Introduction: Intra-tumoral generation of adenosine (ADO), a potent inhibitor of T-cell activation, requires the coordinated and sequential cleavage of extracellular adenosine triphosphate (ATP) by the ecto-nucleotidases CD39 (which produces adenosine monophosphate, AMP) and CD73 (which hydrolyzes AMP to ADO). For this reason, various anti-CD73 antibodies are being advanced into clinical trials; however, there are few reports of potent, selective, small-molecule CD73 inhibitors, such as those described here.

Methods: Enzymatic assays: Ecto-nucleotidase activity was calculated based on the amount of inorganic phosphate (malachite green assay; absorbance at 620 nm) produced after 50-min incubation with 25μM AMP or ATP, in the presence of varying concentrations of test compound(s). The following systems were used. Endogenous expression: hCD73/SKOV-3 cells; mCD73/E771 cells. Stable over-expression: hCD73/CHO; hCD39/CHO. Transient expression: mCD73/CHO; NTPDase2/CHO; NTPDase3/CHO; NTPDase8/CHO. CD8+ T cell functional assays: Human (enriched from buffy coats or leukopaks) and mouse (isolated from spleen of C57BL/6 mice) CD8+ T cells were pretreated with CD73 inhibitor or buffer control. One hour later, cells were stimulated (anti-CD3/CD28; +/- 50 μM AMP) in the presence of ADO deaminase inhibitor (10 μM). 3 days after stimulation, cell activation (CD25), proliferation (CFSE) and effector function (IFN-γ and granzyme B) were quantified by flow cytometry. Tumor model: CT26 cells were subcutaneously implanted into Balb/c mice. When tumor volumes were ∼100 mm3, mice were enrolled into cohorts (1: vehicle control; 2: A000830; 3: PD-1 Ab; 4: A000830 + PD-1 Ab) of ≥13 mice each. Interim immune phenotyping was performed when Group 1 tumor volumes were 800-1,000 mm3. Single-cell suspensions were generated from tumors and spleens (gentleMACS). 106 cells were blocked using purified CD16/32 antibodies (clone 2.4G2) and stained with the appropriate antibody cocktails.

Results: We have designed a series of potent and specific small-molecule inhibitors of human and mouse CD73, represented by A000830 (IC50 against human and mouse CD73 of 1.0 nM and 3 nM, respectively). A000830 blocked ADO generation from AMP by human ovarian cancer cells (SKOV-3) with IC50: 0.2 nM and >10,000-fold selectivity relative to other ecto-nucleotidases (CD39, NTPDase2, NTPDase3, NTPDase8) and a large panel of unrelated enzymes, receptors and ion channels. In in vitro models of ADO-driven inhibition of human and mouse CD8+ T-cell activation, A000830 showed robust rescue of proliferation, CD25 expression, and IFNγ and granzyme B production. A000830 was easily administered (and well tolerated) to mice, resulting in sustained plasma concentrations at least 1,000 times higher than its potency against mouse CD73. In the spleen of CT26 tumor-bearing Balb/c mice, CD39 expression was found mainly on natural killer (NK) cells and myeloid-lineage cells, while in the tumor CD39 was highly expressed on myeloid cells (myeloid-derived suppressor cells (MDSC) and dendritic cells (DC)) as well as CD4+ and CD8+ T cells. CD73 expression was limited (spleen and tumor) to NK cells and CD4+ and CD8+ T cells. Therapeutic dosing of A000830 to these mice (tumors ∼100 mm3 in size) in combination with anti-PD1 antibody produced robust tumor growth inhibition, greater than either treatment alone. This effect was associated with increased CD8:Treg ratios in tumors (but not spleens).

Conclusions: We describe a novel class of potent and selective small-molecule CD73 inhibitors that effectively block the generation of ADO from extracellular ATP, reverse the ADO-driven inhibition of human and mouse T-cell activation, and display promising anti-tumor activity when dosed in combination with PD-1 blockade.

Citation Format: Juan C. Jaen, Jay P. Powers, Ada Chen, Manmohan R. Leleti, Jarek Kalisiak, Ulrike Schindler, Joanne Bl Tan, Steve Young, Shin-Heng Chiou, Laurent Debien. Small-molecule inhibitors of CD73 promote activation of human CD8+ T cells and have profound effects on tumor growth and immune parameters in experimental tumors [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr PR10.