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ATLANTA, June 3 — Five years of breast cancer adjuvant therapy with , an aromatase inhibitor, can lead to a 6% to 7% loss of bone mineral density, a trigger for osteoporosis for women who are borderline osteopenic when they begin treatment, researchers reported here.

For women with normal bone density at baseline, however, five years of is unlikely to cause osteoporosis, said investigators with the Arimidex, Tamoxifen, Alone or in Combination (ATAC) trial, who reported the findings today at the American Society of Clinical Oncology meeting here.

Women taking Arimidex had about a 40% increase in risk of fractures while on the study, said Robert E. Coleman, M.D., Weston Park Hospital in Sheffield, England. "When the drug was discontinued the excess fracture risk declined," he added.

The overall fracture rate was 7.7% among women taking tamoxifen in the study versus 11% for women taking Arimidex. Dr. Coleman said the difference was significant, but did not report the P value. There was, he said, no statistically significant difference in the rate of hip fractures between women taking Arimidex and those taking tamoxifen.

The rate of bone mineral density loss decelerates over time so that the rate at two to five years was significantly less than the loss over the first two years of treatment, (mean difference in annual rate of change 0.0113, 95% CI 0.006-0.0017; P=0.0002).

Five patients with osteopenia at baseline developed osteoporosis by five years, he said.

After five years, women taking Arimidex lost 6.1% of bone mineral density at the hip and 7.2% at the spine versus losses of 2.8% at the hip and 0.7% at the spine in the tamoxifen group (P<0.0001).

The ATAC trial recruited postmenopausal women with invasive primary breast cancer and randomized them to adjuvant treatment with Arimidex, tamoxifen, or combination therapy. The trial reported that Arimidex was superior to tamoxifen or combination therapy for preventing recurrence of breast cancer.

This 68-month subset analysis compared bone mineral density between 81 women treated with Arimidex and 86 women treated with tamoxifen. Lumbar spine and total hip bone mineral density were measured by dual-energy x-ray absorptiometry at baseline, one year, two years, and five years.

On the basis of these results, Dr. Coleman recommended that women taking Arimidex should have their bone mineral density monitored every one to two years. "They should also take calcium and vitamin D supplements during Arimidex therapy."

Oncologist Julie Gralow, M.D., of the University of Washington and Fred Hutchinson Cancer Center in Seattle said that women should also consider other ways to improve bone strength, "such as exercise and use of bisphosphonates."

J. Leonard Lichtenfeld, M.D., deputy medical director at the American Cancer Society, said the results should not be surprising because oncologists since have been long concerned about possible bone loss with aromatase inhibitors.

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