An optimal antitumor response by a novel CEA/CD3 bispecific antibody for colorectal cancers

CD3-based bispecific T cell engagers (bsTCEs) are one of the most promising bispecific antibodies for effective cancer treatments. To elicit target-specific T cell-mediated cytotoxicity, these bsTCEs contain at least one binding unit directed against a tumor antigen and another binding unit targeting CD3 in T cell receptor complex. Development of CD3-based bsTCEs, however, has been severely hampered by dose-limiting toxicities due to cytokine release syndrome. To address this limitation, we developed a novel functionally trivalent T cell engager (t-TCE) antibody containing affinity-reduced CD3 binding unit positioned to ensure monovalent CD3 engagement, in combination with bivalent tumor antigen binding of carcinoembryonic antigen (CEA).

Because of its effective tumor cell killing in vitro and in vivo with reduced cytokine release, CEA/CD3 bsTCE may greatly benefit in CEA-positive cancer immunotherapy.

We modeled the variable region of anti-CD3 in the complementarity-determining regions of the heavy chain and obtained CD3 binders with reduced binding affinity. Two optimized versions CEA/CD3-v1 and CEA/CD3-v2 were identified and generated in tetravalent format, characterized and compared in vitro and in vivo for functional activity.

Our lead candidate, CEA/CD3-v2, demonstrated subnanomolar binding and picomolar potency against a panel of CEA-expressing cancer cell lines. In addition, we detected reduced T cell cytokine release with potent cytotoxic activity. Our t-TCE CEA/CD3-v2 molecule demonstrated strong antitumor effect in a dose-dependent manner in human peripheral blood mononuclear cell (PBMC) xenograft model. Furthermore, combination of CEA/CD3-v2 with atezolizumab provided synergistic antitumor effect. Conclusions: Because of its effective tumor cell killing in vitro and in vivo with reduced cytokine release, CEA/CD3 bsTCE may greatly benefit in CEA-positive cancer immunotherapy.