Redox Cycling Dioxonaphthoimidazoliums Disrupt Iron Homeostasis in Mycobacterium bovis Bacillus Calmette-Guerin

The dioxonaphthoimidazolium scaffold is a novel, highly bactericidal redox cycling antituberculosis chemotype that is reliant on the respiratory enzyme Type II NADH dehydrogenase (NDH2) for the generation of reactive oxygen species (ROS). Here, we employed Mycobacterium bovis Bacillus Calmette-Guerin (M. bovis BCG) reporter strains to show that ROS generated by the redox cycler SA23 simulated an iron deficient state in the bacteria, which led to a compensatory increase in the expression of the iron acquisition mbtB gene while collaterally reducing the expression of the iron storage bfrB gene. Exacerbating the iron deficiency via the inclusion of an iron chelator or aggravating oxidative stress by deploying a catalase (KatG) loss-of-function mutant strain enhanced the activity of SA23, whereas a combined approach of treating the katG mutant strain with an iron chelator led to even greater gains in activity. Our results support the notion that the activity of SA23 pivots on a vicious cycle of events that involve the derailment of iron homeostasis toward greater acquisition of the metal, overwhelmed oxidative stress defenses due to enhanced Fenton reactivity, and, ultimately, self-inflicted death. Hence, we posit that redox cyclers that concurrently perturb the iron equilibrium and cellular respiration are well-positioned to be potent next-generation anti-tubercular drugs.

Automated summary

The research demonstrates that the redox cycler SA23, by generating reactive oxygen species (ROS), simulates an iron-deficient state in bacteria and enhances its bactericidal activity by increasing the expression of iron acquisition genes. Further exacerbating iron deficiency or oxidative stress enhances the activity of SA23. This study suggests that redox cyclers that perturb iron homeostasis and cellular respiration could be potent next-generation anti-tubercular drugs.