Abstract 4547: Clustering CD137 via cell-expressed PD-L1 crosslinking avoided Fc-mediated agonism and resulted in safe and potent conditional lymphocyte activation

Background Activating T cells via clustering tumour necrosis factor receptor superfamily (TNFRSF) has shown promise in cancer therapy. This has typically been achieved by crosslinking of targeting antibodies via Fc-gamma receptor (FcγR). TNFRSF member CD137 is expressed by activated lymphocytes, and its clustering results in lymphocyte proliferation and agonism. First generation CD137 antibodies induced liver immune pathology, causing clinical toxicity and death. It remains a promising target for bispecific antibodies designed to limit unwanted toxicities. Methods An anti-human CD137/PD-L1 mAb2 (FS222) was generated by introducing a CD137-binding specificity into a human IgG1 targeting PD-L1 mAb with reduced FcγR binding. Cell binding and in vitro activity was used to evaluate FS222 agonism conditional on PD-L1 crosslinking. An anti-mouse CD137/PD-L1 mAb2 was generated to assess anti-tumour activity, liver pharmacology and PK/PD in murine models. To predict the clinical toxicity and PK/PD profiles of FS222, studies were performed in non-human primates. Results FS222 is designed to bind human PD-L1 with sub-nanomolar affinity and crosslink to enable CD137 clustering and conditional agonism (low nM EC50). The toxicity, PK and PD of FS222 was tested in cynomolgus monkey. FS222 was well tolerated in a GLP toxicity study up to the maximal dose (30mg/kg QW) and showed no evidence of liver toxicity. The molecule has a t½ of 211h in monkeys, with treatment resulting in pharmacology in peripheral blood which included lymphocyte proliferation and increases in soluble receptors. The anti-mouse CD137/PD-L1 mAb2 significantly reduced tumor growth in mouse tumor models. This correlated with a significant survival benefit and increases in tumor and peripheral activated lymphocytes. An anti-mouse CD137 antibody (clone 3H3) showed crosslink-independent agonism and sustained increases in activated T cells in liver whereas the liver effects of the anti-mouse CD137/PD-L1 mAb2 were of similar magnitude but peaked earlier and resolved. Conclusions FS222 was observed to be a potent anti-human CD137/PD-L1 tetravalent conditional agonist. A favourable safety profile and immunopharmacology was observed with FS222 in a primate GLP toxicity study. FS222 did not show toxicity and the anti-mouse CD137/PD-L1 mAb2 had self-limiting immunopharmacology outside the tumor microenvironment, suggesting a well-tolerated and effective mechanism of action with a broad therapeutic window. Citation Format: Matthew A. Lakins, Alexander Koers, Raffaella Giambalvo, Robert Hughes, Sylwia Marshall, Mateusz Wydro, Cristian Gradinaru, Sarah Batey, Daniel Gliddon, Michelle Morrow, Neil Brewis. Clustering CD137 via cell-expressed PD-L1 crosslinking avoided Fc-mediated agonism and resulted in safe and potent conditional lymphocyte activation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4547.