Abstract 4126: Anti-tumor immunity of M9657, a conditional CD137 immune agonist, is correlated with mesothelin expression on tumor cells

Abstract Background: In clinical trials, systemic administration of first-generation CD137 agonist monotherapies was suspended due to either low anti-tumor efficacy or hepatotoxicity mediated by the epitope recognized on CD137 and Fc gamma receptor (FcγR) ligand-dependent clustering. M9657 is a bispecific conditional agonist that has been developed to bind simultaneously to mesothelin (MSLN) and CD137 to stimulate an anti-tumor immune response in the tumor microenvironment. M9657 was engineered in a tetravalent bispecific antibody (mAb2™) format with a human IgG1 backbone with LALA mutations, which abrogates binding to FcγRs but retains FcRn binding for IgG-like pharmacokinetics. M9657 is expected to have enhanced anti-tumor efficacy while avoiding systemic immune activation. Methods: MSLN surface copy number was quantified in a series of cancer cell lines with a broad range of MSLN expression. CD8+ T cell-mediated tumor cell cytotoxicity and cytokine release from CD8+ T cells were investigated in a series of in vitro functional assays. The receptor occupancy (RO) of MSLN on the tumor cell surface and of CD137 on CD8+ T cells was determined by flow cytometry. MSLN in EMT-6 cells was knocked out using CRISPR and confirmed by immunohistochemistry. The anti-tumor efficacy of FS122m, a murine-reactive surrogate of M9657, was investigated in EMT-6 parental and MSLN knockout syngeneic tumor models. Results: M9657 displayed MSLN-dependent cytokine release and tumor cell cytotoxicity. IFNγ release by CD8+ T cells correlated with MSLN copy number on tumor cells. MSLN copy number of 3000 was found to be the minimum required to trigger CD137 agonism. CD137 RO by M9657 on activated human CD8+ T cells and MSLN RO by M9657 on tumor cells increased with increasing concentrations of M9657. M9657 caused full agonism (EC100) when CD137 RO on activated human CD8+ T cells was approximately 30-38%. The mouse surrogate FS122m demonstrated potent dose-dependent anti-tumor efficacy in the EMT-6 tumor model, while no anti-tumor efficacy was observed in MSLN knockout tumors, which further confirmed that MSLN expression is required for M9657 anti-tumor immunity. Conclusion: M9657 exhibits promising and potent MSLN-dependent conditional immune agonism, supporting its clinical investigation. Citation Format: Chunxiao Xu, Xueyuan Zhou, Sireesha Yalavarthi, Rene Schweickhardt, Andree Blaukat, Laura Helming. Anti-tumor immunity of M9657, a conditional CD137 immune agonist, is correlated with mesothelin expression on tumor cells. [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 4126.