Treatment with 5 years of anastrozole reduced the risk of developing a first breast cancer by 53% and reduced the risk of developing estrogen-receptor positive (ER+) invasive cancer by 58% in women at high risk for developing the disease. These results from the IBIS-II trial were reported at the San Antonio Breast Cancer Symposium.1
“We believe these results provide strong support for chemoprevention of breast cancer in high-risk women. Longer-term follow-up is needed to determine if the preventive effect is sustained after treatment,” said lead author Jack Cuzick, PhD, Queen Mary University, Wolfson Institute of Preventive Medicine, London, United Kingdom.
IBIS-II randomized 3864 postmenopausal women at high risk of breast cancer, either because of family history and other risk factors (including atypia, lobular carcinoma in situ, or breast density), to treatment with anastrozole or use of placebo for 5 years.
At 7 years of follow-up, primary breast cancers (including ductal carcinoma in situ) developed in 5.6% of women in the placebo group compared with 2.8% of the anastrozole group, representing a 53% decrease (P <.0001). ER+ invasive breast cancers developed in 3.3% of the placebo group versus 1.4% of the anastrozole group, representing a 58% decrease in risk (P = .001). Anastrozole had no protective effect against developing ER-negative tumors.
The full 5 years of the study was completed by 72% of placebo patients and 68% of those taking anastrozole. Cuzick said this 4% difference in treatment continuation suggests that dropouts due to anastrozole-specific adverse effects were quite low.
Anastrozole was associated with bone fractures in 7.7% of placebo patients and 8.5% of those taking anastrozole. Participants had a dual-energy X-ray absorptiometry (DEXA) scan at study initiation, and women with low bone mass were prescribed bisphosphonate treatment, which could account for the smaller-than-expected incidence of musculoskeletal/fracture adverse effects seen in this trial, Cuzick said.
Musculoskeletal aches and pains were 10% higher with anastrozole than in placebo patients. Cuzick said that at baseline, many women in both groups reported joint pain. Other joint symptoms observed more frequently in the anastrozole group were joint stiffness and carpal tunnel syndrome.
Surprisingly, the incidence of other primary cancers was reduced in the group taking anastrozole: 40 patients in the anastrozole group developed other cancers while 70 patients in the placebo group developed other cancers, mainly skin and colorectal cancers. “This merits further study, and the reasons for this effect are unclear,” Cuzick noted.
Study results were published online in the Lancet to coincide with Cuzick’s presentation at the 2013 San Antonio Breast Cancer Symposium.2
References
- Cuzick J, Sestak I, Forbes JF, et al. Breast cancer prevention using anastrozole in postmenopausal women at high risk. Presented at: San Antonio Breast Cancer Symposium; December 12, 2013; San Antonio, TX. Abstract S3-01.
- Cuzick J, Sestak I, Forbes JF, et al; IBIS-II Investigators. Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): an international, double-blind, randomised placebo-controlled trial. Lancet. December 12, 2013. Epublished ahead of print.