutations in genes for the F420 biosynthetic pathway and a nitroreductase enzyme are the primary resistance determinants in spontaneous in vitro-selected PA-824-resistant mutants of Mycobacterium tuberculosis

Alleviating the burden of tuberculosis (TB) requires an understanding of the genetic basis that determines the emergence of drug-resistant mutants. PA-824 (pretomanid) is a bicyclic nitroimidazole class compound presently undergoing the phase III STAND clinical trial, despite lacking identifiable genetic markers for drug-specific resistant Mycobacterium tuberculosis. In the present study, we aimed to characterize the genetic polymorphisms of spontaneously generated PA-824-resistant mutant strains by surveying drug metabolism genes for potential mutations. Of the 183 independently selected PA-824-resistant M. tuberculosis mutants, 83% harbored a single mutation in one of five nonessential genes associated with either PA-824 prodrug activation (ddn, 29%; fgd1, 7%) or the tangential F420 biosynthetic pathway (fbiA, 19%; fbiB, 2%; fbiC, 26%). Crystal structure analysis indicated that identified mutations were specifically located within the protein catalytic domain that would hinder the activity of the enzymes required for prodrug activation. This systematic analysis conducted of genotypes resistant to PA-824 may contribute to future efforts in monitoring clinical strain susceptibility with this new drug therapy.

Delamanid when other anti-tuberculosis-treatment regimens failed due to resistance or tolerability

INTRODUCTION

The limited availability of effective drugs causes difficulties in the management of multidrug-resistant tuberculosis (MDR-TB) and novel therapeutic agents are needed. Delamanid , a new nitro-hydro-imidazooxazole derivative, inhibits mycolic acid synthesis. This review covers the efficacy and safety of delamanid for MDR-TB.

AREA COVERED

This paper reviews the pharmacological profile of delamanid and the results of clinical trials evaluating its efficacy for treating MDR-TB in combination with other anti-TB drugs. The drug's safety and tolerability profiles are also considered.

EXPERT OPINION

Delamanid showed potent activity against drug-susceptible and -resistant Mycobacterium tuberculosis in both in vitro and in vivo studies. In clinical trials, the drug showed significant early bactericidal activity in pulmonary TB patients, and increased culture conversion after 2 months of treatment in combination with an optimized background regimen in MDR-TB patients. In addition, decreased mortality was observed in MDR-TB patients who received > 6 months of delamanid treatment. The drug was generally tolerable, but QT prolongation should be monitored carefully using electrocardiograms and potassium levels. Therefore, delamanid could be used as part of an appropriate combination regimen for pulmonary MDR-TB in adult patients when an effective treatment regimen cannot otherwise be composed for reasons of resistance or tolerability.

Nitrotriazole- and imidazole-based amides and sulfonamides as antitubercular agents

Twenty-three 3-nitrotriazole-based and 2-nitroimidazole-based amides and sulfonamides were screened for antitubercular (anti-TB) activity in aerobic Mycobacterium tuberculosis H37Rv by using the BacTiter-Glo (BTG) microbial cell viability assay. In general, 3-nitrotriazole-based sulfonamides demonstrated anti-TB activity, whereas 3-nitrotriazole-based amides and 2-nitroimidazole-based amides and sulfonamides were inactive. Three 3-nitrotriazole-based sulfonamides (compounds 4, 2, and 7) demonstrated 50% inhibitory concentration (IC50), IC90, and MIC values of 0.38, 0.43, and 1.56 μM (compound 4), 0.57, 0.98, and 3.13 μM (compound 2), and 0.79, 0.87, and 3.13 μM (compound 7), respectively. For 3-nitrotriazole-based sulfonamides, anti-TB activity increased with lipophilicity, whereas the one-electron reduction potential (E1/2) did not play a role. 2-Nitroimidazole-based analogs, which were inactive in the BTG assay, were significantly more active in the low-oxygen assay and more active than the 3-nitrotriazoles. All active nitrotriazoles in the BTG assay were similarly active or more potent (lower MIC values) against resistant strains, with the exception of compounds 2, 3, 4, and 8, which demonstrated greater MIC values against isoniazid-resistant strains. Five 3-nitrotriazole-based sulfonamides demonstrated activity in infected murine J774 macrophages, causing log reductions similar to those seen with rifampin. However, some compounds caused toxicity in uninfected macrophages. In conclusion, the classes of 3-nitrotriazole-based amides and sulfonamides merit further investigation as potential antitubercular agents.

Pentacyclic nitrofurans with in vivo efficacy and activity against nonreplicating Mycobacterium tuberculosis

The reductively activated nitroaromatic class of antimicrobials, which include nitroimidazole and the more metabolically labile nitrofuran antitubercular agents, have demonstrated some potential for development as therapeutics against dormant TB bacilli. In previous studies, the pharmacokinetic properties of nitrofuranyl isoxazolines were improved by incorporation of the outer ring elements of the antitubercular nitroimidazole OPC-67683. This successfully increased stability of the resulting pentacyclic nitrofuran lead compound Lee1106 (referred to herein as 9a). In the current study, we report the synthesis and antimicrobial properties of 9a and panel of 9a analogs, which were developed to increase oral bioavailability. These hybrid nitrofurans remained potent inhibitors of Mycobacterium tuberculosis with favorable selectivity indices (>150) and a narrow spectrum of activity. In vivo, the pentacyclic nitrofuran compounds showed long half-lives and high volumes of distribution. Based on pharmacokinetic testing and lack of toxicity in vivo,9a remained the series lead. 9a exerted a lengthy post antibiotic effect and was highly active against nonreplicating M. tuberculosis grown under hypoxia. 9a showed a low potential for cross resistance to current antitubercular agents, and a mechanism of activation distinct from pre-clinical tuberculosis candidates PA-824 and OPC-67683. Together these studies show that 9a is a nanomolar inhibitor of actively growing as well as nonreplicating M. tuberculosis.

Spectinamides: a new class of semisynthetic antituberculosis agents that overcome native drug efflux

Although the classical antibiotic spectinomycin is a potent bacterial protein synthesis inhibitor, poor antimycobacterial activity limits its clinical application for treating tuberculosis. Using structure-based design, we generated a new semisynthetic series of spectinomycin analogs with selective ribosomal inhibition and excellent narrow-spectrum antitubercular activity. In multiple murine infection models, these spectinamides were well tolerated, significantly reduced lung mycobacterial burden and increased survival. In vitro studies demonstrated a lack of cross resistance with existing tuberculosis therapeutics, activity against multidrug-resistant (MDR) and extensively drug-resistant tuberculosis and an excellent pharmacological profile. Key to their potent antitubercular properties was their structural modification to evade the Rv1258c efflux pump, which is upregulated in MDR strains and is implicated in macrophage-induced drug tolerance. The antitubercular efficacy of spectinamides demonstrates that synthetic modifications to classical antibiotics can overcome the challenge of intrinsic efflux pump-mediated resistance and expands opportunities for target-based tuberculosis drug discovery.

Progress in targeting cell envelope biogenesis in Mycobacterium tuberculosis

Most of the newly discovered compounds showing promise for the treatment of TB, notably multidrug-resistant TB, inhibit aspects of Mycobacterium tuberculosis cell envelope metabolism. This review reflects on the evolution of the knowledge that many of the front-line and emerging products inhibit aspects of cell envelope metabolism and in the process are bactericidal not only against actively replicating M. tuberculosis, but contrary to earlier impressions, are effective against latent forms of the disease. While mycolic acid and arabinogalactan synthesis are still primary targets of existing and new drugs, peptidoglycan synthesis, transport mechanisms and the synthesis of the decaprenyl-phosphate carrier lipid all show considerable promise as targets for new products, older drugs and new combinations. The advantages of whole cell- versus target-based screening in the perpetual search for new targets and products to counter multidrug-resistant TB are discussed.

Establishment and validation of whole-cell based fluorescence assays to identify anti-mycobacterial compounds using the Acanthamoeba castellanii-Mycobacterium marinum host-pathogen system

Tuberculosis is considered to be one of the world’s deadliest disease with 2 million deaths each year. The need for new antitubercular drugs is further exacerbated by the emergence of drug-resistance strains. Despite multiple recent efforts, the majority of the hits discovered by traditional target-based screening showed low efficiency in vivo. Therefore, there is heightened demand for whole-cell based approaches directly using host-pathogen systems. The phenotypic host-pathogen assay described here is based on the monitoring of GFP-expressing Mycobacterium marinum during infection of the amoeba Acanthamoeba castellanii. The assay showed straight-forward medium-throughput scalability, robustness and ease of manipulation, demonstrating its qualities as an efficient compound screening system. Validation with a series of known antitubercular compounds highlighted the advantages of the assay in comparison to previously published macrophage-Mycobacterium tuberculosis-based screening systems. Combination with secondary growth assays based on either GFP-expressing D. discoideum or M. marinum allowed us to further fine-tune compound characterization by distinguishing and quantifying growth inhibition, cytotoxic properties and antibiotic activities of the compounds. The simple and relatively low cost system described here is most suitable to detect anti-infective compounds, whether they present antibiotic activities or not, in which case they might exert anti-virulence or host defense boosting activities, both of which are largely overlooked by classical screening approaches.

Safety and Efficacy of Delamanid in the Treatment of Multidrug-Resistant Tuberculosis (MDR-TB)

Globally, the incidence of tuberculosis (TB) is declining but the proportion of drug-resistant cases has increased. Strains resistant to both isoniazid and rifampin, and possibly other antibiotics, called multidrug-resistant (MDR), are particularly difficult to treat. Poorer outcomes, including increased mortality, occur in patients infected with MDR strains and the costs associated with treatment of MDR-TB are substantially greater. The recent recognition of MDR-TB and strains with more complex resistance patterns has stimulated the development of new TB medications including fluoroquinolones, oxazolidinones, diarylquinolines, nitroimidazopyrans, ethylenediamines, and benzothiazinones. Bedaquiline, a diarylquinoline, was approved for the treatment of MDR-TB in 2012. Addition of delamanid to WHO-approved treatment improved outcomes for MDR-TB and for extensively drug-resistant TB in a large randomized, controlled phase II clinical trial and is undergoing evaluation in a large international phase III study. This review will focus on MDR-TB and the role of delamanid in its treatment.

Advances in the development of new tuberculosis drugs and treatment regimens

Despite the introduction 40 years ago of the inexpensive and effective four-drug (isoniazid, rifampicin, pyrazinamide and ethambutol) treatment regimen, tuberculosis (TB) continues to cause considerable morbidity and mortality worldwide. For the first time since the 1960s, new and novel drugs and regimens for all forms of TB are emerging. Such regimens are likely to utilize both repurposed drugs and new chemical entities, and several of these regimens are now progressing through clinical trials. This article covers current concepts and recent advances in TB drug discovery and development, including an update of ongoing TB treatment trials, newer clinical trial designs, TB biomarkers and adjunct host-directed therapies.

Anti-mycobacterial activity of plumericin and isoplumericin against MDR Mycobacterium tuberculosis

BACKGROUND AND OBJECTIVES

Because of the developing resistance of Mycobacterium species against currently available anti-mycobacterial drugs, there is an urgent need for new drug development. In this study, we have evaluated the in vitro anti-mycobacterial activity of Plumeria bicolor extract and its phytoconstituents - plumericin and isoplumericin against multi-drug resistance Mycobacterium tuberculosis.

METHODS

The in vitro anti-mycobacterial activity of chloroform extract of P. bicolor, plumericin and isoplumericin were tested against M. tuberculosis (H37Rv) and four multi-drug resistant (MDR) clinical isolates by measuring the minimum inhibitory concentration (MIC) using MTT (Tetrazolium bromide [3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide]) assay. The extract and both compounds were further evaluated by standard assay procedures to determine their minimum bactericidal concentration (MBC). Cytotoxicity of these compounds was performed against J774G8 murine macrophage cell lines. The activity was represented in the mean (±SD) of duplicate samples from three independent assays.

RESULTS

Plumericin showed better activity against pan sensitive as well as four MDR strains of M. tuberculosis with MIC values of 2.1 ± 0.12, 1.3 ± 0.15, 2.0 ± 0.07, 1.5 ± 0.13 & 2.0 ± 0.14 μg/mL and MBC values of 3.6 ± 0.22, 2.5 ± 0.18, 3.8 ± 0.27, 2.9 ± 0.20 & 3.7 ± 0.32 μg/mL than isoplumericin, respectively. Interestingly, both isolated active compounds showed an advantage over rifampicin (80 times) and isoniazid (8 times) by being highly active against the MDR strains. The extract and both compounds were found to be non-toxic against J774G8 macrophages up to the used concentrations.

CONCLUSION

Plumericin showed more potent activity than isoplumericin. The excellent activity of these compounds against MDR strains opens a possibility of obtaining new anti-mycobacterial drug candidate in near future.

Syntheses and evaluation of macrocyclic engelhardione analogs as antitubercular and antibacterial agents

The natural product engelhardione is an underexplored chemotype for developing novel treatments for bacterial infections; we therefore explored this natural product scaffold for chemical diversification and structure-activity relationship studies. Macrocyclic engelhardione and structural regioisomers were synthesized using a series of aldol condensations and selective hydrogenations to generate the 1,7-diarylheptan-3-one derivatives, followed by microwave-assisted intramolecular Ullmann coupling to afford a series of macrocyclic diaryl ether analogs. An extended macrocyclic chemical library was then produced by oxime formation, reductive amination and O-alkylation. Antibacterial evaluation revealed that the reductive amination derivatives 7b and 7d showed moderate activities (minimum inhibitory concentrations: 12.5-25 μg ml(-1)) against Mycobacterium tuberculosis and Gram-positive pathogens, as well as anti-Gram-negative activity against an efflux impaired Escherichia coli strain. These results provide validated leads for further optimization and development.

Design, synthesis and anti-tuberculosis activity of 1-adamantyl-3-heteroaryl ureas with improved in vitro pharmacokinetic properties

Out of the prominent global ailments, tuberculosis (TB) is still one of the leading causes of death worldwide due to infectious disease. Development of new drugs that shorten the current tuberculosis treatment time and have activity against drug resistant strains is of utmost importance. Towards these goals we have focused our efforts on developing novel anti-TB compounds with the general structure of 1-adamantyl-3-phenyl urea. This series is active against Mycobacteria and previous lead compounds were found to inhibit the membrane transporter MmpL3, the protein responsible for mycolic acid transport across the plasma membrane. However, these compounds suffered from poor in vitro pharmacokinetic (PK) profiles and they have a similar structure/SAR to inhibitors of human soluble epoxide hydrolase (sEH) enzymes. Therefore, in this study the further optimization of this compound class was driven by three factors: (1) to increase selectivity for anti-TB activity over human sEH activity, (2) to optimize PK profiles including solubility and (3) to maintain target inhibition. A new series of 1-adamantyl-3-heteroaryl ureas was designed and synthesized replacing the phenyl substituent of the original series with pyridines, pyrimidines, triazines, oxazoles, isoxazoles, oxadiazoles and pyrazoles. This study produced lead isoxazole, oxadiazole and pyrazole substituted adamantyl ureas with improved in vitro PK profiles, increased selectivity and good anti-TB potencies with sub μg/mL minimum inhibitory concentrations.

Allosteric competitive inhibitors of the glucose-1-phosphate thymidylyltransferase (RmlA) from Pseudomonas aeruginosa

Glucose-1-phosphate thymidylyltransferase (RmlA) catalyzes the condensation of glucose-1-phosphate (G1P) with deoxy-thymidine triphosphate (dTTP) to yield dTDP-d-glucose and pyrophosphate. This is the first step in the l-rhamnose biosynthetic pathway. l-Rhamnose is an important component of the cell wall of many microorganisms, including Mycobacterium tuberculosis and Pseudomonas aeruginosa. Here we describe the first nanomolar inhibitors of P. aeruginosa RmlA. These thymine analogues were identified by high-throughput screening and subsequently optimized by a combination of protein crystallography, in silico screening, and synthetic chemistry. Some of the inhibitors show inhibitory activity against M. tuberculosis. The inhibitors do not bind at the active site of RmlA but bind at a second site remote from the active site. Despite this, the compounds act as competitive inhibitors of G1P but with high cooperativity. This novel behavior was probed by structural analysis, which suggests that the inhibitors work by preventing RmlA from undergoing the conformational change key to its ordered bi-bi mechanism.