Aromatase inhibitors are hormonal therapy medicines used to treat hormone-receptor-positive breast cancer and help stop the cancer from coming back (recurrence) in postmenopausal women. Doctors have been interested in using aromatase inhibitors to reduce breast cancer risk in postmenopausal women at high risk who haven’t been diagnosed.
Results from the IBIS-II trial found that the aromatase inhibitor Arimidex (chemical name: anastrozole) was good at lowering the risk of hormone-receptor-positive breast cancer in high-risk postmenopausal women.
The research was presented on Dec. 12, 2013 at the 2013 San Antonio Breast Cancer Symposium and published online on Dec. 12 by The Lancet. Read the abstract of "Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): an international, double-blind, randomized placebo-controlled trial."
Aromasin (chemical name: exemestane), another aromatase inhibitor, also has been shown to reduce the risk of first-time hormone-receptor-positive breast cancer in postmenopausal women at high risk. Tamoxifen and Evista (chemical name: raloxifene), both SERMs (selective estrogen receptor modulators), another type of hormonal therapy medicine, also have been shown to reduce the risk of hormone-receptor-positive breast cancer in high-risk postmenopausal women. Tamoxifen also reduces the risk of hormone-receptor-positive breast cancer in high-risk premenopausal women.
Doctors call giving medicine to reduce the risk of a disease a person hasn’t been diagnosed with “prophylactic” or “preventive.”
While Aromasin and Arimidex haven’t been approved by the U.S. Food and Drug Administration to be used preventively for breast cancer, doctors may consider them good alternatives to tamoxifen or Evista.
SERMs block the action of estrogen in breast and certain other cells by sitting in the cells' estrogen receptors. SERMs don't affect all estrogen receptors the same way because they're selective (as the name says). In bone cells, SERMs interact with the receptors the way estrogen does and strengthen bones. In breast cells, SERMs block the receptors' interaction with estrogen and limit cell growth.
Aromatase inhibitors work by stopping the body from producing estrogen, which limits the amount of estrogen available to stimulate hormone-receptor-positive breast cancer cells to grow.
More than 3,800 postmenopausal women who had not been diagnosed with breast cancer but were considered at high risk for the disease were in the IBIS-II study. The women were considered at high risk for breast cancer because they:
- had two or more blood relatives who had been diagnosed
- had a mother or sister who had been diagnosed before age 50
- had a mother or sister who had the disease in both breasts
Half the women were randomly assigned to take Arimidex for 5 years and the other half were assigned to take a placebo (a sugar pill that looked just like Arimidex).
After about 5 years of follow-up, the researchers found that women taking Arimidex were about 50% less likely to have developed breast cancer compared to women taking the placebo:
- 2% of the women taking Arimidex developed breast cancer (40 women)
- 4% of the women taking the placebo developed breast cancer (85 women)
The researchers plan to follow the women for at least 5 more years (for a total of at least 10 years) to see if Arimidex continues to reduce breast cancer risk even after the women complete the 5 years of treatment.
While it’s great news to have another medicine that can help reduce breast cancer risk in high-risk postmenopausal women, the reality is that many high-risk women don’t take preventive tamoxifen, Evista, or Aromasin because of side effects.
Tamoxifen’s side effects include:
- blood clots
- a higher risk of endometrial cancer
- leg cramps
- weight gain
- hot flashes
- joint pain
While Evista also may cause blood clots and stroke, women taking Evista are less likely to have these two serious side effects compared to women taking tamoxifen. Evista also may cause:
- hot flashes
- joint pain
- weight gain
Aromasin and Arimidex also cause hot flashes and night sweats, though they’re more common with tamoxifen and Evista. Joint pain is a more common side effect of Aromasin and Arimidex. Aromasin and Arimidex also may weaken bones and make women more likely to break a bone.
If you have a higher-than-average risk of breast cancer, it makes sense to do everything you can to keep your risk as low as it can be. There are lifestyle choices you can make, including:
- maintaining a healthy weight
- exercising regularly at the highest intensity possible
- limiting or avoiding alcohol
- limiting processed foods and foods high in sugar
- eating healthy, nutrient-dense food
- not smoking
You and your doctor also may be considering medicine to reduce your risk. Talk to your doctor about your preferences as well as the risks and benefits of each medicine. If you’re already taking tamoxifen or Evista and having unacceptable side effects, you may want to ask if Aromasin or Arimidex might be a better option for you. Together, you can make the best choice for your unique situation.
You can learn more about tamoxifen, Evista, Arimidex, and Aromasin in the Breastcancer.org Hormonal Therapy section.
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