Anastrozole (Arimidex) is more effective than tamoxifen at preventing breast cancer recurrence for periods of at least 10 years in women with hormone-responsive cancers. In addition, the increased risk of fractures that has been seen in some patients being treated with anastrozole disappears after treatment ends.
Speaking at the San Antonio Breast Cancer Symposium in December, Professor John Forbes of the University of Newcastle presented the much-anticipated long-term results of the ATAC (Anastrozole, Tamoxifen, Alone or in Combination) trial that compared 5 years of tamoxifen with 5 years of the aromatase inhibitor anastrozole for the treatment of postmenopausal women with invasive breast cancer.
Hormone-responsive breast cancers require estrogen stimulation in order to grow. Tamoxifen, a synthetic variation of estrogen, binds to hormone receptors on hormone-sensitive cancer cells, preventing estrogen from attaching to the cells. Anastrozole, on the other hand, prevents the manufacture of estrogen by body tissues other than the ovaries (eg, fat cells), so it is useful for the treatment of hormone-sensitive cancers in postmenopausal women whose ovaries are no longer producing estrogen.
Relative Benefit Increased at 10 Years
Although previous reports at 33 and 68 months demonstrated that anastrozole was more effective, has fewer serious side effects, and was better tolerated than tamoxifen during the period of treatment, it was not known whether the benefits or side effects associated with anastrozole would persist long term after patients were no longer taking the drug. After 10 years (5 years after the completion of treatment), patients who had taken anastrozole continued to show reduced rates of tumor recurrence and metastasis compared with patients who had taken tamoxifen. In fact, the relative benefit of anastrozole actually increased compared with results seen immediately after the completion of treatment.
Most strikingly, the incidence of a new cancer occurring in the other breast was significantly lower at 10 years in women who took anastrozole compared with those who received tamoxifen. Tamoxifen has been approved for several years for use in preventing new breast cancers, and it now appears that anastrozole may be even more effective.
Many patients have been reluctant to begin treatment with anastrozole because of concern about the potential for increased bone loss and risk of fracture that can occur with the use of aromatase inhibitors. It was not known whether these effects would persist after the end of treatment. With the new data, it appears that they do not. Although the incidence of fractures was nearly 30% higher in patients treated with anastrozole while they were taking the drug, that difference disappeared after treatment was completed. In addition, there do not appear to be any other important side effects related to anastrozole treatment at this time.