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Anastrozole: A Promising Breakthrough in Preventing Breast Cancer

9th November 2023 - 0 comments

Anastrozole: A Promising Breakthrough in Preventing Breast Cancer

By Bree Foster, Science Writer.

Every year, around 56,000 women in the UK are diagnosed with breast cancer – about 150 a day [1]. While significant progress has been made in diagnosis, treatment, and survival rates, this disease still claims around 11,500 lives annually. However, there’s promising news on the horizon, particularly for women at an increased risk of breast cancer. Anastrozole, a well-established breast cancer treatment, has now been licensed for a new purpose: prevention [2]. In this blog, we’ll explore the concept of cancer chemoprevention, how these drugs work, and why anastrozole is different to other breast cancer prevention drugs.

Breast cancer prevention drugs

Cancer chemoprevention involves using drugs to delay or prevent the development of cancer, particularly in individuals at high risk. This practice primarily targets individuals without a cancer diagnosis but who possess specific risk factors that heighten their vulnerability. Various risk factors increase the likelihood of breast cancer development including advancing age, a strong family history of breast cancer, and having a precancerous breast condition, such as lobular carcinoma in situ (LCIS) or atypical hyperplasia.

Based upon a careful risk assessment, healthcare providers sometimes recommend therapy with medications to reduce the chance of developing breast cancer for women at increased risk. These medications include aromatose inhibitors (AIs) and selective oestrogen receptor modulators (SERMs), of which there are two: tamoxifen and raloxifene.

The role of oestrogen in breast cancer

Oestrogen, the principle female sex hormone, plays an essential role in breast cancer development and progression, acting as “fuel” for about 80% of breast cancers in post-menopausal women [3]. Breast cancers that respond to oestrogen are termed “oestrogen-sensitive” or ” oestrogen receptor-positive”. Lowering oestrogen levels can prevent or delay breast cancer development.

Both tamoxifen and raloxifene act as anti-oestrogen medications, binding to oestrogen receptors in breast cancer cells instead of natural oestrogen in the body. By reducing the amount of oestrogen available to cancer cells, these drugs starve the cancer cells of the fuel they need to grow.

While both drugs have been shown to effectively reduce the risk of cancer development, they are also associated with potentially harmful side effects. Tamoxifen, in particular, can increase the risk of venous thromboembolic events, cataracts, and endometrial cancer. Due to this, there is a continuous pursuit for more effective and safer preventative medications.  

What makes anastrozole different?

Anastrozole is a hormone therapy drug primarily used to treat breast cancer in postmenopausal women, specifically those with oestrogen-sensitive breast cancer. Anastrozole acts as an aromatase inhibitor, blocking the enzyme responsible for converting androgens into oestrogen. In doing so, it reduces the availability of oestrogen by as much as 95%, contributing to the prevention or slowing of cancer growth.

Unlike SERMs, AIs do not affect oestrogen production in the ovaries, which is the primary source of oestrogen in premenopausal women. For that reason, AIs work best for women who have already reached menopause. Instead, AIs turn off the production of small amounts of oestrogen from fat cells and adrenal glands that continues after menopause.

Evidence from the extensive IBIS-II clinical trial showed a remarkable 49% reduction in breast cancer incidence among women who took anastrozole compared to a placebo group [4]. Additionally, the study found that the reduced risk of breast cancer seen whilst women were taking anastrozole is durable, and remains – even when women had stopped the drug – for an average of seven years.

The side effects associated with anastrozole are generally considered to be mild and can be effectively managed by most women. Notably, in contrast to tamoxifen, anastrozole does not elevate the risk of blood clots or uterine cancer, making it a safer option for preventive treatment. Furthermore, clinical trials have demonstrated that AIs, including anastrozole, are approximately 30% more effective in preventing breast cancer recurrence compared to tamoxifen or raloxifene [5].

One drug, twice the effect

Anastrozole marks a significant milestone as the first medication repurposed through a new multi-agency national programme that looks at using existing medicines in new ways to benefit patients and the NHS. The Medicines Repurposing Programme, set up in 2021, is hosted by NHS England and supported by organisations like the MHRA, NICE, and the NIHR. It follows in the footsteps of successful repurposing efforts seen during the COVID-19 pandemic, where drugs like tocilizumab, an arthritis drug, and dexamethasone, a widely available steroid, were repurposed as treatments for the virus.

By repurposing existing drugs, the drug development timeline and costs are substantially reduced, given that these drugs have already established their safety in human use. This accelerated process allows for quicker approvals of drugs for new applications, ultimately contributing to the timely saving of lives.

A Better Tomorrow

The licensing of anastrozole as a preventive measure for breast cancer is a key moment in the fight against this formidable disease. With its proven effectiveness in reducing the risk of breast cancer in postmenopausal women, anastrozole has the potential to save thousands of lives and critical healthcare resources. Anastrozole serves as a great example of how the repurposing of existing medications can yield breakthroughs in healthcare, providing new hope to women at an increased risk of breast cancer and reinforcing the commitment to using innovation to improve our health service.  

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[1] Breast cancer statistics. (2015, May 14). Cancer Research UK.

[2] Anastrozole: Thousands to be offered drug to prevent breast cancer in England. (2023, November 7). BBC News.

[3] Hormone therapy for breast cancer. Retrieved 8 November 2023, from

[4] Cuzick, J., Sestak, I., Forbes, J. F., Dowsett, M., Cawthorn, S., et al. (2020). Use of anastrozole for breast cancer prevention (IBIS-II): Long-term results of a randomised controlled trial. The Lancet, 395(10218), 117–122.

[5] Early Breast Cancer Trialists’ Collaborative Group. (2015). Aromatase inhibitors versus tamoxifen in early breast cancer: Patient-level meta-analysis of the randomised trials. The Lancet, 386(10001), 1341–1352.

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