Ethionamide biomimetic activation and an unprecedented mechanism for its conversion into active and non-active metabolites

A Review of Antibacterial Candidates with New Modes of Action

There is a lack of new antibiotics to combat drug-resistant bacterial infections that increasingly threaten global health. The current pipeline of clinical-stage antimicrobials is primarily populated by "new and improved" versions of existing antibiotic classes, supplemented by several novel chemical scaffolds that act on traditional targets. The lack of fresh chemotypes acting on previously unexploited targets (the "holy grail" for new antimicrobials due to their scarcity) is particularly unfortunate as these offer the greatest opportunity for innovative breakthroughs to overcome existing resistance. In recognition of their potential, this review focuses on this subset of high value antibiotics, providing chemical structures where available. This review focuses on candidates that have progressed to clinical trials, as well as selected examples of promising pioneering approaches in advanced stages of development, in order to stimulate additional research aimed at combating drug-resistant infections.

Decoding drug resistance in Mycobacterium tuberculosis complex: genetic insights and future challenges

INTRODUCTION

Tuberculosis (TB), particularly its drug-resistant forms (MDR-TB and XDR-TB), continues to pose a significant global health challenge. Despite advances in treatment and diagnosis, the evolving nature of drug resistance in Mycobacterium tuberculosis (MTB) complicates TB eradication efforts. This review delves into the complexities of anti-TB drug resistance, its mechanisms, and implications on healthcare strategies globally.

AREAS COVERED

We explore the genetic underpinnings of resistance to both first-line and second-line anti-TB drugs, highlighting the role of mutations in key genes. The discussion extends to advanced diagnostic techniques, such as Whole-Genome Sequencing (WGS), CRISPR-based diagnostics and their impact on identifying and managing drug-resistant TB. Additionally, we discuss artificial intelligence applications, current treatment strategies, challenges in managing MDR-TB and XDR-TB, and the global disparities in TB treatment and control, translating to different therapeutic outcomes and have the potential to revolutionize our understanding and management of drug-resistant tuberculosis.

EXPERT OPINION

The current landscape of anti-TB drug resistance demands an integrated approach combining advanced diagnostics, novel therapeutic strategies, and global collaborative efforts. Future research should focus on understanding polygenic resistance and developing personalized medicine approaches. Policymakers must prioritize equitable access to diagnosis and treatment, enhancing TB control strategies, and support ongoing research and augmented government funding to address this critical public health issue effectively.

Designing New Magic Bullets to Penetrate the Mycobacterial Shield: An Arduous Quest for Promising Therapeutic Candidates

Mycobacterium spp. intimidated mankind since time immemorial. The triumph over this organism was anticipated with the introduction of potent antimicrobials in the mid-20th century. However, the emergence of drug resistance in mycobacteria, Mycobacterium tuberculosis, in particular, caused great concern for the treatment. With the enemy growing stronger, there is an immediate need to equip the therapeutic arsenal with novel and potent chemotherapeutic agents. The task seems intricating as our understanding of the dynamic nature of the mycobacteria requires intense experimentation and research. Targeting the mycobacterial cell envelope appears promising, but its versatility allows it to escape the lethal effect of the molecules acting on it. The unique ability of hiding (inactivity during latency) also assists the bacterium to survive in a drug-rich environment. The drug delivery systems also require upgradation to allow better bioavailability and tolerance in patients. Although the resistance to the novel drugs is inevitable, our commitment to the research in this area will ensure the discovery of effective weapons against this formidable opponent.

Perchlozone Resistance in Clinical Isolates of Mycobacterium tuberculosis

The emergence of drug-resistant tuberculosis forced the development of new drugs and the screening of more effective or less toxic analogues. Mycolic acid biosynthesis is targeted by several antituberculosis drugs, isoniazid being one of the most important in tuberculosis therapy. Recently, perchlozone, acting on another step in the FAS-II cycle, was officially approved for tuberculosis treatment in the Russian Federation and was included in the Russian national clinical guidelines. Using the serial dilution method on 7H10 agar plates for perchlozone and a Sensititre MYCOTB microdilution plate, we analyzed the phenotypic properties of primary clinical isolates of M. tuberculosis and analyzed the molecular determinants of resistance to isoniazid, ethionamide, and perchlozone. We found a wide variation in the MIC of perchlozone from 2 to 64 mg/L, correlating with the overall resistance profile: the MIC was higher for MDR and pre-XDR isolates. The cross-resistance between ethionamide and perchlozone was driven by mutations in the ethA gene encoding monooxygenase responsible for the activation of both drugs. The presumably susceptible to perchlozone and wild-type strains had MICs ranging from 2 to 4 mg/L, and the breakpoint was estimated to be 4 or 8 mg/L. In conclusion, susceptibility to perchlozone is retained for a part of the MDR strains, as is susceptibility to ethionamide, providing the possibility of therapy for such cases based on phenotypic or molecular analysis.

The Invasive Anemone Condylactis sp. of the Coral Reef as a Source of Sulfur- and Nitrogen-Containing Metabolites and Cytotoxic 5,8-Epidioxy Steroids

The Condylactis-genus anemones were examined for their proteinaceous poisons over 50 years ago. On the other hand, the current research focuses on isolating and describing the non-proteinaceous secondary metabolites from the invasive Condylactis anemones, which help take advantage of their population outbreak as a new source of chemical candidates and potential drug leads. From an organic extract of Condylactis sp., a 1,2,4-thiadiazole-based alkaloid, identified as 3,5-bis(3-pyridinyl)-1,2,4-thiadiazole (1), was found to be a new natural alkaloid despite being previously synthesized. The full assignment of NMR data of compound 1, based on the analysis of 2D NMR correlations, is reported herein for the first time. The proposed biosynthetic precursor thionicotinamide (2) was also isolated for the first time from nature along with nicotinamide (3), uridine (5), hypoxanthine (6), and four 5,8-epidioxysteroids (7–10). A major secondary metabolite (−)-betonicine (4) was isolated from Condylactis sp. and found for the first time in marine invertebrates. The four 5,8-epidioxysteroids, among other metabolites, exhibited cytotoxicity (IC50 3.5–9.0 μg/mL) toward five cancer cell lines.

Molecular Determinants of Ethionamide Resistance in Clinical Isolates of Mycobacterium tuberculosis

Background: Ethionamide and prothionamide are now included in group C of the WHO recommended drugs for the treatment of tuberculosis resistant to rifampicin and multidrug-resistant tuberculosis. The clinical relevance of ethionamide and prothionamide has increased with the wide spread of resistant tuberculosis. Methods: We retrospectively analyzed 349 clinical isolates obtained between 2016 and 2020. The susceptibility to ethionamide was tested using both the Bactec^TM MGIT^TM 960 system and the Sensititre^TM MYCOTB plate. Results: The MIC of ethionamide increases with the total resistance of the isolates in a row from susceptible to XDR strains. A significant part of the isolates have a MIC below the breakpoint: 25%, 36%, and 50% for XDR, pre-XDR, and MDR strains. Sensitivity and specificity of detection of mutations were 96% and 86% using MGIT resistance as a reference. Conclusions: Phenotypic methods for testing ethionamide are imperfectly correlated, and the isolates with MIC of 5 mg/L have the intermediate resistance. A significant proportion of resistant TB cases are susceptible and eligible for ethionamide treatment. Resistance could be explained using only analysis of loci ethA, P_fabG1, and inhA for most isolates in the Moscow region. The promoter mutation P_fabG1 c(-15)t predicts resistance to ethionamide with high specificity but low sensitivity.

Antibacterial Activity of Pharmaceutical-Grade Rose Bengal: An Application of a Synthetic Dye in Antibacterial Therapies

Rose bengal has been used in the diagnosis of ophthalmic disorders and liver function, and has been studied for the treatment of solid tumor cancers. To date, the antibacterial activity of rose bengal has been sporadically reported; however, these data have been generated with a commercial grade of rose bengal, which contains major uncontrolled impurities generated by the manufacturing process (80-95% dye content). A high-purity form of rose bengal formulation (HP-RBf, >99.5% dye content) kills a battery of Gram-positive bacteria, including drug-resistant strains at low concentrations (0.01-3.13 μg/mL) under fluorescent, LED, and natural light in a few minutes. Significantly, HP-RBf effectively eradicates Gram-positive bacterial biofilms. The frequency that Gram-positive bacteria spontaneously developed resistance to HP-RB is extremely low (less than 1 × 10-13). Toxicity data obtained through our research programs indicate that HP-RB is feasible as an anti-infective drug for the treatment of skin and soft tissue infections (SSTIs) involving multidrug-resistant (MDR) microbial invasion of the skin, and for eradicating biofilms. This article summarizes the antibacterial activity of pharmaceutical-grade rose bengal, HP-RB, against Gram-positive bacteria, its cytotoxicity against skin cells under illumination conditions, and mechanistic insights into rose bengal's bactericidal activity under dark conditions.

Further Insights into the Oxidative Pathway of Thiocarbonyl-Type Antitubercular Prodrugs: Ethionamide, Thioacetazone, and Isoxyl

A chemical activation study of the thiocarbonyl-type antitubercular prodrugs, ethionamide (ETH), thioacetazone (TAZ), and isoxyl (ISO), was performed. Biomimetic oxidation of ethionamide using H₂O₂ (1 equiv) led to ETH-SO as the only stable S-oxide compound, which was found to occur in solution in the preferential form of a sulfine (ETH═S═O vs the sulfenic acid tautomer ETH-S-OH), as previously observed in the crystal state. It was also demonstrated that ETH-SO is capable of reacting with amines, as the putative sulfinic derivative (ETH-SO₂H) was supposed to do. Unlike ETH, oxidation of TAZ did not allow observation of the mono-oxygenated species (TAZ-SO), leading directly to the more stable sulfinic acid derivative (TAZ-SO₂H), which can then lose a SO_xH group after further oxidation or when placed in a basic medium. It was also noticed that the unstable TAZ-SO intermediate can lead to the carbodiimide derivative as another electrophilic species. It is suggested that TAZ-SOH, TAZ-SO₂H, and the carbodiimide compound can also react with NH₂-containing nucleophilic species, and therefore be involved in toxic effects. Finally, ISO showed a very complex reactivity, here assigned to the coexistence of two mono-oxygenated structures, the sulfine and sulfenic acid tautomers. The mono- and dioxygenated derivatives of ISO are also highly unstable, leading to a panel of multiple metabolites, which are still reactive and likely contribute to the toxicity of this prodrug.

New insights into ethionamide metabolism: influence of oxidized methionine on its degradation path

Ethionamide (ETH) is a commercial drug, used as a second-line resource to neutralize Mycobacterium tuberculosis infections. It is proven that its metabolization in the organism leads to the formation of the active form of the drug, but some metabolic pathways may lead to the loss of its activity. Our work proved that the presence of oxidized methionine in cells could influence ETH's degradation, leading to the appearance of an inactive metabolite that is detectable by HPLC and mass spectrometry. In addition, it was found this process increases with the degree of methionine oxidation. This study contributes to a better understanding of ethionamide's metabolism in living organisms, and can help in the design of new drugs or ethionamide boosters for the combat of multidrug resistant tuberculosis.

Xanthates: Metabolism by Flavoprotein-Containing Monooxygenases and Antimycobacterial Activity

Ethionamide (ETH) plays a central role in the treatment of tuberculosis in patients resistant to the first-line drugs. The ETH, thioamide, and thiourea class of antituberculosis agents are prodrugs that are oxidatively converted to their active S-oxides by the mycobacterial flavin-dependent monooxygenase (EtaA) of Mycobacterium tuberculosis, thus initiating the chain of reactions that result in inhibition of mycolic acid biosynthesis and cell lysis. As part of a search for new lead candidates, we report here that several xanthates are oxidized by purified EtaA to S-oxide metabolites (perxanthates), which are implicated in the antimycobacterial activity of these compounds. This process, which is analogous to that responsible for activation of ETH, is also catalyzed by human flavoprotein monooxygenase 3. EtaA was not inhibited in a time-dependent manner during the reaction. Xanthates with longer alkyl chains were oxidized more efficiently. EtaA oxidized octyl-xanthate (K_m = 5 µM; V_max = 1.023 nmolP/min; k_cat = 5.2 molP/min/molE) more efficiently than ETH (194 µM; 1.46 nmolP/min; 7.73 nmolP/min/molE, respectively). Furthermore, the in vitro antimycobacterial activity of four xanthates against M. tuberculosis H37Hv was higher (minimum inhibitory concentration of around 1 µM) than that of ETH (12 µM).

Is IQG-607 a Potential Metallodrug or Metallopro-Drug With a Defined Molecular Target in Mycobacterium tuberculosis?

The emergence of strains of Mycobacterium tuberculosis resistant to isoniazid (INH) has underscored the need for the development of new anti-tuberculosis agents. INH is activated by the mycobacterial katG-encoded catalase-peroxidase, forming an acylpyridine fragment that is covalently attached to the C4 of NADH. This isonicotinyl-NAD adduct inhibits the activity of 2-trans-enoyl-ACP(CoA) reductase (InhA), which plays a role in mycolic acid biosynthesis. A metal-based INH analog, Na₃[Fe^II(CN)₅(INH)]·4H₂O, IQG-607, was designed to have an electronic redistribution on INH moiety that would lead to an intramolecular electron transfer to bypass KatG activation. HPLC and EPR studies showed that the INH moiety can be oxidized by superoxide or peroxide yielding similar metabolites and isonicotinoyl radical only when associated to IQG-607, thereby supporting redox-mediated drug activation as a possible mechanism of action. However, IQG-607 was shown to inhibit the in vitro activity of both wild-type and INH-resistant mutant InhA enzymes in the absence of KatG activation. IQG-607 given by the oral route to M. tuberculosis-infected mice reduced lung lesions. Experiments using early and late controls of infection revealed a bactericidal activity for IQG-607. HPLC and voltammetric methods were developed to quantify IQG-607. Pharmacokinetic studies showed short half-life, high clearance, moderate volume of distribution, and low oral bioavailability, which was not altered by feeding. Safety and toxic effects of IQG-607 after acute and 90-day repeated oral administrations in both rats and minipigs showed occurrence of mild to moderate toxic events. Eight multidrug-resistant strains (MDR-TB) were resistant to IQG-607, suggesting an association between katG mutation and increasing MIC values. Whole genome sequencing of three spontaneous IQG-607-resistant strains harbored katG gene mutations. MIC measurements and macrophage infection experiments with a laboratorial strain showed that katG mutation is sufficient to confer resistance to IQG-607 and that the macrophage intracellular environment cannot trigger the self-activation mechanism. Reduced activity of IQG-607 against an M. tuberculosis strain overexpressing S94A InhA mutant protein suggested both the need for KatG activation and InhA as its target. Further efforts are suggested to be pursued toward attempting to translate IQG-607 into a chemotherapeutic agent to treat tuberculosis.

An overview on crystal structures of InhA protein: Apo-form, in complex with its natural ligands and inhibitors

The enoyl-ACP reductase InhA from the mycobacterial fatty acid biosynthesis pathway has become a target of interest for the development of new anti-tubercular drugs. This protein has been identified as essential for the survival of Mycobacterium tuberculosis, the causative agent of tuberculosis, and as the main target of two pro-drugs: isoniazid, the frontline anti-tubercular drug, and ethionamide, a second-line medicine. Since most cases of resistance to isoniazid and ethionamide result from mutations in the mycobacterial activating enzyme (KatG for isoniazid and EthA for ethionamide), research of direct InhA inhibitors, avoiding the activation step, has emerged as a promising strategy for combating tuberculosis. Thereby, InhA is drawing much attention and its three-dimensional structure has been particularly studied. A better understanding of key sites of interactions responsible for InhA inhibition arises thus as an essential tool for the rational design of new potent inhibitors. In this paper, we propose an overview of the 80 available crystal structures of wild-type and mutant InhA, in its apo form, in complex with its cofactor, with an analogue of its natural ligands (C16 fatty acid and/or NADH) or with inhibitors. We will first discuss structural and mechanistic aspects in order to highlight key features of the protein before delivering thorough inventory of structures of InhA in the presence of synthetic ligands to underline the key interactions implicated in high affinity inhibition.