Breast cancer prevention drug may decrease bone density

01 February 2012

February 2012 – A new study has found that a drug shown to be highly effective in preventing breast cancer in women who are at high risk for the disease appears to worsen age-related bone loss, although the clinical implications of the findings are unclear.

In June 2011, the results of a Canadian-led clinical trial showed that, for postmenopausal women at increased risk of developing breast cancer, the drug exemestane reduced the risk by 65 %, compared with placebo. The new study found that exemestane worsens age-related bone loss by about three-fold compared to placebo.

The new study involved 351 postmenopausal women, with a median age of 61 years, who were not previously diagnosed with osteoporosis, were not on bone medications and were taking calcium and vitamin D supplements. The research team measured bone mineral density using high-resolution CT scans to assess the density or bone strength. After two years, the researchers found a 7.9 % loss of bone density in the exemestane group compared to a 1.1 % loss in the placebo group.

The findings were published in the February 7 online edition of The Lancet Oncology.

“These findings provide important additional information about the risks and benefits of potentially using exemestane for breast cancer prevention,” says Dr Christine Williams, Vice-President Research, Canadian Cancer Society. “However, follow-up studies will need to be done to investigate the risk versus benefits of taking exemestane. In particular we need to determine what clinical implications these observed changes in bone density may have for women, specifically whether women are at higher risk for developing osteoporosis or fractures if they take exemestane.”

Exemestane belongs to a class of drugs called aromatase inhibitors. Aromatase inhibitors prevent the conversion of androgens to estrogens, which is the primary source of estrogen production in postmenopausal women. Estrogen is a major growth factor in hormone-receptor positive breast cancers.

In Canada, exemestane is approved for reduction of risk of breast cancer recurrence but not as a breast cancer prevention drug. The new study provides additional evidence to inform the development of clinical recommendations for use of exemestane in breast cancer prevention.

The study was funded by the Canadian Breast Cancer Research Alliance (CBCRA), of which the Canadian Cancer Society was a founding and funding partner. The CBCRA partnership ended in April 2010.

The Society is currently funding this investigator team to do additional research about the longer term effects of exemestane on bone density.