Abstract 893: A novel approach to treating HER2-low breast cancer: surface-engineered NK/tz cells targeted immunotherapy

Abstract Background: The majority (60-70%) of metastatic breast cancers (MBC) are HER2-low expressing, and in distinction to HER2+ disease, is not addicted for growth to this oncogene. Trastuzumab-based treatments have failed to improve the outcomes of patients with HER2-low breast cancer. Despite promising results with trastuzumab-deruxtecan (T-DXd), significant systemic toxicity and disease progression were observed highlighting the need to develop safer and more effective, and accessible therapies. This study evaluates the therapeutic potential of surface-engineered NK cells (SE-NK) modified with trastuzumab (Tz) for targeting HER2-low breast cancer cells. Methods: To establish a preclinical model, the triple-negative murine breast cancer cell line 4T1 was transduced with a lentiviral vector encoding human HER2, generating clones with HER2 expression comparable to HER2-low human breast cancer cells (MDA-MB-175). HER2 expression was validated using flow cytometry, confocal microscopy, and Western blot analysis. SE-NK/Tz cells were generated by embedding 25 ug Tz onto the NK cell surface using a hydrophobic linker. These cells were tested in vitro for HER2-specific cytotoxicity against the engineered 4T1-HER2-low cells. Results: Transduced 4T1-HER2-low cells exhibited HER2 expression levels consistent with HER2-low human breast cancer cells. In vitro, SE-NK/Tz cells demonstrated significantly enhanced cytotoxicity against 4T1-HER2-low cells (24%, p <0.05) compared to NK cells alone (0%) when an effector to target ratio of 1.25:1 was used. Conclusions: This study establishes the feasibility of using SE-NK/Tz cells as a novel immunotherapeutic approach for HER2-low breast cancer. We created for the firsttime to our knowledge a murine cell line with human HER2-low status comparable to expression values reported in patients. The preclinical findings support the potential of this strategy to overcome the limitations of current therapies by offering an effective and accessible treatment option. Future studies will explore mechanisms of immune activation and therapeutic efficacy in diverse in vivo HER2-low models. Citation Format: Maria Jose Godoy Calderon, Eric Gauchat, VK Gadi. A novel approach to treating HER2-low breast cancer: surface-engineered NK/tz cells targeted immunotherapy [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 893.