A clinical-stage oncology compound selectively targets drug-resistant cancers.

Re-evaluating existing clinical compounds can uncover previously unrecognized mechanisms that reshape a drug's therapeutic potential. The small molecule Procaspase-Activating Compound 1 (PAC-1) entered oncology testing as a proposed activator of caspase-driven apoptosis. Here, we show that PAC-1-driven cytotoxicity occurs in the absence of executioner caspase expression, demonstrating that its anti-cancer activity occurs via an alternative mechanism. We provide genetic, biochemical, and biophysical evidence demonstrating that PAC-1 functions as a highly selective iron chelator that eliminates cancer cells by disrupting iron homeostasis. Unexpectedly, we discovered that expression of the key chemotherapy-resistance pump MDR1 confers marked hypersensitivity to PAC-1 treatment. While PAC-1 is only weakly effluxed by MDR1 under basal conditions, this process is potentiated when PAC-1 is bound to iron. Consequently, PAC-1 induces progressive iron depletion and selective cytotoxicity in otherwise drug-resistant MDR1-expressing cancer cells. Together, these findings redefine PAC-1's mechanism-of-action and establish a framework for exploiting multidrug resistance as a therapeutic vulnerability through targeted iron starvation.