The Radiosensitizing Potential of CC-115, PIK-75 and M3814 DNA-PK Inhibitors for Breast Cancer Targeted Radiotherapy

NHEJ is one of the preferred DNA-damage repairing mechanisms for human cells, due to its rapid onset and activity throughout the cell cycle. It is, nevertheless, error prone and highly relevant in numerous oncological malignancies. In the recent years, it gained a lot of attention as a target either for cancer treatment or synergetic strategies, as pharmacological- and radio-sensitization approaches. This study evaluated the genotoxicity and radiosensitization potential of three DNA-PK inhibitors (PIK-75, M3814, and CC-115) in breast cancer by comparing MDA-MB-231 (NHEJ reliant) and MCF-7 (NHEJ and HR reliant) cell lines. Cells were preincubated with the DNA-PK inhibitors, using four different concentrations either in monotherapy or in combination with a DNA damage-inducer (doxorubicin or [64Cu]CuCl2). Viability was measured by MTS assays at 24, 48, and 72 h, while the DNA damage by γH2AX and flow cytometry. DNA-PK blockade preferentially sensitized NHEJ-reliant breast cancer cells to doxorubicin and 64Cu effect. Combination treatments generally reduced viability relative to inhibitor’s monotherapy, with clearer concentration dependence and stronger effects in MDA-MB-231. In the case of [64Cu]CuCl2 alone, MDA-MB-231 viability was reduced to 70–75% at 24 h, whereas several inhibitor combinations reduced it further; MCF-7 showed weaker or inconsistent potentiation. Delayed cytotoxicity was most obvious at 72 h, showing persistence of unrepaired DNA damage after DNA-PK inhibition. Overall, the results highlight the potential of exploiting repair-pathway dependence to improve targeted radiotherapy in breast cancer.