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CD137 (4-1BB/TNFRSF9) is a costimulatory T-cell receptor belonging to the TNF receptor superfamily which is transiently expressed on T cell and NK cells. Targeting CD137 or its natural ligand 4-1BB ligand (4-1BBL) has important implications in many clinical conditions, including cancer. Anti-CD137 agonistic anti-CD137 antibodies have shown potent, often curative anti-tumor activity in preclinical models. In depth analysis revealed that 4-1BB-mediated anti-cancer effects are based on its ability to induce activation of cytotoxic T lymphocytes and high amounts of cytokines, including IFN-γ. However, this approach has suffered significant setback in the clinic due to either dose-dependent severe liver toxicity (e.g. Urelumab), or lack of single agent activity in the case of Utomilumab. Multiple therapeutics that attempt to restrict 4-1BB agonism to the tumor microenvironment and minimize systemic exposure have emerged. Here we present a novel antibody targeting 4-1BB receptor, named HLX25 with engineered Fc region to enhance the binding to FcγR IIB. In vitro, HLX25 induces comparable or superior CD137 activation and IL-2 release compared to Urelumab and Utomilumab, suggesting potentially a better safety profile. We showed that HLX25 binds to unique non-ligand competing epitope, which is different from both Urelumab and Utomilumab. In vivo, HLX25 shows potent dose-dependent single agent anti-tumor activity as low as 0.3mg/kg in MC38 model without sign of liver toxicity up to 20 mg/kg, suggesting superior efficacy and better safety profile. Citation Format: Jie Xue, Jen-Kuan Chang, Chen Dong, Yi-Ting Mao, Pallavi Raghavendra, Ou Li, Hassan Issafras, Wenfeng Xu, Weidong Jiang. A FcgR IIB tumor enriching anti-CD137 novel antibody to avoid liver Immuno-toxicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2786.