R406 (TAMATINIB) is an ATP-competitive inhibitor of spleen tyrosine kinase (Syk), which plays a key role in the signaling of activating Fc receptors and the B-cell receptor (BCR). R406 blocked Syk-dependent FcR-mediated activation of monocytes/macrophages and neutrophils and BCR-mediated activation of B lymphocytes. R406 was selective as assessed using a large panel of Syk-independent cell-based assays representing both specific and general signaling pathways. Consistent with Syk inhibition, oral administration of R406 to mice reduced immune complex-mediated inflammation in a reverse-passive Arthus reaction and two antibody-induced arthritis models. R406 is the active compound of pro-drug Fostamatinib (R-788). Fostamatinib is being developed by Rigel Pharmaceuticals for the treatment of immune thrombocytopenic purpura (ITP) and IgA nephropathy.