As add-on therapy for early-stage breast cancer, the drug anastrozole, known by the brand name Arimidex, is less likely than tamoxifen to cause adverse effects, including serious events and those leading to treatment discontinuation, new research suggests.

In 2005, the results of the “Arimidex, Tamoxifen, Alone or in Combination (ATAC) trial were published and showed a benefit for Arimidex over tamoxifen in terms of disease-free survival,” Dr. Joan Houghton, from University College London, told Reuters Health. “With the current analysis, we just wanted to make sure Arimidex was as safe as tamoxifen.”

Arimidex does increase the risk of bone fractures, which Houghton said was anticipated, but the risk does not extend beyond the treatment phase and is easily managed. However, the newer agent has a better safety profile than tamoxifen.

The current ATAC analysis, which appears in the medical journal Lancet Oncology, includes follow-up data for 3125 women who completed 5 years of treatment with anastrozole and 3116 similar women treated with tamoxifen. The average follow-up period was 68 months.

The overall rate of side effects was significantly lower with anastrozole than with tamoxifen: 61 percent vs. 68 percent. Serious side effects were nearly half as common with anastrozole: 5 percent vs. 9 percent.

Eleven percent of anastrozole-treated patients experienced side effects leading to treatment discontinuation compared with 14 percent of tamoxifen-treated patients.

Tamoxifen has received much praise since it first became available in the 1970s, but, Houghton said, the newer types of drugs such as anastrozole “have really knocked tamoxifen off its perch.”

The study was funded by AstraZeneca, which developed both Arimidex and tamoxifen (Nolvadex), although the latter agent has been available in a generic form for several years.

SOURCE: Lancet Oncology, August 2006.