Background: Tamoxifen, a selective estrogen receptor mod- ulator (SERM), is converted to 4-hydroxy-tamoxifen and other active metabolites by cytochrome P450 (CYP) en- zymes. Selective serotonin reuptake inhibitors (SSRIs), which are often prescribed to alleviate tamoxifen-associated hot flashes, can inhibit CYPs. In a prospective clinical trial, we tested the effects of coadministration of tamoxifen and the SSRI paroxetine, an inhibitor of CYP2D6, on tamoxifen metabolism. Methods: Tamoxifen and its metabolites were measured in the plasma of 12 women of known CYP2D6 genotype with breast cancer who were taking adjuvant ta- moxifen before and after 4 weeks of coadministered parox- etine. We assessed the inhibitory activity of pure tamoxifen metabolites in an estradiol-stimulated MCF7 cell prolifera- tion assay. To determine which CYP isoforms were involved in the metabolism of tamoxifen to specific metabolites, we used CYP isoform-specific inhibitors. All statistical tests were two-sided. Results: We separated, purified, and identi- fied the metabolite 4-hydroxy-N-desmethyl-tamoxifen, which we named endoxifen. Plasma concentrations of endoxifen statistically significantly decreased from a mean of 12.4 ng/mL before paroxetine coadministration to 5.5 ng/mL af- terward (difference 6.9 ng/mL, 95% confidence interval (CI) 2.7 to 11.2 ng/mL) (P.004). Endoxifen concentra- tions decreased by 64% (95% CI 39% to 89%) in women with a wild-type CYP2D6 genotype but by only 24% (95% CI 23% to 71%) in women with a variant CYP2D6 genotype (P.03). Endoxifen and 4-hydroxy-tamoxifen inhibited estradiol-stimulated MCF7 cell proliferation with equal potency. In vitro, troleandomycin, an inhibitor of CYP3A4, inhibited the demethylation of tamoxifen to N-desmethyl-tamoxifen by 78% (95% CI 65% to 91%), and quinidine, an inhibitor of CYP2D6, reduced the subse- quent hydroxylation of N-desmethyl-tamoxifen to endoxifen by 79% (95% CI 50% to 108%). Conclusions: Endoxifen is an active tamoxifen metabolite that is generated via CYP3A4-mediated N-demethylation and CYP2D6-mediated hydroxylation. Coadministration of paroxetine decreased the plasma concentration of endoxifen. Our data suggest that CYP2D6 genotype and drug interactions should be consid- ered in women treated with tamoxifen. (J Natl Cancer Inst 2003;95:1758 - 64)