一种新型抗 CD3×CD28×STEAP1 三特异性 T 细胞衔接器在前列腺癌中具有增强且持久的抗肿瘤反应

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.