The mechanism of action of PA-824: Novel insights from transcriptional profiling.

The bicyclic nitroimidazole PA-824 is a pro-drug with a very complex mechanism of action active against both replicating and hypoxic, non-replicating Mycobacterium tuberculosis. Microarray analysis of the mode of action of PA-824 showed a puzzling mixed effect both on genes responsive to both cell wall inhibition (like isoniazid) and respiratory poisoning (like cyanide). The aerobic killing mechanism of this drug appears to involve inhibition of cell wall mycolic acid biosynthesis through an as yet unknown molecular mechanism. However, the structure-activity relationships governing aerobic activity do not parallel the relationships determining anaerobic activity. Based on the metabolite profiling of PA-824 and various derivatives by Ddn-mediated activation, we have shown that PA-824 acts directly as an NO donor.1 This respiratory poisoning through nitric oxide release seemed to be a crucial element of anaerobic activity by PA-824. The effect of PA-824 on the respiratory complex under hypoxic non-replicating conditions was also manifested in a rapid drop in intracellular ATP levels, again similar to that observed by cyanide treatment. Thus, transcriptional profiling provided valuable clues to elucidating the molecular mechanism of mycobacterial killing.