Active tamoxifen metabolite plasma concentrations after coadministration of tamoxifen and the selective serotonin reuptake inhibitor paroxetine

J Natl Cancer Inst. 2003 Dec 3;95(23):1758-64. doi: 10.1093/jnci/djg108.


Background: Tamoxifen, a selective estrogen receptor modulator (SERM), is converted to 4-hydroxy-tamoxifen and other active metabolites by cytochrome P450 (CYP) enzymes. 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 tamoxifen before and after 4 weeks of coadministered paroxetine. We assessed the inhibitory activity of pure tamoxifen metabolites in an estradiol-stimulated MCF7 cell proliferation 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 identified 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 afterward (difference = 6.9 ng/mL, 95% confidence interval [CI] = 2.7 to 11.2 ng/mL) (P =.004). Endoxifen concentrations 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 subsequent 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 considered in women treated with tamoxifen.

Publication types

  • Clinical Trial
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Aged
  • Antineoplastic Agents, Hormonal / administration & dosage
  • Antineoplastic Agents, Hormonal / blood
  • Antineoplastic Agents, Hormonal / pharmacokinetics*
  • Antineoplastic Combined Chemotherapy Protocols / blood
  • Antineoplastic Combined Chemotherapy Protocols / pharmacokinetics*
  • Antineoplastic Combined Chemotherapy Protocols / therapeutic use
  • Breast Neoplasms / blood*
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / enzymology
  • Chemotherapy, Adjuvant
  • Cytochrome P-450 CYP2D6 Inhibitors
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors / pharmacology
  • Female
  • Humans
  • Microsomes, Liver / metabolism
  • Middle Aged
  • Paroxetine / administration & dosage
  • Paroxetine / blood
  • Paroxetine / pharmacokinetics*
  • Prospective Studies
  • Selective Estrogen Receptor Modulators / administration & dosage
  • Selective Estrogen Receptor Modulators / blood
  • Selective Estrogen Receptor Modulators / pharmacokinetics*
  • Serotonin Uptake Inhibitors / administration & dosage
  • Serotonin Uptake Inhibitors / blood
  • Serotonin Uptake Inhibitors / pharmacokinetics*
  • Tamoxifen / administration & dosage
  • Tamoxifen / analogs & derivatives*
  • Tamoxifen / blood
  • Tamoxifen / pharmacokinetics*
  • Treatment Outcome


  • Antineoplastic Agents, Hormonal
  • Cytochrome P-450 CYP2D6 Inhibitors
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Selective Estrogen Receptor Modulators
  • Serotonin Uptake Inhibitors
  • Tamoxifen
  • afimoxifene
  • Paroxetine
  • CYP3A protein, human
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human