Abstract LB280: Resistance to trastuzumab deruxtecan (T-DXd) in breast cancer via loss of HER2 expression and binding

Abstract Background: T-DXd is an antibody-drug conjugate (ADC) designed to deliver a topoisomerase I inhibitor to cancer cells expressing HER2. It is effective in breast cancer (BC) but resistance emerges over time. For many antibody-based drugs, loss of the antibody-binding epitope is a common mechanism of resistance. We investigated the potential for loss of HER2 binding to promote T-DXd resistance and pharmacologic strategies that could overcome this mechanism. Methods: Clinical samples of tissue biopsies from a cohort of BC patients treated with T-DXd were examined for HER2 expression by immunohistochemistry (IHC) per ASCO/CAP Guidelines, and ERBB2 mutation by next generation sequencing (MSK-IMPACT). Inducible cell line models were made to evaluate the impact of HER2 expression and mutation on responses to T-DXd using immunoblotting, FACS, and cell proliferation assays. Molecular dynamics (MD) simulations were used to model the impact of ERBB2 mutation on drug binding. Results: From a cohort of 375 metastatic BC patients treated with T-DXd, we identified 50 cases with paired pre- and post-treatment samples, among which loss of HER2 expression from IHC 1-2+ down to 0 was observed in 5 cases (10%). We identified one patient with a somatic ERBB2 mutation (V597M) in the trastuzumab binding region who showed no response to T-DXd despite being HER2 IHC 2+. MD simulation for this mutation predicted reduced trastuzumab binding (ΔΔG=0.75±0.10 kcal/mol). We experimentally tested the impact of (1) lower HER2 levels or (2) V597M mutation in BC cells. Pharmacologic reduction of HER2 expression led to a 83.5% reduction of HER2 surface levels, which corresponded to a 215-fold reduction in T-DXd internalization, and a 700-fold increase in IC50 for T-DXd-mediated inhibition of proliferation. Similarly, the V597M mutation suppressed T-DXd internalization and antitumor effects. By contrast, cells with reduced HER2 levels and relative T-DXd resistance showed unchanged sensitivity to the DXd payload. We thus investigated whether the combination with the TROP2-directed ADC Datopotamab deruxtecan (Dato-DXd), might be effective. We found that combining low doses of T-DXd and Dato-DXd achieved similar efficacy to full dose monotherapies. Data will be presented on the effectiveness of this combination approach in patient derived xenografts (PDXs). Conclusion: Loss of the HER2 binding epitope is observed in a subset of patients with resistance to T-DXd either through HER2 mutation or loss of expression. Preclinical models with varying HER2 expression levels reveal that such losses in T-DXd target engagement may be sufficient to suppress cell uptake and cytotoxicity of T-DXd. The combination of T-DXd and Dato-DXd offers the potential to overcome these resistance mechanisms by providing a backup mechanism for payload delivery. Citation Format: Wanyi Chen, Avantika Gupta, Nicholas Mai, Joshua Lau, Sukrit Singh, John Chodera, Bo Liu, Joshua Drago, Sarat Chandarlapaty. Resistance to trastuzumab deruxtecan (T-DXd) in breast cancer via loss of HER2 expression and binding [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 LB280.