San Antonio, TX—Five years of treatment with the aromatase inhibitor anastrozole (Arimidex) cut the risk of breast cancer by 50% in high-risk postmenopausal women who had not developed a first cancer. At a median follow-up of 5 years, only 2% of 1920 women taking anastrozole and 4% of 1944 women using placebo developed breast cancer.
Thus far, anastrozole has had no impact on survival, according to the results of the International Breast Cancer Intervention Study (IBIS)-II, which were presented by Jack Cuzick, PhD, Head, Centre for Cancer Prevention, Wolfson Institute of Preventive Medicine, Queen Mary University of London, United Kingdom, at the 2013 San Antonio Breast Cancer Symposium and published simultaneously online (Cuzick J, et al. Lancet. 2013 Dec 12 [Epub ahead of print]).
The magnitude of the reduction in risk of recurrence at 7 years is greater than that recorded with tamoxifen. “The effect of tamoxifen has been shown to persist for at least 10 years, and further follow-up is needed to establish whether anastrozole has such a sustained effect,” wrote Dr Cuzick and colleagues. “Our results strongly support the use of anastrozole for preventive treatment of high-risk postmenopausal women.”
Between February 2, 2003, and January 31, 2012, IBIS-II enrolled 3864 postmenopausal women with an increased risk for breast cancer related to family history or other factors (median age, 59 years). Women were randomized to 5 years of treatment with anastrozole (N = 1920) or to placebo (N = 1944). Approximately 47% of patients used previous hormone replacement therapy, and 33% had a hysterectomy.
Approximately 50% of the patients had 2 or more first-degree relatives with breast or ovarian cancer, and 35% had 1 first-degree relative with breast cancer under age 50. Approximately 9% of patients had previous lobular carcinoma in situ or atypical hyperplasia.
The full 5-year adherence rate was estimated to be 72% in the placebo group and 68% in the anastrozole group. Treatment discontinuations as a result of adverse events were reported in 15% of patients receiving placebo and in 20% of patients receiving anastrozole.
At 5 years, 5.6% of patients receiving placebo and 2.8% of patients receiving anastrozole developed breast cancer. Of these, 3.3% and 1.4% of patients, respectively, developed an invasive estrogen receptor–positive breast cancer. The predicted cumulative incidence of all breast cancers after 7 years was 5.6% in the placebo group and 2.8% in the anastrozole group. There were 17 deaths in the placebo group and 18 in the anastrozole group, but no pattern of causes was apparent in either group.
Musculoskeletal complaints were reported by 64% of the women taking anastrozole versus in 58% of those assigned to placebo (P = .001). The number of fractures and sites of fracture did not differ between the 2 groups. Carpal tunnel syndrome and joint stiffness were more common in the anastrozole group.
In an accompanying editorial in the Lancet (Cameron DA. Lancet. 2013 Dec 12 [Epub ahead of print]), David A. Cameron, MD, MSc, Clinical Director, Edinburgh Cancer Centre, Western General Hospital, United Kingdom, was less enthusiastic about the use of anastrozole for chemoprevention than Dr Cuzick. Dr Cameron noted that without a proven survival benefit, women may be reluctant to take 5 years of an antiestrogen therapy with all of its perceived and actual toxicities.
