Abstract 557: COBRA: A novel conditionally active bispecific antibody that regresses established solid tumors in mice

Despite clinical success with bispecific antibodies (bsAbs) targeting hematological malignancies (e.g. blinatumomab, a CD19xCD3 bsAb), efficacy in solid tumor indications remains a significant challenge. Because T cell redirecting bsAbs are so potent, even very low levels of cell surface target antigen expression on normal tissues may quickly become a safety liability and severely restrict the dose levels that can be achieved in patients. This limits the likelihood of reaching efficacious concentrations and reduces the therapeutic potential of these highly active molecules. Additionally, identifying “clean” target antigens that are uniquely expressed on the tumor and not on normal tissues has been very difficult at best. To overcome these challenges, we have developed a novel recombinant bsAb platform called COBRA™ (Conditional Bispecific Redirected Activation). COBRAs are engineered to enable targeting of more widely expressed and validated tumor cell surface antigens by focusing T cell engagement within the tumor microenvironment. COBRA molecules are designed to bind to target antigen, which may be expressed on both tumor and normal cells, yet not engage T cells unless exposed to a proteolytic microenvironment, which is common in tumors but not in normal healthy tissues. Once bound to the tumor target antigen, protease-dependent linker cleavage allows COBRAs to convert an inactive anti-CD3 scFv to an active anti-CD3 scFv binding domain. Upon conversion, COBRAs are then able to simultaneously co-engage T cells and target antigen, resulting in a potent cytolytic T cell response against the tumor cells. In addition, COBRAs are designed with a half-life extension moiety that is removed from the active molecule upon proteolytic cleavage. This allows for a sustained presence in the circulation of the inactive COBRA prior to tumor target binding, and more rapid clearance of unbound active COBRA molecules, thereby decreasing the potential for cytotoxic activity in normal tissues. Here we reveal the novel design of the COBRA molecule and demonstrate its ability to engage CD3 and Epidermal Growth Factor Receptor (EGFR) to elicit potent cytotoxic activity in T cell culture and in human T cell implanted tumor-bearing mice. We report low-to-sub-picomolar T cell activation and cytotoxicity in vitro, and COBRA linker cleavage dependent T cell mediated regression of established solid tumor xenografts in NSG mice in vivo. Citation Format: Danielle Dettling, Eilene Kwok, Lucy Quach, Aakash Datt, Jeremiah D. Degenhardt, Maia Vinogradova, Anand Panchal, Pui Seto, Jessica L. Krakow, Russell Wall, Brian J. Hillier, Ying Zhu, Robert B. DuBridge, Chad May. COBRA: A novel conditionally active bispecific antibody that regresses established solid tumors in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 557.