Abstract 6012: PSMA x CD3 T-cell engagers show preclinical efficacy for the treatment of prostate cancer

Abstract Prostate-specific membrane antigen (PSMA) is a clinically validated target in metastatic castration-resistant prostate cancer (mCRPC) that is being prosecuted by a number of different modalities in the clinic. CD3 T-cell engagers (TCEs) targeting PSMA have shown promise in preclinical and early clinical studies, but generating a molecule with a therapeutic window that enables efficacious dosing in patients has been a barrier to development. Here, we present preclinical in vitro and in vivo data on novel PSMA x CD3 TCEs developed using our TCE platform. To address the challenges of TCE development for mCRPC, we screened and identified hundreds of diverse PSMA- and CD3-binding antibodies with different affinities, epitopes, and biophysical properties using our proprietary antibody screening platform. From there, we engineered large panels of OrthomabTM PSMA x CD3 bispecifics, varying TCE parameters that impact function. Detailed in vitro functional assessment and biophysical characterization assays were conducted to identify antibodies with promising functional and developability profiles. IgG-like bispecifics comprised of PSMA- and CD3-binding arms with finely tuned affinity for each target were generated. PSMA binding epitopes were assessed using cryo-electron microscopy, and TCE function was measured using in vitro T cell co-culture assays. TCEs binding membrane-proximal epitopes drove optimal immune synapse formation, leading to potent killing of cells expressing high (C4-2) and low (22Rv1) levels of PSMA with EC50 values in the picomolar range. Molecules show target-dependent T-cell activation with no killing of a low PSMA-expressing cell line (DU-145) in vitro. Further, select molecules show robust CD4+/CD8+ T-cell activation and proliferation in the presence of target cells, as well as sustained killing of target cells over time in a repeat challenge assay. Finally, molecules evaluated in vivo in a humanized C4-2 xenograft mouse model demonstrated anti-tumor activity and a favorable IgG-like pharmacokinetic profile. In summary, we engineered and assessed hundreds of PSMA x CD3 TCEs at high-throughput, conducted detailed in vitro functional and biophysical characterization, and identified molecules with promising preclinical in vivo efficacy that supports further evaluation and development towards the clinic. Citation Format: Peter Bergqvist, Alaa Amash, Kelly Bullock, Lauren Clifford, Patrick Farber, Jessica Fernandes Scortecci, Ingrid Knarston, Tallie Kuang, Ahn Lee, Amy Lee, Cindy-Lee Crichlow, Franco Li, Matt Mai, Stephanie K. Masterman, Janice Reimer, Eduardo Solano Salgado, Raffi Tonikian, Christopher Williamson, Allison Goodman, Lindsay DeVorkin. PSMA x CD3 T-cell engagers show preclinical efficacy for the treatment of prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6012.