Abstract 1864: FS222, a tetravalent bispecific antibody targeting CD137 and PD-L1, is designed for optimal CD137 interactions resulting in potent T cell activation without toxicity
Authors: Lakins Matthew A. , Munoz-Olaya Jose , Veyssier Christel , Jones Daniel , Goodman Emma , Kaka Quincy , Ofoedu Jennifer , Hughes Robert , Gliddon Daniel , Morrow Michelle , Brewis Neil 
Published: July 2021

Abstract
Background: CD137 (4-1BB, TNFRSF9) is expressed on activated lymphocytes, and its clustering leads to agonism of the receptor resulting in lymphocyte proliferation and pro-inflammatory cytokine release. First-generation CD137 antibodies for cancer therapy were high affinity and enabled for FcγR engagement with either severe toxicity or weak activity limiting their therapeutic benefit. However, CD137 remains a promising target for bispecific antibodies designed to re-direct T cell activity to the tumor whilst limiting unwanted toxicities. We have rationally designed and developed a unique tetravalent bispecific antibody targeting CD137 and PD-L1 with reduced FcγR binding for safe and efficacious cancer therapy (Lakins MA et al. 2020).
Methods: A CD137/PD-L1 bispecific mAb2 antibody (FS222) was generated by introducing a bivalent affinity-optimized CD137-binding Fcab into a human IgG1 bivalent PD-L1 mAb. LALA mutations were introduced to abrogate FcγR activity. To elucidate its mechanism of action, FS222's binding valency was assessed by chemically-crosslinked mass spectrometry mapping (XL/MS). Immune pharmacology models were also used to evaluate PD-L1 dependent FS222 agonism against variants with differing valency for each target.
Results: FS222 was specifically designed to bind CD137 with moderate affinity, and without PD-L1-mediated crosslinking it does not cluster or activate the receptor on immune cells. When PD-L1 was bound by FS222, this lead to crosslinking and subsequent clustering and activation of CD137. This clustering was enabled by avid bivalent binding as demonstrated by tetravalent FS222 being biologically more active than variants with reduced valency. Moreover, XL/MS demonstrated all four binding sites were able to concurrently bind target antigens. FS222 was well tolerated in a GLP toxicity study up to the maximal dose (30mg/kg QW). It was pharmacologically active in peripheral blood and lymphoid tissues and had a predicted t½ of 286 hours in humans. A surrogate anti-mouse CD137/PD-L1 mAb2 significantly reduced tumor growth in a murine syngeneic model and this correlated with a significant survival benefit and T cell activation.
Conclusions: FS222 was designed to be a potent anti-human CD137/PD-L1 tetravalent conditional agonist with a unique combination of high affinity PD-L1 binding and moderate monovalent affinity, and highly avid binding, to CD137 on activated T cells. A favorable safety profile and immunopharmacology was observed with FS222 in a primate GLP toxicity study. Tetravalent binding by FS222 was required for optimal activity in pharmacology studies. Preclinically, FS222 was well-tolerated with an effective mechanism of action.
Citation Format: Matthew A. Lakins, Jose Munoz-Olaya, Christel Veyssier, Daniel Jones, Emma Goodman, Quincy Kaka, Jennifer Ofoedu, Robert Hughes, Daniel Gliddon, Michelle Morrow, Neil Brewis. FS222, a tetravalent bispecific antibody targeting CD137 and PD-L1, is designed for optimal CD137 interactions resulting in potent T cell activation without toxicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1864.