Certain drugs can help prevent breast cancer in women who are high risk. From previous studies, breast cancer researchers know that a class of drugs called aromatase inhibitors (AIs) are the most effective medications to prevent estrogen receptor-positive breast cancer in women after menopause. These breast cancers make up 80 percent of all breast cancers. The female hormone estrogen fuels cancer cell growth and development.

A new study from Queen Mary University of London’s Wolfson Institute of Population Health finds that measuring estrogen with a blood test is a simple and inexpensive way of predicting which women will benefit the most from AIs. This is important because these medications can have side effects. By knowing who may benefit, women and their doctors can have a more reliable way of measuring the risks versus advantages of treatment.

What Makes a Woman High Risk

Currently, the UK’s National Institute of Clinical Care and Excellence (NICE) recommends AIs for post-menopausal women at high risk for breast cancer. According to the American Cancer Society, two AIs have been shown in studies to lower breast cancer risk: anastrozole (Arimidex) and exemestane (Aromasin). AIs work by blocking an enzyme that converts other hormones into estrogen. They do not block the ovaries from producing estrogen, so they work best in post-menopausal women.

High risk may include a previous breast biopsy that was suspicious or precancerous but not cancer, or a strong family history of breast cancer. The new study is published in the journal The Lancet Oncology. The research team analyzed data from a large breast cancer prevention study that included women from 18 countries.

Significant Benefit from AIs

There were more than 3,800 women in the study. All the women had risk factors for breast cancer but had never been diagnosed. All the women were post-menopausal with ages ranging from 40 to 70. They were equally divided into two groups. One group received one dose of oral anastrozole every day for five years. The other group received a placebo pill. Neither the women nor the researchers knew who was given which pill until the end of the study. The women were followed for an average of about 11 years.

Based on their estrogen blood levels drawn at the start of the study and during the follow-up period, the women were divided into four groups from the lowest 25 percent to the highest 25 percent of estrogen levels. These were the key results…

  • There were 85 cases of breast cancer diagnosed in the AI group compared to 165 in the placebo group.
  • For women in the highest 75 percent of estrogen blood levels the risk reduction from AI was 55 percent compared to women not taking the AI.
  • For women in the bottom 25 percent of estrogen blood level, the risk reduction was not significant.

The most common side effects of AIs include symptoms that are similar to menopausal symptoms and may include hot flashes, vaginal dryness, headache, and fatigue. More serious but less common risks are osteoporosis, high cholesterol, and joint or muscle pain.

The researchers conclude that testing estrogen blood levels should be included in the weighing of the risks versus benefits of AI intake for breast cancer prevention. Women with low levels of estrogen are less likely to benefit, in which case the risks would most likely outweigh the benefits.

Source: Study titled “Effect of Baseline Oestradiol Serum Concentration on the Efficacy of Anastrozole for Preventing Breast Cancer in Postmenopausal Women at High Risk: a Case-Control Study of the IBIS-II Prevention Trial” led by researchers at Queen Mary University of London Wolfson Institute of Population Health, published in The Lancet Oncology.

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