Hot flashes are the most common side effect of tamoxifen. New antidepressant drugs, such as the selective serotonin reuptake inhibitors (SSRIs) paroxetine and fluoxetine, are effective in treating tamoxifen-associated hot flashes. However, these drugs have also been shown to inhibit the enzyme that converts tamoxifen to its known most potent metabolite, 4-hydroxy-tamoxifen, and other active metabolites. The implication is that co-prescription of SSRIs with tamoxifen may reduce the effectiveness of tamoxifen.

Vered Stearns, M.D., now at Johns Hopkins University in Baltimore, Md., David A. Flockhart, M.D., Ph.D., of the Indiana University School of Medicine, Indianapolis, and their colleagues examined the effect of paroxetine--which inhibits CYP2D6, an enzyme that is primarily responsible for the metabolism of tamoxifen into its active metabolites--on levels of tamoxifen and its metabolites in blood samples from 12 women with breast cancer who were being treated with adjuvant tamoxifen. The women received paroxetine for 4 weeks during their standard course of tamoxifen. They were also tested to determine their CYP2D6 genotype.

The researchers identified that a previously unrecognized active metabolite, which they named endoxifen, was present in substantially higher concentrations than 4-hydroxy-tamoxifen. They observed that, after paroxetine treatment, endoxifen levels decreased, whereas levels of 4-hydroxy-tamoxifen did not. Treatment with paroxetine reduced the concentration of endoxifen from 12.4 ng/mL before paroxetine administration to 5.5 ng/mL after paroxetine administration. Endoxifen concentrations decreased by 64% i
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Contact: Katherine Arnold
jncimedia@oupjournals.org
301-841-1287
Journal of the National Cancer Institute
2-Dec-2003