Abstract 7268: A novel immunocompetent tumor model reveals the immunomodulatory effects of trastuzumab deruxtecan

Abstract The αHER2 antibody-drug conjugate, trastuzumab deruxtecan (T-DXd), has revolutionized the treatment of breast cancer. However, it is being combined with immunotherapy in clinical trials without rigorous preclinical validation. Few studies have explored T-DXd’s immunologic impact, and comprehensive preclinical data supporting immunotherapy combinations is lacking. This gap stems from trastuzumab’s inability to bind to rodent HER2. Consequently, many existing models rely on human-derived cell lines or xenografts transplanted into immune-deficient mice, thus overlooking T-DXd’s adaptive immunologic effects. Other strategies involve inoculating murine tumors expressing human HER2 into immunocompetent mice; however, this approach increases the immunogenicity of the model and often leads to tumor rejection or delayed growth due to xenoantigens arising from sequence differences between mouse and human HER2. To address this, we have developed a robust, immunocompetent tumor model to study trastuzumab-based therapies by minimally editing the mouse HER2 trastuzumab binding region. This tumor model, which we termed HER2X, binds trastuzumab, is nonimmunogenic, and is sensitive to T-DXd treatment. Here we demonstrate the use of this HER2X tumor model to elucidate the immunologic effects of T-DXd. Cytometry by time of flight (CyTOF) analysis revealed significant shifts to proinflammatory, antitumor immune populations, including T cells, NK cells, and M1-like macrophages, in T-DXd treated versus untreated EMT6 HER2X tumors from immunocompetent mice. Accordingly, the proportion of M2-like macrophages, neutrophils, and monocytes were significantly lower in the T-DXd treated tumors. Notably, 60% of T-DXd complete responder mice were also resistant to rechallenge with EMT6 HER2X tumor cells, indicating immunologic memory and potential involvement of adaptive immunity. Furthermore, T-DXd treatment potentiated EMT6 HER2X antitumor responses in immunocompetent mice compared to athymic nude mice. Finally, co-administration of αPD-L1 and T-DXd treatment further increased survival and response compared to either monotherapy in HER2X tumors. Together, these data suggest T-DXd induces a proinflammatory tumor-immune microenvironment, necessitates adaptive immunity for effective antitumor response, and may provide combinatorial benefit with immunotherapy in clinical settings. Further work is needed to determine the mechanism behind these immunomodulatory effects, and how we can leverage this knowledge to inform and improve treatment strategies in patients. Citation Format: Julia Steele, Justin Balko. A novel immunocompetent tumor model reveals the immunomodulatory effects of trastuzumab deruxtecan [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 7268.