Synthesis, oxidation potential and anti-mycobacterial activity of isoniazid and analogues: insights into the molecular isoniazid activation mechanism

Targeting Mycobacterium tuberculosis iron-scavenging tools: a recent update on siderophores inhibitors

Among the various bacterial infections, tuberculosis (TB) remains a life-threatening infectious disease responsible as the most significant cause of mortality and morbidity worldwide. The co-infection of human immunodeficiency virus (HIV) in association with TB burdens the healthcare system substantially. Notably, M.tb possesses defence against most antitubercular antibiotic drugs, and the efficacy of existing frontline anti-TB drugs is waning. Also, new and recurring cases of TB from resistant bacteria such as multidrug-resistant TB (MDR), extensively drug-resistant TB (XDR), and totally drug-resistant TB (TDR) strains are increasing. Hence, TB begs the scientific community to explore the new therapeutic class of compounds with their novel mechanism. M.tb requires iron from host cells to sustain, grow, and carry out several biological processes. M.tb has developed strategic methods of acquiring iron from the surrounding environment. In this communication, we discuss an overview of M.tb iron-scavenging tools. Also, we have summarized recently identified MbtA and MbtI inhibitors, which prevent M.tb from scavenging iron. These iron-scavenging tool inhibitors have the potential to be developed as anti-TB agents/drugs.

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.

Targeting mycobacterial membranes and membrane proteins: Progress and limitations

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

Isonicotinic acid hydrazide (INH) versus extra-amniotic saline infusion (EASI) for cervical ripening at term: a randomised controlled trial

The purpose of this trial was to compare extra-amniotic saline infusion (EASI) and intravaginal isoniazid (INH) for cervical ripening. This randomised clinical trial included 150 pregnant women who were undergoing induction of labour and who required pre-induction cervical ripening. Patients were randomly assigned to receive EASI or intravaginal INH. Bishop's score at the beginning of the study and before oxytocin infusion was not significantly different between INH and EASI groups. However, the time from first intervention to the beginning of the induction and also to the beginning of the active phase were significantly shorter in EASI group (p value ≤.001). Moreover, INH did not influence the labour process after the beginning of the active phase of labour. In conclusion, INH could be used for cervical ripening especially in the outpatient setting; however, it is a slower ripening agent compared to EASI.Impact StatementWhat is already known on this subject? To date there has been only one study about the safety and effectiveness of isoniazid (INH) in cervical ripening at term pregnancy which has compared INH with misoprostol.What do the results of this study add? The results of this study showed that vaginal INH is an effective agent for cervical ripening at term but in comparison to extra-amniotic saline infusion (EASI) it takes a longer time.What are the implications of these findings for clinical practice and/or further research? INH can be used in outpatient settings for cervical ripening at term pregnancy which makes it convenient for patient and cost effective for both patient and health system. Further studies are needed to discover the clinical efficacy of INH in comparison to other ripening methods and also the best dosage of INH for cervical ripening.

Emerging impact of triazoles as anti-tubercular agent

Tuberculosis, a disease of poverty is a communicable infection with a reasonably high mortality rate worldwide. 10 Million new cases of TB were reported with approx 1.4 million deaths in the year 2019. Due to the growing number of drug-sensitive and drug-resistant tuberculosis cases, there is a vital need to develop new and effective candidates useful to combat this deadly disease. Despite tremendous efforts to identify a mechanism-based novel antitubercular agent, only a few have entered into clinical trials in the last six decades. In recent years, triazoles have been well explored as the most valuable scaffolds in drug discovery and development. Triazole framework possesses favorable properties like hydrogen bonding, moderate dipole moment, enhanced water solubility, and also the ability to bind effectively with biomolecular targets of M. tuberculosis and therefore this scaffold displayed excellent potency against TB. This review is an endeavor to summarize an up-to-date innovation of triazole-appended hybrids during the last 10 years having potential in vitro and in vivo antitubercular activity with structure activity relationship analysis. This review may help medicinal chemists to explore the triazole scaffolds for the rational design of potent drug candidates having better efficacy, improved selectivity and minimal toxicity so that these hybrid NCEs can effectively be explored as potential lead to fight against M. tuberculosis.

Residues of helix ɑ2 are critical for catalytic efficiency of mycobacterial alkylhydroperoxide reductase subunit C

The ability of Mycobacteria to overcome oxidative stress is of paramount importance for its survival within the host. One of the key enzymes that are involved in protecting the bacterium from reactive oxygen species is the catalase-peroxidase (KatG). However, in strains resistant to the antibiotic isoniazid, KatG is rendered ineffective, which is associated with an increased expression of alkylhydroperoxide reductase subunit C (AhpC). Mycobacterial AhpC possesses a unique helical displacement when compared to its bacterial counterparts. Here, via mutagenesis studies, we demonstrate the importance of this helix for redox modulation of the catalytic activity of AhpC. Along with structural insights from crystallographic data, the impact of critical residues on the structure and flexibility of the helix and on AhpC oligomerization is described.

Mechanistic and thermodynamic aspects of a pyrene-based fluorescent probe to detect picric acid

Thermodynamic investigation of picric acid sensing using a pyrene-based fluorescent probe.

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.

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

Ethionamide (ETH), a second-line anti-tubercular drug that is regaining a lot of interest due to the increasing cases of drug-resistant tuberculosis, is a pro-drug that requires an enzymatic activation step to become active and to exert its therapeutic effect. The enzyme responsible for ETH bioactivation in Mycobacterium tuberculosis is a monooxygenase (EthA) that uses flavin adenine dinucleotide (FAD) as a cofactor and is NADPH- and O₂-dependant to exert its catalytic activity. In this work, we investigated the activation of ETH by various oxygen-donor oxidants and the first biomimetic ETH activation methods were developed (KHSO₅, H₂O₂, and m-CPBA). These simple oxidative systems, in the presence of ETH and NAD⁺, allowed the production of short-lived radical species and the first non-enzymatic formation of active and non-active ETH metabolites. The intermediates and the final compounds of the activation pathway were well characterized. Based on these results, we postulated a consistent mechanism for ETH activation, not involving sulfinic acid as a precursor of the iminoyl radical, as proposed so far, but putting forward a novel reactivity for the S-oxide ethionamide intermediate. We proposed that ETH is first oxidized into S-oxide ethionamide, which then behaves as a "ketene-like" compound via a formal [2 + 2] cycloaddition reaction with peroxide to give a dioxetane intermediate. This unstable 4-membered intermediate in equilibrium with its open tautomeric form decomposes through different pathways, which would explain the formation of the iminoyl radical and also that of different metabolites observed for ETH oxidation, including the ETH-NAD active adduct. The elucidation of this unprecedented ETH activation mechanism was supported by the application of isotopic labelling experiments.