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A class of drugs called aromatase inhibitors may be more effective than the standard drug tamoxifen at treating breast cancer, according to studies presented at the 24th annual San Antonio Breast Cancer Symposium. In one study, the aromatase inhibitor Femara (generic name, letrozole) was found to shrink cancerous tumors and increase survival time more so than tamoxifen. Another study found that another aromatase inhibitor called Arimidex (generic name, anastrozole) appeared to reduce the risk of a breast cancer recurrence more than tamoxifen. The results of both studies offer hope that more effective breast cancer treatments may soon be available to women.

Tamoxifen is the standard drug used to treat breast cancer (and the only drug approved by the U.S. Food and Drug Administration to prevent the disease in high-risk women). Tamoxifen is an anti-estrogen and is effective against breast cancer cells that require the hormone estrogen in order to survive (so-called estrogen-receptor positive breast cancers). Tamoxifen works by blocking estrogen from estrogen receptors in cancer cells, thereby starving the cells. However, tamoxifen can also mimic the effects of estrogen in the body and therefore has been associated with side effects such as hot flashes and vaginal problems, as well as a slight increased risk of endometrial cancer (cancer of the uterine lining).

Post-menopausal women who take aromatase inhibitors, on the other hand, do not make much estrogen and therefore are not subject to many of the side effects associated with tamoxifen. Aromatase inhibitors are a class of drugs that work by also starving cancer cells of estrogen. However, the way in which they accomplish this is different from tamoxifen. Aromatase inhibitors block the production of an enzyme called aromatase, which converts the hormone androgen into estrogen. Researchers have been studying aromatase inhibitors in recent years as possible alternatives to tamoxifen.

At the annual San Antonio Breast Cancer Symposium, researchers presented findings on new studies on aromatase inhibitors. The study on Femara was conducted by Matthew Ellis, MD, of Duke University Medical Center, and his colleagues. The researchers compared the effects of Femara and tamoxifen (brand name, Nolvadex) in 324 post-menopausal women prior to breast cancer surgery. Dr. Ellis and his team found that after four months of treatment, 60% of women who were given Femara experienced shrinkage in the size of their cancer tumors compared to 41% of women who were given tamoxifen. Moreover, taking Femara allowed tumors to shrink enough in some women so they could avoid mastectomies and have less invasive surgery instead (however, in most instances, Femara would be given to women after surgery, not before surgery, as it was in the study). Femara also increased patient survival time over tamoxifen.

According to Dr. Ellis and his colleagues, the findings are exciting because Femara appears to be more effective at treating breast cancer than the current standard, tamoxifen. Femara may also work in women who become resistant to tamoxifen, said Dr. Ellis at the San Antonio Breast Cancer Symposium. Feasibly, Femara could also be used in conjunction with other breast cancer drugs, such as Herceptin (generic name trastuzumab), for more of an effect, or Femara could be a viable replacement for more toxic chemotherapy drugs. However, Dr. Ellis and his team cautioned that before Femara becomes standard treatment, its benefits must be confirmed in larger clinical trials.

In another study presented at the San Antonio Breast Cancer Symposium, Dr. Michael Baum of the University College Hospital in London, England and his colleagues found that the aromatase inhibitor Arimidex was more effective than tamoxifen at preventing a recurrence of breast cancer by 17%. The results were part of a large study called ATAC (Arimidex and Tamoxifen Alone or in Combination). After 30 months of treatment and 33 months of monitoring, the researchers found that 317 of 3,125 women who were given Arimidex experienced a cancer recurrence compared to 379 of 3,116 women who were given tamoxifen. Arimidex also worked better at preventing breast cancer in the opposite breast compared to tamoxifen.

Tamoxifen is the standard drug treatment for breast cancer and has been used in practice for over 20 years. Until future studies can conclusively show that other drugs are more effective than tamoxifen, it will continue to be the most common drug prescribed to breast cancer patients. However, the Femara and Arimidex studies show that emerging new therapies are on the horizon that appear to be promising advances in breast cancer treatment.

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