A popular antidepressant could interfere with a breast cancer drug
and lead to a greater chance of dying from the disease, researchers
said today.
Women taking the antidepressant paroxetine (brand name Seroxat)
alongside tamoxifen for breast cancer were more likely to die from the
disease than women on other antidepressants, a study found.
The researchers, writing in the British Medical Journal (BMJ), said
their results had "major implications for clinical practice".
Tamoxifen is taken by thousands of British women each year and works
by blocking the female sex hormone oestrogen, which can fuel tumour
growth.
It is generally given for up to five years following initial treatment or surgery.
The latest research was led by a team from the Sunnybrook Health Sciences Centre and the University of Toronto in Canada.
Up to one in four women with breast cancer can experience some
degree of depression and antidepressants are also prescribed for hot
flushes, they said.
A total of 2,430 women aged over 66 took part in the research, which looked at those having treatments between 1993 and 2005.
All the women were taking tamoxifen and one of five anti-depressants
known as selective serotonin reuptake inhibitors (SSRIs), including
paroxetine, which was the most commonly prescribed drug.
Over a typical follow-up of 2.4 years, 374 women died from breast cancer.
Analysis of health records showed women taking paroxetine were far
more likely to die from breast cancer and were slightly more likely to
die from any other cause when compared with women not on paroxetine.
The researchers, who believe paroxetine interferes with the
metabolism of tamoxifen, found no evidence that other SSRIs increased
the risk of death.
They concluded: "After adjustment for age, duration of tamoxifen
treatment, and other potential confounders, absolute increases of 25%,
50%, and 75% in the proportion of time on tamoxifen with overlapping
use of paroxetine were associated with 24%, 54%, and 91% increases in
the risk of death from breast cancer, respectively.
"We estimate that use of paroxetine for 41% of tamoxifen treatment
(the median overlap in our sample) would result in one additional
breast cancer death within five years of cessation of tamoxifen for
every 19.7 patients so treated; the risk with more extensive overlap
would be greater.
"In conclusion, our findings indicate that the choice of
antidepressant can significantly affect survival in women receiving
tamoxifen for breast cancer."
The authors stressed that women should not stop taking tamoxifen and
said their study does not imply that paroxetine itself causes or
influences the course of breast cancer.
"This is simply a situation in which paroxetine impairs the effectiveness of tamoxifen," they wrote.
Dr David Juurlink, one of the study's authors, added: "These results
highlight a drug interaction that is extremely common, widely
under-appreciated and potentially life-threatening, yet uniformly
avoidable.
"Tamoxifen is a crucial element of therapy for patients with hormone
receptor-positive breast cancer regardless of age or breast cancer
stage.
"When co-prescription of tamoxifen with an antidepressant is
necessary, preference should be given to antidepressants that exhibit
little or no impact on tamoxifen's metabolism."
More than 45,000 women are diagnosed with breast cancer each year in the UK and just under 12,000 die from the disease.
Meg McArthur, senior policy and information officer at Breakthrough
Breast Cancer, said: "This research is welcome because it gives an
insight into how the effectiveness of tamoxifen could be influenced by
taking antidepressants.
"Tamoxifen remains a beneficial treatment for breast cancer and, as
several antidepressants are available, doctors should be able to find
the right combination for patients.
"This should not put patients off taking tamoxifen and any concerns should be discussed with their doctor."
A spokeswoman for GlaxoSmithKline (GSK), which makes Seroxat, said:
"GSK is aware of a reported interaction between paroxetine and
tamoxifen and, in early 2008, following a thorough review of the
scientific literature, the company submitted updated prescribing
information to regulatory agencies on a worldwide basis.
"This interaction is currently included as a Warning and Precaution
in the European label (Summary of Product Characteristics) and in the
equivalent section of the US Prescribing Information.
"Following publication of this new study, GSK will review these
additional data and will work with regulatory authorities to determine
next steps."
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