2-((3,5-Dinitrobenzyl)thio)quinazolinones: Potent Antimycobacterial Agents Activated by Deazaflavin (F420)-Dependent Nitroreductase (Ddn)

Swapping the substituents in positions 2 and 4 of the previously synthesized but yet undisclosed 5-cyano-4-(methylthio)-2-arylpyrimidin-6-ones 4, ring closure, and further optimization led to the identification of the potent antitubercular 2-thio-substituted quinazolinone 26 . Structure–activity relationship (SAR) studies indicated a crucial role for both meta -nitro substituents for antitubercular activity, while the introduction of polar substituents on the quinazolinone core allowed reduction of bovine serum albumin (BSA) binding ( 63c, 63d ). While most of the tested quinazolinones exhibited no cytotoxicity against MRC-5, the most potent compound 26 was found to be mutagenic via the Ames test. This analogue exhibited moderate inhibitory potency against Mycobacterium tuberculosis thymidylate kinase, the target of the 3-cyanopyridones that lies at the basis of the current analogues, indicating that the whole-cell antimycobacterial activity of the present S -substituted thioquinazolinones is likely due to modulation of alternative or additional targets. Diminished antimycobacterial activity was observed against mutants affected in cofactor F 420 biosynthesis ( fbiC ), cofactor reduction ( fgd ), or deazaflavin-dependent nitroreductase activity ( rv3547 ), indicating that reductive activation of the 3,5-dinitrobenzyl analogues is key to antimycobacterial activity.