Aromatase inhibitors can cause significant bone loss over the long term, according to new research presented in Atlanta at the annual meeting of the American Society of Clinical Oncology.

Although researchers have known that this new class of hormone therapy for breast cancer is harder on bones than tamoxifen, the new results are helping them understand how serious the differences are -- and will lead to ways to better manage this side effect.

The new findings come from a large international clinical trial known as ATAC (Arimidex and Tamoxifen, Alone or in Combination), which in 2004 showed that the aromatase inhibitor Arimidex (anastrozole) was better than tamoxifen at preventing breast cancer recurrences in women past menopause. The current study involved about 300 women from the original trial.

The researchers measured bone density in the women on both drugs at the beginning of the trial and after 5 years of hormone therapy. They found that women in the tamoxifen group had slightly improved bone density.

But women in the Arimidex group had about 6% reduced bone density in the lower back (lumbar spine) and about 7% reduced bone density in the hip. Normal bone loss for women in this age group over this period of time is only about 2%-3%, said lead study author Robert Coleman, MD, professor of medical oncology at Weston Park Hospital in Sheffield, England.