O-Desmethyl gefitinib is the major metabolite of gefitinib in human plasma, formed by the cytochrome P450 isoform CYP2D6. It is an active metabolite that inhibits EGFR similarly to gefitinib in subcellular assays (IC50s = 36 and 22 nM, respectively) but is less active in whole cell assays (IC50s = 760 and 49 nM, respectively). In a LoVo tumor mouse xenograft model, the tumor concentration of O-desmethyl gefitinib was 6.8-fold lower than that of gefitinib and did not significantly reduce tumor growth. A high plasma concentration of O-desmethyl gefitinib in patients homozygous for CYP2D6 was not associated with an increase in adverse effects.