Introduction. Extracellular adenosine (ADO) suppresses immune function via inhibition of T cell and NK cell activation and is present in high concentrations in the tumor micro-environment (TME). Intra-tumoral generation of ADO depends on the sequential catabolism of ATP by two ecto-nucleotidases, CD39 (ATP→AMP) and CD73 (AMP→ADO). Inhibition of CD73 eliminates a major, non-redundant, pathway of ADO production in the TME and can reverse ADO-mediated immune suppression. Here we present the characterization of AB680, a novel, highly potent, reversible and selective small molecule inhibitor of CD73, currently in preclinical development as a potential anti-tumor agent.

Methods. The potency of CD73 inhibitors was evaluated by measuring AMP hydrolysis by CHO-CD73 cells using a malachite green assay. Potency was also measured using human T-cells and soluble recombinant CD73. Selectivity against related ecto-nucleotidases was also assessed. In the presence of human serum, CD73 inhibition was measured by quantitation of AMP hydrolysis via luminescence. The ability of AB680 to reverse AMP-mediated immune suppression of human CD4+/CD8+ T cells was determined by adding exogenous AMP during T cell activation by anti-CD3/CD28/CD2 beads. The pharmacokinetic properties of AB680 were evaluated in multiple preclinical species. A projected human dosing schedule for AB680 was determined via allometric scaling.

Results. AB680 is a highly potent, reversible and selective inhibitor of CD73 (IC50 < 0.01 nM on human CD8+ T-cells), which retains high potency in the presence of human serum. AB680 is > 10,000-fold selective against related ecto-nucleotidases and a large panel of unrelated enzymes, receptors, and ion channels. AB680 does not show significant inhibition of the major CYP450 isoforms or the hERG potassium channel. AB680 potently reverses AMP and ADO-mediated suppression of immune function in vitro. In the presence of high concentrations of AMP, AB680 robustly restored CD25 expression, IFN-γ production and proliferation of human CD4+ and CD8+ T-cells. The pharmacokinetics (PK) of AB680 were assessed in rodent and non-rodent species. The PK properties of AB680 are characterized by very low clearance and long half-lives in preclinical species, resulting in projected human PK properties suitable for intravenous (i.v.) dosing on a schedule consistent with typical mAb dosing cycles. High-dose infusions of AB680 in preclinical species were well tolerated.

Conclusions. AB680 is a highly potent and selective small-molecule inhibitor of CD73 which can mitigate AMP and ADO-mediated tumor immunity by potently blocking the production of adenosine in the TME. AB680 exhibits a favorable projected human PK profile suitable for parental administration and is expected to enter clinical development in 2018.

Citation Format: Kenneth V. Lawson, Lixia Jin, Jenna L. Jeffrey, Jarek Kalisiak, Fangfang Yin, Kristen Zhang, Ada Chen, Debbie Swinarski, Matt J. Walters, Steve Young, Ulrike Schindler, Powers P. Jay. Discovery and characterization of AB680, a potent and selective small-molecule CD73 inhibitor for cancer immunotherapy [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 1756.