Abstract 4876: Avidity engineered multispecific antibodies that are highly efficacious in immune cell stimulation in patient tissue and in mice: significantly improved tumor control and turning cold tumors hot
Authors: Sophie Poznanski, Erik Slinger, Jarek Juraszek, Fatemeh Vahedi, Marcelo Pereira, Loreto Parga-Vidal, Dean Lee, Ali Ashkar, Robert Friesen
Journal: Cancer Research (2025)

Abstract Introduction: Antibodies and ADCs are mainstays in the treatment of cancer. However, given difficulties in achieving a deep and sustained response, significant improvements are desirable. We report on first in class “Booster” molecules, based on clinically validated ADCC-competent antibodies, equipped with two immunomodulatory domains that are affinity engineered to be functional only when in contact with a tumor cell. We see strong expansion and increased cytotoxicity of immune cells in the presence of cancer cells in vitro, activation of relevant immune cell types ex vivo, and reduction of tumor burden in vivo, with significantly better activity than adoptive cell therapy or control antibody without fusion domains. Methods: NRG mice were engrafted with luciferase expressing SKOV3 cells via intraperitoneal injection 7 days prior to treatment. Mice received 1 million NK cells isolated from healthy donors. Compounds were administered biweekly, and low dose IL2 thrice weekly. Blood was collected weekly, and tumor burden monitored weekly via bioluminescence. ex vivo: in situ activation of tumor infiltrating immune populations was evaluated by nanostring in freshly isolated tumor tissue. in vitro: cytotoxicity was measured by quantifying the number of alive tumor cells using automated microscopy. Expansion was performed by stimulating NK cells weekly with tumor cells that were opsonized with Booster or antibody, NK cells were counted weekly to determine expansion. Results: Mice treated with a HER2 targeting Booster demonstrated superior tumor control than trastuzumab treated mice, with near tumor remission by day 35. High NK counts were observed in the blood of Booster treated mice, and no NK cells were detected in that of trastuzumab treated mice. In the peritoneal cavity, NK counts were up to 600x higher in Booster treated mice than in trastuzumab treated mice. Our Booster reprograms the immune microenvironment ex vivo in freshly isolated tumor tissue, transforming a cold tumor into a hot tumor. It activates multiple cytotoxic and IFN-γ pathways, stimulates CD8+ T cell activation, downregulates pro-tumor pathways in Tregs and induces a phenotypic shift in macrophages from the immunosuppressive M2 to the pro-inflammatory M1 phenotype. In separate in vitro assays we saw sustained expansion and enhanced cytotoxicity of NK cells for at least 6 weeks. Cells stimulated with HER2 Booster showed prolonged tumor control, whereas trastuzumab stimulated cells failed to sustain tumor control beyond 21 days. Conclusion: In correlation with extensive in vitro and ex vivo data, we observe a prolonged and significant improvement in tumor control in mice treated with Boosters compared to mice treated with trastuzumab. Work is ongoing to develop these molecules, with the first clinical trial expected to start in 2026. Citation Format: Sophie Poznanski, Erik Slinger, Jarek Juraszek, Fatemeh Vahedi, Marcelo Pereira, Loreto Parga-Vidal, Dean Lee, Ali Ashkar, Robert Friesen. Avidity engineered multispecific antibodies that are highly efficacious in immune cell stimulation in patient tissue and in mice: significantly improved tumor control and turning cold tumors hot [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 4876.