A potent and highly selective small-molecule inhibitor of Bruton's tyrosine kinase (BTK) (IC₅₀ = 0.5 nM) with potential antineoplastic activity. It.binds to and irreversibly inhibits BTK activity, thereby preventing both B-cell activation and B-cell-mediated signaling. This leads to an inhibition of the growth of malignant B cells that overexpress BTK.