Overactivity of epidermal growth factor receptor (EGFR) tyrosine kinase activity has been associated with a number of cancers. CL-387785 is a potent, irreversible inhibitor of EGFR kinase activity (IC50 = 370 pM). It blocks EGF-stimulated autophosphorylation of receptors in cells (IC50 = 5 nM) and halts cell cycling in cells that overexpress EGFR or c-ErbB2 (IC50s = 31-125 nM). CL-387785 profoundly blocks the growth of EGFR-overexpressing tumors in nude mice when given orally at 80 mg/kg/day for 10 days.