Many postmenopausal women take hormonal therapy medicine -- either an aromatase inhibitor or tamoxifen -- after breast cancer surgery and other treatments for hormone-receptor-positive, early-stage breast cancer. Hormonal therapy can reduce the risk of the cancer coming back (recurrence). Hormonal therapy used in this way is called adjuvant hormonal therapy.
The aromatase inhibitors are:
- Arimidex (chemical name: anastrozole)
- Aromasin (chemical name: exemestane)
- Femara (chemical name: letrozole)
Most women take adjuvant hormonal therapy for 5 years.
The American Society of Clinical Oncology (ASCO) has issued new guidelines on adjuvant hormonal therapy medicines. ASCO is a national organization of oncologists and other cancer care providers.
The new ASCO recommendations for adjuvant hormonal therapy treatment for postmenopausal women diagnosed with early-stage, hormone-receptor-positive breast cancer are:
An aromatase inhibitor is preferred over tamoxifen. This recommendation is supported by a number of studies showing that women treated with an aromatase inhibitor are somewhat less likely than those treated with tamoxifen to have the cancer come back.
Most women should take adjuvant hormonal therapy for a total of 5 years. Options include:
- taking the same hormonal therapy for all 5 years (monotherapy)
- taking tamoxifen for 2 or 3 years and then switching to an aromatase inhibitor until hormonal therapy has been taken for a total of 5 years (sequential therapy)
- taking tamoxifen for 5 years if a woman started taking an aromatase inhibitor but had to stop taking the aromatase inhibitor before completing 5 full years of treatment (perhaps because of intolerable side effects)
While not routine, some women may benefit from taking adjuvant hormonal therapy for 8 to 10 years (called extended adjuvant therapy). In these cases, ASCO recommends 5 years of tamoxifen followed by 3 to 5 years of an aromatase inhibitor.
ASCO doesn't recommend one aromatase inhibitor over another -- they're considered interchangeable. If side effects from one aromatase inhibitor are intolerable, switching to a different aromatase inhibitor rather than tamoxifen may make sense.
ASCO didn't recommend routine genetic testing for the CYP2D6 enzyme when deciding which hormonal therapy medicine to use. The body uses the CYP2D6 enzyme to turn tamoxifen into its active form. Women who make low levels of this enzyme may not get all the benefits of tamoxifen treatment.
ASCO recommends that premenopausal women take only tamoxifen as adjuvant hormonal therapy.
Research shows that aromatase inhibitors are generally somewhat better than tamoxifen for reducing the risk of recurrence in postmenopausal women diagnosed with early-stage, hormone-receptor-positive breast cancer. Still, for a number of reasons, including side effects and cost, tamoxifen may be a better choice for some women.
Hot flashes and night sweats -- called vasomotor symptoms -- are side effects of both tamoxifen and the aromatase inhibitors, though they're more common with tamoxifen. Joint pain is a more common side effect of the aromatase inhibitors. Both tamoxifen and the aromatase inhibitors can cause serious side effects. Treatment with either tamoxifen or an aromatase inhibitor can lead to dangerous blood clots in rare cases. This complication is more common with tamoxifen. Also, aromatase inhibitors can weaken bones and make women more likely to break a bone.
Tamoxifen and Arimidex are available as generic medicines, so these may be much less expensive than the other two aromatase inhibitors (depending on your insurance coverage).
If you're a postmenopausal woman who's been diagnosed with hormone-receptor-positive, early-stage breast cancer, keep two things in mind when you and your doctor are deciding on an adjuvant hormonal therapy plan:
- 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 isn't written in stone. You can always switch medicines if another treatment has greater benefits and/or fewer side effects.
Ask your doctor about the differences in benefits and side effects between aromatase inhibitors and tamoxifen, as well as the pros and cons of monotherapy vs. sequential therapy.
If you're already taking tamoxifen, ask your doctor if switching to an aromatase inhibitor would be a good idea. Together, you can decide on a treatment plan that's best for YOU.
Editor’s note: In 2018, the Clinical Pharmacogenetics Implementation Consortium, an international group of scientists that issues guidelines on the effects of genetic factors on reactions to drugs, issued a guideline on using CYP2D6 genotype information to make decisions about prescribing tamoxifen after surgery to treat hormone-receptor-positive breast cancer. The guideline strongly recommends that people with an abnormal CYP2D6 genotype that makes them less able to metabolize tamoxifen be treated with a different type of hormonal therapy, such as an aromatase inhibitor.