Structure-activity relationships of antitubercular nitroimidazoles. 1. Structural features associated with aerobic and anaerobic activities of 4- and 5-nitroimidazoles.

The 4-nitroimidazole PA-824 is active against aerobic and anaerobic Mycobacterium tuberculosis (Mtb) while 5-nitroimidazoles like metronidazole are active against only anaerobic Mtb. We have synthesized analogues of both 4- and 5-nitroimidazoles and explored their antitubercular activities. The nitro group is required for both activities in all compounds. The key determinants of aerobic activity in the 4-nitroimidazoles include the bicyclic oxazine, the lipophilic tail, and the 2-position oxygen. For the 5-nitroimidazoles, neither the corresponding bicyclic analogue nor addition of a lipophilic tail conveyed aerobic activity. Incorporation of a 2-position oxygen atom into a rigid 5-nitroimidazooxazine provided the first 5-nitroimidazole with aerobic activity. Across both series, anaerobic and aerobic activities were not correlated and Mtb mutants lacking the deazaflavin-dependent nitroreductase (Ddn) retained anaerobic sensitivity to some compounds. Aerobic activity appears to be correlated with efficiency as a substrate for Ddn, suggesting a means of structure-based optimization of improved nitroimidazoles.