SAN ANTONIO—The estrogen-blocking drug anastrozole is associated with reducing breast cancer risk by half among postmenopausal women, according to initial results from the double-blind, randomized, placebo-controlled IBIS II trial. The findings were presented at the 2013 San Antonio Breast Cancer Symposium and published in The Lancet.

“Our initial results show that for postmenopausal women who do not have breast cancer, but are at high risk for developing the disease, anastrozole reduced breast cancer incidence by 53% with very few side effects,” said Jack Cuzick, MD, chairman of the study’s Steering Committee, head of the Cancer Research UK Centre for Cancer Prevention, and director of the Wolfson Institute of Preventive Medicine at Queen Mary University of London.

“The results are about as good as we could have hoped for,” Dr. Cuzick said. “This drug was very well tolerated. Over 5 years, only 4% additional drop-out was seen with the drug.”

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Anastrozole was associated with nonsignificant “very tiny 7.7% [placebo] to 8.5% increases in bone fractures,” when a bone density scan was performed before treatment, Dr. Cuzick noted.

Approximately 80% of women diagnosed with breast cancer have estrogen receptor (ER)–positive tumors. Anastrozole, a nonsteroidal aromatase inhibitor, blocks estrogen production and is already in widespread use for treating ER-positive breast cancer in postmenopausal women.

“Two other anti-hormone therapies, tamoxifen and raloxifene, are used by some women to prevent breast cancer, but these drugs are not as effective and can have adverse side effects, which limit their use,” Dr. Cuzick noted. “Hopefully, our findings will lead to an alternative prevention therapy with fewer side effects.”

A total of 3,864 postmenopausal women at high risk of developing breast cancer (eg, having two or more blood relatives with breast cancer, or a mother or sister diagnosed with breast cancer before age 50 or in both breasts) were enrolled in the study between 2003 and 2012. Participants were randomly assigned to receive 1 mg anastrozole daily (n = 1,920) or placebo (n = 1,944).

After a 5-year follow-up, 40 (2%) women in the study’s anastrozole arm had developed breast cancer compared with 85 women in the placebo group (4%; hazard ratio [HR], 0.47; 95% CI: 0.32-0.68; P < 0.0001), Dr. Cuzick reported.

Adverse effects included “small increases” in muscle ache and hot flashes, he noted. Side effects “were only slightly higher than in the placebo arm,” he was quick to add. “This means most symptoms were not drug related, and the concern about side effects for this type of drug may have been overstated in the past.”

Anastrozole was not associated with better survival among women who developed breast cancer, however (18 vs. 17 deaths among women in the placebo arm; P = 0.836). That early result is “unsurprising,” wrote David A. Cameron, MD, Edinburgh Cancer Centre, Western General Hospital, in a commentary on the study published in The Lancet.

The reduced risk of developing breast cancer reported among women in IBIS II’s anastrozole arm “is in keeping with those of other similar studies,” although longer follow-up will be important for IBIS II, Dr. Cameron said.

“Our priority now is ensuring that as many women as possible can benefit from these new findings,” Dr. Cuzick said. “Prevention is an important tool in the fight against breast cancer.”

The researchers also found “about a 40% reduction in other cancers, mostly skin cancers,” Dr. Cuzick said, calling the finding an “exciting possibility that we will continue to explore.”

The study’s funders included AstraZeneca and Sanofi-Aventis but was independently conducted by Cancer Research UK.


  1. Cuzick J, Sestak I, Forbes JF et al.  Lancet. 2013;doi:10.1016/S0140-6736(13)62292-8.
  2. Cuzick J et al. S3-01. Presented at: San Antonio Breast Cancer Symposium 2013. Dec. 10-14, 2013; San Antonio.