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 recent years, it has gained a lot of attention as a target either for cancer treatment or 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 and MCF-7 breast cancer cell lines, selected based on reported differences in DNA-repair characteristics and relative NHEJ dependence. 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 was measured by γH2AX and flow cytometry. DNA-PK inhibitor combinations produced greater reductions in MTS viability in MDA-MB-231 than in MCF-7 under experimental conditions when combined with doxorubicin and 64Cu. Combination treatments generally reduced viability relative to inhibitor 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. The reduction in metabolic viability became more pronounced at 72 h, particularly in MDA-MB-231 cells. Overall, the results highlight the potential of exploiting repair-pathway dependence to improve targeted radiotherapy in breast cancer.