Abstract 3820: Novel PD-1/GITRL bispecific fusion protein delivering concurrent T cell co-stimulation and checkpoint pathway inhibition

Glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) is a co-stimulatory receptor, and programmed cell death protein 1 (PD-1) is an important co-inhibitory receptor involved in controlling T-cell activation and function. Preclinical studies using murine cross-reactive molecules demonstrate that the combination of an αPD-1 monoclonal antibody (mAb) with a GITR-ligand (GITRL) fusion protein can elicit synergistic antitumor activity and long-term survival benefit in syngeneic mouse tumor models. Here we describe the activity of two bispecific fusion proteins, each comprising two GITR ligand trimers fused to the Fc region of an αPD1 mAb, in an IgG1 (MEDI5771) or IgG4P (MEDI3387) format. We show that these bispecific molecules are able to concurrently engage both targets and are effective in cell-based PD-1 and GITR reporter assays, and in a range of human primary T-cell assays. Furthermore, the PD-1/GITRL FP bispecific constructs elicit T-cell responses in vitro that are at least equivalent to a combination of a) the parental molecules and b) MEDI1873 (a GITR agonist currently undergoing clinical evaluation) and an αPD-1 mAb. Moreover, using surrogate molecules we demonstrate that IV administration leads to dose-dependent pharmacodynamic responses on CD4 + and CD8 + T cells for both isotypes of the bispecific molecule. These data demonstrate that the antitumor activity for these novel molecules is equivalent to the combination of GITRL-FP and PD-1 mAb in the B16 model. In summary, the simultaneous dual targeting of PD-1 and GITR pathways mediated by these two novel bispecific molecules induces robust T-cell activity that is at least equivalent to a combination of molecules targeting these pathways, thereby demonstrating strong potential as a novel anticancer therapy. Citation Format: Simon J. Dovedi, Jonathan Seaman, Ines Osma-Garcia, Katharina Deschler, Edmund Poon, Ronald Herbst, Robert Wilkinson. Novel PD-1/GITRL bispecific fusion protein delivering concurrent T cell co-stimulation and checkpoint pathway inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3820.