In the clinical practice setting, almost a quarter of women treated for breast cancer stop tamoxifen within 1 year, a rate twice as high as indicated by previous studies. The new study, to be published in the March 1, 2007, issue of CANCER, says early termination of this highly effective breast cancer drug may negatively affect treatment efficacy. At 3.5 years, the study also reveals that over one-third of women have ceased tamoxifen treatment. Scientists estimate that over 40,000 breast cancer recurrences are prevented each year worldwide by tamoxifen. However, the optimal duration of therapy is 5 years. Less than 5 years of continual treatment is associated with higher rates of recurrence and higher mortality rates. The most common reason for discontinuation is adverse effects, including mood swings and hot flashes; these are often successfully treated with a specific antidepressant.

Flawed Data

Discontinuation or nonpersistence rates estimated by clinical trials range from 16% to 32%. Studies of usage outside clinical trials report nonpersistence rates of only 17% at 2 years and 31% at 5 years. However, these rates are compiled from self-reporting data collection methods and target elderly patients—an inherently biased data collection method in a population accustomed to chronic medications. Led by Thomas I. Barron, MSC, of the Department of Pharmacology & Therapeutics at Trinity College Dublin and St. James's Hospital, Dublin, researchers reviewed pharmaceutical data from a national database of 2,816 women aged 35 years and older who started tamoxifen for breast cancer.

The researchers found that at 12 months, 22% of women had ceased using the drug. At 24 months, 28% had stopped tamoxifen, and at 3.5 years, 35% had stopped the treatment without commencing an alternative therapy. Analysis for risk factors for discontinuing tamoxifen identified age and history of antidepressant use. Women between the ages of 35 and 44 or over 75 as well as women who reportedly used an antidepressant within 1 year of starting tamoxifen were more likely to stop tamoxifen.

"This study demonstrates that persistence with tamoxifen cannot be assumed and raises concerns about persistence with other oral hormonal therapies for breast cancer and oral antineoplastics in general," the authors write. "This is of particular importance," they conclude, "as longer durations of adjuvant therapy may be recommended for breast cancer in the future and as cancer survivorship becomes a priority area in clinical practice and research."