SAN FRANCISCO—A surprisingly high percentage of women with breast cancer who start adjuvant therapy with an aromatase inhibitor stop taking these drugs because of associated musculo-skeletal side effects, researchers reported at the first annual ASCO Breast Cancer Symposium (abstract 227).

While this finding is unlikely to change prescribing habits, it should spur physicians to pay more attention to pain complaints in these patients.

The study found that 13% of women with estrogen-receptor-positive, stage I-III breast cancer assigned to treatment with either exemestane(Drug information on exemestane) (Aromasin) or letrozole(Drug information on letrozole) (Femara) stopped AI treatment entirely due to musculoskeletal side effects an average of 6.1 months after starting treatment (range, 2.2 to 13 months). (The study did not compare the rate of side effects between the two AIs.)

"This was the most surprising finding of our study," said lead investigator N. Lynn Henry, MD, PhD, of the University of Michigan Comprehensive Cancer Center, Ann Arbor. "Thirteen percent is approximately the percentage of patients who typically discontinue a drug for all reasons combined, and this was just for musculoskeletal complaints," she said.

COBRA substudy

This NIH-funded Consortium on Breast Cancer Pharmacogenomics (COBRA) substudy included 100 patients; about half had previously taken tamoxifen(Drug information on tamoxifen). Patients completed a Health Assessment Questionnaire (HAQ) and Visual Analog Scale (VAS) at baseline, 1, 3, 6, 12, and 24 months to assess changes in pain and function. Those who exceeded a predefined threshold based on HAQ and/or VAS score were referred for detailed rheumatology evaluation.

"It is important to realize that these patients were referred due to responses to a questionnaire, not because the oncologist felt their pain was severe," Dr. Henry said.

Of the 100 patients, 42 (42%) reported musculoskeletal side effects and were referred for rheumatology evaluation. Detailed data were available for 38 of these women. The most common musculo-skeletal side effects reported were rotator cuff tendonitis (8 patients), carpal tunnel syndrome (9), and osteoarthritis (12).