Isoniazid Reborn: Computational Design Yields a Schiff Base Lead That Outperforms a First-Line TB Drug

Tuberculosis remains one of the most stubborn infectious diseases on the planet, and the rise of drug-resistant strains has turned a once-manageable infection into a moving target. Against this backdrop, a team of researchers at Modern College of Pharmacy in Pune, India, has taken a deliberately modern approach to an old drug: they started with isoniazid, a first-line antitubercular agent whose clinical usefulness is increasingly eroded by resistance and toxicity, and systematically rebuilt it into a family of Schiff base derivatives designed to bind the tuberculosis enzyme InhA more tightly than the parent molecule. The result, published in Discover Chemistry, is a lead compound that not only outscored isoniazid in molecular docking but also outperformed it in a direct biological assay against Mycobacterium tuberculosis H37Rv.

The logic behind the study begins with the pharmacology of the starting material. Isoniazid is a prodrug that must be activated by the bacterial KatG enzyme before it can inhibit enoyl-acyl carrier protein reductase, or InhA, the enzyme responsible for building the mycolic acids that form the waxy protective coat of the tuberculosis bacillus. Mutations in KatG are a principal route by which the bacterium escapes the drug. Schiff bases, compounds defined by an azomethine linkage of the form C=N, have long attracted medicinal chemists because they are easy to synthesize, interact strongly with biological targets, and display a wide range of pharmacological activities. By attaching substituted aromatic aldehydes to the hydrazide portion of isoniazid, the researchers hoped to create molecules that could engage InhA directly and retain activity even where the classical activation pathway fails.

The team designed a focused virtual library of 30 novel isoniazid-based Schiff base derivatives by condensing isoniazid with structurally diverse substituted aromatic aldehydes. The aldehydes were chosen to span both electron-withdrawing and electron-donating substituents in different positional orientations, allowing a preliminary structure-activity relationship assessment of how these electronic changes influence lipophilicity and predicted interactions with the InhA active site. Structures were drawn in ChemDraw, converted into three-dimensional conformations, and geometry-optimized with the MMFF94 force field. Before any synthesis was attempted, the entire library was subjected to a sequential computational funnel designed to eliminate weak candidates early and concentrate laboratory effort on the most promising molecules.

That funnel is the methodological heart of the study. First, drug-likeness and pharmacokinetic properties were estimated with the SwissADME and pkCSM web servers, evaluating molecular weight, lipophilicity as LogP, hydrogen bond donors and acceptors, and topological polar surface area against Lipinski’s rule of five. Of the 30 designed compounds, 13 passed this filter and advanced. Next, toxicity prediction using the ProTox web server trimmed the field to 9 compounds with acceptable safety profiles. The Prediction of Activity Spectra for Substances, or PASS, analysis then estimated the probability of antitubercular activity, and 5 compounds emerged with favorable predicted activity. This progressive reduction, from 30 to 13 to 9 to 5, illustrates how an integrated in silico workflow can spare chemists the expense of synthesizing and testing compounds that are unlikely to survive downstream development.

The five survivors, designated SLB-1, SLB-3, SLB-5, SLB-7, and SLB-10, were then docked into the active site of InhA using AutoDock Vina. The crystal structure of the enzyme from M. tuberculosis, retrieved from the Protein Data Bank under accession 6EP8, was prepared by removing water molecules and co-crystallized ligands, adding polar hydrogens, and assigning Kollman charges. A 20 by 20 by 20 angstrom grid box centered on the active site defined the search space, and identical docking parameters were applied to all ligands, the reference drug isoniazid, and the co-crystallized ligand to ensure a fair comparison. The five Schiff base derivatives achieved binding affinities ranging from minus 8.7 to minus 9.5 kcal per mole, substantially more favorable than the minus 5.8 kcal per mole score recorded for isoniazid under the same conditions.

SLB-5, subsequently identified as N’,N”’-((5-nitro-1,3-phenylene)bis(methaneylylidene))di(isonicotinohydrazide), topped the ranking with a binding affinity of minus 9.5 kcal per mole. Interaction analysis revealed hydrogen bonds, hydrophobic contacts, and pi-pi stacking with key active-site residues including SER94, LYS165, and PHE149. The docking score also compared favorably with several known InhA inhibitors reported in the literature, including luteolin and piperine at minus 9.1 kcal per mole and tiliacorinine at minus 8.3 kcal per mole. A database and literature search across PubChem, Reaxys, and related resources found no previously reported identical structure, supporting the novelty of the compound and justifying its selection for synthesis. The authors are careful to note, however, that molecular dynamics simulations and RMSD-based redocking validation were not performed, leaving formal computational validation of the predicted binding modes for future work.

Synthesis of SLB-5 followed a classic Schiff base condensation. A substituted nitrobenzene-1,3-dicarbaldehyde was dissolved in ethanol, and a warm ethanolic solution of isoniazid was added dropwise with a few drops of glacial acetic acid as catalyst. The mixture was refluxed at 70 to 80 degrees Celsius for three to four hours, with reaction progress tracked by thin-layer chromatography. Cooling the reaction mixture produced a precipitate that was filtered, washed with cold ethanol, and dried. Structural characterization then confirmed the transformation: the FT-IR spectrum displayed a prominent azomethine C=N stretching band in the 1600 to 1650 per centimeter region alongside nitro group absorptions near 1550 and 1350 per centimeter, while the disappearance of the aldehydic C=O band confirmed consumption of the starting aldehyde.

Proton nuclear magnetic resonance spectroscopy provided further confirmation. A singlet at 8.80 parts per million corresponded to the azomethine proton, the signature of successful Schiff base formation, and a downfield singlet at 12.17 parts per million was assigned to the amide NH of the hydrazide moiety. Aromatic and pyridyl protons appeared between 7.59 and 8.68 parts per million, their deshielding consistent with the electron-withdrawing nitro and azomethine groups. Crucially, the absence of aldehydic proton signals around 9.5 to 10.0 parts per million verified complete conversion. High-resolution mass spectrometry sealed the case, showing a molecular ion at m/z 417.1722 and a characteristic fragment at m/z 371.1828 corresponding to loss of the nitro group, along with a stable base peak at m/z 285.1381 arising from cleavage of one pyridine carbohydrazide unit and the azomethine linkage.

The decisive test came in the biology. Using the Alamar Blue assay against M. tuberculosis H37Rv, SLB-5 demonstrated concentration-dependent growth inhibition, reaching 17.68, 35.48, 49.47, 60.16, and 75.14 percent inhibition at concentrations of 20, 40, 60, 80, and 100 micrograms per milliliter respectively. Isoniazid tested under identical conditions produced 16.27, 25.85, 43.95, 54.94, and 65.63 percent inhibition across the same range. At every concentration, the Schiff base derivative outperformed the standard drug, and at the highest dose the margin was nearly ten percentage points. Although complete inhibition was not achieved, the consistent superiority over a first-line agent suggests that Schiff base modification can genuinely enhance the antitubercular potential of the isoniazid scaffold rather than merely decorate it computationally.

The authors are appropriately measured about what these results do and do not prove. Enzyme inhibition assays and mechanistic studies are still needed to experimentally confirm InhA as the molecular target of SLB-5, since the docking evidence, however encouraging, remains predictive. Expanded testing against drug-sensitive and drug-resistant clinical strains, additional structural characterization by carbon-13 NMR, synthesis and evaluation of other high-ranking library members, and eventually in vivo studies of pharmacokinetics, bioavailability, and safety all lie ahead. Yet the study stands as a tidy demonstration of a workflow that many laboratories are now embracing: design broadly, filter computationally through drug-likeness, ADMET, activity prediction, and docking, and synthesize only the compound that survives every gate. In an era when multidrug-resistant and extensively drug-resistant tuberculosis continue to spread, that kind of efficiency is not just elegant chemistry; it is a practical strategy for getting new antitubercular candidates to the bench, and eventually perhaps to the clinic, faster than the bacterium can adapt.

Subject of Research: Computer-aided design and biological evaluation of isoniazid-based Schiff base derivatives as antitubercular agents targeting InhA

Article Title: Design and in silico screening of novel isoniazid-based Schiff base derivatives followed by synthesis and biological evaluation of the lead compound against Mycobacterium tuberculosis H37Rv

Article References: Bhuse, S. L., Patil, P. M., & Durbule, R. R. (2026). Design and in silico screening of novel isoniazid-based Schiff base derivatives followed by synthesis and biological evaluation of the lead compound against Mycobacterium tuberculosis H37Rv. Discover Chemistry, 3(1), Article 516. https://doi.org/10.1007/s44371-026-00909-w

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00909-w

Keywords: tuberculosis, isoniazid, Schiff bases, molecular docking, InhA, drug resistance, ADMET, virtual screening, medicinal chemistry, H37Rv, Alamar Blue assay, lead compound