A Robust Multiplex Mass Spectrometric Assay for Screening Small-Molecule Inhibitors of CD73 with Diverse Inhibition Modalities

CD73/Ecto-5'-nucleotidase is a membrane-tethered ecto-enzyme that works in tandem with CD39 to convert extracellular adenosine triphosphate (ATP) into adenosine. CD73 is highly expressed on various types of cancer cells and on infiltrating suppressive immune cells, leading to an elevated concentration of adenosine in the tumor microenvironment, which elicits a strong immunosuppressive effect. In preclinical studies, targeting CD73 with anti-CD73 antibody results in favorable antitumor effects. Despite initial studies using antibodies, inhibition of CD73 catalytic activity using small-molecule inhibitors may be more effective in lowering extracellular adenosine due to better tumor penetration and distribution. To screen small-molecule libraries, we explored multiple approaches, including colorimetric and fluorescent biochemical assays, and due to some limitations with these assays, we developed a mass spectrometry (MS)-based assay. Only the MS-based assay offers the sensitivity and dynamic range required for screening small-molecule libraries at a substrate concentration close to the K_m value of substrate and for evaluating the mode of binding of screening hits. To achieve a throughput suitable for high-throughput screening (HTS), we developed a RapidFire-tandem mass spectrometry (RF-MS/MS)-based multiplex assay. This assay allowed a large diverse compound library to be screened at a speed of 1536 reactions per 40-50 min.