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Introduction: In the tumor micro-environment, extracellular ATP is sequentially hydrolyzed to adenosine by the ecto-nucleotidases CD39 (ATP→AMP) and CD73 (AMP→adenosine). Adenosine, through activation of the A2a receptor (A2aR), is a potent inhibitor of T-cell activation, resulting in an immunosuppressed phenotype. Thus, inhibition of A2aR has recently generated great interest in immuno-oncology. We present the characterization of a novel, selective, and highly potent small molecule antagonist of A2aR which is slated to enter the clinic in 2017. Methods: The cellular potency of A2aR antagonists was assessed as a function of decreased cAMP levels in CHO cells stably over-expressing hA2aR, a Gs coupled receptor, following stimulation with the agonist NECA. Experiments were conducted in the presence and absence of human serum. Selectivity against the Gi-coupled receptor A1R was assessed similarly as a function of cAMP elevation in CHO cells stably expressing hA1R, following pretreatment with forskolin and stimulation with NECA. The ability of AB928 to reverse adenosine-mediated immune suppression (25 μM) of human or mouse CD8+ T-cells was determined using standard CD3/CD28 activation conditions. CD25 expression and cytokine release were measured by flow cytometry and ELISA, respectively. The pharmacokinetic characteristics of AB928 were assessed in rodent and non-rodent species to facilitate calculation of a projected human dose. Results: AB928 represents a novel series of potent and selective compounds designed to inhibit adenosine-mediated A2aR activation. This molecule is different from most known A2aR antagonists in that it does not cross the blood brain barrier. AB928 inhibited NECA-mediated A2aR activation with a potency of Conclusions: AB928 is a potent, selective and peripherally restricted antagonist of the A2aR receptor which is slated to enter clinical development in 2017. Citation Format: Matt J. Walters, Joanne B. Tan, Annette Becker, Fangfang Yi, Tim Park, Manmohan R. Leleti, Brandon Rosen, Ehesan Sharif, Laurent Debien, Steve Young, Wan Hsin Lim, Stefan Garrido-Shaqfeh, Juan C. Jaen, Jay P. Powers. Characterization of the potent and selective A2aR antagonist AB928 for the treatment of cancer [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 4572. doi:10.1158/1538-7445.AM2017-4572