Tamoxifen: Does It Work For You?

If you’re one of the tens of thousands of women taking tamoxifen to prevent new breast cancer or a recurrence, listen up: it may not be doing you a bit of good.

Tamoxifen has been the gold standard of breast cancer treatment for over 30 years; reportedly, it’s the world’s most widely prescribed breast cancer drug. Effective for women with hormone-receptive cancer—up to about 75% of all women with breast cancer—it’s been shown to reduce a woman’s risk of recurrence by 40%, and her risk of death from breast cancer by 30%. In addition, the risk of new cancer in the opposite breast is reduced by about 50%.

Originally prescribed only for women with advanced breast cancer, studies over the years have enlarged its sphere of effectiveness. Now, a 5-year course of tamoxifen is routinely prescribed for women with early, non-invasive cancer: DCIS and LCIS. And it’s received FDA approval as a breast cancer preventive drug for women who haven’t had the disease, but who are at moderate to high risk.

Recently, tamoxifen has been prescribed less often for post-menopausal women with hormone-receptive breast cancer.  Aromatase inhibitors (Arimidex et. al.) have been shown to be more effective for this group, and many oncologists are starting women on tamoxifen, then switching them to an AI.

Still, if you have hormone-receptive breast cancer, there’s a good chance you’ll be taking tamoxifen, at least for a couple of years.

But the drug’s not perfect; women DO have recurrences while taking it. Is there a way to predict if you’ll fall into that unlucky category?

Maybe.

Scientists at the Mayo Clinic revealed some interesting research at the annual San Antonio Breast Cancer Symposium last month. First, they shared studies that conclude tamoxifen doesn’t work the way they’d always believed: by filling estrogen receptors on cancer cells, thus depriving those cells of the estrogen they need to grow.

Instead, it turns out a chemical that makes tamoxifen work—a metabolizer, endoxifen—breaks down the cancer cells’ hormone receptors. So it’s endoxifen, rather than tamoxifen, that’s the actual hero here.

And how might that information be used to predict whether tamoxifen will work for YOUR cancer?

In the course of their research, scientists also discovered that a certain gene, CYP2D6, is critical for metabolizing tamoxifen, and getting the whole process started. Women without this gene are four times more likely to have a breast cancer recurrence while on tamoxifen than women with the gene.

Matthew Goetz, M.D., the study’s lead author, recommends that, “…going forward, post-menopausal patients being considered for tamoxifen therapy should be tested for CYP2D6 before beginning therapy.”

If you’re a post-menopausal survivor on tamoxifen, or about to start it, ask your oncologist about this genetic test.

In a separate study presented in San Antonio, researchers showed that it’s possible to do a simple test to determine whether tamoxifen is working for high- and moderate-risk women taking the drug as a preventive therapy.