FS222, a CD137/PD-L1 Tetravalent Bispecific Antibody, Designed for Optimal CD137 Engagement Resulting in Potent Conditional T-Cell Activity

Introduction: Agonists of CD137, a co-stimulatory receptor expressed on acti-vated lymphocytes, are under investigation, including their use in combination with immune checkpoint blockade. FS222 is a tetravalent bispecific antibody targeting both CD137 and programmed death-ligand 1 (PD-L1). Methods: Fragment crystallizable regions with antigen-binding (Fcab™) domains with high avidity to CD137 were engineered and matured. A resulting Fcab was introduced into a human high-affinity PD-L1 monoclonal antibody, generating FS222. Valency variants of FS222 and its mouse surrogate (FS222m) were generated. Antibody properties were as-sessed in vitro, while antitumor efficacy and T-cell proliferation were evaluated in vivo. Results: FS222 could bind to two copies of each antigen concurrently (‘2+2’ binding). In T-cell activation assays, FS222 and a variant that is bivalent for CD137 and monovalent for PD-L1 activated T cells, whereas monovalent variants for CD137 did not. FS222 had no hook effect up to 300 nM and was superior to a heterodimeric ‘1+1’ CD137/PD-L1 bispecific antibody. In mixed lymphocyte reaction assays, FS222 and another bivalently linked antibody for PD-L1 were more potent than monovalent antibodies for PD-L1. In syngeneic mouse models, FS222m was associated with more peripheral T-cell proliferation and antitumor efficacy than all MC38 or most CT26.WT valency variants. Discussion: FS222 could potently target and activate high CD137-expressing tumor-infil-trating lymphocytes due to its high avidity for CD137. FS222 demonstrates activity when PD-L1 levels are limited, highlighting its potential activity in tumors insensitive to PD-1 blockade, and in situations requiring both PD-L1 and CD137 binding. This may have clini-cal relevance. Conclusion: In this study, it was found that the ‘2+2’ structure of FS222 is superior to that of a ‘1+1’ CD137/PD-L1 antibody.