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Abstract

Introduction

Agonists of CD137, a co-stimulatory receptor expressed on activated 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 assessed , while antitumor efficacy and T-cell proliferation were evaluated .

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-infiltrating 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 clinical 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.

© 2025 The Author(s). Published by Bentham Science Publishers.
This is an open access article published under CC BY 4.0 https://creativecommons.org/licenses/by/4.0/legalcode
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2025-10-09
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FS222, a CD137/PD-L1 Tetravalent Bispecific Antibody, Designed for Optimal CD137 Engagement Resulting in Potent Conditional T-Cell Activity
Bentham Science Publishers ; https://doi.org/10.2174/0115680096362922250918112113
/content/journals/ccdt/10.2174/0115680096362922250918112113
/content/journals/ccdt/10.2174/0115680096362922250918112113
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  • Article Type:     Research Article
Keywords: T cell ; FS222 ; immunotherapy ; tetravalent ; Fcab ; CD137 ; Bispecific antibody ; PD-L1

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