Objective: PA-824 (Pretomanid), a bicyclic nitroimidazole drug, exhibits significant bactericidal activity toward Mycobacterium tuberculosis  (MTB) in vitro and in vivo, but not against Mycobacterium smegmatis . Through catalytic bioreduction, deazaflavin-dependent nitroreductase (Ddn) within MTB directly converts PA-824 to potent bactericidal products. This study aimed to identify key MTB Ddn residues involved in PA-824 conversion toward development of in vitro surrogate markers for detection of mycobacterial resistance to PA-824.
Methods: We evaluated in vitro activity of PA-824 toward MTB and nontuberculous mycobacterial species using antimicrobial susceptibility testing. Ddn amino acid sequence alignments and phylogenetic analysis revealed putative key enzyme active site residues. Candidate MTB Ddn residues required for PA-824 conversion activity were evaluated for loss-of-function using recombinantly cloned Ddn mutant proteins expressed in Mycobacterium smegmatis .
Results: PA-824 minimum inhibitory concentrations of 90% of bacterial growth (MIC ₉₀s) against MTB and Mycobacterium kansasii  were 0.12 mg/L and 8 mg/L, respectively, but > 32 mg/L for Mycobacterium  spp. M. avium, M. intracellulare, M. abscessus  and M. fortuitum . MTB Ddn and M. kansasii  Ddn homologous sequences shared the greatest similarity (89.3% amino acid identity). M. smegmatis  expressing Ddn proteins with Y65L, A76V or Y133F substitutions (but not V75L, Q125K or V148I) were resistant to PA-824.
Conclusion: Our data demonstrated that PA-824 exhibited excellent and moderate levels of in vitro activity against MTB and M. kansasii , respectively. Substitutions of Ddn residues Y65, A76 or Y133 conferred mycobacterial resistance to PA-824.
Keywords: mycobacteria, PA-824, susceptibility, Ddn