FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits low toxicity and anti-tumor activity in colorectal cancer models.

PURPOSE: With the increased prevalence in checkpoint therapy resistance, there remains a significant unmet need for additional therapies for relapsing or refractory cancer patients. We have developed FS222, a bispecific tetravalent antibody targeting CD137 and PD-L1 to induce T cell activation to eradicate tumours without the current toxicity and efficacy limitations seen clinically. EXPERIMENTAL DESIGN: A bispecific antibody (FS222) was developed by engineering CD137 binding sites into the Fc region of a PD-L1 IgG1 mAb. T cell activation by FS222 was investigated using multiple in vitro assays. The anti-tumour efficacy, survival benefit, pharmacodynamics and liver pharmacology of a murine surrogate molecule were assessed in syngeneic mouse tumour models. Toxicology and the pharmacokinetic/pharmacodynamic profile of FS222 was investigated in a non-human primate dose-range finding study. RESULTS: We demonstrated simultaneous binding of CD137 and PD-L1 and showed potent T cell activation across CD8(+) T cell activation assays in a PD-L1-dependent manner with FS222. FS222 also activated T cells in a human primary mixed lymphocyte reaction assay, with greater potency than the monospecific mAb combination. FS222 showed no signs of liver toxicity up to 30 mg/kg in a non-human primate dose-range finding study. A surrogate molecule caused significant tumour growth inhibition and survival benefit, concomitant with CD8(+) T cell activation, in CT26 and MC38 syngeneic mouse tumour models. CONCLUSIONS: By targeting CD137 agonism to areas of PD-L1 expression, predominantly found in the tumour microenvironment, FS222 has the potential to leverage a focused, potent and safe immune response augmenting the PD-(L)1 axis blockade.