Antidepressant 'could block breast cancer drug'

Scientists have said a popular antidepressant could block the beneficial effects of a breast cancer drug and cause an "increased risk of death".

Women who take the antidepressant paroxetine whilst also taking tamoxifen could find the two interfere with each other; but the authors of today's study stressed that patients should not stop taking tamoxifen and the results of the study do not imply that paroxetine itself causes breast cancer.

"This is simply a situation in which paroxetine impairs the effectiveness of tamoxifen," the authors of the study reported.

Breast cancer is the most commonly diagnosed cancer in women worldwide and the drug tamoxifen significantly improves survival. In order to work tamoxifen must be converted into an active metabolite (endoxifen) by the liver, but some drugs can interfere with this process.

Antidepressants are of particular importance because they are commonly used in women with breast cancer, often for long periods of time. Although many antidepressants have little or no impact on tamoxifen's metabolism, paroxetine, a member of the selective serotonin reuptake inhibitor (SSRI) class of drugs, is a potent inhibitor of the metabolic step that converts tamoxifen to endoxifen.

Dr Catherine Kelly and colleagues at the Institute for Clinical Evaluative Sciences (ICES) in Toronto examined the healthcare records of 2,430 women aged 66 years or older with breast cancer who received tamoxifen between 1993 and 2005. About 30 per cent of these women also received an antidepressant at some time during their treatment with tamoxifen, and paroxetine was the most commonly used agent.

The researchers said in their conclusion that use of paroxetine, but not other SSRIs, in combination with tamoxifen, was associated with an increased long-term risk of breast cancer death, "in a fashion that correlated with the extent of drug overlap".

"Our findings indicate that the choice of antidepressant can significantly influence survival in women receiving tamoxifen for breast cancer," said Dr David Juurlink, one of the study's authors and a scientist at ICES.

"These results highlight a drug interaction that is extremely common, widely underappreciated and potentially life-threatening, yet uniformly avoidable."

In an accompanying editorial, Frank Andersohn and Stefan Willich from Charité University Medical Center in Berlin said that clinicians should avoid co-prescribing paroxetine and tamoxifen in women with breast cancer, but warn against abrupt withdrawal of SSRI treatment.