Isoniazid–phytochemical conjugation: A new approach for potent and less toxic anti‐TB drug development

Mycobacterium tuberculosis (Mtb) causes one of the most grievous pandemic infectious diseases, tuberculosis (TB), with long‐term morbidity and high mortality. The emergence of drug‐resistant Mtb strains, and the co‐infection with human immunodeficiency virus, challenges the current WHO‐TB stewardship programs. The first‐line anti‐TB drugs, isoniazid (INH) and rifampicin (RIF), have become extensively obsolete in TB control from chromosomal mutations during the last decades. However, based on clinical trial statistics, the production of well‐tolerated anti‐TB drug(s) is miserably low. Alternately, semi‐synthesis or structural modifications of first‐line obsolete antitubercular drugs remain as the versatile approach for getting some potential medicines. The use of any suitable phytochemicals with INH in a hybrid formulation could be an ideal approach for the development of potent anti‐TB drug(s). The primary objective of this review was to highlight and analyze available INH–phytochemical hybrid research works. The utilization of phytochemicals through chemical conjugation is a new trend toward the development of safer/non‐toxic anti‐TB drugs.     

A Note on Derivatives of Isoniazid,Rifampicin, and Pyrazinamide Showing Activity Against Resistant Mycobacterium tuberculosis

Drug‐resistant tuberculosis (DR‐TB) is a serious problem that impedes the success of the TB control program. Of note, multidrug‐resistant (MDR)‐TB and extensively drug‐resistant (XDR)‐TB have certainly complicated the scenario. One of the possible strategies to overcome drug resistance in an economic and simple manner would involve modification of existing anti‐TB drugs to obtain derivatives that can work on resistant TB bacilli. These may have improved half‐life and increased bioavailability, be more efficacious, and serve as cost‐effective alternatives, as compared to new drugs identified through conventional methods of drug discovery and development. Although extensive literature is available on the activity of various derivatives of first‐line drugs (isoniazid, rifampicin and pyrazinamide) on drug‐susceptible Mycobacterium tuberculosis (MTB), reports on the activity of derivatives on resistant MTB are very limited, to our knowledge. In light of this, the present review aims to provide a concise report on the derivatives of first‐line drugs that have the potential to overcome the resistance to the parental drug and could thus serve as effective alternatives.     

New Compounds Hybrids 1H‐1,2,3‐Triazole‐Quinoline Against Plasmodium falciparum

Malaria is one of the most prevalent parasitic diseases in the world. The global importance of this disease, current vector control limitations, and the absence of an effective vaccine make the use of therapeutic antimalarial drugs the main strategy to control malaria. Chloroquine is a cost‐effective antimalarial drug with a relatively robust safety profile, or therapeutic index. However, chloroquine is no longer used alone to treat patients with Plasmodium falciparum due to the emergence and spread of chloroquine‐resistant strains, which have also been reported for Plasmodium vivax. However, the activity of 1,2,3‐triazole derivatives against chloroquine‐sensitive and chloroquine‐resistant strains of P. falciparum has been reported in the literature. To enhance the anti‐P. falciparum activity of quinoline derivatives, we synthesized 11 new quinoline‐1H‐1,2,3‐triazole hybrids with different substituents in the 4‐positions of the 1H‐1,2,3‐triazole ring, which were assayed against the W2‐chloroquine‐resistant P. falciparum clone. Six compounds exhibited activity against the P. falciparum W2 clone, chloroquine‐resistant, with IC₅₀ values ranging from 1.4 to 46 μm . None of these compounds was toxic to a normal monkey kidney cell line, thus exhibiting good selectivity indexes, as high 351 for one compound ( 11 ).     

Quinoxaline and quinoxaline‐1,4‐di‐N‐oxides: An emerging class of antimycobacterials

Tuberculosis (TB) is a highly dreaded, infectious, chronic, airborne disease affecting more than two million people all around the world, with more than eight million cases every calendar year. TB is the second leading infectious cause of death after HIV/AIDS. Over the past few decades, numerous efforts have been undertaken to develop new anti‐TB agents. The current frontline therapy for TB consists of administering three or more different drugs (usually isoniazid, rifampin, pyrazinamide, and ethambutol) over an extended period of time. But these drugs will take 6–12 months to cure TB, along with many side effects; hence, there is an urgent need to explore new anti‐TB agents. Quinoxaline derivatives are a class of compounds that show a spectrum of biological properties and the interest in these compounds is exponentially growing within the field of medicinal chemistry. Quinoxaline‐1,4‐di‐N‐oxide derivatives have shown to improve the biological results and are endowed with anti‐viral, anti‐cancer, anti‐bacterial, and anti‐protozoal activities with application in many other therapeutic areas. Since quinoxaline derivatives are regarded as a new class of effective anti‐TB candidates, their 1,4‐di‐N‐oxide analogues may show promising in vitro and in vivo anti‐TB activities and might be able to prevent the drug resistance to a certain extent. Therefore, the main aim of this review is to focus on important quinoxaline and quinoxaline‐1,4‐di‐N‐oxide analogues that have shown anti‐TB activities, and their structure–activity relationships for designing anti‐TB agents with better efficacies. The present review will be helpful in providing insights for rational designs of more active and less toxic quinoxaline‐based anti‐TB prodrugs.

     

Bioassay‐Guided Isolation and Structural Modification of the Anti‐TB Resorcinols from Ardisia gigantifolia

Tuberculosis (TB) is a highly contagious disease mainly caused by Mycobacterium tuberculosis H₃₇R_V. Antitubercular (anti‐TB) bioassay‐guided isolation of the CHCl₃ extract of the leaves and stems of the medicinal plant Ardisia gigantifolia led to the isolation of two anti‐TB 5‐alkylresorcinols, 5‐(8Z‐heptadecenyl) resorcinol ( 1 ) and 5‐(8Z‐pentadecenyl) resorcinol ( 2 ). We further synthesized 15 derivatives based on these two natural products. These compounds (natural and synthetic) were evaluated for their anti‐TB activity against Mycobacterium tuberculosis H₃₇R_V. Resorcinols 1 and 2 exhibited anti‐TB activity with MIC values at 34.4 and 79.2 μm in MABA assay, respectively, and 91.7 and 168.3 μm in LORA assay, respectively. Among these derivatives, compound 8 was found to show improved anti‐TB activity than its synthetic precursor ( 2 ) with MIC values at 42.0 μm in MABA assay and 100.2 μm in LORA assay. The active compounds should be regarded as new hits for further study as a novel class of anti‐TB agents. The distinct structure–activity correlations of the parent compound were elucidated based on these derivatives.     

Design,synthesis and biological evaluation of novel quinoline‐based carboxylic hydrazides as anti‐tubercular agents

In this study, seventeen novel quinoline‐based carboxylic hydrazides were designed as potential anti‐tubercular agents using molecular hybridization approach and evaluated in‐silico for drug‐likeness behavior. The compounds were synthesized, purified, and characterized using spectral techniques (like FTIR, ¹H NMR, and Mass). The in‐vitro anti‐tubercular activity (against Mycobacterium tuberculosisH37Ra) and cytotoxicity against human lung fibroblast cells were studied. Among the tested hydrazides, four compounds ( 6h , 6j , 6l, and 6m ) exhibited significant anti‐tubercular activity with MIC values below 20 μg/mL. The two most potent compounds of the series, 6j and 6m exhibited MIC values 7.70 and 7.13 μg/mL, respectively, against M. tuberculosis with selectivity index >26. Structure–activity relationship studies were performed for the tested compounds in order to explore the effect of substitution pattern on the anti‐tubercular activity of the synthesized compounds.     

6‐Hydrogen‐8‐Methylquinolones Active Against Replicating and Non‐replicating Mycobacterium tuberculosis

The screening of an in‐house quinolones library against Mycobacterium tuberculosis (Mtb) H₃₇Rv, followed by a first cycle of optimization, yielded 6‐hydrogen‐8‐methyl derivatives endowed with good potency. The antitubercular activity also encompassed the bacteria in a non‐replicating state (NRP‐TB) with minimum inhibitory concentration values lower than those of the reference agent, moxifloxacin. Among the best compounds, 11w and 11ai , characterized by a properly substituted piperidine at the C‐7 position, were active against single‐drug‐resistant (SDR‐TB) Mtb strains, maintaining overall good potency also against ciprofloxacin‐resistant Mtb. This study expands the body of SAR around antitubercular quinolones leading to reconsider the role played by the usual fluorine atom at the C‐6 position. Further elaboration of the 6‐hydrogen‐8‐methylquinolone scaffold, with a particular focus on the C‐7 position, is expected to give even more potent congeners holding promise for shortening the current anti‐TB regimen.     

4‐Aminoquinoline‐Triazine‐Based Hybrids with Improved In Vitro Antimalarial Activity Against CQ‐Sensitive and CQ‐Resistant Strains of Plasmodium falciparum

A systematic chemical modification in the triazine moiety covalently attached via suitable linkers to 4‐amino‐7‐chloroquinolines yielded a series of new 7‐chloro‐4‐aminoquinoline‐triazine hybrids exhibiting high in vitro activity against W2 (chloroquine‐resistant) and D6 (chloroquine‐sensitive) strains of Plasmodium falciparum without any toxicity against mammalian cell lines (Vero, LLC‐PK11, HepG2). Many of the compounds ( 6, 8, 10, 11, 13, 14, 16, 27, 29 and 33 ) showed excellent potency against chloroquine sensitive and resistant strains. In particular, compounds 6, 8 , 14 , 16 and 29 were found to be significantly more active than chloroquine against the chloroquine‐resistant strains (W2 clone) of P. falciparum.     

Rational design of isonicotinic acid hydrazide derivatives with antitubercular activity: Machine learning,molecular docking,synthesis and biological testing

The problem of designing new antitubercular drugs against multiple drug‐resistant tuberculosis (MDR‐TB) was addressed using advanced machine learning methods. As there are only few published measurements against MDR‐TB, we collected a large literature data set and developed models against the non‐resistant H37Rv strain. The predictive accuracy of these models had a coefficient of determination q² = .7–.8 (regression models) and balanced accuracies of about 80% (classification models) with cross‐validation and independent test sets. The models were applied to screen a virtual chemical library, which was designed to have MDR‐TB activity. The seven most promising compounds were identified, synthesized and tested. All of them showed activity against the H37Rv strain, and three molecules demonstrated activity against the MDR‐TB strain. The docking analysis indicated that the discovered molecules could bind enoyl reductase, InhA, which is required in mycobacterial cell wall development. The models are freely available online ( http://ochem.eu/article/103868 ) and can be used to predict potential anti‐TB activity of new chemicals.     

Synthesis,biological evaluation,and molecular docking studies of novel 3‐aryl‐5‐(alkyl‐thio)‐1H‐1,2,4‐triazoles derivatives targeting Mycobacterium tuberculosis

A small library of new 3‐aryl‐5‐(alkyl‐thio)‐1H‐1,2,4‐triazoles was synthesized and screened for the antimycobacterial potency against Mycobacterium tuberculosis H₃₇Ra strain and Mycobacterium bovis BCG both in active and dormant stage. Among the synthesized library, 25 compounds exhibited promising anti‐TB activity in the range of IC₅₀0.03–5.88 μg/ml for dormant stage and 20 compounds in the range of 0.03–6.96 μg/ml for active stage. Their lower toxicity (>100 μg/ml) and higher selectivity (SI = >10) against all cancer cell lines screened make them interesting compounds with potential antimycobacterial effects. Furthermore, to rationalize the observed biological activity data and to establish a structural basis for inhibition of M. tuberculosis, the molecular docking study was carried out against a potential target MTB CYP121 which revealed a significant correlation between the binding score and biological activity for these compounds. Cytotoxicity and in vivo pharmacokinetic studies suggested that 1,2,4‐triazole analogues have an acceptable safety index, in vivo stability and bio‐availability.     

Two‐ and Three‐dimensional Rings in Drugs

Using small, flat aromatic rings as components of fragments or molecules is a common practice in fragment‐based drug discovery and lead optimization. With an increasing focus on the exploration of novel biological and chemical space, and their improved synthetic accessibility, 3D fragments are attracting increasing interest. This study presents a detailed analysis of 3D and 2D ring fragments in marketed drugs. Several measures of properties were used, such as the type of ring assemblies and molecular shapes. The study also took into account the relationship between protein classes targeted by each ring fragment, providing target‐specific information. The analysis shows the high structural and shape diversity of 3D ring systems and their importance in bioactive compounds. Major differences in 2D and 3D fragments are apparent in ligands that bind to the major drug targets such as GPCRs, ion channels, and enzymes.     

Multi‐target,ensemble‐based virtual screening yields novel allosteric KRAS inhibitors at high success rate

RAS mutations account for >15% of all human tumors, and of these ~85% are due to mutations in a particular RAS gene: KRAS. Recent studies revealed that KRAS harbors four druggable allosteric sites. Here, we have (a) used molecular simulations to generate ensembles of wild type and four major oncogenic KRAS mutants (G12V, G12D, G13D, and Q61H); (b) characterized the druggability of each allosteric pocket in each protein; (c) conducted extensive ensemble‐based virtual screening using pocket‐tailored ligand libraries; (d) prioritized hits through hierarchical postdocking analysis; and (e) validated predicted hits with NMR. Of the 785 diverse potential hits identified by our in silico analysis, we tested 90 for their ability to bind KRAS using NMR and found that nine cause backbone amide chemical shift perturbations of residues near the functionally responsive switch loops, suggesting potential binding. We conducted detailed biophysical analyses on a novel indole‐based compound to demonstrate the potential of our workflow to yield lead compounds. We believe the detailed information documented in this work regarding the druggability profile of each allosteric site and the chemical fingerprints of compounds that target them will serve as vital resources for future structure‐based drug design efforts against KRAS, a high‐value target for cancer therapy.     

Synthesis and Antimycobacterial Activity of Azetidine‐, Quinazoline‐, and Triazolo‐thiadiazole‐containing Pyrazines

The re‐emergence of tuberculosis (TB) as a global health problem over the past few decades, accompanied by the rise of drug‐resistant strains of Mycobacterium tuberculosis, emphasizes the need for the discovery of new therapeutic drugs against this disease. The emerging serious problem both in terms of TB control and clinical management prompted us to synthesize a novel series of N‐[2‐(substituted aryl)‐3‐chloro‐4‐oxoazetidin‐1‐yl]‐2‐(pyrazin‐2‐yloxy)acetamide, 6‐(substituted aryl)‐3‐[(pyrazin‐2‐yloxy)methyl][1,2,4]triazolo[3,4‐b][1,3,4]thiadiazole, and N‐[6‐({2‐[(pyrazin‐2‐yloxy)acetyl] hydrazino}sulfonyl)‐2‐methyl‐4‐oxo‐1,4‐dihydroquinazolin‐3(2H)yl]‐substituted aryl sulfonamides. The compounds were synthesized using the appropriate synthetic route. All synthesized compounds were assayed in vitro for antimycobacterial activity against the H37 Rv strain of Mycobacterium tuberculosis. The minimum inhibitory concentration (MIC) was determined for the test compounds as well as for the reference standards. The compound which exhibited good antimycobacterial activity contains the substituents fluorine and methoxy. These electron‐withdrawing or ‐donating substituents amend the lipophilicity of the test compounds which, in turn, alter the permeability across the bacterial cell membrane. Compounds 28 , 37 , and 43 showed good antimycobacterial activity while compound 51 showed a promising antimycobacterial activity.     

N‐hydroxy‐substituted 2‐aryl acetamide analogs: A novel class of HIV‐1 integrase inhibitors

An in silico method has been used to discover N‐hydroxy‐substituted 2‐aryl acetamide analogs as a new class of HIV‐1 integrase inhibitors. Based on the molecular requirements of the binding pocket of catalytic active site, two molecules (compounds 2 and 4b ) were designed as fragments. These were further synthesized and biologically evaluated. In vitro potency along with docking studies highlighted compound 4b as an active fragment which was further used to synthesize new leads as HIV‐1 integrase inhibitors. Finally, six promising compounds (compounds 5b , 5c , 5e, 6–2c, 6–3b, and 6–5b ) were identified by integrase inhibition assay (>50% inhibition). Based on in vitro anti‐HIV‐1 activity in a reporter gene‐based cell assay system, compounds 5d , 6s , and 6k were found as novel HIV‐1 integrase inhibitors due to its better selectivity index. Additionally, docking study revealed the importance of H‐bond as well as hydrophobic interactions with Asn155, Lys156, and Lys159 which were required for their anti‐HIV‐1 activity.     

Synthesis and In Vitro Activity of Polyhalogenated 2‐phenylbenzimidazoles as a New Class of anti‐MRSA and Anti‐VRE Agents

A series of novel polyhalogenated 2‐phenylbenzimidazoles have been synthesized and evaluated for in vitro antistaphylococcal activity against drug‐resistant bacterial strains (methicillin‐resistant Staphylococcus aureus, and vancomycin‐resistant Enterococcus faecium. Certain compounds inhibit bacterial growth perfectly. 11 was active than vancomycin (0.78  μ g/mL) with the lowest MIC values with 0.19  μ g/mL against methicillin‐resistant Staphylococcus aureus, 8 and 35 exhibited best inhibitory activity against vancomycin‐resistant Enterococcus faecium (1.56  μ g/mL). The mechanism of action for this class of compounds appears to be different than clinically used antibiotics. These polyhalogenated benzimidazoles have potential for further investigation as a new class of potent anti‐methicillin‐resistant Staphylococcus aureus and anti‐vancomycin‐resistant Enterococcus faecium agents.     

Novel linear diamine disubstituted polycyclic 'cage' derivatives as potential antimycobacterial candidates

As a part of an ongoing project to develop highly potent antituberculosis therapeutics, a series novel polycyclic ‘cage’ tetra‐amines were synthesized and screened for in‐vitro antituberculosis activities against the H₃₇Rv strain of tuberculosis. Three disubstituted polycyclic moieties, namely pentacyclodecane, pentacycloundecane, and tricyclodecane, were used in this study. Compounds 5 and 7 showed similar activity to SQ109 at a MIC of 1 μm while compounds 4 , 6 and 8 displayed MIC activity at 1 < MIC<10 μm against H₃₇Rv strain of tuberculosis. Compounds 5 , 7 and SQ109 were selected for further screening against, multi‐drug resistant, (R1097) and extensively drug resistant, (X149) strains of tuberculosis. Compound 5 showed anti‐TB activity of a MIC = 1 μm against multi‐drug resistant strain (R1097) and <1 μM against extensively drug resistant strain (X149) while compound 7 and SQ109 showed excellent anti‐TB activity against both drug‐resistant strains at a MIC <1 μm . This study demonstrates the first reported analysis of pentacyclo[5.3.0.0^2,5.0^3,9.0^4,8]decane as a potential therapeutic agent.     

Click chemistry based regioselective one‐pot synthesis of coumarin‐3‐yl‐methyl‐1,2,3‐triazolyl‐1,2,4‐triazol‐3(4H)‐ones as newer potent antitubercular agents

Coumarin‐3‐yl‐methyl‐1,2,3‐triazolyl‐1,2,4‐triazol‐3(4H)‐ones ( 8k‐z ) were synthesized via copper(I)‐catalyzed azide‐alkyne cycloaddition click chemistry. The synthesized hybrid molecules were characterized by spectral studies. Compounds 8k‐z were screened for their in vitro anti‐TB activity by using the Microplate Alamar Blue assay and for cytotoxicity using the MTT assay. Some of the compounds were found to be most potent against the tested Mycobacterium tuberculosis H37Rv strain with a MIC of 1.60 µg/ml. Further, docking the compounds into the InhA binding pocket showed strong binding interactions and effective overall docking scores were recorded. The drug‐likeness and toxicity studies were computed using Molinspiration and Protox, respectively.