Aromatase Inhibitors to Treat Breast Cancer

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Drugs that Lower Estrogen Levels
(Arimidex, Febara and Aromasin)

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Aromatase inhibitors are a newer class of hormonal treatments for postmenopausal women. Aromatase is an enzyme the body uses in the production of estrogen. Aromatase inhibitors slow or stop the growth of breast cancers controlled by estrogen by lowering estrogen levels in a woman's body. In women who have gone through menopause, they can block estrogen production and reduce estrogen levels by more than 90%.

There are 3 types of aromatase inhibitors approved to treat breast cancer: anastrozole (Arimidex), letrozole (Febara) and exemestane (Aromasin). They cannot stop the ovaries of premenopausal women from making estrogen. For this reason they are only effective in postmenopausal women. For premenopausal women, tamoxifen remains the best drug to take.

Aromatase Inhibitors Compared with Tamoxifen

Aromatase inhibitors are at least as effective as tamoxifen in preventing breast cancer from coming back in postmenopausal women. Based on recent studies, aromatase inhibitors may be used as first line (first approach) therapy or after tamoxifen therapy in the adjuvant setting.

Aromatase inhibitors have been compared with tamoxifen as adjuvant hormone therapy. They have different side effects than tamoxifen. They may cause osteoporosis and bone fractures because they remove all estrogen from a postmenopausal woman. Other side effects include increased cholesterol levels, hot flashes and joint pain.

Long-term side effects of aromatase inhibitors are not fully known. Early study results suggest that when compared with tamoxifen, they may reduce the chance of blood clots and uterine cancer but may increase the risk of osteoporosis and fractures.

Who should take aromatase inhibitors?

Women with hormone receptor-positive breast cancer who have gone through menopause at the time of diagnosis have 2 options for adjuvant hormone therapy:

  • They can begin treatment with tamoxifen for 2-5 years and switch to an aromatase inhibitor for another 2-5 years.
  • They can begin treatment with an aromatase inhibitor for 5 years.

At this time, it is not known which treatment option is better. It is reasonable for women who have been taking tamoxifen for 2 or more years to switch to an aromatase inhibitor.

What This Means for Women with Breast Cancer

Women with breast cancer and their doctors must weigh the risks and benefits of all treatment options. For some women, the risks and inconvenience of an aromatase inhibitor may outweigh the potential benefits.

Those who might be advised to consider using an aromatase inhibitor as initial adjuvant therapy include:

  • Women who develop invasive hormone receptor-positive breast cancer while taking an agent such as tamoxifen for breast cancer risk reduction
  • Women who cannot take tamoxifen because of a high risk of side effects
  • Women who have tried tamoxifen and had to stop because of side effects
  • Any woman who has gone through menopause and has hormone receptor-positive breast cancer who would prefer to take an aromatase inhibitor instead of tamoxifen as initial treatment

Those who might be advised NOT to consider using an aromatase inhibitor include:

  • Women with hormone receptor-negative breast cancer (These women should also not take tamoxifen.)
  • Women who experience side effects while taking an aromatase inhibitor
  • Women who have not gone through menopause and have functioning ovaries
  • Women who have gone through menopause and have hormone receptor-positive breast cancer who would prefer to start with tamoxifen and then switch to an aromatase inhibitor after a number of years

Women who have not gone through menopause at the time of diagnosis should generally not use an aromatase inhibitor as initial adjuvant hormonal therapy outside of a clinical trial, even if they have gone through menopause because of cancer treatment (such as chemotherapy or the suppression or removal of the ovaries). In fact, women whose monthly menstrual periods have stopped may not have gone through menopause, so women should discuss their menopausal status with their doctor. At this time, clinical trials have not yet shown that aromatase inhibitors are effective in this group of women.

Hormones Used to Treat Breast Cancer

Aromatase inhibitor medications are taken orally, once a day, for up to 5 years. For women with advanced (metastatic) disease, the medicine is continued as long as it is effective.

  • Arimidex is approved for women with early-stage disease right after surgery.
  • Aromasin is approved for women with early-stage disease who have completed 2-3 years of tamoxifen.
  • Femara is approved for women with early-stage disease right after surgery, as well as for women with early-stage disease who have completed 5 years of tamoxifen.

Other Hormonal Therapies for Breast Cancer

Other treatments include megestrol acetate (a progesterone-like drug), fluoxymesterone (a male hormone like testosterone) and ethinylestradiol (an estrogen drug that is effective if it is given in high doses). Other hormones used include:

  • Fareston (generic: toremifene)
  • Faslodex (generic: fulvestrant)
  • Lupron (generic: luprolide)
  • Megace (generic: megesterol)
  • Nolvadex (generic: tamoxifen)
  • Zoladex (generic: goserelin)

For more information about breast cancer treatment and prevention at Fox Chase Cancer Center or to make an appointment, call 1-888-FOX CHASE (1-888-369-2427). The breast cancer scheduling department can be reached at 215-728-3001.

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