Immunocompetent Mouse Models of Cancer Reveal the Superiority of Cellular Targets over Stromal Targets for the Development of Anticancer Bispecific Antibodies.

Bispecific antibodies (BsAb) are a rapidly advancing class of biopharmaceuticals with substantial potential for cancer immunotherapy. Although BsAbs have shown notable success in treating certain hematologic malignancies, their application for solid tumors remains limited. The extra domain B (EDB) of fibronectin represents a promising pan-tumoral stromal target, offering an attractive alternative to conventional cellular tumor antigens, which often face limitations with respect to specificity in solid tumors. In this study, we describe the generation and characterization of a T cell-engaging BsAb that targets murine CD3 using the 2C11 clone and EDB with the L19 clone. Specifically, the BsAb consists of a Fab fragment (targeting CD3) fused with two single-chain Fv fragments (targeting EDB) at the C-terminus. The BsAb was produced in Chinese hamster ovary cells and purified to homogeneity. To compare stromal and cellular targeting, two murine tumor cell lines naturally secreting EDB in the stroma were transduced to express the target on the cell surface. In both cell lines, biodistribution analysis revealed increased tumor uptake in the cellular model compared with the stromal one. Similarly, treating immunocompetent cellular EDB tumor-bearing mice with the BsAb improved anticancer activity. By contrast, no significant therapeutic benefit was observed in the stromal model. These findings underscore the importance of direct tumor cell targeting compared with stromal targeting for effective BsAb therapy.