Cell-permeable. A potent, ATP-competitive and reversible inhibitor of spleen tyrosine kinase (Syk) (IC₅₀ = 10 nM). Displays excellent selectivity against many other kinases including Btk, Fyn, Itk, Lyn and Src up to concentrations of 4.7 µM. Shows good in vivo efficacy in the treatment of various allergy and asthma conditions in rat models.