IBIS-II is an international multi-centre breast cancer prevention study that is split into prevention and DCIS components.

IBIS-II Prevention is studying the use of anastrozole versus placebo in postmenopausal women at increased risk of breast cancer. It aims to determine whether anastrozole can help prevent breast cancer in these patients.

IBIS-II DCIS is investigating the effects of tamoxifen versus anastrozole in postmenopausal women with Ductal Carcinoma in Situ (DCIS). It aims to determine which breast cancer treatment drug, anastrozole or tamoxifen, is better at preventing the disease from returning in women who have had DCIS.

Long-term follow-up results of the International Breast cancer Intervention Study IBIS-II has demonstrated a 49% overall reduction in breast cancer occurrence with anastrozole. This was based on a 61% reduction during the 5-year treatment period and an additional 36% reduction in the 5-12 year post-treatment follow-up period. These results indicate a long-term preventive benefit with the aromatase inhibitor anastrozole based mostly on ER-positive breast cancer in postmenopausal women at increased risk of developing breast cancer, suggesting that anastrozole should be the first option in breast cancer prevention for most women with a higher risk of disease occurrence.

These results are very important, and substantially strengthen the findings from the initial report after a 5-year median follow-up, with no evidence of new late side effects. They provide crucial information about the effects of anastrozole in preventing breast cancer in women at high risk of the disease. It is hoped that they will lead to a change in clinical practice with anastrozole being routinely prescribed as preventive medicine for post-menopausal women at high risk of breast cancer.  The results of the study were published in The Lancet on 12 December 2019

In total 6,844 women worldwide joined this important study (3,864 in Prevention and 2,980 in DCIS).

For the Prevention study, 3,864 high-risk postmenopausal women aged 40-70 years from 153 centres in 19 countries were recruited between February 2003 and January 2012. 

The IBIS-II Prevention trial is run by the International Breast Cancer Intervention Study group (IBIS), under the umbrella of the Breast International Group (BIG) network.

Pharmaceutial partner: AstraZeneca

Sponsor: Queen Mary University of London

• Cuzick J, Sestak I, Forbes JF et al. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results of a randomised controlled trial. Lancet. 2020 Jan 11;  doi: 10.1016/S0140-6736(19)32955-1.
• Forbes JF, Sestak I, Howell A et al. Anastrozole versus tamoxifen for the prevention of locoregional and contralateral breast cancer in postmenopausal women with locally excised ductal carcinoma in situ (IBIS-II DCIS): a double-blind, randomised controlled trial. Lancet. 2016 Feb 27;387(10021):866-73. doi: 10.1016/S0140-6736(15)01129-0. Epub 2015 Dec 11.
• Cuzick J, Sestak I, Forbes JF et al. Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): an international, double-blind, randomised placebo-controlled trial. Lancet. 2014 Mar 22;383(9922):1041-8. doi: 10.1016/S0140-6736(13)62292-8. Epub 2013 Dec 12. 

The study was funded by Cancer Research UK, the National Health and Medical Research Council Australia, Breast Cancer Research Foundation, Sanofi Aventis, and AstraZeneca. 

Prevention ClinicalTrials.gov Identifier: NCT00078832

DCIS ClinicalTrials.gov Identifier: NCT00072462