Abstract 6021: Development of JLF025, an anti-mesothelin T cell engager optimized for efficacy and safety

Abstract Mesothelin (MSLN) is a GPI-anchored glycoprotein that is overexpressed in several solid tumors including mesothelioma, pancreatic cancer, ovarian cancer, non-small cell lung cancer, biliary cancer, and triple negative breast cancer with limited expression in normal tissues. This expression profile makes MSLN an attractive target for cancer immunotherapy. m912 antibody that has demonstrated positive effect in CAR T format in clinics is derived from a fully human Fab library. It has a 100% germline sequence, but its binding affinity is relatively poor (KD around 60 nM). To expand applications of the m912 to include the T-cell engager format, we used phage display to improve m912 affinity, and further, developed a bivalent T-cell engaging bispecific format by fusing the humanized OKT3 scFv to the C-terminus of the light chain. When the final lead with a KD of 1.4 nM was incorporated into this format, the resulting bispecific antibody JLF025 showed high potency in T-cell activation and T-cell dependent cytotoxicity in vitro. Interestingly, the cytotoxicity induced by MSLN-490 was not affected by soluble MSLN at a level found in cancer patient sera. Using a co-grafting mouse model in which a mixture of MSLN-positive NCI-N87 cancer cells and human PBMCs was co-injected with JLF025, we demonstrated that JLF025 exhibited a remarkable tumor suppressing activity. The potent tumor suppressing activity was also observed in a xenograft model in which PBMCs were not co-grafted with tumor cells but delivered from the periphery. In addition, combination of MSLN-490 with Atezolizumab or a chemotherapy drug paclitaxel produced a synergistic effect on tumor inhibition and more T cells were found infiltrated into tumors in the combo groups. In hMSLN and hCD3 double knock-in mouse, JLF025 demonstrated acceptable safety profiles. Thus, combination of JLF025 with immune checkpoint inhibitors and chemotherapy drugs represents a promising cancer treatment strategy and could be considered for future clinical trials. Citation Format: Xiaodong Xiao, Zhidi Pan, Junjun Liu, Yali Yue, Fengzhi Li, Shusheng Wang, Haiqiu Huang, Kaiyong Yang, Zhangyi Song, Zhenyu Wang, Hua Jiang, Yueqing Xie, Jianwei Zhu. Development of JLF025, an anti-mesothelin T cell engager optimized for efficacy and safety [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6021.