Potent and selective ErbB receptor family inhibitor (IC50 values are 21, 22 and 38 nM for ErbB4, EGFR and ErbB2 respectively). Displays potent growth inhibition of human cancer cell lines overexpressing ErbB2 in vitro (IC50 = 60 - 168 nM) and inhibits growth of human tumor xenografts in vivo. Brain penetrant and orally active.