Development of gallic acid formazans as novel enoyl acyl carrier protein reductase inhibitors for the treatment of tuberculosis

The enoyl acyl carrier protein reductase (InhA) of Mycobacterium tuberculosis (MTB) is an attractive target for developing novel antitubercular agents. A series of gallic acid formazans, were computationally designed and docked into the active site of InhA to understand their binding mode and potential to inhibit InhA. Nine compounds from the designed series were identified as potential InhA inhibitors, on the basis of good Glide score. These compounds were synthesized in the laboratory and evaluated for in vitro antitubercular activity against drug-sensitive and multi-drug resistant strains of MTB. Out of nine compounds, three compounds exhibited the most promising MIC of <2 μM against the sensitive strain of MTB, H37Rv. The compounds were evaluated against five resistant strains of MTB. Most of the compounds exhibited activity superior to the standard, linezolid, against all these resistant strains. The mechanism of action of these compounds was concluded to be InhA inhibition, through InhA enzyme inhibition study. Insignificant cytotoxicity of these compounds was observed on RAW 264.7 cell line. Inactivity of all these compounds against gram positive and gram negative bacteria indicated their specificity against MTB. The compounds were further analyzed for ADME properties and showed potential as good oral drug candidates. The results clearly identified some novel, selective and specific InhA inhibitors against sensitive and resistant strains of MTB.     

Design,synthesis, and biological evaluation of 4-(5-nitrofuran-2-yl)prop-2-en-1-one derivatives as potent antitubercular agents

Based on stereoelectronic feature analysis using density functional theory (DFT) at B3LYP/3-211G level, a series of 4-(5-nitrofuran-2-yl)prop-2-en-1-one derivatives with low LUMO energies (<?0.10 eV); concentrated over the nitro group, furan moiety and α,β-unsaturated carbonyl bridge were envisaged as potential antitubercular agents. The target compounds were prepared by condensation of 5-nitro-2-furaldehyde with various ketones under acidic condition. The compounds were evaluated for antitubercular activity against Mycobacterium tuberculosis H37Rv and their cytotoxicity in VERO cell line. Several synthesized compounds showed good antitubercular activity of <5 μM along with low cytotoxicity. In particular, compound ((E)-3-(5-nitrofuran-2-yl)-1-(4-(piperidin-1-yl)phenyl)prop-2-en-1-one) (3v) was found to be very potent (MIC: 0.19 μM) with good selectivity index (MIC₉₀/CC₅₀: >1800). Thus, this study shows the potential of stereoelectronic property analysis in developing improved nitroaromatics as antitubercular agents.     

Design,synthesis, and biological evaluation of dihydroartemisinin–fluoroquinolone conjugates as a novel type of potential antitubercular agents

Tuberculosis remains a global public health problem in recent years. To develop novel type of potential antitubercular agents, twelve novel dihydroartemisinin–fluoroquinolone (DHA–FQ) conjugates (three types of molecules) were gradually designed and conveniently synthesized. All the newly synthesized conjugates were well characterized and evaluated against different Mycobacterium tuberculosis strains in vitro. The screening results showed that five DHA–FQ conjugates were active toward M. tuberculosis H37Rv, and compound 3a exhibited the strongest inhibitory activity (MIC = 0.0625 μg/mL), which was comparable to the positive control Moxifloxacin and even stronger than Ofloxacin. Conjugates 2a and 3a also displayed comparable activities against various clinically isolated sensitive and resistant M. tuberculosis strains (MIC = 0.125–16 μg/mL) to Moxifloxacin. All target compounds possessed selective anti-M. tuberculosis ability. Preliminary structure–activity relationship demonstrated that short linker between DHA and FQ was favorable for strong antitubercular activity. This study provides a new clue for the development of novel antitubercular lead molecules.     

Discovery of dipiperidines as new antitubercular agents

As part of our ongoing research effort to develop new therapeutics for treatment of tuberculosis (TB), we synthesized a combinatorial library of 10,358 compounds on solid support using a pool-and-split technique and tested the resulting compounds for activity against Mycobacterium tuberculosis. Structure–activity relationship (SAR) evaluation identified new compounds with antitubercular activity, including a novel hit series that is structurally unrelated to any existing antitubercular drugs, dipiperidines. Dipiperidine representatives exhibited MIC values as low as 7.8 μM, the ability to induce promoter Rv0341 activated in response to cell wall biosynthesis inhibition, relatively low nonspecific cellular toxicity in the range of 30–162 μM, and log P values less than 4.     

Identification and development of 2-methylimidazo[1,2-a]pyridine-3-carboxamides as Mycobacterium tuberculosis pantothenate synthetase inhibitors

In the present study, we used crystal structure of mycobacterial pantothenate synthetase (PS) bound with 2-(2-(benzofuran-2-ylsulfonylcarbamoyl)-5-methoxy-1H-indol-1-yl) acetic acid inhibitor for virtual screening of antitubercular compound database to identify new scaffolds. One of the identified lead was modified synthetically to obtain thirty novel analogues. These synthesized compounds were evaluated for Mycobacterium tuberculosis (MTB) PS inhibition study, in vitro antimycobacterial activities and cytotoxicity against RAW 264.7 cell line. Among the compounds tested, N′-(1-naphthoyl)-2-methylimidazo[1,2-a]pyridine-3-carbohydrazide (5b) was found to be the most active compound with IC₅₀ of 1.90 ± 0.12 μM against MTB PS, MIC of 4.53 μM against MTB with no cytotoxicity at 50 μM. The binding affinity of the most potent inhibitor 5b was further confirmed biophysically through differential scanning fluorimetry.     

Synthesis and antimycobacterial activities of some new thiazolylhydrazone derivatives

This Letter reports the synthesis and evaluation of some thiazolylhydrazone derivatives for their in vitro antimycobacterial activities against Mycobacterium tuberculosis H37Rv. The cytotoxic activities of all compounds were also evaluated. The compounds exhibited promising antimycobacterial activity with MICs of 1.03–72.46 μM and weak cytotoxicity (8.9–36.8% at 50 μg/mL). Among them, 1-(4-(1H-1,2,4-triazol-1-yl)benzylidene)-2-(4-(4-nitrophenyl)thiazol-2-yl)hydrazine 10 was found to be the most active compound (MIC of 1.03 μM) with a good safety profile (16.4% at 50 μg/mL). Molecular modeling studies were done to have an idea for the mechanism of the action of the target compounds. According the docking results it can be claimed that these compounds may bind most likely to TMPK than InhA or CYP121.     

Development of benzo[d]oxazol-2(3H)-ones derivatives as novel inhibitors of Mycobacterium tuberculosis InhA

A series of twenty seven substituted 2-(2-oxobenzo[d]oxazol-3(2H)-yl)acetamide derivatives were designed based on our earlier reported Mycobacterium tuberculosis (MTB) enoyl-acyl carrier protein reductase (InhA) lead. Compounds were evaluated for MTB InhA inhibition study, in vitro activity against drug-sensitive and -resistant MTB strains, and cytotoxicity against RAW 264.7 cell line. Among the compounds tested, 2-(6-nitro-2-oxobenzo[d]oxazol-3(2H)-yl)-N-(5-nitrothiazol-2-yl)acetamide (30) was found to be the most promising compound with IC₅₀ of 5.12 ± 0.44 μM against MTB InhA, inhibited drug sensitive MTB with MIC 17.11 μM and was non-cytotoxic at 100 μM. The interaction with protein and enhancement of protein stability in complex with compound 30 was further confirmed biophysically by differential scanning fluorimetry.     

Synthesis and SAR studies of 1,4-benzoxazine MenB inhibitors: Novel antibacterial agents against Mycobacterium tuberculosis

Menaquinone is an essential component of the electron transport chain in many pathogens and consequently enzymes in the menaquinone biosynthesis pathway are potential drug targets for the development of novel antibacterial agents. In order to identify leads that target MenB, the 1,4-dihydroxy-2-naphthoyl-CoA synthase from Mycobacterium tuberculosis, a high-throughput screen was performed. Several 1,4-benzoxazines were identified in this screen and subsequent SAR studies resulted in the discovery of compounds with excellent antibacterial activity against M. tuberculosis H37Rv with MIC values as low as 0.6 μg/ml. The 1,4-benzoxazine scaffold is thus a promising foundation for the development of antitubercular agents.     

Antimycobacterial activity evaluation,time-kill kinetic and 3D-QSAR study of C-(3-aminomethyl-cyclohexyl)-methylamine derivatives

A series of C-(3-aminomethyl-cyclohexyl)-methylamine derivatives were synthesized and evaluated for their antitubercular activity. Some of the compounds exhibited potent activity against Mycobacterium tuberculosis H37Rv. One of the compound having t-butyl at para position of the benzene ring showed excellent activity even better than the standard drug ethambutol with MIC value 1.1 ± 0.2 μM. The time-kill kinetics study of two most active compounds showed rapid killing of the M. tuberculosis within 4 days. Additionally atom-based quantitative structure–activity relationship (QSAR) model was developed that gave a statistically satisfying result (R²) = 0.92, Q² = 0.75, Pearson-R = 0.96 and effectively predicts the anti-tuberculosis activity of training and test set compounds.     

A regio- and stereoselective 1,3-dipolar cycloaddition for the synthesis of new-fangled dispiropyrrolothiazoles as antimycobacterial agents

A series of dispiropyrrolothiazoles compounds were synthesized using 1,3-dipolar cycloaddition and were screened for antimycobacterial activity against Mycobacterium tuberculosis H₃₇Rv and INH resistant M. tuberculosis strains. Two of them were showing good activity with MIC of less than 1 μM. Compound (5f) was found to be the most active with MIC of 0.210 and 8.312 μM respectively.     

Synthesis of non-purine analogs of 6-aryl-9-benzylpurines,and their antimycobacterial activities. Compounds modified in the imidazole ring

Purine analogs modified in the five-membered ring have been synthesized and examined for antibacterial activity against Mycobacterium tuberculosis H₃₇Rv in vitro employing the microplate alamar blue assay (MABA). The 9-deaza analogs were only found to be weak inhibitors, but the 8-aza-, 7-deaza- and 8-aza-7-deazapurine analogs studied displayed excellent antimycobacterial activities, some even substantially better than the parent purine. In the 7-deazapurine series, MIC values between 0.08 and 0.35 μM, values comparable or better than the reference drugs used in the study (MIC rifampicin 0.09 μM, MIC isoniazid 0.28 μM and MIC PA-824 0.44 μM). The five most active compounds were also examined against a panel of drug-resistant Mtb strain, and they all retained their activity. The compounds examined were significantly less active against M. tuberculosis in a state of non-replicating persistence (NRP). MIC in the low-oxygen-recovery assay (LORA) ?60 μM. The 7-deazapurines were somewhat more toxic towards mammalian cells, but still the selectivity indexes were excellent. The non-purine analogs exhibit a selective antimycobacterial activity. They were essentially inactive against Staphylococcus aureus and Escherichia coli.     

Development of antitubercular compounds based on a 4-quinolylhydrazone scaffold. Further structure–activity relationship studies

A series of 4-quinolylhydrazones was synthesized and tested in vitro against Mycobacterium tuberculosis. At a concentration of 6.25 μg/mL, most of the newly synthesized compounds displayed 100% inhibitory activity against M. tuberculosis in cellular assays. Further screening allowed the identification of very potent antitubercular agents. Compound 4c was also tested in a time-course experiment and against mtb clinical isolates, displaying interesting results.     

Novel molecular hybrids of cinnamic acids and guanylhydrazones as potential antitubercular agents

In an attempt to identify potential new agents active against tuberculosis, 20 novel phenylacrylamide derivatives incorporating cinnamic acids and guanylhydrazones were synthesized using microwave assisted synthesis. Activity of the synthesized compounds was evaluated using resazurin microtitre plate assay (REMA) against Mycobacterium tuberculosis H37Rv. Based on empirical structure–activity relationship data it was observed that both steric and electronic parameters play major role in the activity of this series of compounds. Compound 7s (2E)-N-((-2-(3,4-dimethoxybenzylidene) hydrazinyl) (imino) methyl)-3-(4-methoxyphenyl) acrylamide showed MIC of 6.49 μM along with good safety profile of >50-fold in VERO cell line. Thus, this compound could act as a potential lead for further antitubercular studies.     

Novel isoniazid embedded triazole derivatives: Synthesis,antitubercular and antimicrobial activity evaluation

In the present study, a series of new isoniazid embedded triazole derivatives have been synthesized. These compounds were evaluated for their in vitro antitubercular and antimicrobial activities. Among the screened compounds, six have exhibited potent antitubercular activity against Mycobacterium tuberculosis H37Rv strain with MIC value 0.78 μg/mL, whereas, three compounds have displayed activity with MIC value ranging from 1.56 to 3.125 μg/mL. The cytotoxicity of the active compounds was studied against RAW 264.7 cell line by MTT assay and no toxicity was observed even at 25 μg/mL concentration. The five compounds have displayed good antimicrobial activities. Molecular docking have been performed against mycobacterial InhA enzyme to gain an insight into the plausible mechanism of action which could pave the way for our endeavor to identify potent antitubercular candidates. We believe that further optimization of these molecules may lead to potent antitubercular agents.     

Bacterial Peptide deformylase inhibition of cyano substituted biaryl analogs: Synthesis,in vitro biological evaluation,molecular docking study and in silico ADME prediction

Herein, we report the synthesis and screening of cyano substituted biaryl analogs 5(a–m) as Peptide deformylase (PDF) enzyme inhibitors. The compounds 5a (IC₅₀ value = 13.16 μM), 5d (IC₅₀ value = 15.66 μM) and 5j (IC₅₀ value = 19.16 μM) had shown good PDF inhibition activity. The compounds 5a (MIC range = 11.00–15.83 μg/mL), 5b (MIC range = 23.75–28.50 μg/mL) and 5j (MIC range = 7.66–16.91 μg/mL) had also shown potent antibacterial activity when compared with ciprofloxacin (MIC range = 25–50 μg/mL). Thus, the active derivatives were not only potent PDF inhibitors but also efficient antibacterial agents. In order to gain more insight on the binding mode of the compounds with PDF, the synthesized compounds 5(a–m) were docked against PDF enzyme of Escherichia coli and compounds exhibited good binding properties. In silico ADME properties of synthesized compounds were also analyzed and showed potential to develop as good oral drug candidates.     

New class of methyl tetrazole based hybrid of (Z)-5-benzylidene-2-(piperazin-1-yl)thiazol-4(%H)-one as potent antitubercular agents

In search of potential therapeutics for tuberculosis, we describe here the synthesis and in vitro antitubercular activity of a novel series of thiazolone piperazine tetrazole derivatives. Among all the synthesized derivatives, four compounds (10, 14, 20 and 33) exhibited more potent activity (MIC = 3.08, 3.01, 2.62 and 2.51 μM) than ethambutol (MIC = 9.78 μM) and pyrazinamide (MIC = 101.53 μM) against Mycobacterium tuberculosis. Furthermore, they displayed no toxicity against Vero cells (C1008) and mouse bone marrow derived macrophages (MBMDMϕ). These investigated analogues have emerged as possible lead molecule to enlarge the scope of the study.     

Anti-tubercular agents. Part 8: Synthesis,antibacterial and antitubercular activity of 5-nitrofuran based 1,2,3-triazoles

A series of 5-nitrofuran–triazole conjugates were synthesized and evaluated for their antimicrobial activity against both Gram-positive and Gram-negative bacterial strains. All the compounds exhibited promising inhibition towards Gram-positive pathogenic strains, while mild inhibitory effects were observed towards Gram-negative bacterial strains. Some of the compounds 8a, 8b, 8e, 8f, 8h are most active among the series exhibiting MIC value of 1.17 μg/ml against different bacterial strains. The bactericidal activity is found to be in accordance with the bacterial growth inhibition data. Compound 8e was found to be equipotent to the standard drug Ciprofloxacin displaying MBC value of 1.17 μg/ml against the bacterial strain Bacillus subtilis. The compounds have also demonstrated promising antibacterial activity against the resistant strain MRSA and were found to be effective inhibitors of biofilm formation. The compound 8b exhibited excellent anti-biofilm activity with IC₅₀ value as low as 0.8 μg/ml. These conjugates were also screened for antitubercular activity against Mycobacterium tuberculosis H₃₇Rv strain. Compound 8e showed promising antitubercular activity with MIC value of 0.25 μg/ml. Most of these compounds are less toxic to normal mammalian cells than the widely used antibacterial drug Ciprofloxacin.     

A facile three-component [3+2]-cycloaddition for the regioselective synthesis of highly functionalised dispiropyrrolidines acting as antimycobacterial agents

A series of fourteen dispiropyrrolidines were synthesized using [3+2]-cycloaddition reactions and were screened for their antimycobacterial activity against Mycobacterium tuberculosis H₃₇Rv in HTS (High Throughput Screen). Most of the compounds showed moderate to good activity with MIC of less than 20 μM. Compound 4′-(4-bromophenyl)-1′-methyldispiro[acenaphthylene-1,2′-pyrrolidine-3′,2″-indane]-2,1″(1H)-dione (4c) was found to be the most active with MIC of 12.50 μM.     

Microwave-assisted synthesis,molecular docking and antitubercular activity of 1,2,3,4-tetrahydropyrimidine-5-carbonitrile derivatives

Based on bioisosteric similarities with isoniazid, a series of 1,2,3,4-tetrahydropyrimidine-5-carbonitrile derivatives has been designed. The target compounds have been synthesized by multicomponent reaction which involves one-pot organic reactions using ethylcyanoacetate, urea/thiourea and arylaldehydes in presence of ethanolic K₂CO₃. Two methodologies, conventional and microwave-assisted, have been adopted for the synthesis. The later strategy gave high yields in less than 10 min as compared to long hours using the former approach. Molecular docking of the target compounds into the enzyme Mycobacterium tuberculosis enoyl reductase (InhA) revealed important structural information on the plausible binding interactions. Major binding interactions were found to be of dispersion type (residues Tyr158, Ile215, Met103 and Met199) and a hydrogen bond with Tyr158. Binding poses of the all the compounds were energetically favorable and showed good interactions with the active site residues. Few selected compounds were also evaluated for antitubercular activity in vitro against drug-sensitive M. tuberculosis H37Rv strain and clinically isolated S, H, R and E resistant M. tuberculosis by luciferase reporter phage (LRP) assay method. Some compounds displayed promising antimycobacterial activity comparable or less than the standard drugs isoniazid and rifampicin.     

Development of novel tetrahydrothieno[2,3-c]pyridine-3-carboxamide based Mycobacterium tuberculosis pantothenate synthetase inhibitors: Molecular hybridization from known antimycobacterial leads

Twenty six 2,6-disubstituted 4,5,6,7-tetrahydrothieno[2,3-c]pyridine-3-carboxamide derivatives were designed by molecular hybridization approach using and synthesized from piperidin-4-one by five step synthesis. Compounds were evaluated for Mycobacterium tuberculosis (MTB) pantothenate synthetase (PS) inhibition study, in vitro activities against MTB, cytotoxicity against RAW 264.7 cell line. Among the compounds, 6-(4-nitrophenylsulfonyl)-2-(5-nitrothiophene-2-carboxamido)-4,5,6,7-tetrahydrothieno[2,3-c]pyridine-3-carboxamide (11) was found to be the most active compound with IC₅₀ of 5.87 ± 0.12 μM against MTB PS, inhibited MTB with MIC of 9.28 μM and it was non-cytotoxic at 50 μM. The binding affinity of the most potent inhibitor 11 was further confirmed biophysically through differential scanning fluorimetry.