Abstract 6088: The bispecific antibody KA-5377, targeting CD19 and 4-1BB, specifically enhances T cell activity against lymphoma

Abstract KA-5377 is a novel bispecific antibody (BsAb) developed to target CD19 and human 4-1BB, addressing the challenge of dose-limiting toxicity associated with traditional 4-1BB agonist cancer immunotherapy. CD19, which is highly expressed in lymphoma, serves as an ideal target for this therapy. By restricting 4-1BB activation to the tumor microenvironment, KA-5377 aims to mitigate systemic toxicity. The 4-1BB nanobody used in KA-5377 was developed through our nanobody antibody discovery platform. KA-5377 is designed in an IgG1 format with a LALA mutation in the Fc region to eliminate ADCC (antibody-dependent cellular cytotoxicity) and ADCP (antibody-dependent cellular phagocytosis) activity, ensuring that the antibody’s activity is focused on stimulating 4-1BB in T cells. The bispecific antibody contains two 4-1BB nanobodies at the C-terminal that engage 4-1BB signaling upon binding, and two Fabs at the N-terminal that bind to CD19 expressed on tumor cells. KA-5377 demonstrates high avidity for lymphoma tumor cells expressing CD19, while binding to 4-1BB expressed on 293T cells with significantly lower avidity. Notably, 4-1BB signaling is strongly activated only when the antibody binds to CD19 on tumor cells, with much weaker activation observed when CD19 binding is absent. The difference in EC50 (effective concentration for 50% activation) between the CD19-bound and non-bound states is approximately 100-fold, compared to only a 2-fold difference observed with the benchmark bispecific antibody Englumafusp Alfa.When combined with a low concentration of CD3 antibody to stimulate T cell activation, KA-5377 significantly enhances T cell activation and proliferation in co-culture assays using primary T cells isolated from human PBMCs. In PBMC-humanized immunodeficient mouse tumor models, KA-5377 improves the cytotoxic activity of bispecific antibodies targeting CD20 and CD3 against WSU-DLCL2 tumors, which highly express CD19. Additionally, KA-5377 exhibits a normal half-life in the blood of Balb/c mice and demonstrates good developability. Toxicity studies are currently ongoing. In conclusion, KA-5377, developed using our nanobody antibody discovery platform, represents a promising preclinical candidate for the treatment of Relapsed/Refractory B-cell lymphoma. Citation Format: Guojin Wu, Jiabei Liang, Hao Peng, Feng Hao, Jinying Ning. The bispecific antibody KA-5377, targeting CD19 and 4-1BB, specifically enhances T cell activity against lymphoma [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 6088.