Abstract 6340: Novel conditionally active tetravalent B7-H3 x CD3 T-cell engagers targeting solid tumors

Abstract B7-H3, also known as CD276, is an immune-checkpoint molecule and a type 1 transmembrane protein that exerts a variety of biological functions. In cancer biology, the overexpression of B7-H3 is correlated with tumor cell invasion and metastasis, decreased T-cell infiltration and function, poor prognosis, and resistance to therapy. As B7-H3 is expressed in multiple solid tumors and plays an important role in modulating anti-tumor immune response as well as promoting cancer cell formation, it became an important target for the development of antibody-based immunotherapy. Many forms of bispecific antibodies targeting B7-H3 have shown antitumor response in preclinical studies. However, bispecific T-cell engagers (TCE) targeting B7H3 and CD3 have been correlated with cytokine release syndrome and hepatic adverse events, a common toxicity observed using TCEs which has limited the development of this immunotherapeutic in clinic. We leveraged BioAtla’s Conditionally Active Biologic (CAB) platform (1) to develop a DualCAB T-cell engager targeting B7-H3 and CD3. This DualCAB-TCE molecule, BA3142, was engineered to bind with high affinity to both recombinant B7-H3 and CD3 receptor/antigens under conditions that mimic the acidic tumor microenvironment, but with low affinity in alkaline physiological conditions. In vitro and in vivo efficacy and toxicity data for the clone BA3142 will be presented. Our data demonstrated that BA3142 promotes cytotoxicity of B7-H3-expressing cancer cells in vitro and induces significant tumor regression in vivo. In Non-Human Primates BA3142 bispecific antibody was tolerated up to 25 mg/kg with minimal cytokine release and no clinical and histopathological signs of toxicities. The CAB technology offers the opportunity to expand the number of targets suitable for drug development in a new class of DualCAB TCEs with superior safety margin and potency in the clinic. (1) Chang HW, Frey G, Liu H, Xing C, Steinman L, Boyle WJ, and Short, J. M. Generating tumor-selective conditionally active biologic anti-CTLA4 antibodies via protein-associated chemical switches. Proc Natl Acad Sci U S A 2021;118. Citation Format: Ana Paula Cugnetti, PhD, Haizhen Liu, Patricia McNeeley, Kathryn Woodard, Cathy Chang, Gerhard Frey, William J. Boyle, Jay M. Short. Novel conditionally active tetravalent B7-H3 x CD3 T-cell engagers targeting solid tumors abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6340.