A major active metabolite of gefitinib in human plasma; inhibits EGFR in subcellular assays similarly to gefitinib (IC50s = 36 and 22 nM, respectively); less active than gefitinib in whole cell assays (IC50s = 760 and 49 nM, respectively); does not reduce tumor growth as well as gefitinib in a LoVo tumor mouse xenograft model,