Rational Substitution-Driven Enhancement of Anti-Mycobacterium tuberculosis Activity in a Quinoline-Isonicotinic Acid Hydrazide Hybrid.

The obstinatproblem of drug resistance in tuberculosis (TB) requires sustained efforts in the search for new drug molecules. Building on our previously developed synthetic hybrid compound, UH-NIP-16, which demonstrated anti-Mycobacterium tuberculosis (M.tb) activity, we synthesized and evaluated 15 new derivatives to improve its potency. Among these, two lead compounds, UN-7 and UN-14, bearing hydrogen and methyl substitutions at the sixth position, respectively, along with a 2-phenoxy moiety, exhibited a ∼5-fold reduction in MIC50 values across laboratory, clinical, and multidrug-resistant M.tb strains relative to UH-NIP-16. Both compounds displayed a bacteriostatic mechanism of action and exhibited predominantly additive or indifferent interaction profiles in combinatorial antitubercular studies. Importantly, UN-7 and UN-14 showed minimal cytotoxicity toward mammalian cells and effectively reduced intracellular bacterial burden in an in vitro infection model. A coupled enzymatic assay revealed that UN-7 and UN-14 inhibited mycobacterial shikimate kinase (Mt-SK) activity by approximately 65%, a target not inhibited by the parent compound UH-NIP-16. In silico ADMET analyses indicated several favorable drug-likeness properties despite predicted liabilities, including Ames mutagenicity and low aqueous solubility. Collectively, these findings identify UN-7 and UN-14 as structure-activity relationship (SAR)-guided derivatives of the UH-NIP-16 scaffold with improved antimycobacterial activity and promising potential for further antitubercular lead development.