Abstract 2937: Preclinical characterization of CBA-1535, a novel bi-specific tribody, with two binding sites to 5T4 and one site to CD3ε
Authors: Takizawa Fumitake , Yamamoto Kotaro , Nakamura Koji , Richard Ana R. , Cunnah Philip J. , Mertens Nico , Pissarra Pedro D. , Tsukumo Yukihito 
Published: April 2023

Abstract
5T4 oncofetal tumor-associated antigen (also known as TPGB or WAIF1) is over-expressed in a wide range of solid tumors but shows very limited expression in normal adult tissues thus suggesting 5T4 as an attractive target for antibody therapy. CBA-1535, originally developed by Biotecnol (UK), is a novel trivalent biological construct named a Tribody™ with two binding sites to the tumor associated antigen 5T4 and one to the T cell expressed antigen CD3ε. CD-3 based T cell engagers (TCE) is one of the attractive strategy to treat cancer patients resistant to conventional therapies, however, the effects of TCE on solid tumors are limited due to the immunosuppressive tumor microenvironment. To overcome this limitation, CBA-1535 is designed to have stronger binding activity to 5T4 with two binding sites, and to have the lower affinity to CD3 than to 5T4 to prevent non-specific T cell activation.CBA-1535 does not have a Fc region with a molecular weight of ~100 kDa and it has a low nM Kd for bivalent binding to 5T4, a 20 to 30 nM Kd for monovalent binding to CD3ε. CBA-1535 efficiently induced T cell activation in human PBMCs as exemplified as CD25 and CD69 expressions and showed cytokine release, i.e., IL-2, IL-6, TNFα, and IFN-ɤ, at the concentration between 0.01 and 10 nM only when co-cultured with target cancer cells, however, those T cell activation and cytokine release were not observed under the conditions without target cancer cells, even at the concentration of 1 µM. In the presence of activated human PBMCs as effector cells, CB-1535 elicited 5T4 expressing target cell cytotoxicity with an EC50 of approximately 1-30 pM in a variety of cancer cells. The anti-tumor efficacy of CB-1535 has examined in immunocompromised mice bearing human lung mesothelioma (MSTO-211H) and alveolar epithelial adenocarcinoma (A549) xenografts (such as the 5T4 expressing target cell) and human PBMCs (as effector cells). Anti-tumor efficacy was observed with complete suppression of tumor growth at doses as low as 1 mg/kg in both xenograft models. No abnormalities related to CBA-1535 administration were observed regarding to body weight and clinical signs. Importantly, CBA-1535 showed a synergistic cytotoxicity against 5T4 expressing target cancer cells in vitro when combined with PD-1 antibody Pembrolizumab in a co-culture experiment. In conclusion, we demonstrated encouraging preclinical profile of CBA-1535 as a novel CD-3 based T-cell engager antibody. These results provide the strong rationale for further clinical evaluation of
CBA-1535 in 5T4 positive tumors. CBA-1535 is now under the phase 1 trial in Japan (jRCT2031210708), with 2 parts, the monotherapy and the combination with Pembrolizumab.
Citation Format: Fumitake Takizawa, Kotaro Yamamoto, Koji Nakamura, Ana R. Richard, Philip J. Cunnah, Nico Mertens, Pedro D. Pissarra, Yukihito Tsukumo. Preclinical characterization of CBA-1535, a novel bi-specific tribody, with two binding sites to 5T4 and one site to CD3ε [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2937.