For postmenopausal women with hormone-sensitive early breast cancer, switching from tamoxifen (after two years of treatment) to Arimidex may provide more cancer protection than staying on tamoxifen, new research shows.

That finding, published in The Lancet, is based on data from two trials. Researchers reviewing the trials' data included Raimund Jakesz, MD. He is a member of the Austrian Breast and Colorectal Cancer Study Group and a professor at Austria's Vienna Medical University.

Study's Findings

About 3,200 postmenopausal women participated. All had early, hormone-sensitive breast cancer and had taken tamoxifen for two years.

Tamoxifen is an estrogen-blocking drug that has been a cornerstone of breast cancer treatment for more than 20 years.

Having taken tamoxifen for two years, the women were given either tamoxifen or a newer breast cancer drug, Arimidex. After about two years, there were 40% fewer cancer recurrences in the Arimidex group, write the researchers.

There were 67 cancer recurrences in the Arimidex group and 110 in the tamoxifen group. These occurred in either the other breast or in other areas of the body.

Women taking Arimidex had more bone fractures and fewer blood clots than those taking tamoxifen, the researchers note.

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About the Drugs

Tamoxifen and Arimidex both address estrogen, which fuels some breast cancers.

The drugs work differently. Tamoxifen blocks estrogen from docking at receptors on cells. It's like a hefty bodyguard standing by the receptor door, swatting estrogen away.

Arimidex cuts estrogen production. That way, there's less estrogen available to fuel new breast cancer. Arimidex is only approved for women who have completed menopause.

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Five-Year Timeline

Breast cancer treatments include surgery, chemotherapy, and radiation. After that, tamoxifen may be taken for five years to help prevent breast cancer's return.

Why five years? That's how long tamoxifen's benefits seem to outweigh its risks.

Unfortunately, women who have ever had breast cancer stay at higher-than-normal risk of the disease for the rest of their lives. So the hunt has been on for drugs that extend protection.

Arimidex is in a group of breast cancer drugs called aromatase inhibitors. Studies are probing whether those drugs can piggyback on tamoxifen's work or replace tamoxifen. Another aromatase inhibitor, Femara, fared well compared with tamoxifen in a study published earlier this year. Which drug and schedule is best is not yet known.

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Researchers' Verdict

The researchers call for more studies. But they write that their studies "lend support" to switching from tamoxifen to Arimidex in postmenopausal women with early breast cancer after two years of taking tamoxifen.

They don't recommend Arimidex instead of tamoxifen in newly diagnosed patients. The studies only included women who had already taken tamoxifen for two years.

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Second Opinion

The result is "important," writes Anthony Howell, BSc, MBBS, MSc, FRCP, in The Lancet. Howell didn't work on the study, but he wrote an editorial about it. Howell works at the Cancer Research U.K. Department of Medical Oncology at Christie Hospital NHS Trust in Manchester, England.

"The aromatase inhibitors improve substantially on tamoxifen, which is, in turn, substantially superior to placebo," writes Howell. He notes controversy about whether aromatase inhibitors should totally replace tamoxifen.

Howell writes that aromatase inhibitors are regarded as having a better side-effect profile than tamoxifen because of significantly reduced rates of uterine cancers, clotting problems, and hot flashes. This must be set against the increased rates of fractures and joint aches, and vaginal dryness and painful sex, which can be troublesome in a smaller number of patients, he writes.

Howell notes that aromatase inhibitors seem to have broadly similar side effects, apart from apparent small increases in heart toxicity with Femara and Aromasin, an effect not apparent with Arimidex. "These apparent increases might be the play of chance but clearly need to be monitored carefully," writes Howell.

"Until we have answers to all our questions, we should base the decision of when to use an aromatase inhibitor, and which to use, on the availability and maturity of data in the appropriate clinical setting," he writes.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Jakesz, R. The Lancet, Aug. 6, 2005; vol 366: pp 455-462. The Susan G. Komen Breast Cancer Foundation, "Treatment: Tamoxifen, Aromatase Inhibitors, and Other Hormonal Therapies." WebMD Medical News: "A Better Breast Cancer Drug?" WebMD Medical News: "New Breast Cancer Drugs May Beat Tamoxifen." WebMD Medical News: "Breast Cancer Drug Femara Beats Tamoxifen." News release, The Lancet.