Abstract 734: Inhibition of equilibrative nucleoside transporter 1 relieves intracellular adenosine-mediated immune suppression

Abstract The profound benefit of immune checkpoint blockade for cancer therapy is restricted to limited subsets of patients with specific cancers. Many factors contribute to primary and acquired resistance to immune checkpoint blockade, including local accumulation of immunosuppressive metabolites such as the nucleoside adenosine and downstream adenosine receptor signaling. Pharmacological inhibition of adenosine generation and signaling are active areas of clinical investigation. Here we report a novel mechanism whereby adenosine suppresses anti-cancer immune responses by intracellular accumulation. We show that human T cells readily take up adenosine via equilibrative nucleoside transporter 1 (ENT1), suppressing proliferation and effector function by inhibition of de novo pyrimidine nucleotide synthesis in T cells. ENT1 expression increases upon T cell activation, suggesting uptake of extracellular adenosine is a mechanism to limit T cell responses in high adenosine environments. Quantitative mass spectrometry imaging reveals adenosine concentrations in human tumors up to the range of 100 µM - levels significantly greater than previously demonstrated and consistent with suppression of T cell responses. Deletion of ENT1 leads to potent control of tumor growth in syngeneic mouse models including KPC, a poorly immunogenic model of pancreatic ductal adenocarcinoma, and is associated with increased CD8+ T cell frequency, proliferation and cytokine production within tumors. Furthermore, ENT1 expression was observed by flow cytometry on human tumor-infiltrating CD8+ T cells. We have discovered and characterized EOS301984, a potent ENT1 antagonist that is currently being evaluated in patients with advanced solid tumors. EOS301984 blocks intracellular adenosine transport and restores pyrimidine levels in T cells activated in the presence of adenosine, resulting in enhanced tumor cell killing by memory T cells and increased ex vivo expansion of functional human tumor-infiltrating lymphocytes in adenosine-rich environments. Finally, in a humanized mouse model of triple negative breast cancer resistant to anti-PD-1 blockade (MDA-MB-231), combination of EOS301984 with nivolumab synergistically led to control of tumor growth. Thus, ENT1 inhibition represents a novel approach to augment anti-cancer immune responses through restoring pyrimidine nucleotide synthesis in T cells suppressed by adenosine. Citation Format: Theodore J. Sanders, Christopher S. Nabel, Margreet Brouwer, Annelise L. Hermant, Lucas Chaible, Jean-Philippe Deglasse, Angelique Pabois, Wilfried Cathou, Aurore Smets, Michael Deligny, Joao Marchante, Quentin Dubray, Marie-Claire Lettelier, Chiara Martinoli, Reece Marillier, Olivier De Henau, Yvonne McGrath, Matthew G. Vander Heiden, Erica Houthuys. Inhibition of equilibrative nucleoside transporter 1 relieves intracellular adenosine-mediated immune suppression abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 734.