R406 is an orally available small molecule targeting spleen tyrosine kinase (SYK) inhibitor with IC50 ~40 nM. It can block Syk-dependent FcR-mediated activation of monocytes/macrophages and neutrophils and BCR-mediated activation of B lymphocytes. R406 has shown efficacy in a number of animal models of immune disorders. R406’s pro-drug R788 (Fostamatinib) is currently in phase III clinical trials to treat rheumatoid arthritis.