A better chance at remaining cancer-free may be on the horizon for women past menopause who need preventive treatment for early-stage breast cancer, according to results of an ongoing clinical trial.

Presented this week at the 24th San Antonio Breast Cancer Conference, these initial results show the experimental hormone therapy drug anastrozole (Arimidex) may be somewhat better at preventing breast cancer recurrence in such women than tamoxifen, the drug most often used now for that purpose.

Researchers reported that women taking anastrozole were 17% less likely to have their breast cancer return than women taking tamoxifen.

This means that over a three-year period, 90% of patients taking anastrozole remained free of breast cancer compared with 88% of patients taking either tamoxifen or both drugs together, reported Michael Baum, MD, professor emeritus at University College in London, England, and a member of the Arimidex and Tamoxifen Alone or in Combination (ATAC) trial's steering committee.

The drug also caused fewer side effects than tamoxifen, according to the early data Baum presented.

Postmenopausal women in the US with early stage breast cancer usually are treated with surgery followed by radiation therapy. Women whose cancer is hormone sensitive and who have lymph nodes containing cancer cells are frequently given tamoxifen to prevent cancer recurrence.

Tamoxifen prevents the hormone estrogen from attaching to breast cancer cells and fueling their growth.

Available for more than 30 years, tamoxifen cuts the likelihood of recurrence in half and cuts the death rate by almost one-third in those women whose breast cancer is sensitive to estrogen (ER+ breast cancer). About two-thirds of all breast cancers are ER+.

Anastrozle is one of a new class of hormone therapy drugs called aromatase inhibitors — instead of preventing estrogen from attaching to cells, these drugs prevent the body from producing much estrogen to begin with.

"These early results suggest anastrozole may be somewhat better than tamoxifen to prevent recurrence in the short term, but it's too early to know if that will hold true long term, or whether women on anastrozole will live longer," said Debbie Saslow, PhD, director of breast and cervical cancer programs for the American Cancer Society (ACS).

Saslow and Baum both said patients on tamoxifen shouldn't assume switching to anastrozole would be better, noting that the ATAC trial results involved women starting hormone therapy for the first time, and that the combination produced poorer results than either drug alone.

"A trial testing the safety and usefulness of making that switch is still underway, and until it's done, women on either drug should remain on it unless they're having problems with it, in which case they should discuss it with their doctor," noted Saslow.

Saslow also cautioned that women should not assume anastrozole would prevent breast cancer in women who don't have it until studies are done to answer that question.

"It's best to help science answer these questions, or wait for the full answers from those involved in that process before taking action," concluded Saslow.

Anastrozole is relatively new, but had already proven better than some other breast cancer drugs for some purposes, and doctors wanted to know if it might also help prevent recurrence better than tamoxifen.

The researchers enrolled more than 9,000 patients worldwide in the trial, including over 2,000 in the US, making it the largest trial of adjuvant therapy ever done.

About 90% of the women in the trial had ER+ breast cancer. All had surgery before entering the trial, and some also had chemotherapy and radiation therapy. Some of the women had cancer in their lymph nodes at the time of surgery; most did not.

One group of women took tamoxifen alone, another group took anastrozole alone, and a third group took both drugs.

After the women had been taking the drug for about two and a half years, the researchers found:

• Hot flashes developed in about 34% on anastrozole and about 40% on tamoxifen.
• Fatigue developed in about 15% on either drug.
• Vaginal bleeding occurred in about 8% of women taking anastrozole, compared to about 4.5% of those on tamoxifen.
• Vaginal discharge occurred in about 3% of those on anastrozole and about 11% of those on tamoxifen.
• About 6% of women had bone fractures on anastrozole, compared to about 2% on tamoxifen.
• About 3% of the women on either drug developed cataracts.
• Slightly more women on anastrozole developed heart problems, compared to those on tamoxifen — 2.5% compared to 1.9%.
• One-tenth of one percent of the women on anastrozole developed endometrial cancer compared to one-half of one percent of those on tamoxifen.