The anti-protozoan drug nifurtimox preferentially inhibits clonogenic tumor cells under hypoxic conditions.

Tumor hypoxia is an independent prognostic indicator of tumor malignant progression and poor patient survival. Therefore, eradication of hypoxic tumor cells is of paramount importance for successful disease control. In this study, we have made a new discovery that nifurtimox, a clinically approved drug to treat Chagas disease caused by the parasitic protozoan trypanosomes, can function as a hypoxia-activated cytotoxin. We have found that nifurtimox preferentially kill clonogenic tumor cells especially under the hypoxic conditions of ≤0.1% O. Mechanistically, nifurtimox becomes activated after tumor cells enter into a fully hypoxic state, as shown by the stabilization of the Hypoxia-Inducible Factor 1α (HIF-1α). Nifurtimox specifically induces the formation of 53BP1 foci, a hallmark of DNA double-stranded breaks, in hypoxic tumor cells. Hypoxia-dependent activation of nifurtimox involves P450 (cytochrome) oxidoreductase. The anti-protozoan drug nifurtimox holds promise as a new hypoxia-activated cytotoxin with the potential to preferentially eliminates severely hypoxic tumor cells.