Rationally Designed InhA Inhibitors: A Comparative Anti-Tubercular Activity Study of Sulfonate Esters of Isoniazid Hydrazones and Their Structurally Flexible Benzyl Analogues

Molecular hybridization of isoniazid with hydrophobic aromatic moieties represents a promising strategy for the development of novel anti-tubercular therapeutics. In this study, a series of hybrid molecules (5a–i) was synthesized by linking isoniazid with aromatic sulfonate esters via a hydrazone bridge. Molecular docking studies revealed that these compounds interact effectively with the catalytic triad of the InhA enzyme (Y158, F149, and K165), suggesting their potential as InhA inhibitors. To enhance molecular flexibility and improve binding interactions with both NADH and the catalytic residues, a second generation of derivatives (8a–k) was designed and synthesized. All synthesized compounds were structurally characterized using spectroscopic techniques, including nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (IR), and high-resolution mass spectrometry. As anticipated, these new compounds exhibited enhanced anti-tubercular activity compared to their precursors. Notably, compound 8b demonstrated significant potency with an MIC of 0.078 μg/mL, approximately twofold more active than its precursor 5b (MIC = 0.156 μg/mL) against Mycobacterium tuberculosis (Mtb). However, both generations of compounds (e.g., 5a, 5b, 8a, 8b, 8c, and 8 k) lost activity against INH-resistant Mtb strains harboring katG mutations. Importantly, no cytotoxicity was observed for these compounds in THP-1 human monocytic cells at a concentration of 10 μg/mL. The structural integrity of the lead compound 8b was confirmed via ¹H NMR stability studies. The ADME/T parameters (absorption, distribution, metabolism, excretion, and toxicity) were also explored to determine their drug likeness and safety profile. Collectively, these hybrid molecules present valuable scaffolds for further optimization in the pursuit of new anti-tubercular agents.