Abstract 5316: SM2256: A bispecific conditional CD40 and 4-1BB dual agonist for enhanced solid tumor immune response

Abstract Background: Monoclonal antibodies targeting the co-stimulatory receptors CD137 (4-1BB) and CD40 have displayed encouraging activity against a wide range of solid tumors in preclinical animal models. However, their clinical development as monotherapy has been hindered by dose-related liver toxicity, leading to limited efficacy at safe doses. To address this challenge and bolster tumor-specific immunity by orchestrating the interaction between dendritic cells (DCs) and cytotoxic T cells, we introduce SM2256 - a bispecific conditional CD40 and 4-1BB dual agonist. SM2256 physically bridges DCs and T cells, enhancing T cell priming and reactivation. In preclinical animal studies, SM2256 exhibited both excellent tolerability and potent anti-tumor activity. Methods: SM2256 was engineered by fusing high-affinity, non-ligand-blocking VHH domains targeting 4-1BB and CD40 to a silenced IgG1 Fc backbone. This fusion resulted in a tetravalent bispecific antibody capable of conditionally activating CD40 and 4-1BB upon mutual receptor binding. We assessed the binding specificity and affinity of SM2256 to human CD40 and 4-1BB using ELISA and flow cytometry. The target-dependent activation of CD40 and 4-1BB was confirmed via reporter assays and co-culture systems using primary DCs and T cells. In vivo safety and anti-tumor efficacy were evaluated in a syngeneic transplantation model using the MC38 cancer cell line in B-h41BB/B-hCD40 double humanized mice. Results: SM2256 exhibited sub-nanomolar binding affinity to recombinant human and cynomolgus monkey CD40 and 4-1BB, activating 4-1BB-NFκB-luc and CD40-NFκB-luc reporter cell lines at picomolar concentrations in a strictly target-dependent manner. Furthermore, SM2256 physically facilitated the interaction between DCs and T cells, leading to increased expression of DC co-stimulatory ligands and elevated secretion of cytokines by T cells in mixed lymphocyte cultures. In a syngeneic tumor transplantation model, SM2256 significantly restrained the growth of MC38 tumors in B-h41BB/B-hCD40 double humanized mice. Conclusions: SM2256 is a novel conditional CD40 and 4-1BB dual agonist that demonstrates promising preclinical efficacy against solid tumors while boasting an improved safety profile when compared to existing monoclonal antibodies. Citation Format: Wenjing Song, Simin Yang, Yi Wei, Shihao Lu, Huiqin Geng, Xiaodan Liu, Hong Zhou, Siyao Xie, Xing Zhang, Sheila Zhou, Yanbin Liang. SM2256: A bispecific conditional CD40 and 4-1BB dual agonist for enhanced solid tumor immune response abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5316.