Abstract 2874: The tetravalent structure of FS118, a bispecific antibody targeting LAG-3 and PD-L1, is required for its novel mechanism of LAG-3 shedding
Authors: Reader Claire S. , Liao Wenjia , Potter-Landau Beatrice J. , Veyssier Christel Séguy , Rhoades Martyn C. , Seal Claire J. , Morrow Michelle , Brewis Neil 
Published: June 2022

Abstract
FS118 is a tetravalent bispecific antibody targeting LAG-3 and PD-L1 that can overcome immune suppressive signals with greater preclinical activity than a combination of monoclonal antibodies. In vivo, FS118 downregulates LAG-3 on tumor-infiltrating lymphocytes (TILs) and increases soluble LAG-3 (sLAG-3) in the serum of mice. In a Phase 1 trial, FS118 demonstrated a dose-dependent increase in sLAG-3 in the serum of patients with advanced malignancies. Here, we demonstrate that the tetravalent structure of FS118 is required to mediate this novel LAG-3 shedding mechanism.
Human ex vivo assays were performed by co-culturing expanded CD4+ T cells with iDCs in the presence of Staphylococcal enterotoxin B and FS118, or controls. sLAG-3 was measured by ECLIA. The binding valency of FS118 was determined using chemically-crosslinked mass spectrometry mapping (XL/MS). Variants of FS118 and FS118 surrogate molecules were generated with differing valency for LAG-3 and PD-L1. MC38 tumor-bearing C57BL/6 mice were dosed once interperitoneally with FS118 surrogate or valency variants. TILs were analyzed by flow cytometry, and sLAG-3 was measured in the serum by ECLIA.
In patients receiving FS118 treatment, a dose-dependent increase in sLAG-3 was detected in the blood. In an ex vivo human T cell assay, the FS118-mediated increase in sLAG-3 was greater than with the combination of the individual bispecific components. sLAG-3 increase by FS118 was mediated by ADAM10 and ADAM17. FS118 demonstrated tetravalent binding to its target antigens using XL/MS. All four binding sites simultaneously bound to target antigens, with preferential binding to PD-L1 in the presence of equal amounts of both LAG-3 and PD-L1. Variants of FS118 with reduced binding valency for target antigens mediated lower levels of LAG-3 release than tetravalent FS118. Compared to FS118, monovalent PD-L1 binding reduced sLAG-3 release and importantly, a variant of FS118 with monovalent LAG-3 binding generated the lowest level of sLAG-3. Data will be presented showing the role of binding valency on LAG-3 downregulation by TILs and T cell activation in ex vivo murine samples.
FS118 is tetravalent and can bind to two LAG-3 and two PD-L1 molecules simultaneously. Bivalent LAG-3 and PD-L1 binding is required for enhanced shedding of cell surface LAG-3 in vitro. Removing LAG-3 from the surface of exhausted TILs is a novel mechanism attributed to the tetravalent structure of FS118, which may be important to overcome compensatory upregulation of LAG-3 induced by blockade of PD-L1 in patients.
Citation Format: Claire S. Reader, Wenjia Liao, Beatrice J. Potter-Landau, Christel Séguy Veyssier, Martyn C. Rhoades, Claire J. Seal, Michelle Morrow, Neil Brewis. The tetravalent structure of FS118, a bispecific antibody targeting LAG-3 and PD-L1, is required for its novel mechanism of LAG-3 shedding [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2874.