Fostamatinib (R788 disodium salt) is a prodrug of the active metabolite R406, which is a potent, ATP-competitive inhibitor of Syk kinase with Ki/IC50 of 30/41 nM; dose-dependently inhibits anti-IgE-mediated CHMC degranulation with an EC50 of 56 nM, inhibits all phosphorylation events downstream of Syk signaling; specifically inhibits FcR signaling in human mast cells, macrophages, and neutrophils. R788 can inhibit local inflammatory injury mediated by immune complexes; reduces immune complex-mediated inflammation in arthritis models. Rheumatoid ArthritisPhase 3 Clinical; Purity: >98% (HPLC); Molecular Weight: 624.4232, 25 mg