Abstract 4598: Triggering of OX40 on T cells by a novel monoclonal antibody elicits robust antitumor immunity<!–EndFragment–>

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Abstract T cell costimulation is an attractive strategy for cancer treatment in addition to check point inhibitors. Costimulatory molecule OX40 is a member of the TNF receptor superfamily that is transiently expressed on activated T cells. Activation of OX40 leads to enhanced T cell proliferation and cytokine secretion and in turn results in better anti-tumor efficacy as shown by multiple mouse syngeneic tumor models. PF-04518600 (PF-8600) is a fully human IgG2 agonist antibody to human OX40 with high affinity and specificity. We used in vitro and in vivo methods to evaluate the co-stimulatory functions of PF-8600. PF-8600 demonstrated better agonist activity in a luciferase reporter cell line expressing OX40 and NFκB as compared to a human IgG1 antibody. In human primary T cells, PF-8600 dose dependently increased T cell proliferation and cytokine secretion in vitro. Furthermore in a mouse line expressing human OX40, PF-8600 in a mouse IgG1 framework increased the proliferation of OT1 cells and inhibited EG7 tumor growth as compared to isotype control antibody, further confirming the mechanism of action as an OX40 agonist. As regulatory T cells are a major inhibitory population in the tumor microenvironment, their specific depletion in the tumor is highly desirable. In an in vitro assay using monocyte derived macrophages as effector cells, PF-8600 mediated the depletion of OX40 expressing cells similar to a human IgG1 antibody, and this activity is dependent on the binding to hFcγRIIA. In human FcγR knock-in mice, anti-OX40-hIgG1 showed increased Treg reduction in the tumor comparing to anti-OX40-hIgG2, but less T cell proliferation and activation in the periphery. In combination with an anti-PD-L1 antibody, both anti-OX40-hIgG1 and hIgG2 showed similar enhanced anti-tumor activity in the MC38 colon carcinoma model as comparing to anti-PD-L1 alone. Both in vitro and in vivo results demonstrated PF-8600 enhanced T cells functions and inhibited tumor growth in a mouse syngeneic tumor model. The activity of the hIgG2 antibody is similar to that of an hIgG1 in both phagocytosis assays and hFcγR KI in vivo assays. Based on the mechanism of action and robust anti-tumor efficacy in preclinical models, PF-8600 is currently in clinical development in a broad spectrum of malignancies. Citation Format: Hua Long, Ann White, Jie Wei, Brittany Jiang, Reid Feldman, Danielle Pappas, Aymen Al-Shamkhani, John Lin. Triggering of OX40 on T cells by a novel monoclonal antibody elicits robust antitumor immunity<!–EndFragment–> [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 4598. doi:10.1158/1538-7445.AM2017-4598