Abstract 3996: The impact of HER3 dynamics on the efficacy of HER3-DXd, a novel HER3 directed antibody-drug conjugate

Abstract Background: HER3 is broadly expressed in various solid tumor types, and its expression can be upregulated by treatment with receptor tyrosine kinase inhibitors (RTKi) such as EGFR TKIs used to treat EGFR-mutated NSCLC. HER3-DXd, a novel antibody-drug conjugate (ADC) composed of a human anti-HER3 IgG1 monoclonal antibody (patritumab) covalently linked to a topoisomerase I inhibitor payload (DXd), is currently being studied in clinical trials for breast cancer and NSCLC. As previously reported, HER3-DXd treatment transiently decreases HER3 expression levels in tumors and EGFR TKIs increase HER3 membrane expression. However, the impact of HER3 dynamics on payload delivery has not been clarified yet. In this study, we investigated HER3 dynamics including HER3 receptor turnover and payload delivery in cancer cells using HER3-DXd both as a single agent and in combination with RTKi including osimertinib, which is in clinical trials in combination with HER3-DXd. Methods: HER3/ADC internalization was evaluated by using confocal imaging in MDA-MB-453 cells treated with HER3-DXd. Internalization and payload release were quantitatively measured in 3 cancer cell lines treated with HER3-DXd. HER3 turnover on the cell surface was also evaluated upon wash-out of HER3-DXd. In xenograft models, mice were administered two doses of HER3-DXd at different doses and dosing intervals, and membrane HER3 expression and tumor payload concentration were examined over time. NSCLC cell lines harboring EGFR activating mutations, ROS1 fusions, or ALK fusions were used to evaluate the effect of osimertinib, lorlatinib, or ceritinib on cell surface HER3 expression and payload release (osimertinib only). Results: HER3-DXd was rapidly transferred to early endosomes after binding to HER3. HER3 dynamics varied among the cell lines tested in vitro, and payload release reflected cell surface HER3 expression levels, HER3 internalization speed and turnover rates. In xenograft models, a higher dosage of HER3-DXd resulted in a larger decrease in membrane HER3 expression. Dosing interval also affected membrane HER3 expression levels; the degree of tumor payload concentration increase after the second dose was dependent on the recovery of HER3 expression after the first dose. Furthermore, we confirmed that RTKi increased the cell surface HER3 expression in NSCLC cell lines with targetable driver genomic alterations and that osimertinib increased payload delivery in PC-9 cells through the upregulation of cell surface HER3 expression. Conclusion: HER3 expression was dynamically changed by HER3-DXd dosing regimen and by RTKi treatment, resulting in a substantial impact on payload release. These findings support our strategy of clinical studies using HER3-DXd after drugs that increase HER3 expression including EGFR TKI and indicate that HER3 dynamics may play a key role in achieving optimal efficacy of HER3-DXd. Citation Format: Nagiho Komatsu, Saori Sato, Sumie Muramatsu, Ryuichi Nakamura, Kumiko Koyama. The impact of HER3 dynamics on the efficacy of HER3-DXd, a novel HER3 directed antibody-drug conjugate. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3996.