Simple SummaryColorectal cancer is one of the most common cancers worldwide, and novel treatments are urgently needed to improve treatment for cancer patients. In this report, three different designs of humanized EpCAM-CD3 antibodies were engineered and tested against colorectal tumors. The antibodies demonstrated high efficacy and specificity. In addition, the study demonstrates a novel method of delivering bispecific antibodies using mRNA-lipid nanoparticle (LNP) technology. The delivery of EpCAM-CD3 human Fc (hFc) mRNA-LNPs into mice tumors with intravenous T cell injection significantly blocked OVCAR-5 xenograft tumor growth in vivo. The data provide a basis for future clinical studies.AbstractThe epithelial cell adhesion molecule (EpCAM) is often overexpressed in many types of tumors, including colorectal cancer. We sequenced and humanized an EpCAM mouse antibody and used it to develop bispecific EpCAM-CD3 antibodies. Three different designs were used to generate bispecific antibodies such as EpCAM-CD3 CrossMab knob-in-hole, EpCAM ScFv-CD3 ScFv (BITE), and EpCAM ScFv-CD3 ScFv-human Fc designs. These antibody designs showed strong and specific binding to the EpCAM-positive Lovo cell line and T cells, specifically killed EpCAM-positive Lovo cells and not EpCAM-negative Colo741 cells in the presence of T cells, and increased T cells’ IFN-gamma secretion in a dose-dependent manner. In addition, transfection of HEK-293 cells with EpCAM ScFv-CD3 ScFv human Fc mRNA-LNPs resulted in antibody secretion that killed Lovo cells and did not kill EpCAM-negative Colo741 cells. The antibody increased IFN-gamma secretion against Lovo target cells and did not increase it against Colo741 target cells. EpCAM-CD3 hFc mRNA-LNP transfection of several cancer cell lines (A1847, C30, OVCAR-5) also demonstrated functional bispecific antibody secretion. In addition, intratumoral delivery of the EpCAM-CD3 human Fc mRNA-LNPs into OVCAR-5 tumor xenografts combined with intravenous injection of T cells significantly blocked xenograft tumor growth. Thus, EpCAM-CD3 hFc mRNA-LNP delivery to tumor cells shows strong potential for future clinical studies.