Abstract 6343: A multispecific T cell engager binding to both membrane proximal and membrane distal epitopes of MSLN with low affinity CD3 for the treatment of AML and solid tumors

Purpose: The effective treatment of AML and solid tumors remains an unmet medical need. Mesothelin (MSLN) is a glycophosphatidylinositol-linked tumor antigen overexpressed in a variety of malignancies, including AML and solid tumors, such as mesothelioma, cholangiocarcinoma, ovarian, pancreatic, lung, esophageal, gastric, colorectal, endometrial and triple-negative breast cancer. Early signs of clinical efficacy with MSLN-targeting agents have validated MSLN as a promising target for therapeutic intervention, but therapies with improved efficacy are still needed. CD3-based T cell engagers are highly potent therapeutic molecules with T cell cytotoxicity activities in the picomolar range. But high CRS risk and T cell exhaustion has been observed for high affinity CD3-based T cell engagers in clinicals, so we used high affinity MSLN and low affinity CD3 to improve efficacy of T cell engager. Experimental Design: We designed SMTE-001, a 78-kDa, tetra-specific, T-cell-activating protein molecule, which binds to two epitopes (membrane proximal and membrane distal) of MSLN, CD3ε on T cells, and to serum albumin. Experiments were conducted to assess the potency, activity, and half-life of HPN536 in in vitro assays, rodent models, and in nonhuman primates (NHP). This molecule was tested in a cytotoxicity assay using human PBMCs co-cultured with OVCAR3, Hela and NCI-H292 cells, which express different levels of MSLN. Soluble MSLN and CA125 (ligand of MSLN) was added to determine effects on cytotoxicity. In vivo xenograft mouse studies were conducted using a PBMC model. Results: Here we report the design and the promising preclinical activity of SMTE-001 molecule in vitro and in vivo. We demonstrate that the bivalent MSLN T cell engager has increased in vitro potency in T cell activation and tumor cell killing, as compared to a monovalent counterpart on high MSLN expressing cells. We also demonstrate that T cell exhaustion is reduced for the low-affinity CD3, compared to the low-affinity CD3 molecule. Because soluble MSLN and CA125 is shed from cancer cells into cancer patient serum, we also demonstrate that soluble MSLN and CA125 does not interfere with the cytotoxic activity of SMTE-001. Importantly, we demonstrate in vivo that SMTE-001 significantly inhibits tumor growth in a dose-dependent manner, while high-affinity CD3 molecule’ efficacy reduces when higher dosing. In cynomolgus monkeys, SMTE-001 shows pharmacokinetics in support of weekly dosing in humans. Conclusions: Collectively, these data demonstrate strong anti-tumor efficacy by this novel multispecific bivalent T cell engager. These data indicate the therapeutic potential of this molecule to activate T cells and improve the clinical efficacy in AML and MSLN-expressing solid tumors. Citation Format: Cuiqing Yang, Xiaohui Shao, Feng Zhou, Lei Zhou, Qi Deng, Yun Zhang, Guangcun Cheng, Yingying Hu, Huaiyuan Ma, Yadan Wu, Shuai Wang, Jie Zang, Lei Liu, Wenqing Yang, Yang Liu, Chunlei Xia, Jianzhong Hu, Ande Luo, Yayuan Fu, Zhuoxiao Cao, Renhong Tang. A multispecific T cell engager binding to both membrane proximal and membrane distal epitopes of MSLN with low affinity CD3 for the treatment of AML and solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6343.