Aromatase Inhibitors Slightly Better Than Tamoxifen for Postmenopausal Women

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Tamoxifen and aromatase inhibitors, two types of hormonal therapy medicines, usually are taken for 5 years after surgery (and possibly radiation and chemotherapy) to lower the risk of hormone-receptor-positive, early-stage breast cancer coming back in postmenopausal women. Because the hormonal therapy medicines are taken after surgery, they're called adjuvant hormonal therapy.

A study adds more evidence to support the current standard of care for postmenopausal women diagnosed with early-stage hormone-receptor-positive breast cancer: aromatase inhibitors are somewhat better than tamoxifen at reducing the risk of breast cancer coming back, as well as at improving overall survival in some cases. These results were presented at the 2008 San Antonio Breast Cancer Symposium.

Tamoxifen and the aromatase inhibitors work in different ways to lower the risk of breast cancer coming back (recurrence). Tamoxifen blocks the effects of estrogen on breast cancer cells. Aromatase inhibitors lower the amount of estrogen in the body. The aromatase inhibitors are:

  • Arimidex (chemical name: anastrozole)
  • Aromasin (chemical name: exemestane)
  • Femara (chemical name: letrozole)

The researchers looked at the results from nearly 10,000 women who participated in several studies comparing individual aromatase inhibitors to tamoxifen. In the studies, the women usually were given tamoxifen alone or an aromatase inhibitor alone for 5 years. In some cases the women began the study taking tamoxifen and then switched to an aromatase inhibitor.

Overall, aromatase inhibitors were 23% better than tamoxifen at reducing the risk of breast cancer coming back. Aromatase inhibitors offered different amounts of risk reduction depending on the specific location of the cancer coming back. Compared to tamoxifen, aromatase inhibitors were:

  • 30% better at lowering the risk of the cancer coming back in the same breast
  • 41% better at lowering the risk of the cancer coming back in the opposite breast
  • 16% to 18% better at lowering the risk of the cancer coming back in other parts of the body (called distant recurrence)

While aromatase inhibitors improved overall survival compared to tamoxifen, this increase could have been due to chance.

Women who started taking tamoxifen and then switched to an aromatase inhibitor also had a lower risk of the cancer coming back and improved overall survival.

Still, the advantages of aromatase inhibitors compared to tamoxifen diminish over time. While breast cancers that are going to come back usually do so in the first 3 years after diagnosis, recurrence can happen any time after breast cancer treatment. Some doctors recommend that hormonal therapy be taken for more than 5 years to further reduce the risk of the cancer coming back later.

Both tamoxifen and aromatase inhibitors can cause side effects. Tamoxifen may cause hot flashes. Aromatase inhibitors may cause muscle and joint pain and sometimes hot flashes. Aromatase inhibitors also can weaken bones, while tamoxifen actually can strengthen bones.

While aromatase inhibitors have clear advantages, tamoxifen may be a better choice for some women because of side effects and cost. Some doctors will recommend starting on one type of hormonal therapy medicine and then switching to another type after a few years.

When you're deciding on a treatment plan, keep two things in mind:

  • Every woman responds differently to treatment. What works for someone else may not work for you and what works for you may not work for someone else.
  • Your treatment plan is not written in stone. You can always switch to another medicine if it has greater benefits and fewer side effects.

If you're a postmenopausal woman being treated for hormone-receptor-positive, early-stage breast cancer, talk to your doctor about the pros and cons of aromatase inhibitors compared to tamoxifen. If you're currently taking tamoxifen, discuss whether switching to an aromatase inhibitor makes sense for your unique situation. Together, you can decide on a treatment plan that is best for YOU.

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