Abstract 965: A novel agonist of CD137, SA-4-1BBL, as a single agent halts spontaneous breast cancer progression

Abstract Introduction: Breast cancer is the most commonly diagnosed cancer among women globally, and early immunologic intervention could reduce disease burden and improve long-term outcomes. Immunotherapy has demonstrated encouraging efficacy and an acceptable safety profile in both early-stage and metastatic triple-negative breast cancer. However, currently approved immunotherapies such as PD-1/PD-L1 blockade, HER2-targeted antibodies, and tumor vaccines exhibit highly variable responses due to tumor-intrinsic factors and the immunosuppressive tumor microenvironment. A novel CD137 agonist, SA-4-1BBL, as a single agent, prevents the development of various transplantable tumor types as well as a tobacco carcinogen, NNK, induced spontaneous lung cancer in preclinical models with an excellent safety profile. Here, we evaluated the immunoprevention efficacy of SA-4-1BBL in a mouse model of spontaneous breast cancer, MMTV-PyMT, which recapitulates the progression of human breast cancer. Study design: Female MMTV-PyMT C57BL/6J mice, aged 5 (n=12) or 8 (n=17) weeks, were assigned to either the SA-4-1BBL or saline (n=19) treatment groups. SA-4-1BBL was administered every two weeks for a total of four doses. Tumor growth was monitored twice weekly for 120 days, at which time animals were euthanized to collect tumors, tumor-draining lymph nodes, and spleen for tumor weight measurement, histology, and deep immunophenotyping. Results: SA-4-1BBL significantly delayed the onset of tumors and inhibited their progression in both the 5-week and 8-week treatment cohorts. Median tumor appearance was observed at 13 weeks in PBS controls, compared to 16.5 weeks for the 5-week cohort and 15 weeks for the 8-week cohort. Tumor growth was markedly reduced in both SA-4-1BBL-treated groups (p < 0.0001). As compared to saline controls, both SA-4-1BBL treatment groups showed a significant reduction in tumor weight by ∼76% and a decrease in the number of tumor-bearing mammary glands by ∼50%. Deep immunophenotyping of tumor-draining lymph nodes showed substantial increases in multiple immune cell subsets, including NK cells, CD4+ T cells, and CD8+ T cells, demonstrating enhanced antitumor immunity. Conclusion: SA-4-1BBL, as a single agent, effectively suppresses spontaneous breast tumor development and progression in the MMTV-PyMT model. These findings support its potential as a promising biologic for breast cancer immunoprevention. Elucidation of the underlying mechanisms will further guide the optimization and clinical translation of this protocol for immunoprevention of cancer in high-risk individuals. Acknowledgment: Supported in part by the Paula and Rodger Riney Foundation and NCI (5UG3CA290305-02). Citation Format: Yongyong Li, Sarita Rani, Feyza Nur Arguc, Yichen Wang, Daniel Davis, Esma S. Yolcu, Haval Shirwan. A novel agonist of CD137, SA-4-1BBL, as a single agent halts spontaneous breast cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 965.