Abstract 4719: Small-molecule antagonists of the Aryl Hydrocarbon Receptor (AhR) promote activation of human PBMCs in vitro and demonstrate significant impact on tumor growth and immune modulation in vivo

Cancer cells utilize a variety of pathways to generate an immune permissive growth environment and evade the host anti-tumor immune response. One such pathway is production of T-cell inhibitory tryptophan (Trp) metabolites, kynurenine and kynurenic acid (KYN), via up-regulation of indoleamine 2,3-dioxygenase (IDO1) and tryptophan 2,3-dioxygenase (TDO2) and up-regulation of the KYN- receptor (aryl hydrocarbon receptor, AhR) in infiltrating lymphoid and myeloid cells. Through AhR activation, KYN both directly and indirectly, via stimulation of regulatory T-cells (Tregs), dendritic cells (DCs) and tumor-associated macrophages (TAM), modulates CD8+ and CD4+ T-cell function resulting in potent inhibition of anti-tumor immunity. Inhibition of AhR activation therefore represents a novel approach to blocking the immuno-inhibitory effects of Trp metabolites generated by endogenous IDO1 and TDO2. To thwart the effects of KYN and other immunosuppressive AhR agonist ligands, we have generated a series of small-molecule antagonists of the human and mouse AhR. To further analyze the effects of AhR inhibition on immuno-modulation and tumor growth, we selected from our series a highly potent antagonist of AhR (IDB-AHRi). In an XRE-dependent luciferase reporter system, IDB-AHRi blocked activation of AhR by 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) with an IC 50 of 111 and 3 nM in human HEPG2 and mouse HEPA1C1C7 cell lines respectively. IDB-AHRi inhibited translocation of cytoplasmic AhR to the nucleus and protected AhR from TCDD-induced degradation. When human peripheral blood mononuclear cells were treated with IDB-AHRi ex-vivo, expression of IFN-γ and TNF-α was significantly enhanced, an effect that could be reversed with the addition of TCDD. In vivo, IDB-AHRi dosed at 30 mg/kg (PO QD) in the CT26 colon carcinoma syngeneic mouse model resulted in a reduction of tumor growth, an increase in tumor-infiltrated CD45+CD8+Ki67 high T-cells and a significant reduction in the frequency of tumor infiltrated FoxP3+CD4+ regulatory T-cells and tumor associated M2-like macrophages. In summary, this data demonstrates that blocking AhR with a highly potent antagonist can significantly enhance T-cell activity in vitro and drive anti-tumor immunity in vivo, resulting in reduced tumor growth. Citation Format: James Joseph, Marcos Gonzalez-Lopez, Christina Galang, Candy Garcia, Hadia Lemar, Jing Lu, Kedar Vaidya, Marcus Fischer, Christian Frey, Muzaffar Alam, Bing Yao, Michael Dillon, Jeffrey H. Hager, Eleni Venetsanakos, Fred Aswad. Small-molecule antagonists of the Aryl Hydrocarbon Receptor (AhR) promote activation of human PBMCs in vitro and demonstrate significant impact on tumor growth and immune modulation in vivo [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 4719.