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Study Finds Women Stop Taking Breast Cancer Drug Due to Side Effects

A recent study shows that some women taking commonly prescribed drugs to treat breast cancer tend to stop their treatment due to side effects. The study focused two specific drugs, Aromasin (generic name, exemestane) and Femara (generic name, letrozole). These drugs are often prescribed after breast cancer surgery, chemotherapy or radiation therapy to help prevent a recurrence of the disease. In the study, 13% of women stopped taking the drugs due to side effects such as joint and muscle pain. Tamoxifen is another drug used to treat breast cancer, may be another option for women suffering side effects from these drugs, according to the researchers.

Aromatase inhibitors are a class of breast cancer drugs that work by binding to the body's aromastase enzyme, an enzyme responsible for producing estrogen. Many breast cancer cells depend on estrogen to grow and multiply quickly. Once the aromatase inhibitor has binded to the aromastase enzyme, estrogen cannot be produced by the enzyme. This lack of estrogen starves cancer cells, preventing them from growing and dividing. Recent studies suggest that some aromatase inhibitors may be more effective than tamoxifen in treating advanced breast cancer or may be useful after patients become resistant to tamoxifen.

There are several aromatase inhibitors that are used to help treat advanced breast cancer including:

  • Aromasin (generic name, exemestane)
  • Femara (generic name, letrozole)
  • Arimidex (generic name, anastrozole)
  • Megace (generic name, megestrol)

Tamoxifen belongs to a class of drugs called SERMs (selective estrogen-receptor modulators). SERMs mimic estrogen, and are able to bind to estrogen receptors in breast cancer cells. By binding to these receptors, they block estrogen from breast cancer cells, thereby starving the cancer cells. Tamoxifen is the most commonly prescribed drug to treat breast cancer.

In the current study, Lynn Henry, M.D., Ph.D., of the University of Michigan Comprehensive Cancer Center, and colleagues studied how genetics play a role in the way drugs work in patients. One hundred breast cancer patients were given either exemestane or letrozole. The researchers tracked the women for six months, during which time the women completed questionnaires about their health and side effects.

In the study, 13% of the women dropped due to musculoskeletal problems, such as joint and muscle pain. The symptoms tended to appear soon after treatment began and included tendonitis in the shoulder or wrist, inflammation in the knees or arthritis-type symptoms in the hands or hips, in addition to joint and muscle pain. The symptoms almost always improved after treatment was halted.

"We know 25% to 30% of women taking aromatase inhibitors have aches and pains. What was surprising here was the number of people who actually discontinued the drugs because of the side effects," said Dr. Henry, in a Science Daily news release. The results of the study were presented on September 8, 2007 at the annual meeting of the American Society of Clinical Oncology.

Dr. Henry and her colleagues are trying to determine whether switching to another drug might help women who experience these types of symptoms from aromatase inhibitors. They are also examining ways to treat the symptoms.

"Tamoxifen has been around 20-30 years and has a long track record. We know about its benefits and its risks. Aromatase inhibitors are new, and we don't have as much experience with them. We have to see in the long term which one ends up being better," said Dr. Henry, in the news release. She noted that tamoxifen might be a better option for some women who experience these side effects.

Additional Resources and References

  • The study, "Aromatase inhibitor (AI)-induced musculoskeletal toxicity in a prospective clinical trial: A Consortium on Breast Cancer Pharmacogenomics (COBRA) study," was presented at the 2007 annual meeting of the American Society of Clinical Oncology.
  • The September 6, 2007 Science Daily news release, "Thirteen Percent Of Women Stop Taking Breast Cancer Drug Because Of Side Effects, Study Finds," was published on its website,
  • To learn more about breast cancer drugs, please visit