Characterization of the anticancer and immunologic activity of RGX-019, a novel pre-clinical stage humanized monoclonal antibody targeting the MERTK receptor

MERTK, a receptor tyrosine kinase of the TYRO3/AXL/MERTK (TAM) family, is expressed in innate immune cells including macrophages, dendritic cells and NK cells and is overexpressed in a wide variety of cancers, including leukemia and many solid cancers. Activation of MERTK on cancer cells via ligand binding results in activation of several tumor-promoting signaling pathways, which stimulate proliferation, migration and angiogenesis, and decrease apoptosis and chemosensitivity. Furthermore, activation of MERTK on macrophages drives immune evasion through the promotion of an immune-suppressive M2 phenotype. Pre-clinical and clinical studies have shown promising emerging evidence of anti-tumor efficacy upon modulation of TAM receptor signaling. Herein, we report the pre-clinical characterization of RGX-019, a humanized IgG1 antibody with high affinity and specificity for human MERTK. Surface Plasmon Resonance, cell-based binding assays and competition ELISA demonstrate high affinity/avidity binding for RGX-019 to human MERTK. In contrast, binding to human AXL, human TYRO3, or murine MERTK was not detected. Binding of RGX-019 to the MERTK receptor triggered its rapid internalization and degradation from the surface of human cancer cells within 4 hours and prevented Gas6 induced phosphorylation of AKT, a downstream signal transduction pathway that promotes cell growth and survival. RGX-019 treatment of SKMel5 melanoma cells effectively inhibited colony formation. RGX-019-induced degradation of MERTK in in vitro differentiated human M2 macrophages reduced AKT activation and promoted a pro-inflammatory cytokine signature. Furthermore, a related surrogate murine MERTK antibody significantly inhibited growth of MDA-MB-231 breast cancer tumors, demonstrating single agent anti-tumor efficacy in vivo. These findings demonstrate that RGX-019 is a potent and selective inhibitor of MERTK signaling with a unique mechanism-of-action that potently disrupts MERTK signaling in both cancer cells and immune-suppressive macrophages via MERTK receptor degradation. This activity results in robust repression of cancer cell growth both in vitro and in vivo. Overall, these data support further development of RGX-019 as a cancer therapeutic. Citation Format: Shugaku Takeda, Celia Andreu-Agullo, Subhasree Sridhar, Nils Halberg, Ivo C. Lorenz, Sohail Tavazoie, Isabel Kurth, Masoud Tavazoie. Characterization of the anti-cancer and immunologic activity of RGX-019, a novel pre-clinical stage humanized monoclonal antibody targeting the MERTK receptor [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 LB-277.