WASHINGTON (Reuters) - Antidepressants used to treat hot flashes in women taking the breast cancer drug tamoxifen may reduce the effectiveness of the tumor-fighting medication, U.S. doctors said on Tuesday.
Their study, published in the Journal of the National Cancer Institute, also offered a possible explanation for why tamoxifen works better in some women than others.
“Our study suggests that tamoxifen’s metabolism, and possibly its effectiveness, can be modified by the genetic makeup of the person taking the drug and by the use of another drug prescribed to reduce tamoxifen-related hot flashes,” Dr. David Flockhart of the Indiana University School of Medicine, who led the study, said in a statement.
The study included only 12 women, so more research is needed to show whether taking the antidepressants reduces the benefits of taking tamoxifen.
Tamoxifen, sold by AstraZeneca under the brand name Nolvadex and also available generically, is widely used to treat breast cancer and to prevent the disease in women at high risk of developing it.
Up to 80 percent of patients taking the drug, which helps block the effects of the hormone estrogen, experience hot flashes as a side effect.
Antidepressants in a class called selective serotonin reuptake inhibitors (SSRIs) are often used to treat the hot flashes.
Flockhart’s team tested the SSRI paroxetine, made by GlaxoSmithKline under the brand name Paxil.
Such drugs are known to interfere with the enzyme that breaks down tamoxifen into chemical agents called metabolites, including 4-hydroxy-tamoxifen, believed to be tamoxifen’s most active breakdown product.
The researchers studied 12 women with a history of breast cancer who took tamoxifen for a year. Each woman also took paroxetine for four weeks.
Looking at the women’s blood, the researchers discovered a previously unknown metabolite they named endoxifen. Its levels were higher than 4-hydroxy-tamoxifen.
In the women actively taking paroxetine, endoxifen concentrations fell anywhere between 24 percent and 64 percent.
The different levels corresponded to genetic variations in the women, they reported, specifically in a gene called CYP2D6.
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