Abstract 3643: INCAGN1876, a unique GITR agonist antibody that facilitates GITR oligomerization

Glucocorticoid-induced TNFR family related protein (GITR, CD357 or TNFRSF18) is a member of the tumor necrosis factor receptor superfamily (TNFRSF). Like other T cell co-stimulatory TNFR family members, GITR utilizes multiple oligomerization states to regulate the initiation of downstream signaling during T cell activation by antigen presenting cells (APCs). The formation of receptor superclusters, comprised of two or more trimeric molecules, has been defined for multiple TNFRs as a means of regulating downstream signal amplification. For co-stimulatory TNFRs, like GITR, CD137 and OX40, signaling outcomes in T cells are primarily mediated via the NFκB pathway that promotes cell survival and effector cell activities in response to suboptimal T cell receptor (TCR) stimulation. It has been hypothesized that the manipulation of the oligomeric states of co-stimulatory TNFRs using antibodies may have therapeutic utility in enhancing the activity of tumor-reactive T cells, either as single agents or in combination with other immunomodulatory or immune education strategies. Here we describe a structure-based analysis of two functionally distinct classes of anti-human GITR antibodies that stabilize unique conformational states of the receptor. INCAGN1876, a human IgG1 monoclonal anti-GITR antibody, was found to engage a conformational epitope located within a β-turn of the extracellular domain of GITR. This antibody binding site modified the equilibrium of GITR monomer, dimer and trimers to promote receptor oligomerization, resulting in downstream NFκB signaling. Notably, this mode of INCAGN1876 receptor engagement enabled it to effectively activate the GITR pathway in recently primed T cells. By contrast, a second reference anti-GITR antibody required concomitant TCR co-engagement in order to modulate the GITR pathway. High content confocal analysis was used to evaluate the kinetics of GITR clustering by both classes of anti-GITR antibody, confirming our T cell functional analysis. The ability of INCAGN1876 to engage and effectively activate GITR on recently primed T cells may enable them to overcome suppressive features of the tumor microenvironment. Notably, INCAGN1876 was shown to promote T cell co-stimulation both as a single agent and in combination with other antibodies targeting PD-1, CTLA-4 and OX40. Finally, we compared the pharmacologic activity of INCAGN1876 to Fc variants of this antibody with diminished binding to the inhibitory Fcγ receptor (FcγR), CD32B. The superiority of an IgG1 antibody in these assays was consistent with the potential to achieve optimal GITR clustering by FcγRs, while maintaining the potential for FcγR-mediated effector cell activity directed toward intratumoral GITRhigh regulatory T cells. INCAGN1876 is currently under evaluation in Phase 1/2 studies in subjects with advanced metastatic solid tumors (NCT02697591). Citation Format: Ana M. Gonzalez, Mariana L. Manrique, Lukasz Swiech, Thomas Horn, Jeremy Waight, Yuqi Liu, Shiwen Lin, Dennis Underwood, Ekaterina Breous, Olivier Leger, Volker Seibert, Taha Merghoub, Roberta Zappasodi, Gerd Ritter, David Schaer, Kevin N. Heller, Kimberli Brill, Peggy Scherle, Gregory Hollis, Reid Huber, Marc van Dijk, Jennifer Buell, Robert Stein, Nicholas S. Wilson. INCAGN1876, a unique GITR agonist antibody that facilitates GITR oligomerization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3643. doi:10.1158/1538-7445.AM2017-3643