Abstract LB042: Targeting ATR Enhances the Antitumor Efficacy of Patritumab Deruxtecan (Her3-Dxd)in Tamoxifen-Resistant ER+ Breast Cancer Cells by Inducing DNA Damage and Apoptosis

Abstract Background: HER3, a member of the ERBB family of receptor tyrosine kinases that activates multiple oncogenic signaling pathways, is overexpressed in 50-70% of breast cancers (BC). HER3 mRNA expression is highest in luminal (ER+) breast tumors. Approximately 30% of ER+ breast tumors are de novo resistant to tamoxifen. Therefore, therapeutically targeting HER3 with HER3-DXd, an antibody-drug conjugate (ADC) composed of a fully human anti-HER3 IgG1 monoclonal antibody (patritumab) covalently linked to a topoisomerase I (TOP I) inhibitor payload (deruxtecan) via a tetrapeptide-based cleavable linker, can be an effective treatment for tamoxifen-resistant (TMR) ER+ BC. After assessing HER3-DXd’s efficacy as a single agent, we sought to identify a synergistic partner to maximize its antitumor activity in HER3+/ER+ TMR BC. Methods: Whole-genome high-throughput siRNA screening (Ambion Silencer Select Human Genome siRNA Library V4) was performed to identify synergistic partners for maximizing HER3-DXd’s antitumor efficacy. The synergistic antitumor effects were assessed in vitro using a soft agar colony formation assay and a clonogenic assay in TMR HER3+/ER+ MCF7 and T47D BC cells and in vivo using xenograft mouse models of these cells. Targeting specificity was determined using siRNA. Treatment effects on cell cycle progression, DNA damage, apoptosis, and expression of proteins of interest were assessed by flow cytometry, comet assay, staining with annexin V-PE and 7-AAD, and Western blotting, respectively. Results: HER3-DXd inhibited the anchorage-independent growth of HER3+/ER+ cells by >50% at 5 nM and their colony formation at 5-25 nM (P < 0.05). Among the synergistic targets identified by whole-genome high-throughput siRNA screening, inhibiting ATR with siRNA or BAY1895344 showed the greatest synergistic effect with HER3-DXd in TMR HER3+/ER+ BC cells. In contrast, no synergistic effect was observed with the combination of BAY1895344 plus patritumab or control ADC (IgG-DXd), suggesting its dependence on HER3-DXd-mediated delivery of DXd. To further confirm the targeting specificity, we knocked down ATR or TOP I expression using siRNA in TMR HER3+/ER+ BC cells and then treated the cells with HER3-DXd or BAY1895344, respectively. A synergy was also observed, indicating that the drugs achieve the synergy by targeting ATR and TOP I. The combination of HER3-DXd plus BAY1895344 reprogrammed cell cycle progression from G2/M arrest to sub-G1 arrest by inhibiting both ATR/Chk1/cyclin A2/CDK2 and ATR/Chk1/cyclin E/CDK2 signaling. The combination also induced DNA damage, which was further confirmed by the reduced expression of H2AX, an ATR substrate that contributes to DNA repair, and the increased expression of γH2AX (phospho-H2AX at Ser139), an indicator of DNA damage. HER3-DXd and BAY1895344 synergistically inhibited the growth of both TMR HER3+/ER+ MCF7 (P < 0.0001) and T47D (P < 0.01) xenografts in mice. Conclusion: The combination of HER3-DXd plus ATR inhibitors has therapeutic potential for overcoming tamoxifen resistance in HER3+/ER+ BC. Citation Format: Xuemei Xie, Jangsoon Lee, Jon A. Fuson, Huey Liu, Young J. Gi, Thanasis Poullikkas, Pang-Dian Fan, Kumiko Koyama, Debu Tripathy, Naoto T. Ueno. Targeting ATR enhances the antitumor efficacy of patritumab deruxtecan (HER3-DXd)in tamoxifen-resistant ER+ breast cancer cells by inducing DNA damage and apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB042.