FS118, a bispecific antibody targeting LAG-3 and PD-L1, enhances T cell activation resulting in potent anti-tumor activity.

Purpose: Although programmed death-ligand 1 (PD-L1) antibody-based therapy has improved the outcome of patients with cancer, acquired resistance to these treatments limits their clinical efficacy. FS118 is a novel bispecific, tetravalent antibody (mAb(2)) against human lymphocyte activation gene-3 (LAG-3) and PD-L1 with the potential to reinvigorate exhausted immune cells and overcome resistance mechanisms to PD-L1 blockade. Here, using FS118 and a murine surrogate, we characterized the activity and report a novel mechanism of action of this bispecific antibody. Experimental Design: This study characterizes the binding activity and immune function of FS118 in cell lines and human peripheral blood mononuclear cells and further investigates its antitumor activity and mechanism of action using a surrogate murine bispecific antibody (mLAG-3/PD-L1 mAb(2)). Results: FS118 demonstrated simultaneous binding to LAG-3 and PD-L1 with high affinity and comparable or better activity than the combination of the single component parts of the mAb(2) in blocking LAG-3- and PD-L1-mediated immune suppression and enhancing T-cell activity. In syngeneic tumor mouse models, mLAG-3/PD-L1 mAb(2) significantly suppressed tumor growth. Mechanistic studies revealed decreased LAG-3 expression on T cells following treatment with the mouse surrogate mLAG-3/PD-L1 mAb(2), whereas LAG-3 expression increased upon treatment with the combination of mAbs targeting LAG-3 and PD-L1. Moreover, following binding of mLAG-3/PD-L1 mAb(2) to target-expressing cells, mouse LAG-3 is rapidly shed into the blood. Conclusions: This study demonstrates a novel benefit of the bispecific approach over a combination of mAbs and supports the further development of FS118 for the treatment of patients with cancer.