Two new studies suggest that women who take the drugs, called bisphosphonates, are about one-third less likely to develop breast cancer than women who do not.
One study, an analysis of data on over 150,000 women involved in the Women's Health Initiative, showed that there were 31% fewer cases of breast cancer among women who took Fosamax or other oral bisphosphonates than among women who didn't. Other commonly used oral bisphosphonates are Boniva and Actonel.
The second study, involving more than 4,000 postmenopausal women in Israel, showed that those who had breast cancer were 29% less likely to have taken oral bisphosphonates for at least one year than women who did not have breast cancer.
The two studies used different methods to arrive at the same basic result, adding to the strength of the findings, says Indiana University's Theresa Guise, MD, who moderated a news briefing on the findings at the San Antonio Breast Cancer Symposium.
Still, the studies do not prove that the drugs prevent breast cancer, doctors here say. More definitive clinical trials in which half the women are given bisphosphonates and half are not and then they are followed over time to see how many in each group develop breast cancer should offer a clearer picture of the drugs' benefits within the year.
But "the idea that bisphosphonates may protect against breast cancer incidence is very exciting because there are about 30 million prescriptions for them written annually in the United States," says Rowan Chlebowski, MD, PhD, a medical oncologist at Harbor-University of California, Los Angeles Medical Center, who headed the new U.S. study.
"By protecting their bone health, women may also be protecting themselves against cancer," he tells WebMD.
Bisphosphonates and Breast Cancer Recurrence
The new research builds on a study presented at last year's breast cancer meeting showing that the intravenous bisphosphonate Zometa appears to prevent breast cancer from coming back.
Animal and lab research suggests that the drugs may fight breast cancer in a number of ways -- by directly killing tumor cells, by cutting off their blood supply, or by stimulating the immune system to mount an attack against the tumor, Chlebowski says.
In fact, there's every reason to expect that the drugs may protect against other types of cancer as well, he says, adding that future study testing that hypothesis is planned.
Using Women's Health Initiative data, Chlebowski and colleagues researchers compared breast cancer rates in 2,816 women who reported using oral bisphosphonates at the start of the study with those of 151,592 women who said they did not take the drugs.
The women were followed for an average of 7.8 years. During that time, 5,156 of the women developed breast cancer, with 64 of the cases among bisphosphonates users.
After taking into account the women's bone mineral density, this translated to a 31% lower risk of cancer among bisphosphonates users.
"Since bisphosphonates are prescribed for women with low bone mineral density -- and low bone mineral density has been associated with lower breast cancer incidence -- it's important to correct for that," Chlebowski says.
Israeli Breast Cancer Study
The second study involved more than 4,000 women in Israel, about half of whom had breast cancer. Using pharmacy records, the researchers determined whether the women had taken bisphosphonates.
After taking into account a variety of risk factors for breast cancer including age, ethnicity, fruit consumption, sports activity, family history of breast cancer, and body mass index, women who took the drugs for over a year were 29% less likely to have breast cancer than women who did not.
Taking the drugs for longer didn't appear to offer any more protection than taking them for one year, the study showed.
"Importantly, the tumors that developed among bisphosphonates users were more likely to be estrogen-receptor-positive," says study head Gad Rennert, MD, PhD, of the Technion-Israel Institute of Technology. Tumors that are fueled by estrogen have a better prognosis.