Role of Aromatase Inhibitors in Management of Early Stage Breast Cancer
by Dr. C.H. Weaver M.D. updated 10/2020
According to a combined analysis of previous studies, adjuvant (post-surgery) treatment with an anti-aromatase drug or aromatase inhibitor (AI) results in fewer recurrences than treatment with tamoxifen (Nolvadex®) among post-menopausal women with early, hormone receptor-positive (HR+) breast cancer.
Treatment of HR+ breast cancer often involves hormonal therapies that suppress or block the action of estrogen. These therapies include tamoxifen as well as agents known as aromatase inhibitors. Tamoxifen acts by blocking estrogen receptors, whereas aromatase inhibitors suppress the production of estrogen in postmenopausal women.
Several large trials comparing AI's to tamoxifen have established the efficacy and safety of aromatase inhibitors for the treatment of hormone receptor-positive breast cancers among postmenopausal women. Follow-up of these trials continues.
To further explore the effects of aromatase inhibitors, researchers affiliated with the Aromatase Inhibitors Overview Group conducted a combined analysis of previous studies, including the following six randomized trials:
- Arimidex, Tamoxifen, Alone or in Combination (ATAC)
- Breast International Group (BIG) 1-98/International Breast Cancer Study Group (IBCSG) 18-98
- Austrian Breast and Colorectal Cancer Study Group (ABCSG)
- German Austrian Breast Cancer Group (GABG)/Arimidex-Nolvadex (ARNO)
- Intergroup Exemestane Study (IES)/BIG 2-97
- Italian Tamoxifen Anastrozole (ITA)
Some of these studies compared five years of treatment with an aromatase inhibitor to five years of treatment with tamoxifen, and others evaluated the effect of switching to an aromatase inhibitor after two to three years of tamoxifen.
The aromatase inhibitors that were evaluated were
In the studies that compared five years of tamoxifen to five years of an aromatase inhibitor, the following results were reported:
- Over the course of five years, 9.6% of women treated with an aromatase inhibitor experienced a cancer recurrence compared with 12.6% of women treated with tamoxifen.
- There was no statistically significant difference in survival between the two groups.
In the studies that evaluated a switch from tamoxifen to an aromatase inhibitor, the following results were reported:
- Over the course of six years, 12.6% of women who switched to an aromatase inhibitor experienced a cancer recurrence compared with 16.1% of women who remained on tamoxifen.
- Women who switched to an aromatase inhibitor also had modestly improved survival compared with women who remained on tamoxifen.
The researchers concluded that these results provide clear evidence that aromatase inhibitors significantly reduce recurrences in early, hormone receptor-positive, postmenopausal breast cancer. There was also some evidence that switching to an aromatase inhibitor after two to three years of tamoxifen improved survival compared with continued treatment with tamoxifen. The duration of follow-up in these studies is still somewhat short, however; longer follow-up may provide additional information about effects on survival.
- Ingle J, et al. Aromatase inhibitors versus tamoxifen as adjuvant therapy for postmenopausal women with estrogen receptor positive breast cancer: meta-analysis of randomized trials of monotherapy and switching strategies. San Antonio Breast Cancer Symposium. December 10-14, 2008. Abstract 12.