Abstract 5397: A novel anti-CD3×CD28×STEAP1 tri-specific T-cell engager with enhanced and durable antitumor responses in prostate cancer.

Abstract Introduction: Limited efficacy has been achieved with immunotherapy for prostate cancer, largely due to an immunosuppressive or “immune desert” tumor microenvironment, poor T-cell infiltration, or lack of robust tumor-specific targets. To overcome these barriers, we have developed an anti-CD3×CD28×STEAP1 tri-specific T-cell engager (TCE) that redirects T cells to prostate tumors with high levels of STEAP1 and improves T cells through co-delivering CD3 and CD28 signals. Methods: T cell-dependent cellular cytotoxicity was assessed using a luciferase assay or flow cytometry. Concurrently, cytokine secretion in cell culture supernatants was analyzed using a multiplex bead-based flow cytometric assay. To evaluate T-cell proliferation and memory formation, peripheral blood mononuclear cells (PBMCs) were co-cultured with C4-2B cells and subsequently analyzed by flow cytometry. The sustained cytotoxic potency of T cells was further examined in a repeated antigen challenge model, and cytotoxicity was measured after each cycle using a luciferase assay. In vivo, antitumor efficacy was investigated in humanized PBMC/C4-2, LNCaP and patient-derived xenograft (PDX) xenograft models. Additionally, a pilot toxicology was studied in cynomolgus monkeys. Results: Anti-CD3×CD28×STEAP1 TCE exhibited a target-dependent T-cell activation and cytotoxicity, especially under conditions with a low T effector to tumor cell ratio (1:5-1:10). HLX3902 demonstrated enhanced T cell function and sustained in vitro cytotoxicity compared with bispecific TCEs and their combinations, showing superior T-cell proliferation and enhanced expansion of memory T cells. Furthermore, Anti-CD3×CD28×STEAP1 TCE demonstrated superior and sustained antitumor activity in both humanized PBMC/cell line-derived xenograft and PDX models at a single 0.01 mg/kg dose, with increased T-cell infiltration, activation, and persistent intratumoral CD8+ T cells resulting from CD28 co-stimulation compared to AMG509. Anti-CD3×CD28×STEAP1 TCE was well tolerated in cynomolgus monkeys after weekly intravenous administration at a dose of 45 μg/kg/QW Conclusion: We developed a novel tri-specific TCE with optimized CD3 and CD28 signaling, effectively co-engages T cells and STEAP1-expressing tumor cells. It elicits potent, target-dependent T-cell activation and cytotoxicity, The incorporation of CD28 signaling significantly improved T-cell persistence and sustained functional activity against repeated antigen challenges in vitro and in vivo. It was well tolerated in cynomolgus monkeys, supporting its potential for clinical development, and positions as a best-in-class molecule with Investigational New Drug submission anticipated in Q1 2026. This tri-specific co-stimulatory platform may be extended to treat immunosuppressive or “immune desert” solid tumors. Citation Format: Ge Song, Aiying Xiong, Seng Zhu, Xuemei Tian, Xiaoling Yuan, Wanli Zhang, Wan-Jen Yang, Xiansong Xiong, Ling Ding, Haixiang Yu, Xiaoli Hou, Pingao Yan, Qingyu Wang, Xiao Zou, Chen Hu, Jijun Yuan, . A novel anti-CD3×CD28×STEAP1 tri-specific T-cell engager with enhanced and durable antitumor responses in prostate cancer 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 5397.