Doctors said the finding that exemestane, a Pfizer drug, could prevent breast cancer will provide a new option for women who have mostly shied away from taking other drugs aimed at preventing breast cancer because of rare, but serious side effects. Today, only a tiny fraction of the women who could potentially benefit from such medications take advantage of them, and researchers said increased adoption of risk-reducing drugs could benefit millions of women.
"There was a 65 percent reduction in the risk of breast cancer -- a pill that can do that to the commonest cancer that affects women globally and kills women globally; there's no such pill that I know of for any kind of cancer," Dr. Paul E. Goss, director of the breast cancer research program at Mass. General and lead author of the study, said in an interview before presenting the findings at the meeting of the American Society of Clinical Oncology in Chicago.
"We haven't seen any serious toxicity that might stifle someone's decision to try and take this drug," said Goss, who has received honoraria from several pharmaceutical companies, including a one-time speakers fee from Pfizer. The paper was also published online today by the New England Journal of Medicine.
The seven-year-long study followed 4,560 women from the United States, Canada, Spain, and France who were post-menopausal and had at least one risk factor for breast cancer, which could include being over age 60 or having a breast biopsy result that showed they were at higher risk. Women were randomly assigned to one of two groups: one group received a placebo, and the other took exemestane, which is already approved to treat women after they're diagnosed with breast cancer.
After a median follow up of three years, there were 11 invasive breast cancers among the women receiving exemestane compared with 32 in the placebo group, and there were also fewer precursor lesions found in women taking the drug. There were some side effects: More women on exemestane than placebo suffered hot flashes, fatigue, sweating, and insomnia. The rate of bone fractures, osteoporosis, and cardiovascular effects was the same in both groups. The study was funded by the Canadian Cancer Society and Pfizer.
Already, tamoxifen and raloxifene have been approved for prevention of breast cancer, but both carry rare but serious side effects: uterine cancer and blood clots for tamoxifen, and clots for raloxifene. Doctors said those risks have been a major barrier to women who could benefit from the drugs. Both drugs reduce risk of developing invasive breast cancer by about half. This study did not compare exemestane head-to-head with tamoxifen and raloxifene, so it isn't certain that exemestane is superior. Tamoxifen is approved for preventing breast cancer in women at high risk, not just post-menopausal women.
"I think it is very significant," said Dr. Therese Bevers, director of the cancer prevention center at MD Anderson Cancer Center, commenting on the new study. Because of the rare, though serious side effects associated with existing therapies, "it's a hard sell, and this is exciting -- that this drug appears to be more effective than what tamoxifen and raloxifene do, and doesnít have serious toxicities."
Bevers, who was not involved in the research, cautioned that it would be important to follow the women for longer to watch for longer-term complications, especially because the drug is known to affect bone density.
Still, another barrier to the use of preventive drugs in breast cancer is yet to be overcome. Both Goss and Bevers pointed out that while many people are comfortable taking drugs to reduce their cardiovascular risks by taking pills that control their blood pressure or cholesterol, patients may be more willing to take those drugs because they can measure their own progress.
Patients can return six months later and find out that they're doing well, whereas there is not yet a way for doctors to show patients they are reducing their risk for breast cancer.
"We donít have in breast cancer what they have -- numbers they can follow. They can see, 'Oh my goodness, itís working,' " Bevers said. "What do I have to show the woman when she comes back in six months or a year other than that she doesnít have breast cancer."
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