Abstract 3388: One-stop solution for preclinical evaluation of bispecific T-cell engagers targeting STEAP1

Abstract STEAP1 (Six-Transmembrane Epithelial Antigen of the Prostate 1) is a membrane protein highly expressed in prostate and other epithelial cancers, making it an attractive target for immune-based therapeutics. To support the development of STEAP1-directed agents, GemPharmatech established a comprehensive in vitro-in vivo evaluation platform designed to characterize STEAP1 expression, functional relevance, and T-cell engager (TCE)-mediated pharmacology. AMG509 (Xaluritamig), a Phase III STEAP1×CD3 TCE currently in clinical development, was used as a benchmark molecule to validate platform performance. Methods: In vitro assays were developed to quantify endogenous STEAP1 expression across prostate cancer cell lines (e.g., LNCaP, 22Rv1) using flow cytometry, followed by evaluation of T-cell dependent cytotoxicity (TDCC) with both luciferase-based and flow-based readouts. T-cell activation markers (CD69, CD25) and cell conjugation frequency were analyzed to characterize the efficiency of T-cell recruitment to STEAP1-positive tumor cells. Cytokine profiling (IL-2, IFN-γ, TNF-α, IL-6) was performed via Cytometric Bead Array (CBA) to monitor T-cell activation and cytokine release potential. Additionally, GemPharmatech engineered 22Rv1-hSTEAP1 and RM-1-hSTEAP1 cell lines to expand the translational scope of the STEAP1 platform across human and murine tumor contexts. In vivo, a panel of humanized immune mouse models (PBMC-NCG, NCG-MHC-dKO, and HSC-NCG) were employed to evaluate STEAP1-targeted TCE pharmacology under distinct immune settings. AMG509 treatment led to potent, antigen-dependent tumor suppression in STEAP1-positive xenografts, achieving 90% tumor growth inhibition and, in several cases, complete regression. Tumor analyses revealed robust infiltration of human CD3+ T-cells and increased local IFN-γ expression, consistent with effective STEAP1-directed immune engagement. Treated mice maintained stable body weight and displayed no signs of cytokine release syndrome (CRS) or graft-versus-host disease (GVHD), confirming the models’ suitability for translational safety assessment. Collectively, these results demonstrate the establishment of a robust and modular evaluation workflow for STEAP1-targeted immunotherapies. By integrating antigen quantification, immune functional assays, and humanized in vivo systems, GemPharmatech’s platform enables comprehensive and reproducible assessment of emerging TCE modalities. This work provides valuable preclinical insight into STEAP1 biology and establishes an experimental foundation for future therapeutic programs targeting this clinically relevant antigen. Citation Format: Hongyan Sun, Yuqing Han, Fang Zhu, Yujing Zhang, Huixin Yang, Xiang Gao. One-stop solution for preclinical evaluation of bispecific T-cell engagers targeting STEAP1 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 3388.