Blackwell said the importance of those proteins has long been recognized, but until now, there hasn't been a drug that could safely block them.
"This represents a brand-new way in slowing down breast cancer progression," Blackwell said.
The combination did cause side effects, including fatigue and neutropenia -- a decrease in important disease-fighting white blood cells. But these "were manageable side effects," Finn said.
Still, Blackwell said, the safety and ultimate effectiveness of the therapy "remain to be validated."
According to the American Cancer Society, the average U.S. woman has a 12 percent chance of developing breast cancer in her lifetime. Death rates from the disease have dropped in recent decades because of better treatments and earlier detection, experts said.
Learn more about breast cancer from the American Cancer Society.
SOURCES: Richard Finn, M.D., associate professor, medicine, University of California, Los Angeles, Jonsson Comprehensive Cancer Center; Kimberly Blackwell, M.D., director, Duke Breast Cancer Clinic, and professor, medicine, Duke University School of Medicine, Durham, N.C.; Dec. 5, 2012, presentation, San Antonio Breast Cancer Symposium, San Antonio, Texas
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