The DNA repair pathway as a therapeutic target to synergize with trastuzumab deruxtecan in HER2-targeted antibody-drug conjugate-resistant HER2-overexpressing breast cancer

Background: Anti-HER2 therapies, including the HER2-targeting antibody-drug conjugates (ADCs) trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd), have significantly improved survival outcomes for patients with HER2-overexpressing (HER2+) metastatic breast cancer. However, resistance—whether intrinsic or acquired—remains a major clinical challenge, with no established standard of care once disease progression occurs. This study aimed to uncover the mechanisms underlying resistance to T-DM1 and T-DXd in both patients and preclinical models, and to identify therapeutic targets whose inhibition could enhance the efficacy of T-DXd in resistant HER2+ breast cancer, both in vitro and in vivo.

Methods: We performed targeted DNA sequencing and whole transcriptome analysis on breast cancer tissue samples from patients to identify genetic changes that develop following anti-HER2 therapy. Additionally, we developed T-DM1 and T-DXd-resistant HER2+ breast cancer cell lines. To investigate resistance mechanisms and discover synergistic kinase targets for boosting T-DXd efficacy, we utilized a range of techniques including fluorescence in situ hybridization, droplet digital PCR, Western blotting, whole-genome sequencing, cDNA microarrays, and kinome RNA interference (RNAi) screening. Further, cell viability, colony formation, and xenograft models were used to evaluate the antitumor effects of T-DXd in combination with other agents.

Results: Our analysis revealed reduced HER2 expression and amplification of DNA repair-related genes in post-therapy patient samples. In the T-DM1 and T-DXd-resistant cell lines, resistance was linked to decreased ERBB2 gene amplification, driven by DNA damage and epigenetic changes. Through unbiased RNAi screening, the DNA repair pathway emerged as a promising target to enhance the activity of T-DXd. We confirmed that combining T-DXd with elimusertib, an inhibitor of ataxia telangiectasia and Rad3-related protein (ATR), significantly increased cancer cell death both in vitro (P < 0.01) and in vivo (P < 0.01) compared to monotherapy.

Conclusions: DNA repair pathways play a crucial role in resistance to HER2-targeted ADCs. Our findings support further clinical investigation of combination treatments using T-DXd and DNA repair-targeting agents, such as ATR inhibitors, to overcome resistance in HER2+ breast cancer.