ATAC trial

17 November 2010


This international trial investigated which was the best treatment; Arimidex (anastrozole) alone, tamoxifen alone of anastrozole and tamoxifen in combination (taken daily over five years).

Scientific Title

A randomised double-blind trial comparing Arimidex alone with Tamoxifen alone with Arimidex and Tamoxifen in combination, as adjuvant treatment in postmenopausal women with breast cancer.

Short title: Arimidex (anastrozole), Tamoxifen alone or in Combination (ATAC) trial.


The dependence of many breast cancers on oestrogens for their continued growth has long been recognised and many current therapies involve hormonal treatment.   Hormonal therapy can achieve this goal, either by reducing circulating concentrations of the female hormone oestrogen, or by blocking the effects of oestrogens on a breast tumour. At the time of commencement of the ATAC trial tamoxifen was currently the drug of choice as hormonal/endocrine therapy for breast cancer. Aromatase inhibitors, a class of drugs that prevented oestrogen production in tissues had shown benefit in trials for advanced breast cancer.  Anastrozole, an aromatase inhibitor, was shown to be well tolerated and effective in treating advanced breast cancer and was thought to be more effective than tamoxifen.  Alternatively, it was thought to be synergistic with tamoxifen. The ATAC trial was an international trial that introduced anastrozole to early breast cancer treatment in the early 2000s. It is still prescribed for many women.


Trial information confirms the long-term superior efficacy and safety of anastrozole over tamoxifen as initial adjuvant therapy for postmenopausal women with hormone-sensitive early breast cancer.

Main publication: Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trial. Cuzick J, Sestak I, Baum M, Buzdar A, Howell A, Dowsett M, Forbes JF, on behalf of the ATAC/LATTE investigators. Lancet Oncol 2010; 11(12):1135-1141.

ATAC quality of life sub-study


The Quality of Life sub-study showed that anastrozole was associated with a lower risk of endometrial (womb lining) abnormalities than tamoxifen resulting in significantly fewer gynaecological investigations and operations.  There was also a lower incidence of blood clotting events with anastrozole compared with tamoxifen after five years of treatment. Menopausal side effects such as hot flushes were similar as was the quality of life for women receiving anastrozole or tamoxifen.