Structural Modification of Indole Derivatives as PknB Inhibitors for Enhanced Enzymatic Inhibition and Antimycobacterial Activity

Abstract

Protein kinase B (PknB) of Mycobacterium tuberculosis has emerged as a promising target for antituberculosis drug development. In our previous study, virtual screening combined with biological validation identified indole derivatives as novel PknB inhibitors with inhibitory activity against the growth of M. tuberculosis H37Rv, supporting a role for PknB inhibition in their antitubercular activity. Accordingly, the present study employed a structure-based drug design strategy using the previously identified indole scaffold as a template for optimization of PknB inhibitory activity and antimycobacterial potency. Twenty-seven indole derivatives were rationally designed and subjected to molecular docking calculations, which identified four candidates for chemical synthesis and evaluation of their inhibitory activity against M. tuberculosis growth and PknB. Three synthesized compounds (4b, 4c, and 4d) exhibited enhanced inhibition of M. tuberculosis growth, with minimum inhibitory concentration (MIC) values of 3.1 μg/mL, whereas compound 4a showed activity comparable to that of the parent indole (MIC = 6.2 μg/mL). All derivatives inhibited PknB activity with IC<inf>50</inf> values ranging from 0.06 to 0.42 μM, which are comparable to that of the parent indole (IC<inf>50</inf> = 0.45 μM). Notably, compound 4b demonstrated the highest potency against both M. tuberculosis growth and PknB activity, with an MIC value of 3.1 μg/mL and an IC<inf>50</inf> value of 0.06 μM. Cytotoxicity evaluation against Caco-2 cells indicated low toxicity and favorable safety profiles for all indole derivatives at effective concentrations. These results highlight the indole scaffold as a promising structural template for the further development of new PknB inhibitors with potential antitubercular activity.