Abstract A129: Bispecific antibody Ubamatamab induces personalized anti-tumor immune responses in an ex vivo platform for ovarian cancer

Abstract Introduction. Immunotherapies have shown significant clinical benefit in cancers with high unmet medical need. As next-generation bispecific antibodies are entering the clinic, early identification of patients likely to respond to immunotherapy remains both critical and challenging. We have previously demonstrated that our ex vivo 3D micro-tumor platform, which preserves the native tumor microenvironment (TME), is predictive for platinum-based chemotherapy and correlates with clinical immune biomarkers. Here, we show that the ex vivo platform also enables patient-specific profiling of drug sensitivity and immune activity for bispecific antibody ubamatamab. Methods. This study analyzed 14 tissue samples obtained from 12 ovarian cancer patients with primary, recurrent or progressive disease. Micro-tumors including the native TME were isolated from fresh ascites (n=12) or pleural fluid (n=2), seeded, and treated with pembrolizumab (single dose) and dose ranges of chemotherapeutics and ubamatamab (CD3xMUC16 bispecific T cell engager in Phase 1/2 clinical trials). Drug response was assessed through morphological features extracted from high-throughput 3D imaging. Sensitivity was defined by statistically significant immune cell expansion and micro-tumor killing. Supernatants collected after treatment were analyzed for secreted cytokines, chemokines and cytotoxic mediators using proximity extension assay (Olink) immune biomarker profiling for 5 patients. Quantification of immune cell populations was performed via immunofluorescence staining. Results. Treatment with ubamatamab resulted in a significant expansion of immune cells in 43% (6/14) of ex vivo patient samples. Of these, 33% (2/6) also showed significant tumor killing, demonstrating complete functional responses. Responses to ubamatamab were reproducible in successive samples from the same patients, showing assay consistency. Baseline cytokine profiling confirmed substantial inter-patient heterogeneity, and patient samples with low baseline levels of CD8+ T cell cytokines showed ex vivo resistance to ubamatamab. Notably, responders with effective tumor killing exhibited robust and simultaneous induction of effector T cell cytokines and cytotoxic mediators (IFN-γ, IL-2, granzyme B, perforin) as well as IFN-γ-induced chemokines CXCL9 and CXCL11 upon ubamatamab treatment. Importantly, ex vivo sensitivity to ubamatamab did not align with responses to pembrolizumab or chemotherapy, indicating that this therapy may be effective in a distinct subset of patients. Conclusions. This study demonstrates the classification and functional characterization of patient-specific sensitivity to ubamatamab, a bispecific T cell engager in clinical development for ovarian cancer. Our ex vivo 3D micro-tumor platform facilitates quantification of immune responses and the discovery of predictive biomarkers to support patient stratification for emerging immunotherapies. Citation Format: Esmee Koedoot, Felix Behr, Timothy Sijsenaar, Léa Le Large, Cor D. de Kroon, Anne van Altena, Sander Bins, Dieudonné van der Meer, Willemijn Vader, Ingrid Boere, Nelleke Ottevanger, Judith Kroep. Bispecific antibody Ubamatamab induces personalized anti-tumor immune responses in an ex vivo platform for ovarian cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A129.