Almost 93 percent of women on exemestane (Aromasin) remained free of breast cancer after five years, compared to about 89 percent of the women on tamoxifen. That's according to the study of nearly 4,700 women with breast cancer who all had their ovarian function suppressed.
However, the researchers found no differences in overall survival between the two drugs.
And both drugs came with significant side effects. For Aromasin, those side effects included osteoporosis, vaginal dryness and decreased sex drive, among others. For tamoxifen, side effects included blood clots, hot flashes and urinary incontinence, according to the study.
The study is published in the July 10 issue of the New England Journal of Medicine. The findings were also presented at the American Society of Clinical Oncology annual meeting in June.
Some breast cancers -- dubbed hormone receptor-positive -- are fueled by the hormone estrogen. Tamoxifen works by blocking estrogen from getting into cancer cells. Aromasin -- part of a class of drugs known as aromatase inhibitors -- works by stopping other hormones from changing into estrogen, according to the American Cancer Society. Aromatase inhibitors are only helpful if the ovaries aren't still producing estrogen.
That's why aromatase inhibitors have traditionally been used in postmenopausal women, and have been shown to work better for them than tamoxifen. Tamoxifen is typically used in breast cancer patients who have not yet gone through menopause because it can block estrogen that's circulating in the body.
Now, the new study suggests that aromatase inhibitors may be an option for premenopausal women, although not everyone thinks the new research will greatly change practice.
"It's another option," said Dr. Joanne Mortimer, director of women's cancer programs at the City of Hope Comprehensive Cancer Center in Duarte, Calif. She reviewed the findings and said that the aromatase inhibitor would be a trade-off. "Compared to aromatase inhibitors, the quality of life is better with tamoxifen for premenopausal women," she said.
The study was funded by the U.S. National Cancer Institute, Pfizer (which makes Aromasin), Ipsen (makers of an ovarian suppression drug) and the International Breast Cancer Study Group.
The study researchers combined the results of two phase 3 clinical trials, known as the Tamoxifen and Exemestane Trial (TEXT) and the Suppression of Ovarian Function Trial (SOFT).
All of the women in the studies had hormone receptor-positive breast cancer. They also all had their ovarian function suppressed to further thwart cancer growth.
Both studies included analysis of five years of treatment over a median follow-up of more than 5.5 years.
Compared to tamoxifen, Aromasin reduced the risk of breast cancer recurrence by 34 percent.
But, women taking Aromasin experienced more osteoporosis -- a condition caused by thinned bones. The rate of osteoporosis was 13.2 percent in the Aromasin group versus 6.4 percent in the tamoxifen group. Women taking Aromasin also reported more fractures, vaginal dryness, decreased sex drive and painful sexual intercourse. But, women on tamoxifen had more blood clots, hot flashes, sweating and urinary incontinence, according to the study.
Dr. Hope Rugo, clinical professor of medicine and director of the breast oncology and clinical trials program at the Helen Diller Family Comprehensive Cancer Center, University of California, San Francisco, wrote an editorial to accompany the study. The aromatase inhibitor led to more sexual problems, osteoporosis, muscle and joint symptoms, she said. These "may be more noxious to young women than the hot flushes experienced with tamoxifen," she wrote.
About half of the women in both treatment groups reported depression, she noted.
While the researchers didn't find a difference in overall survival, it may be too soon to see it, Rugo said. She believes a difference in overall survival might become evident, because the aromatase inhibitor was better at lowering recurrence or a second cancer.
The best use of the aromatase inhibitors in premenopausal women might be in those who have "higher-risk disease," she said, such as those with bigger tumors or more involvement of the lymph nodes.
Cost differences also could play a role in deciding on one drug over another, Rugo added. "Tamoxifen is very cheap," she said, but exemestane is more expensive.
To learn more about hormone treatment for breast cancer, visit the U.S. National Cancer Institute.