Abstract 4346: Tumor-localized monoclonal antibody targeting 4-1BB for superior safety and efficacy in cancer treatment

Abstract 4-1BB is a potent co-stimulatory receptor that enhances CD8+ T-cell activity and drives robust anti-tumor immunity. However, systemic activation by conventional anti-4-1BB agonist antibodies (Abs) leads to severe toxicities—including hepatotoxicity, thrombocytopenia, and cytokine-mediated inflammation—resulting in multiple terminated clinical trials. To overcome these limitations, we engineered Lock-Urelumab, a tumor-selective pro-agonist form of the clinical anti-4-1BB Ab Urelumab. Lock-Urelumab incorporates an autologous hinge-based antibody lock and an MMP-cleavable peptide linker, enabling spatial hindrance under normal physiological conditions and selective activation within the tumor microenvironment where MMP is overexpressed. Biochemical characterization showed that the antibody lock reduced 4-1BB binding by ∼400-fold, suppressing T-cell co-stimulation and preventing pro-inflammatory cytokine secretion. Upon MMP cleavage, Lock-Urelumab fully restored 4-1BB agonist activity. In a human T-cell transfer mouse model, Lock-Urelumab avoided the 4-1BB “antigen sink” effect, exhibited no detectable organ toxicity, and achieved 100% survival, whereas all Urelumab-treated mice succumbed to treatment-related toxicity within 14 days. Importantly, Lock-Urelumab maintained potent anti-tumor activity, inducing 77% tumor growth inhibition (TGI) compared with 45% for Urelumab, and significantly enhanced intra-tumoral T-cell activation. These findings demonstrate that tumor-localized 4-1BB activation can uncouple efficacy from systemic toxicity. Lock-Urelumab represents a next-generation immune checkpoint agonist with strong potential to revitalize 4-1BB-targeted immunotherapy and broaden its clinical applicability. Citation Format: Yun-Chi Lu, Yi-An Cheng, Tzu-Yi Liao. Tumor-localized monoclonal antibody targeting 4-1BB for superior safety and efficacy in cancer treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4346.