A newer drug clearly outperforms tamoxifen at preventing breast cancer from returning and should become the first-choice treatment for most women who have had the disease, doctors are reporting.
The drug, Arimidex, might be able to prevent 70 to 80 percent of the most common type of tumors that occur in women after menopause, compared to the 50 percent that tamoxifen is credited with warding off, their research suggests.
Women who took Arimidex for five years after they were treated for early-stage breast cancer were less likely to have cancer recur, develop in the other breast or spread throughout their bodies than women who took tamoxifen.
"Arimidex is a more effective treatment. This is a better drug," said Dr. Aman Buzdar, a specialist at the University of Texas' M.D. Anderson Cancer Center. He headed the U.S. portion of the study, which involved nearly 2,000 American women and an additional 7,300 from 20 other countries.
It was funded by Arimidex's maker, AstraZeneca PLC. Results were reported Wednesday at a meeting in Texas of breast cancer experts and were published online by the British medical journal The Lancet.
Tamoxifen revolutionized breast cancer treatment when it came into use some three decades ago. It blunts the effects of estrogen, a hormone that promotes the growth of about three-fourths of the tumors that occur in postmenopausal women.
Arimidex, known generically as anastrozole, is one of three newer drugs called aromatase inhibitors, which prevent estrogen from being made in the first place instead of just keeping it from entering cells. Excitement for Arimidex grew three years ago, when early results from this same large international study suggested it was better at preventing recurrence.
But many doctors were reluctant to recommend it instead of tried-and-true tamoxifen, which has long been available in cheap, generic form, on early results alone. Just last month, the world's largest group of cancer specialists, the American Society for Clinical Oncology, published new guidelines saying aromatase inhibitors were promising drugs that at some point should be part of most breast cancer patients' treatment, but not spelling out which drugs should be used for which women and when.
The new five-year results on Arimidex are the most definitive, finding that the drug improved disease-free survival by 26 percent over tamoxifen.
Yet they fall short of meeting the toughest standard for proving a drug's value — improving overall survival. Doctors in and outside the study say that women in the study had very early cancers and therefore the best possible prognosis, so seeing a difference between groups getting one or the other drug likely will take longer than five years.
In fact, 13 percent fewer cancer deaths occurred among Arimidex users, but the trend wasn't strong enough to say it couldn't have resulted from chance alone, Buzdar said.
Nevertheless, "I don't think you have to show a survival advantage to change practice habits," because of Arimidex's many other benefits, said Dr. Paul Goss, a Massachusetts General Hospital breast cancer expert who had no role in this study but has led others involving different aromatase inhibitors.
They include fewer cases of endometrial cancer, blood clots, hot flashes and vaginal bleeding and discharge than among tamoxifen users. Women on Arimidex had more joint pain and bone fractures, though the latter can be treated with other drugs.
The new findings on Arimidex for postmenopausal women don't change tamoxifen's status as the drug of choice for breast cancer cases that occur before menopause, because aromatase inhibitors aren't thought to be effective against them. Women whose tumors are not sensitive to the effects of estrogen also may be best off with tamoxifen.