Abstract 1613: Expanded therapeutic window of potent bispecific PD-L1:CD3 T cell engager using a novel best-in-class CD3 binder

Abstract Introduction: Bispecific T cell engagers (TCE) are potent, off-the-shelf immune modulators that have been hugely successful against hematologic malignancies and are beginning to demonstrate utility in solid tumors. PD-L1 targeting bispecific TCE with an SP34 variant CD3 binder showed exceptionally potent activity against intracranial tumor models in mice. To further expand the potential therapeutic index, we engineered SCRI-6, a novel, high-affinity, non-polyreactive CD3 binder, into a next generation PD-L1 TCE and compared the SP34 and SCRI-6 versions for efficacy and tolerability. Methods: The efficacious dose range of systemically administered PD-L1:CD3 TCE was interrogated using SP34 or SCRI-6 CD3 binders against intracranially implanted NCI-H1975-GFP/ffLuc, modeling metastatic lung cancer. Activity of SP34 or SCRI-6 TCE with a murine PD-L1 binder was evaluated in mice transgenic for human CD3ε. Results: Bioluminescent tumor burden of mice treated with the SP34 TCE was eliminated in 85-100% of mice across the entire dose range of 0.3 to 8.5 mg/kg. However, despite the loss of luminescent signal at 2.1, 4.2, and 8.4 mg/kg, 28-85% of these mice met humane end point criteria and were removed from study early, suggesting that these doses exceeded the maximum tolerated dose. SCRI-6 TCE eliminated bioluminescent tumor signal and 100% of mice remained healthy at doses from 0.03 to 2.12 mg/kg (the highest dose tested). Doses of 0.0005-0.008 mg/kg did not alter tumor growth, and the median survival was similar to the negative control group, establishing the lower limit of efficacy for this TCE. A dose titration from 0.01 to 5 mg/kg of both the SP34 and SCRI-6 TCE in human CD3ε transgenic mice showed pharmacodynamic responses with similar, modest, and transient loss of body weight and transient depletion of circulating lymphocytes at doses of 0.4-5mg/kg. Cytokines associated with cytokine release syndrome (IL-6, IFNγ and TNFα) were found to be elevated at higher levels in SP34 TCE treated mice compared to those in SCRI-6 TCE mice. INFγ was significantly higher in the SP34 TCE at 5, 2.5, and 0.1mg/kg (p=0.03, 0.05, and 0.01 respectively), TNFα at 5mg/kg (p=0.003), and IL-6 at 1.3mg/kg (p=0.03), suggesting a higher tolerated threshold for the SCRI-6-based TCE. In the NCI-H1975 brain metastases model, the SCRI-6 TCE induced complete and durable remissions for the entirety of the study, whereas remissions tended to be variable and transient with the SP34 version. Conclusion: Incorporation of the novel SCRI-6 CD3 binder into a potent PD-L1 targeted TCE showed superior efficacy with lower cytokine secretion, demonstrating potential to expand the therapeutic index. Citation Format: Andrew J. Mhyre, Emily J. Girard, Sinduja Marx, Shelli M. Morris, Kristina Pilat, Alison M. Williams, Parvathi Muthuraman, Ray Ruff, Hailey Hentschel, Steven Chen, Chunfeng Yin, Zachary Crook, Jason Price, James M. Olson, . Expanded therapeutic window of potent bispecific PD-L1:CD3 T cell engager using a novel best-in-class CD3 binder [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 1613.