Clinical trial data confirmed that tamoxifen reduces the risk of breast cancer in healthy women who are at increased risk of breast cancer; however, long term follow-up data were lacking. International Breast Cancer Intervention-I (IBIS-I) trial investigators in the United Kingdom, Finland, Australia and other countries conducted a randomized controlled trial of premenopausal and postmenopausal women aged 35–70 years who were deemed to be at an increased risk of developing breast cancer. The women were randomly assigned to receive tamoxifen 20 mg daily or a matching placebo for 5 years. The primary endpoint was the occurrence of breast cancer (invasive breast cancer and ductal carcinoma in situ).
Between 4/14/92 and 3/30/01, 7,154 eligible women were recruited from genetics clinics and breast care clinics in eight countries and randomly assigned to the two treatment groups. The median age at enrollment was 50.8 years in each group. After a median follow up of 16 years, 601 breast cancers were reported, with 251 (7%) occurring in the 3,579 women taking tamoxifen and 350 (9.8%) occurring in the 3,575 women in the placebo group. The researchers concluded that tamoxifen offers a very long period of protection after treatment cessation and substantially improves the benefit-to-harm ratio of the drug for breast cancer prevention.
Cuzick J et al. Tamoxifen for prevention of breast cancer: extended long-term follow-up of the IBIS-I breast cancer prevention trial.Published Online: 11 December 2014 The Lancet Oncology 2015; 16(1): 67-75.