Discovery of a Series of Potent, Selective, and Orally Bioavailable Nucleoside Inhibitors of CD73 That Demonstrates In Vivo Antitumor Activity

CD73 (ecto-5'-nucleotidase) has emerged as an attractive target for cancer immunotherapy of many cancers. CD73 catalyzes the hydrolysis of adenosine monophosphate (AMP) into highly immunosuppressive adenosine that plays a critical role in tumor progression. Herein, we report our efforts in developing orally bioavailable and highly potent small-molecule CD73 inhibitors from the reported hit molecule 2 to lead molecule 20 and then finally to compound 49. Compound 49 was able to reverse AMP-mediated suppression of CD8⁺ T cells and completely inhibited CD73 activity in serum samples from various cancer patients. In preclinical in vivo studies, orally administered 49 showed a robust dose-dependent pharmacokinetic/pharmacodynamic (PK/PD) relationship that correlated with efficacy. Compound 49 also demonstrated the expected immune-mediated antitumor mechanism of action and was efficacious upon oral administration not only as a single agent but also in combination with either chemotherapeutics or checkpoint inhibitor in the mouse tumor model.

Abstract 4134: Reversal of adenosine-mediated immune suppression by CB-708, an orally bioavailable and potent small molecule inhibitor of CD73

Introduction: High adenosine (ADO) in the tumor microenvironment suppresses the immune response against cancer cells by inhibiting immune effector functions and promoting the development of immunosuppressive cells. Extracellular ADO can be generated from ATP released by cells undergoing stress or death through the combined actions of the ecto-nucleotidases CD39 (ATP to AMP) and CD73 (AMP to ADO). Inhibition of ADO production via CD73 is a promising therapeutic approach for the treatment of cancer.

Methods: The potency of CB-708 was evaluated against recombinant CD73 and CD73-expressing cells using a malachite green assay. Selectivity against related ecto-nucleotidases was also assessed. Inhibition of CD73 in plasma was measured using LC/MS to assess conversion of 15N5-AMP into 15N5-ADO. The ability to reverse AMP-mediated immune suppression of human CD8+ T cells was determined by adding exogenous AMP during T cell activation. T cell proliferation was assayed by flow cytometry and cytokine levels were measured by ELISA. The EG7 syngeneic tumor model was used to assess the therapeutic effect of CB-708.

Results: CB-708 is an orally bioavailable small molecule inhibitor that can potently inhibit both soluble human CD73 (IC50 = 170 pM) and cell-bound human CD73 (IC50 = 210 pM), but does not inhibit human CD39 (IC50>10 µM), ENTPD2 (IC50>10 µM), nor ENTPD3 (IC50>10 µM). CB-708 retained high potency in the presence of plasma and reversed AMP-mediated suppression of human CD8+ T cell proliferation and production of IFNγ (EC50 = 4.5 nM) and granzyme B (EC50 = 5.6 nM) in vitro. Orally administered CB-708 had dose-dependent single-agent tumor growth inhibition in the EG7 mouse syngeneic tumor model and that was associated with pharmacodynamic inhibition of plasma CD73. Enhanced tumor growth inhibition was observed when anti-PD-L1 was combined with a highly related analog of CB-708 in the EG7 model.

Conclusion: CB-708 is an orally bioavailable and highly potent small molecule inhibitor of CD73. CB-708 reverses the immunosuppressive effects of AMP-derived ADO in vitro and in vivo and leads to anti-tumor activity as a monotherapy. CB-708 is expected to enter clinical development in 2019.

Citation Format: Clarissa C. Lee, Deepthi Bhupathi, Roland J. Billedeau, Jason Chen, Lijing Chen, Ethan D. Emberley, Matthew I. Gross, Tony Huang, Weiqun Li, Yong Ma, Andrew L. Mackinnon, Amani Makkouk, Gisele M. Marguier, Silinda Neou, Francesco Parlati, Natalija Sotirovska, Timothy F. Stanton, Susanne M. Steggerda, Jing Zhang, Winter Zhang, Jim Li. Reversal of adenosine-mediated immune suppression by CB-708, an orally bioavailable and potent small molecule inhibitor of CD73 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4134.