Abstract 1620: ATG-112, a novel ALPP/G x CD3 bispecific T cell engager, for the treatment of ALPP/G+ solid tumors

Abstract Background Placental alkaline phosphatase (ALPP) and related placental-like/germ-cell isoforms (ALPPL2 / ALPG) are cell-surface alkaline phosphatases that are aberrantly expressed in various solid tumors (e.g., ovarian, endometrial, germ cell, gastric, and pancreatic cancers), while being virtually absent from normal adult tissues except the placenta, making them highly promising tumor-selective targets for immunotherapy. T-cell engagers (TCEs) demonstrate robust clinical activity by co-engaging T cells with tumor cells, thereby inducing T cell activation and T cell-dependent cellular cytotoxicity (TDCC) against tumors. Here we developed ATG-112, an ALPP/G × CD3 bispecific TCE using the AnTenGager™ platform, featuring bivalent antigen binding for improved low-antigen tumor recognition and a sterically-masked CD3 binding arm to restrict T-cell activation to the tumor microenvironment. Methods ATG-112 was engineered in a 2+1 IgG-based format, enabling bivalent binding to ALPP/G and monovalent engagement of CD3. We characterized its binding kinetics, TDCC, immunogenicity, and cytokine release profile using ALPP/G-positive cell lines, recombinant proteins, and human PBMCs. Antitumor efficacy was assessed in humanized mice with HPAC and NCI-H1650 xenografts. Results Tissue microarray (TMA) IHC analysis revealed that ALPP/G expression was restricted to placental tissue in healthy organs and not detected in other normal tissues. In human tumors, ALPP/G was frequently expressed in endometrial (58.50%) and ovarian cancers (51.01%), with lower prevalence in bladder (26.14%), gastric (25.00%), and pancreatic cancers (16.67%). ATG-112 showed high binding affinity to both ALPP/G-positive tumor cells and recombinant protein, with EC50 and KD values in the sub-nanomolar range. ATG-112 induced robust TDCC activity against target positive cells with pico-molar range EC50, confirming potent antigen-dependent T-cell redirection. In vitro cytokine-release assays demonstrated minimal cytokine secretion from human PBMCs, suggesting low risk of excessive immune activation. Immunogenicity assessment showed that humanized ATG-112 molecules exhibited low immunogenic potential. In vivo, ATG-112 achieved potent tumor suppression across multiple dose levels, demonstrating strong antitumor activity and dose responsiveness. Safety studies revealed a favorable in vivo safety profile, including low cytokine release and controllable CRS risk at efficacious doses. ATG-112 also exhibited excellent developability. Conclusion ATG-112 is a 2+1 T-cell engager that potently and selectively targets ALPP/G and CD3. It demonstrates potent in vitro and in vivo efficacy with minimal cytokine release. These findings support ATG-112 as a differentiated and promising therapeutic candidate for ALPP/G-positive solid tumors and justify its advancement toward clinical development. Citation Format: Jishun Chen, Yu Bai, Suya Bai, Huiling Liu, Zaoshun Hu, Jay Mei, Peng Chen, Bing Hou. ATG-112, a novel ALPP/G x CD3 bispecific T cell engager, for the treatment of ALPP/G+ solid tumors [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 1620.