Tamoxifen and Some Antidepressants

Tamoxifen is a popular medication that may help reduce the risk of certain types of breast cancer by interfering with estrogen activity, which can promote tumor growth. Although there are several side effects that may cause problems (blood clots and cancer of the uterus, for example), it’s been found that tamoxifen’s benefits tend to outweigh the risks. (National Cancer Institute )

However, we know that people don’t get sick in a vacuum. Many women (and men) who have breast cancer may be ill with other diseases or disorders, which means they may be taking other medications that could end up interacting with tamoxifen. For this reason, research into tamoxifen and its interaction with other medications is essential.

This research has led to a finding that women who take tamoxifen and some types of antidepressants may be increasing their risk of developing breast cancer, rather than decreasing it. A study presented at the 2009 Annual Meeting of the American Society of Clinical Oncology (ASCO) found that women who took certain popular antidepressants, such as Prozac (fluoxetine), Paxil (paroxetine) and Zoloft (sertraline), taken along with tamoxifen could more than double the risk of a breast cancer recurrence.

The study reviewed the cases of 1,300 women who were newly prescribed tamoxifen to treat breast cancer between 2003 and 2005. They were then monitored for at least two years.

Antidepressant Types

There are different types of antidepressants; those in this study are selective serotonin reuptake inhibitors (SSRIs) and are part of a group of medications called CYP2D6 inhibitors. The first of its kind was Prozac, which came onto the US market in the late 1980s.

Although researchers aren’t exactly sure how SSRIs work. MayoClinic.com explains it like this:

Some research suggests that abnormalities in neurotransmitter activity affect mood and behavior. SSRIs seem to relieve symptoms of depression by blocking the reabsorption (reuptake) of serotonin by certain nerve cells in the brain. This leaves more serotonin available in the brain. Increased serotonin enhances neurotransmission — the sending of nerve impulses — and improves mood. SSRIs are called selective because they seem to affect only serotonin, not other neurotransmitters.

The Study

The 1,300 women were divided into two groups. Three hundred fifty three women took a CYP2D6 inhibitor in addition to their tamoxifen treatment. The second group,  945 patients who took tamoxifen were not using a CYP2D6 inhibitor during the study period.

In the group of women taking the CYP2D6 inhibitors, 60% were taking an SSRI. A following analysis looked specifically at patients taking any SSRI in conjunction with tamoxifen. This subset was divided into yet another two groups. Two hundred thirteen women were on so-called moderate to potent CYP2D6 inhibitors (Prozac, Paxil, and Zoloft), while 137 women took weaker forms: Celexa (citalopram), Lexapro (escitalopram) and Luvox (fluvoxamine).

Results

Recurrent breast cancer rates among women who take tamoxifen is generally about 7.5%. According to this study’s findings, women taking weak CYP2D6 inhibitors had similar recurrence rates, but those taking potent to moderate CYP2D6 inhibitors almost doubled their risk of developing recurrent breast cancer to 13.9%, and those who took moderate to potent CYP2D6 inhibitors SSRIs had a rate as high as 16%.

Why is this such an important issue?

• As many as 28% of women who take tamoxifen also take an antidepressant.
• Antidepressants aren’t prescribed only for depression, but may be used to help reduce a common side effect of tamoxifen, hot flashes.
• Almost half a million women take tamoxifen – up to 80,000 new patients start taking it each year
• According to ASCO, tamoxifen is the first choice treatment for women who have not yet gone through menopause and is also a popular option for women who are post-menopausal.
• Tamoxifen is a very effective medication for the right women, reducing the risk of cancer recurrence by almost half in women after menopause, and between 30 to 50% in women before menopause.

Dr. Robert Epstein, chief medical officer of Medco Health Solutions, Inc,  and study co-author says, “We’ve known that these CYP2D6 inhibitor drugs block the activation of tamoxifen chemically, but this is the first time there’s evidence that these drugs are putting women at a much higher risk for recurrent breast cancer.”

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