Abstract 2983: Tumor-specific CD28 costimulatory bispecific antibodies enhance T cell activation in solid tumors

Abstract T cells in the tumor micro-environment require TCR/MHC engagement and co-stimulatory receptor engagement to achieve optimal activation. Solid tumor cells lack expression of CD28 ligands, so we hypothesized that activation of CD28 signaling at the T cell/tumor cell interface could enhance anti-tumor activity. We developed a CD28 bispecific platform that allows for the rapid generation of tumor-associated antigen (TAA) x CD28 bispecific antibodies that conditionally provide CD28 costimulation only in the presence of TAA and TCR engagement. Here, we present results for a set of bispecific antibodies with broad applicability across a range of solid tumors, including CEACAM5 x CD28, Trop-2 x CD28, STEAP1 x CD28, and mesothelin x CD28. We developed optimized CD28 scFv and Fabs that cover a range of affinities and are only agonistic in the context of TCR engagement. A matrix of bispecifics pairing these CD28 binders with CEACAM5, Trop-2, STEAP1, and mesothelin antibodies of varying affinities and epitopes were generated using Xencor’s XmAb® bispecific antibody platform. Activities of these bispecifics were assessed in vitro by measuring T cell proliferation, cytokine production, and cytotoxicity in co-cultures of human cancer cell lines mixed with primary human T cells. CEACAM5 x CD28, Trop-2 x CD28, STEAP1 x CD28, and mesothelin x CD28 bispecifics enhanced T cell degranulation, cytokine secretion, and cancer cell cytotoxicity only in concert with TCR engagement. IHC was used to compare TAA expression on cancer cell lines versus that found on cancer tissues, allowing categorization of cell lines as high density (tumor surrogate) or low density (normal tissue surrogate). CD28 bispecifics were identified with selective potency on high versus low expressing cell lines, suggesting a favorable therapeutic index. CD28 bispecific antibodies co-targeting CEACAM5, Trop-2, STEAP1, and mesothelin show promising activity and warrant further development across a range of solid tumors. Citation Format: Matthew A. Dragovich, Viralkumar Davra, Matthew S. Faber, Yoon Kyung Kim, Alex Nisthal, Veronica G. Zeng, Jonathan Jacinto, Juan E. Diaz, Thuy Truong, Jing Qi, Kendra N. Avery, Rumana Rashid, Sung-Hyung Lee, Seung Y. Chu, Christine Bonzon, Ruschelle Love, Matthew J. Bernett, James A. Ernst, Rena Bahjat, Norman J. Barlow, John R. Desjarlais, Michael Hedvat, Gregory L. Moore. Tumor-specific CD28 costimulatory bispecific antibodies enhance T cell activation in solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2983.