Breast cancer prevention drug may cause bone loss

A new study shows that exemestane, an aromatase inhibitor used to prevent breast cancer in postmenopausal women, worsens age-related decreases in bone mineral density.

Researchers from the University Health Network in Toronto led by Angela Cheung, MD, FRCPC, PhD, reported the results of a bone-mineral and bone structure substudy of participants in the Mammary Prevention 3 (MAP.3) trial.

The MAP.3 randomized trial examined the effect of exemestane on preventing breast cancer in more than 4,500 healthy, postmenopausal women who had a high risk for developing the cancer, according to a press release. Three hundred fifty-one women without osteoporosis were included in the bone substudy. They were separated into a control group who received a placebo and a test group who received exemestane. Bone mineral density was measured by x-ray and CT scans.

Exemestane reduced the risk of developing cancer by 65% compared with the placebo group. However, after taking the drug for 2 years, the test group had a significant loss of bone mineral density at the distal radius and distal tibia compared with their levels at the start of the study. Additionally, the cortical thickness and area decreased by almost 8% in the test group compared with a 1% decrease in the control group. This finding is especially important because 80% of fractures in old age are the result of a loss of cortical bone.

“Women considering exemestane for the primary prevention of breast cancer should weigh their individual risks and benefits. For women taking exemestane, regular bone monitoring plus adequate calcium and vitamin D supplementation are important. Long-term studies are needed to assess the effect of our findings on fracture risk,” the authors stated in the release.

The study was published online ahead of print in Lancet Oncology.

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