Early functional reactivity to a CEACAM5 x CD3 T cell bispecific antibody in solid tumors associates with baseline immune contexture rather than target antigen density alone.

Clinical translation of T cell bispecific (TCB) therapies in solid tumors is hindered by the immunosuppressive tumor microenvironment and intratumoral heterogeneity, features poorly captured by standard preclinical models. We applied an integrated translational framework, combining humanized mouse models, a patient-derived ex vivo organ culture (EVOC) platform, and Phase 1 clinical trial data (ClinTrials.gov identifier NCT03539484), to define the biologic determinants of functional reactivity to a CEACAM5 x CD3 TCB. In humanized mice bearing CEA-positive tumors, the TCB inhibited tumor growth and increased intratumoral T cell infiltration. EVOC, which preserves native 3D tumor architecture and endogenous tumor immune contexture, revealed rapid TCB-induced secretion of IFNγ, IL-2, TNFα, and IL-10 across colorectal, pancreatic, and non-small cell lung cancer patient tumors, with substantial inter-patient heterogeneity. Higher baseline CD3+ T cell infiltration trended with a greater magnitude of cytokine release in patient tumors treated with the TCB ex vivo versus tumors with lower CD3+ T cell infiltration. Baseline tumor CEA expression did not associate with functional response in EVOC. In the Phase 1 trial, on-treatment peripheral cytokine induction was greater in patients with immune-inflamed or -excluded tumors as compared to those with immune-desert tumors. EVOC recapitulated the high inter-patient functional heterogeneity observed clinically, with a coefficient of variation for IFNγ of 135% in EVOC versus 190% in patients, compared with 52% in humanized mice. These findings suggest that early pharmacodynamic responses to CEACAM5 x CD3 TCB associate with a pre-existing tumor immune state rather than target antigen expression alone and support integration of patient-derived ex vivo models for modeling clinically-relevant pharmacodynamics.