Selective inhibitor of Bruton's tyrosine kinase (BTK). Inhibits the interaction between PKCbII and BTK (IC50 ~ 30 mM) and the catalytic activity of BTK but does not affect the activity of PKC. Has little effect on the activities of Lyn, Syk, PKA, casein kinase I, ERK1, ERK2 and p38 kinases.