Cannabidiol potentiates olaparib-induced cytotoxicity through cell cycle arrest and DNA damage modulation in breast cancer cells.

Triple-negative breast cancer (TNBC) remains a major clinical challenge due to its aggressive nature and limited treatment options, while therapeutic resistance in estrogen receptor-positive (ER+) breast cancer continues to limit treatment efficacy. Although olaparib is primarily effective in BRCA-mutated cancers, its activity in BRCA-wild type (BRCA-wt) tumors is limited. Therefore, this study aimed to investigate whether cannabidiol (CBD) can enhance the response of BRCA-wt breast cancer cells to PARP inhibition. The effects of olaparib (OLAP) and CBD, alone and in combination, were evaluated in MDA-MB-231 (TNBC) and MCF-7 (ER+) cell lines using comprehensive two-dimensional (2D) mechanistic analyses and three-dimensional (3D) spheroid models, including HCC-70 cells to extend TNBC validation. The results demonstrate that combined OLAP and CBD treatment enhanced cytotoxic effects compared to single treatments, with more pronounced responses observed in 3D spheroid models, particularly in TNBC models. Flow cytometry and caspase 3/7 assays indicated increased apoptosis and G2​/M phase arrest following combination treatment. Gene expression analysis revealed downregulation of key DNA damage response and cell cycle-related genes (ATM, ATR, BRCA1/2, RAD51, and CDK1/2/4/6), supporting a role for cell cycle arrest and DNA damage modulation in mediating these effects. Functional assays showed reduced colony formation and migratory capacity, although these effects may reflect both cytotoxic and cytostatic responses under the selected experimental conditions. Overall, these findings suggest that CBD may enhance the efficacy of olaparib in BRCA-wt breast cancer models and highlight its potential as a combinational therapeutic strategy in breast cancer treatment.