A major international trial has confirmed the long-term benefit of anastrozole to help stop breast cancer coming back or developing in the other breast in postmenopausal women with hormone sensitive early breast cancer, according to a study published in Lancet Oncology today (Wednesday).

The trial – led by Professor Jack Cuzick, Cancer Research UK epidemiologist based at Queen Mary, University of London – is the longest ever follow up of a particular class of drugs called aromatase inhibitors1.

These replaced tamoxifen as the gold standard hormone treatment in 2009, following earlier trial results showing they are the best treatment for preventing breast cancer recurrences in women who have gone through the menopause2.

More than 9,000 post-menopausal women in 21 countries were involved in the trial, including 3,000 from Britain. The women took either anastrozole or tamoxifen as an adjuvant treatment after surgery – to help stop their breast cancer from coming back.

Both drugs were shown to be highly effective at preventing the disease returning in this high risk group of women. But over a ten year period the newer drug – anastrozole – was shown to be more effective, with around 80 per cent of patients still cancer-free ten years later, compared to 76 per cent of those given tamoxifen.

Professor Cuzick said: “Tamoxifen has been in use for at least 25 years and remains one of the most effective breast cancer treatments ever developed. But these results provide further support for using anastrozole as the first line treatment for postmenopausal women with hormone sensitive breast cancer, as it’s even more likely to stop the disease coming back and was better tolerated than tamoxifen.”

Around 32,000 postmenopausal women are diagnosed with breast cancer every year in the UK3, so the results of this trial could lead to over a thousand fewer women developing a recurrence of their breast cancer each year.

Both anastrozole and tamoxifen work by interfering with the female hormone oestrogen4, which is known to be the most important cause of breast cancer.

Kate Law, Cancer Research UK’s director of clinical research, said: “Drugs like tamoxifen and now anastrozole have made a significant contribution to the impressive survival rates we see for breast cancer today by cutting the risk of the disease coming back.

“Anastrozole is one of three aromatase inhibitors currently prescribed to help prevent breast cancer coming back in postmenopausal women – the highest risk group. But all drugs have side-effects and it’s essential we continue to monitor the long-term performance of these drugs before we can conclusively say which is the safest and best option for this group of women.”


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Cuzick J. et al, ‘Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trial‘, Lancet Oncology (2010).


1. Aromatase inhibitors

This is the longest ever follow-up time of a trial looking at preventing breast cancer with so-called ‘aromatase inhibitors’ – a class of drugs including anastrozole that blocks oestrogen being made in the fatty tissues of the body. This process uses an enzyme known as aromatase.
Tamoxifen by contrast works by stopping oestrogen from attaching to breast cells and stimulating them to divide and grow.

Both drugs are known to have different side effects. The incidence of womb cancer was found to be slightly lower in women taking anastrozole, but they were more likely to experience weakening of the bones due to osteoporosis. There were also fewer cases of blood clots in the veins, known as deep vein thrombosis (DVT) with anastrozole.

This research builds on an earlier study showing breast cancer patients who experienced side effects such as hot flushes, night sweats or painful joints after taking either drug were ten per cent less likely to have a recurrence of breast cancer than women who didn’t develop these symptoms.

2. ATAC (Arimidex, Tamoxifen, Alone or in Combination) was a large phase 3 clinical trial published in December 2004, comparing anastrozole (Arimidex) with tamoxifen for post menopausal women who had early breast cancer. This latest study looks at the long-term outcomes of the trial after a follow-up period of ten years. For more information visit: http://www.cancerhelp.org.uk/about-cancer/cancer-questions/anastrozole-o…

3. ‘Postmenopausal’ includes women aged 55+. This figure is based on a three year average of data collected between 2005 and 2007.

4. Oestrogen

Breast cancer can be oestrogen positive (ER positive) – in which case it is driven by the hormone oestrogen attaching to oestrogen receptors on the breast cancer cells. These tumours are normally treated with hormonal therapy such as tamoxifen or anastrozole. If a breast cancer involves cells which do not have oestrogen receptors – it is ER negative – patients will usually be advised to have chemotherapy.