PA-824 Kills Nonreplicating <i>Mycobacterium tuberculosis</i> by Intracellular NO Release

TLDR

Bicyclic nitroimidazoles such as PA‑824 are promising tuberculosis therapeutics that require intracellular activation to exert their effect. PA‑824 is metabolized by the deazaflavin‑dependent nitroreductase Ddn into a des‑nitro metabolite that generates nitric oxide, and the resulting NO mediates anaerobic killing of Mycobacterium tuberculosis, as shown by a correlation between metabolite levels and bactericidal activity and protection conferred by NO scavengers.

Abstract

Bicyclic nitroimidazoles, including PA-824, are exciting candidates for the treatment of tuberculosis. These prodrugs require intracellular activation for their biological function. We found that Rv3547 is a deazaflavin-dependent nitroreductase (Ddn) that converts PA-824 into three primary metabolites; the major one is the corresponding des-nitroimidazole (des-nitro). When derivatives of PA-824 were used, the amount of des-nitro metabolite formed was highly correlated with anaerobic killing of Mycobacterium tuberculosis (Mtb). Des-nitro metabolite formation generated reactive nitrogen species, including nitric oxide (NO), which are the major effectors of the anaerobic activity of these compounds. Furthermore, NO scavengers protected the bacilli from the lethal effects of the drug. Thus, these compounds may act as intracellular NO donors and could augment a killing mechanism intrinsic to the innate immune system.