R406 (free base) is a potent Syk inhibitor with IC50 of 41 nM, strongly inhibits Syk but not Lyn, 5-fold less potent to Flt3.

R406 is a potent ATP-competitive inhibitor of spleen tyrosine kinase (Syk, Ki = 30 nM).Through this action, R406 blocks FcεRI-dependent mast cell activation (EC50 = 43 nM) and, at 3 μM, reduces the release of IL-10, -12, and -13 by immune complex-pulsed dendritic cells.R406 is orally available and can reduce immune complex-mediated inflammation.In cancers characterized by over-expression of Syk, R406 can prevent signaling downstream of Syk and induce apoptosis.R406 also inhibits Syk-dependent signaling through c-Jun N-terminal kinase in rheumatoid arthritis synoviocytes, suggesting a therapeutic intervention in this autoimmune disease.