Abstract 1634: A conditionally-active, dual-targeting TCE with superior tumor selectivity for the treatment of solid tumors

Abstract Background: T cell engagers (TCEs) represent a rapidly advancing class of immunotherapies that have demonstrated remarkable clinical efficacy in hematologic malignancies. However, their application in solid tumors remains limited by on-target, off-tumor toxicities resulting from antigen expression in healthy tissues. Moreover, off-target, off-tumor effects can arise from systemic T cell engagement and nonspecific activation, leading to cytokine release syndrome (CRS). To overcome these challenges, we have developed an innovative TCE platform incorporating an AND-gated safety mechanism that enables selective killing of tumor cells while sparing healthy cells. Method: The Bi-targeted Tumor-Associated Cytotoxicity (BiTAC) T cell engager builds up on the previously described hemibody concept1 and employs an on-cell assembly mechanism of two complementary compounds to achieve conditional activation. Each compound comprises a tumor-binding domain recognizing a distinct tumor-associated antigen (TAA) and a split T cell-binding moiety. Individually, the compounds do not engage T cells. Productive T cell activation occurs only when both compounds co-bind and accumulate on tumor cells co-expressing the respective antigens, leading to reconstitution of an active CD3 binder. The BiTAC-TCE was optimized for developability, stability, and potent functional activity in both in vitro and in vivo settings. Results: Our bioinformatics pipeline identified a TAA pair highly co-expressed in colorectal and lung cancers. Constitutively active drugs targeting each TAA individually have been tested in humans and revealed target-associated toxicities. An antibody format with favorable manufacturability and developability was subsequently selected. Using in vitro T cell-dependent cellular cytotoxicity (TDCC) assays across multiple cell lines in 2D and 3D cultures, we demonstrate that the combinatorial design of BiTAC-TCE enables T cells to selectively eliminate dual-target-positive tumor cells while sparing single-positive healthy cells. Individual compounds/BiTAC precursors don’t bind T cells and don’t induce TDCC in vitro and in vivo. Furthermore, BiTAC-TCE exhibits strong antitumor efficacy in PBMC-engrafted mice bearing colorectal tumors and displays a favorable pharmacokinetic (PK) profile in preclinical mouse studies. Conclusion: Our BiTAC-TCE platform enables precise targeting of solid tumors through TAAs not druggable by current immunotherapies. Both TAAs addressed are broadly expressed but not co-expressed on healthy cells of different tissues, leading to failure of monotargeting T cell engagers in clinical settings. Our lead BiTAC-TCE for the treatment of colorectal and lung cancers has recently entered formal preclinical development and IND-enabling studies. 1Banaszek, A. et al. Nat Commun 10, 5387 (2019) Citation Format: Steffen Dickopf, Jorge A. Lerma Romero, Jessica Seib, Laura Perucho-Aznar, Steffen Runz, Abdul-Habib Maag, Anna Pryszlak, Anja Schreiber, Martha Gschwandtner, Christoph Erkel, Richard J. Austin, . A conditionally-active, dual-targeting TCE with superior tumor selectivity for the treatment of 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 1634.