Biological and biochemical evaluation of isatin-isoniazid hybrids as bactericidal candidates against Mycobacterium tuberculosis

Tuberculosis remains a leading cause of mortality among infectious diseases worldwide, prompting the need to discover new drugs to fight this disease. We report herein, the design, synthesis and anti-mycobacterial activity of isatin-mono/bis-isoniazid hybrids. Most of the compounds exhibited very high activity against Mycobacterium tuberculosis with minimal inhibitory concentrations in the range of 0.195-0.39 μg/mL and exerted a more potent bactericidal effect than the standard anti-tubercular drug isoniazid (INH). Importantly, these compounds were found to be well tolerated at high doses (>200 μg/mL) on Vero kidney cells, leading to high selectivity indices. Two of the most promising hybrids were evaluated for activity in THP-1 macrophages infected with M. tuberculosis, among which 11e was found to be slightly more effective than INH. Overexpression of InhA along with cross-resistance determination of the most potent compounds, selection of resistant mutants and biochemical analysis allowed us to decipher their mode of action. These compounds effectively inhibited mycolic acid biosynthesis and required KatG to exert their biological effects. Collectively, this suggests that the synthesized isatin-INH hybrids are promising anti-tubercular molecules for further evaluation in pre-clinical settings.