Aromatase inhibitors lower estrogen levels in the body by blocking aromatase, an enzyme that converts other hormones into estrogen.
Like tamoxifen, aromatase inhibitors are used to treat metastatic and early breast cancers that are hormone-receptor positive. Aromatase inhibitors include:
Unlike tamoxifen, aromatase inhibitors are only used to treat postmenopausal women.
Postmenopausal women with hormone receptor-positive breast cancer begin hormone therapy with an aromatase inhibitor or begin with tamoxifen and after a few years, switch to an aromatase inhibitor.
Aromatase inhibitors come in a pill form and are taken every day. When aromatase inhibitors are the only hormone therapy given, they are taken for five years. When women switch to an aromatase inhibitor after taking tamoxifen for a few years, they take these drugs for a combined total of at least five years. More and more, hormone therapy is recommended for a longer period of time for postmenopausal women (five years of tamoxifen followed by five years of an aromatase inhibitor, for a total of 10 years of treatment). The number of years that aromatase inhibitors should be taken to get the most treatment benefit is still under study.
Treatment with an aromatase inhibitor (alone or after several years of tamoxifen) lowers the risk of :
Among postmenopausal women with hormone receptor-positive breast cancer, aromatase inhibitors (alone or after tamoxifen) offer the same or slightly better treatment benefit compared to treatment with tamoxifen alone [57,64-67]. The aromatase inhibitors anastrozole, exemestane and letrozole all have a similar treatment benefit [57,64-67]. They also have similar side effects. However, you may tolerate one drug better than another.
For a summary of research studies on aromatase inhibitors and early breast cancer, visit the Breast Cancer Research section.
The side effects of aromatase inhibitors differ from those of tamoxifen.
If you have hormone receptor-positive breast cancer, hormone therapy with tamoxifen and/or an aromatase inhibitor (such as anastrozole, letrozole and exemestane) is a key part of your treatment.
Aromatase inhibitors are only used to treat postmenopausal women. Premenopausal women should not take aromatase inhibitors. And, women with certain heart problems should not take the aromatase inhibitor anastrozole.
Compared to chemotherapy, aromatase inhibitors have few side effects. For example, they do not cause nausea.
Joint pain (arthralgia – ar-THRAL-juh) and muscle pain (myalgia – my-AL-juh) are common side effects of aromatase inhibitors [70-71]. The pain may be in the hands and wrists, feet and ankles, knees, back or other parts of the body. Up to 36 percent of women in clinical trials of aromatase inhibitors have reported joint pain and up to 15 percent have reported muscle pain (other studies have found even higher rates of these side effects) [70-72]. These symptoms can mimic carpal tunnel syndrome (in rare cases, aromatase inhibitors can cause carpal tunnel syndrome). Although aromatase inhibitors can cause joint and muscle pain, they do not cause permanent joint or muscle damage.
If you have joint or muscle pain while taking an aromatase inhibitor, talk to your health care provider. Anti-inflammatory medications (such as aspirin or ibuprofen), special exercises or acupuncture may ease the pain . Or, your provider may switch you to another aromatase inhibitor (you may have less pain with a different drug) or recommend tamoxifen .
Aromatase inhibitors cause a loss of bone density, which leads to higher rates of osteoporosis and bone fractures compared to tamoxifen . Some medications may help prevent osteoporosis. Your health care provider may prescribe drugs called bisphosphonates or the drug denosumab to help maintain bone density while you are taking aromatase inhibitors. Although mostly a concern for people with metastatic breast cancer who take higher doses of bisphosphonates or denosumab, these drugs can cause severe bone, joint and muscle pain [70,74]. If you have any of these symptoms, report them to your health care provider right away. In rare cases, a serious jawbone disorder called osteonecrosis may occur [46-47]. So, it is important to have a full dental exam before starting treatment and to talk with your provider before getting any dental procedure while you are on bisphosphonates .
Regular exercise can help strengthen and protect your bones . For example, weight-bearing exercise (exercise that involves standing rather than sitting) helps protect bones and lowers the risk of hip fractures . Getting enough calcium and vitamin D (your health care provider can tell you if you are getting enough calcium and vitamin D) and not smoking can also help strengthen your bones .
Hot flashes and night sweats are common in women who take aromatase inhibitors . Although these symptoms may become less frequent and less intense over time, they can still be hard to manage. If you have any symptoms, talk to your health care provider. He/she can help treat many of them.
Learn more about treating menopausal symptoms.
Although both aromatase inhibitors and tamoxifen can cause menopausal symptoms such as hot flashes, many of their side effects differ (see Figure 5.11 below). Your health care provider can help you treat many of these side effects.
Adapted from National Cancer Institute materials [62,64-67].
Learn more about the side effects of tamoxifen.
More time is needed to assess the long-term risks and benefits of aromatase inhibitors.
At this time, we still do not know how long women should take aromatase inhibitors. There are no data on the benefits or safety of taking aromatase inhibitors for longer than five years.
Another question is whether aromatase inhibitors benefit women who were premenopausal when diagnosed with breast cancer, but became menopausal due to treatment. New and ongoing studies should help answer these questions. Because aromatase inhibitors only work in postmenopausal women, tamoxifen should be used if there is any chance you are still premenopausal.
Aromatase inhibitors are being studied in the chemoprevention (risk-lowering drugs) setting as well. Findings from randomized controlled trials have shown the aromatase inhibitors exemestane and anastrozole may lower the risk of developing breast cancer in postmenopausal women at high risk [68-69]. Exemestane and anastrozole do not yet have FDA-approval for use in the chemoprevention setting. Learn more about risk-lowering drugs for women at high risk of breast cancer.
To learn more about aromatase inhibitors, visit the National Institutes of Health's Medline Plus website.
Breast cancer treatment is most effective when all parts of the treatment plan are followed. So, it is important to follow your treatment plan (for medications and other therapies) prescribed by your health care provider in terms of:
Hormone therapy used to treat breast cancer is typically prescribed for at least five years. The length of treatment coupled with side effects can make adherence to hormone therapy difficult. Although the menopausal symptoms related to hormone therapy can be hard to deal with, there are treatments that may ease these side effects. If you have side effects, talk to your health care provider about ways to treat them.
To get the most benefit out of hormone therapy, you need to take the full course of treatment. Women who complete the full course have higher rates of survival .
If you have trouble remembering to take your hormone therapy, a daily pillbox or setting an alarm on your watch or mobile device (you may be able to download a mobile app) may help . However, you do not need to panic if you miss a day or two.
Learn more about the importance of following your breast cancer treatment plan.
Anastrozole, exemestane and letrozole are off-patent and available in generic forms. Generics are cheaper than name brand drugs, but are just as effective.
Medicare and many insurance providers offer prescription drug plans. One may already be included in your policy or you may be able to buy an extra plan for prescriptions. You may also qualify for assistance from programs that help with drug costs or offer low-cost or free prescriptions.
Learn more about insurance plans and prescription drug assistance programs.
Komen Support Resources
Hormone Therapy Video
Facts for Life: Hormone Therapy
Breast Cancer 101 - Aromatase Inhibitors
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