Abstract LB012: Antitumor effect and underlying mechanism on DNA damage response in breast cancer cells treated with HER3-DXd

Abstract Background: HER3 is a human epidermal growth receptor overexpressed in 50-70% of breast cancers, playing a pivotal role in cancer progression across all breast cancer subtypes. HER3-DXd is an antibody-drug conjugate (ADC) that combines patritumab, an anti-HER3 antibody and DXd, a topoisomerase I inhibitor, which is linked via a peptide-cleavable linker. HER3-DXd has shown anti-tumor effects in non-small cell lung, colorectal, and breast cancers, among other tumor types, in vitro and in vivo, and evaluated in phase 2 and 3 clinical trials. However, its effects on DNA damage response (DDR) in breast cancer are largely unknown. Methods: Ten breast cancer cell lines (HCC1428, ZR-75-1, MCF7, T47D, MDA-MB-453, SK-BR-3, BT-549, HCC1937, MDA-MB-157, and MDA-MB-231) representing luminal, HER2-enriched, and triple-negative breast cancer (TNBC) subtypes were used. Colony formation assay (CFA) was performed with HER3-DXd (0-50 nM) for 14 days. HER3-DXd internalization and cell cycle distribution were analyzed by flow cytometry. Apoptotic cell death was assessed by Annexin-V assay, and DNA strand breaks were examined using the alkaline comet assay. Inhibition of signal transduction, DDR, and cGAS activation were analyzed by western blotting and immunofluorescence assays. Results: HER3 protein expression was heterogeneous across the ten breast cancer cell lines. Based on CFA results, seven breast cancer cell lines with varying sensitivities to HER3-DXd were selected. MDA-MB-453, T47D, SK-BR-3, and HCC1937 were sensitive (IC50<10 nM), HCC1428 and MDA-MB-231 were less sensitive (IC50: 10-50 nM), and MCF7 was insensitive (IC50>50 nM). HER3-DXd was efficiently internalized within one hour in all cell lines regardless of its sensitivity. Signal transduction via the HER2 and HER3 receptors through the PI3K-AKT pathway was inhibited in MDA-MB-453 cells. HER3-DXd increased Annexin-V-positive, sub-G1 populations as well as cleavage of PARP and caspase-3 in sensitive cells and HCC1428, but not in MDA-MB-231 and MCF7. In all breast cancer cells except for MDA-MB-231 and MCF7, HER3-DXd induced ɣ-H2AX expression and elongated comet tails, indicating DNA damage. Additionally, HER3-DXd caused micronuclei formation in the cytosol of sensitive cells which were colocalized with cGAS. Conclusion: HER3-DXd induces DNA damage, apoptotic cell death, and micronuclei formation in sensitive breast cancer cells, but not in insensitive cells. Thus, the induction of DNA damage, rather than protein expression levels of HER3 or internalization rates of HER3-DXd, may be associated with sensitivity to HER3-DXd in these cell lines, suggesting the payload could be particularly important for HER3-DXd activity in these models. Citation Format: Hae Min Hwang, Sujin Ham, Youlim Noh, Chaeyoung Lee, Seohyeon Lim, Minyoung Jeong, Yu-Jin Kim, Ahrum Min, Ji-Won Koh, Changhee Park, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Antitumor effect and underlying mechanism on DNA damage response in breast cancer cells treated with HER3-DXd [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB012.