Characterization of MK-4166, a Clinical Agonistic Antibody That Targets Human GITR and Inhibits the Generation and Suppressive Effects of T Regulatory Cells

GITR is a T-cell costimulatory receptor that enhances cellular and humoral immunity. The agonist anti-mouse GITR antibody DTA-1 has demonstrated efficacy in murine models of Cancer primarily by attenuation of T_reg-mediated immune suppression, but the translatability to human GITR biology has not been fully explored. Here, we report the potential utility of MK-4166, a humanized GITR mAb selected to bind to an epitope analogous to the DTA-1 epitope, which enhances the proliferation of both naïve and tumor-infiltrating T lymphocytes (TIL). We also investigated the role of GITR agonism in human antitumor immune responses and report here the preclinical characterization and toxicity assessment of MK-4166, which is currently being evaluated in a phase I clinical study. Expression of human GITR was comparable with that of mouse GITR in tumor-infiltrating T_regs despite being drastically lower in Other human TILs and in many human peripheral blood populations. MK-4166 decreased induction and suppressive effects of T_regsin vitro In human TIL cultures, MK-4166 induced phosphorylation of NFκB and increased expression of dual specificity Phosphatase 6 (DUSP6), indicating that MK-4166 activated downstream NFκB and ERK signaling pathways. Furthermore, MK-4166 downregulated FOXP3 mRNA in human tumor infiltrating T_regs, suggesting that, in addition to enhancing the activation of TILs, MK-4166 may attenuate the T_reg-mediated suppressive tumor microenvironment. Cancer Res; 77(16); 4378-88. ©2017 AACR.