ATR inhibition potentiates the antitumor efficacy of HER3-DXd in HER3-positive/HR-positive breast cancer by increasing DNA damage

Background: Endocrine resistance remains a major challenge in hormone receptor–positive (HR+) breast cancer (BC), where up to 70% of tumours overexpress HER3, a receptor associated with poor prognosis and therapeutic resistance. HER3-DXd (patritumab deruxtecan) is currently under clinical investigation for HER3-expressing metastatic BC. However, strategies to further enhance its efficacy, particularly in endocrine therapy–resistant settings, are urgently needed. We hypothesised that targeting ATR, a key regulator of DNA damage repair (DDR), potentiates HER3-DXd in HER3+/HR+ BC, including tamoxifen–resistant (TMR) disease. Methods: Synergistic partners for HER3-DXd were identified by whole-genome RNAi screening. Treatments’ effects on cell cycle, DNA damage, and protein expression were analysed using flow cytometry, comet assay, and Western blotting, respectively. Treatments’ antitumor efficacy was assessed using xenograft mouse models. TCGA and CPTAC databases were analysed for clinical relevance. Results: HER3-DXd inhibited growth in both parental and TMR MCF7 and T47D cells. Compared to monotherapies, combining HER3-DXd with an ATR inhibitor enhanced DNA damage, sub-G1 arrest, apoptosis, downregulation of DDR and cell cycle regulatory proteins, and tumour growth inhibition. TCGA and CPTAC analyses confirmed high HER3 expression and correlation of ATR, CHEK1, and TOP1 gene expression with poor prognosis in HR+ BC. Conclusion: Combining HER3-DXd with an ATR inhibitor could benefit HER3+/HR+ BC patients with both endocrine–sensitive and –resistant diseases.