WASHINGTON—A long-term study comparing two popular drugs that help to prevent breast cancer suggests the older one is significantly more effective over time.
But both tamoxifen and Eli Lilly & Co.'s newer Evista cut high-risk women's risk of developing breast cancer, according to results presented Monday at a meeting here of the American Association for Cancer Research. Tamoxifen, while more effective, also brought increased risk of side effects.
The federally funded study followed 19,490 women for almost seven years during and after their treatment with tamoxifen, an off-patent drug used to treat cancer, and Evista, which was developed as an osteoporosis drug and is generically known as raloxifene.
The drugs are used to fend off breast cancer in post-menopausal women at high risk of the disease. Only 5% to 20% of the tens of thousands who could benefit from the medicines use them, according to researchers.
In 2006, earlier published results of the study, known as STAR, found that both drugs cut the risk of breast cancer by 50% in high-risk, postmenopausal women. In those results, women taking tamoxifen had increased risk of certain side effects, such as blood clots.
Monday's findings suggest tamoxifen is significantly better than raloxifene at preventing breast cancer several years after treatment, which generally lasts about five years. Over the longer time period, tamoxifen cut the risk of invasive breast cancer by 50%, while raloxifene's effectiveness dropped to 38%. Raloxifene did match tamoxifen in preventing noninvasive breast cancer.
Raloxifene users continued to have significantly fewer side effects, including blood clots and uterine cancer, compared with those who took tamoxifen.
"Tamoxifen is still a little bit better," said D. Lawrence Wickerham, who presented the data and helps leads a federally funded research network on breast and colorectal cancer. But "raloxifene remains an effective way to prevent breast cancer and does it with far less toxicity," said Dr. Wickerham, who serves as a consultant to Eli Lilly.
Unless women have a known risk for blood clots or uterine cancer, they can consider both drugs as breast-cancer prevention options, according to Dr. Wickerham. Raloxifene may be particularly beneficial for those post-menopausal women with fragile bones because it offers them a "two-for-one benefit," he said.
But not all experts agree that raloxifene is just as good an option as tamoxifen.
"Tamoxifen is more protective than raloxifene," said Mary Daly, chairwoman of clinical genetics Fox Chase Cancer Center site in Philadelphia, who ran the study at that site. "Raloxifene is falling behind. The longer we follow these women, the more that divide will grow."
These drugs work by acting like the hormone estrogen in some organs and by blocking the effect of estrogen in others. In the breast, both block estrogen's action, which prevents the progress of breast cancer or pre-malignant changes in the breast.
However, in the lining of the uterus, tamoxifen acts like estrogen. This may be the reason women taking it experience higher rates of endometrial cancer.
Tamoxifen stays in the body much longer than does raloxifene, which might be the reason for the difference in the drugs' effectiveness, said researchers.
Patricia Ganz, another study investigator and director of cancer prevention and control research at the Jonsson Comprehensive Cancer Center at the University of California, Los Angeles, said she would incorporate the new findings into her practice by leaning toward tamoxifen for preventing breast cancer in high-risk women, those with a family history of the disease or who have had a biopsy to look at abnormally growing cells, or those with no uterus.
But if a woman had an elevated risk of blood clots or couldn't tolerate the side effects of tamoxifen, Dr. Ganz said she would suggest raloxifene.
"The updated results of the STAR trial provide important information to help guide patients and health-care professionals in weighing the risks and benefits of medications available to reduce the risk of invasive breast cancer," said Teeresa Shewman, a spokeswoman for Eli Lilly. "Lilly stands behind the safety profile and efficacy of Evista."
Eric Winer, director of the breast oncology center at Dana-Farber Cancer Institute in Boston, who wasn't involved in the study, agrees that there are more women in the U.S. who could benefit from these treatments than currently use them if they understood more about them.
"It doesn't mean that everyone should take it," said Dr. Winer. But, women "deserve to hear more."
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