Researchers found that a combination of drugs increased breast cancer survival in postmenopausal women.
The results of the phase III trial, released at a meeting of breast cancer experts in San Antonio, Texas, show the first benefit for women with hormone receptor-positive metastatic breast cancer in 10 years, study authors said.
Both drugs -- anastrozole (Arimidex) and fulvestrant (Faslodex) -- are currently used to treat breast cancer but they have typically been administered separately, not together.
The results of the randomized, controlled SWOG trial showed that the two anti-estrogen drugs given together extended the median survival time by more than six months compared to standard treatment with anastrozole only.
The study began in 2004 and included 707 postmenopausal women with metastatic hormone receptor-positive breast cancer.
All the women took a daily oral dose of anastrozole, which works by reducing production of tumor-promoting estrogen.
About half of the women also got a 250 milligram injection of fulvestrant every 28 days. Fulvestrant works by interfering with the receptors that allow estrogen to signal cells to grow and reproduce.
Lead study coordinator Rita Mehta of the University of California, Irvine, Medical Center said the two drugs together showed an impressive impact on a subtype of breast cancer that accounts for more than half of all cases.
"If we take away estrogen and the estrogen receptor, the two together should be better than just doing one at a time," Mehta said.
"These patients have not had a new treatment that gave them an overall survival benefit in more than a decade."
Women in the trial who received just anastrozole survived a median of 41.3 months. Women who received the two in combination survived a median of 47.7 months.
Researchers said side effects were generally similar in both groups, though the most severe side effects, including one stroke and two pulmonary embolisms, were seen in the combination group.
"This most likely will change the standard of care for how we treat these patients," said study co-author Kathy Albain of Loyola University Medical Center.