THURSDAY, June 24 (HealthDay News) -- Bone-building drugs known as bisphosphonates appear to reduce the risk of invasive breast cancer by around 30 percent, two new studies show.

"If a woman is considering bisphosphonate use for bone, this might be another potential benefit," said Dr. Rowan T. Chlebowski, a clinical oncologist at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center in Torrance, Calif. He is the lead author of one of the two studies on the topic, published online this week in the Journal of Clinical Oncology.

The findings were first presented late last year at the San Antonio Breast Cancer Symposium, but Chlebowski said the results now have the benefit of having been peer-reviewed before publication for scientific accuracy.

Chlebowski and his colleagues looked at nearly 155,000 women who participated in the Women's Health Initiative (WHI) study, evaluating the 2,816 women who took oral bisphosphonates at the study start and comparing them to women who did not.

Ninety percent of the women who were taking the bone-building drugs took alendronate (Fosamax), according to the study.

After nearly eight years of follow-up, Chlebowski found invasive breast cancer incidence was 32 percent lower in those on bone-building drugs, with ER-positive cancers reduced by 30 percent. The incidence of ER-negative cancers in those on bisphosphonates also decreased, but not by enough to be statistically significant.

The incidence of early, noninvasive breast cancers, known as ductal carcinoma in situ, was 42 percent higher in bisphosphonate users, so the bisphosphonates could somehow be selectively affecting invasive cancers, Chlebowski postulated.

In a second study, conducted in Israel, researchers looked at 4,039 postmenopausal women, including some who took bisphosphonates and some who did not. Those who took the drug longer than a year had a 39 percent reduced risk of breast cancer; after adjusting for factors such as age and family history, there was still a risk reduction of 28 percent.

Exactly how the drugs reduce risk isn't known. Chlebowski speculated that the drugs may block the release of growth factors that would encourage tumors to grow or may block blood vessel formation within a tumor.

It's known that low bone mineral density (BMD) is linked with a reduced risk of breast cancer, and women with low BMD are likely to be on the drugs. So for the study analysis, Chlebowski adjusted for this possible confounding effect by incorporating a hip fracture risk score to take into account the bone mineral differences between drug users and non-users.

Another expert, Dr. Joanne Mortimer, director of the women's cancers program at the City of Hope Comprehensive Cancer Center in Duarte, Calif., pointed out that the studies found an associative link, not a cause-and-effect, so it's not definitive.

However, she said, "for people with osteoporosis, it's one more reason to feel comfortable taking a bisphosphonate."

Like other medications, the drugs have favorable and unfavorable effects. For instance, researchers recently found women on the bone-building drugs can have a higher risk of an uncommon fracture; that research is being evaluated further, Mortimer said.

>From the two studies, however, Mortimer said, it appears that "these drugs change the environment in such a way that cancer cells are less likely to take root and grow, not only in the bone marrow but elsewhere as well."

In an accompanying editorial, Dr. Michael Gnant, of the Medical University of Vienna, said future studies will help pinpoint the benefit of the drugs in breast cancer incidence reduction and supply more answers as to their best use.

Chlebowski reported that he has been a consultant to Novartis and Amgen, which make the bone-building medications.

More information

To learn more about osteoporosis, visit the National Osteoporosis Foundation.