In Vivo Fluorescence Imaging of the Activity of CEA TCB, a Novel T-Cell Bispecific Antibody, Reveals Highly Specific Tumor Targeting and Fast Induction of T-Cell–Mediated Tumor Killing

Purpose: CEA TCB (RG7802, RO6958688) is a novel T-cell bispecific antibody, engaging CD3e upon binding to carcinoembryonic antigen (CEA) on tumor cells. Containing an engineered Fc region, conferring an extended blood half-life while preventing side effects due to activation of innate effector cells, CEA TCB potently induces tumor lysis in mouse tumors. Here we aimed to characterize the pharmacokinetic profile, the biodistribution and the mode of action of CEA TCB by combining in vitro and in vivo fluorescence imaging readouts. Experimental Design: CEA expressing tumor cells (LS174T) and human peripheral blood mononuclear cells (PBMC) were co-cultured in vitro or co-grafted into immunocompromised mice. Fluorescence reflectance imaging and intravital 2-photon (2P) microscopy were employed to analyze in vivo tumor targeting while in vitro confocal and intravital time-lapse imaging were used to assess the mode of action of CEA TCB. Results: Fluorescence reflectance imaging revealed increased ratios of extravascular to vascular fluorescence signals in tumors after treatment with CEA TCB compared to control antibody, suggesting specific targeting, which was confirmed by intravital microscopy. Confocal and intravital 2P microscopy showed CEA TCB to accelerate T-cell dependent tumor cell lysis by inducing a local increase of effector to tumor cell ratios and stable crosslinking of multiple T-cells to individual tumor cells. Conclusions:Using optical imaging we demonstrate specific tumor targeting and characterize the mode of CEA TCB mediated target cell lysis in a mouse tumor model, which supports further clinical evaluation of CEA TCB.