Anastrozole Cuts Breast Cancer Incidence in Half for Women at High Risk

Photo Courtesy © SABCS/Todd Buchanan 2013

Jack Cuzick, PhD

Anastrozole may be a new option for primary prevention of breast cancer in postmenopausal women at high risk for the disease, according to newly reported findings from the International Breast Cancer Intervention Study II (IBIS-II), providing strong support for the use of the aromatase inhibitor in high risk postmenopausal women. The IBIS-II results were presented at the 2013 San Antonio Breast Cancer Symposium and published simultaneously in The Lancet.

“IBIS II was initiated to investigate whether anastrozole can prevent breast cancer. Our initial results suggest that anastrozole reduced the risk of breast cancer by 53% in postmenopausal women who do not have breast cancer but are at high risk with very few side effects,” stated lead author Jack Cuzick, PhD, head of the Cancer Research UK Centre for Cancer Prevention and director of the Wolfson Institute of Preventive Medicine at Queen Mary University in London.

The investigators enrolled 3864 postmenopausal women from 2003 to 2012 who were at increased risk for developing breast cancer. A number of criteria were used to determine eligibility for the study, including having two or more blood relatives with breast cancer, having a mother or sister who developed breast cancer before the age of 50, and having a mother or sister with bilateral breast cancer. Participants were randomized to anastrozole for 5 years (n = 1920) or to placebo (n = 1944).

Median age was 59.3 years, and 47% of the women had used HR therapy in the past.

At a median follow-up of slightly more than 5 years, the incidence of all breast cancers (including ductal carcinoma in situ) was reduced by 53% in the anastrozole-treated group, from 5.6% with placebo to 2.8% in those on the AI (P <.0001). Anastrozole also cut the incidence of estrogen receptor-positive breast cancer by 58%, from 3.3% in placebo patients to 1.4% in those taking anastrozole (P =. 001).

Compliance was 72% in the placebo group versus 68% in the anastrozole cohort. Cuzick interpreted this to mean that only 4% more women were noncompliant with the AI. He said that 90% of women enrolled in the trial reported joint pain at baseline, and only an additional 10% reported arthralgias on AIs.

Cuzick said that the IBIS II data, combined with the preventive effects of anastrozole on developing contralateral breast cancer reported in the ATAC trial, make anastrozole the therapy of choice for prevention in the setting of high-risk women who have not developed breast cancer yet.

Planned follow-up is at least 10 years and hopefully much longer, he continued. “We want to determine if anastrozole has a continued impact on cancer incidence even after stopping treatment, if it reduces deaths from breast cancer, and to ensure that there are no long-term adverse side effects.”

Cuzick J, Sestak I, Forbes JF, et al. First results of the international breast cancer prevention study II (IBIS-II): A multicentre prevention trial of anastrozole versus placebo in postmenopausal women at increased risk of developing breast cancer. Presented at: the 36th Annual San Antonio Breast Cancer Symposium; December 10-14, 2013; San Antonio, TX. Abstract S3-01.

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