A new study has found that 13 per cent of women being treated for breast cancer stopped taking estrogen-blocking aromatase inhibitors because of side effects such as joint and muscle pain.

The findings of the study, conducted by researchers at the University of Michigan, will be presented Saturday at the 2007 Breast Cancer Symposium in San Francisco.

The study examined the first 100 post-menopausal women enrolled in a trial to see how genetics affects the way individuals metabolize drugs and experience side effects.

They had all been treated for early-stage hormone-responsive breast cancer and were taking the aromatase inhibitors exemestane or letrozole, which block the production of estrogen and inhibit tumour growth.

Many breast cancer tumours are fuelled by estrogen.

The study found that over six months, 13 per cent of the women stopped taking the drugs because of muscoskeletal problems, said N. Lynn Henry, a lecturer at the University of Michigan medical school.

"We know 25 per cent to 30 per cent 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," she said.

Side effects painful

The side effects typically came on just before the two-month mark. They included tendonitis in the shoulder or wrist, inflammation in the knees or arthritis-type symptoms in the hands or hips.

Some women reported joint pain while others had muscle pain, which they rated on questionnaires.

The researchers are looking at how to manage these side effects, because failing to continue with treatment can spell trouble, Henry told CBCNews.ca.

"If women stop taking their aromatase inhibitor prematurely and do not switch to a different hormone therapy, then they are at significantly increased risk of having their breast cancer come back," she said.

"However, if they switch to a different therapy, such as Tamoxifen, this increased risk is negligible. Therefore, we strongly recommend that women discuss any problems that they have with their physician rather than just stopping the medication on their own," she said.

Researchers now want to determine whether switching to a different aromatase inhibitor will prevent the side effects or whether taking Tamoxifen, another commonly prescribed breast cancer drug that also blocks estrogen but doesn't cause joint and muscle pain, is a better approach.

Resistance to drug

Since its approval by the U.S. Food and Drug Administration for the treatment of hormone-receptor positive breast cancer in 1977, Tamoxifen has become a mainstay of therapy. However, many women develop resistance to the drug over time, leading to cancer recurrence.

In addition, because Tamoxifen binds directly to the estrogen receptor, it can sometimes activate the signaling pathways it was designed to block, according to the National Cancer Institute.

Aromatase inhibitors, used mainly to treat postmenopausal women, take a different approach to hormone therapy. They prevent the production of estrogen rather than blocking its activity.

In 2007, an estimated 22,300 women will be diagnosed with breast cancer and 5,300 will die of it, according to the Canadian Breast Cancer Foundation.