Evofosfamide Enhances Sensitivity of Breast Cancer Cells to Apoptosis and Natural-Killer-Cell-Mediated Cytotoxicity Under Hypoxic Conditions

Background/objectives: Hypoxia in the tumor microenvironment is linked to aggressiveness, epithelial-mesenchymal transition, metastasis, and therapy resistance. Targeting hypoxia to enhance antitumor immunity is crucial for overcoming therapeutic resistance. Here, we investigated the ability of Evofosfamide, a prodrug that gets activated under hypoxic conditions, to sensitize breast cancer cells to cell death. Evofosfamide is converted into bromo-isophosphoramide mustard, a potent DNA cross-linking agent that is expected to enhance the killing of cancer cells under hypoxic conditions, where these cells typically exhibit resistance. Methods: Representative breast cancer cell lines, MCF-7 and MDA-MB-231, were treated with Evofosfamide under normoxia and hypoxia. Changes in cell viability and the mechanism of cell death were measured using neutral red dye uptake, Annexin-FITC/propidium iodide staining, and Western blot analysis of markers-PARP1 and caspase 3/7. We tested Evofosfamide's ability to counteract hypoxic suppression of type I Interferon signaling genes using quantitative PCR (qPCR), as well as its capacity to trigger natural killer (NK)-cell-mediated cytotoxicity. Results: Evofosfamide enhanced cell killing in both MCF-7 and MDA-MB-231 cells under hypoxic conditions compared to normoxic conditions. Cell killing was accompanied by increased cellular reactive oxygen species (ROS), diminished mitochondrial membrane potential, and induction of apoptosis, as demonstrated by the fragmentation or laddering of genomic DNA, the activation of caspase 3/7, and the cleavage of PARP. qPCR analysis revealed that Evofosfamide was capable of restoring type I interferon signaling in hypoxic breast cancer cells, leading to the subsequent cytolytic activity of NK cells against the tumor cells. Conclusions: Thus, conditioning the breast cancer cells with Evofosfamide resulted in enhanced cell killing under hypoxia, further underscoring its potential as a sensitizer to target hypoxia-driven tumors.