A large landmark study funded by Cancer Research UK has found that the breast cancer drug anastrozole can halve the chances of developing the disease in high-risk women.

The results of the IBIS II trial, led by Queen Mary University of London and published online in the The Lancet this week, show that in nearly 4,000 post-menopausal women at high risk of breast cancer, taking anastrozole for 5 years cut cases by 53%, compared with a placebo.

The researchers note that this amount of reduction is more effective than tamoxifen and the results also showed fewer side effects from anastrozole.

Anastrozole, an aromatase inhibitor, stops the body making estrogen, a hormone that fuels the growth of many breast cancers. It has been in use for many years as a treatment for post-menopausal women with estrogen receptor positive breast cancer.

Randomized trial tested anastrozole against placebo for 5 years

For the international, double-blind, randomized placebo-controlled trial, the researchers recruited just under 4,000 post-menopausal women at high risk of breast cancer.

Women were considered to be at high risk of developing breast cancer if any of the following applied to them:

  • Having certain high-risk types of benign breast disease
  • Having two or more blood relatives with the disease
  • Having a sister or mother who developed breast cancer before reaching the age of 50, or
  • Having a sister or mother with breast cancer in both breasts.

The women were aged 40-70 years and came from 18 countries. Half were randomly assigned to a daily dose of 1 mg of anastrozole, and half to a daily dose of placebo.

"Double-blind" means neither the women nor the health care professionals treating them knew whether they were given placebo or the active drug.

Over a median follow-up of 5 years, 40 (2%) participants in the anastrozole group developed breast cancer, compared with 85 (4%) in the placebo group.

A total of 18 deaths occurred in the anastrozole group and 17 in the placebo group.

The researchers also note that no specific causes (such as invasive cancers or non-invasive ductal carcinoma in situ) were more common in one group than the other.

'Exciting development' in breast cancer prevention

Lead investigator Jack Cuzick, professor and head of the Centre for Cancer Prevention at Queen Mary University of London, describes the trial as an "exciting development" in the prevention of breast cancer, and adds:

"We now know anastrozole should be the drug of choice when it comes to reducing the risk of breast cancer in post-menopausal women with a family history or other risk factors for the disease. This class of drugs is more effective than previous drugs such as tamoxifen and crucially, it has fewer side effects."

He suggests that concerns about side effects for this type of drug, such as acute aches and pains, may have been overstated in the past, since the side effects reported in their study were only slightly higher in the anastrozole than the placebo group.

He says the priority should now be to ensure as many women as possible benefit from these new findings, adding that:

"Prevention is an important tool in the fight against breast cancer and we strongly urge the National Institute for Health and Care Excellence (NICE) to consider adding anastrozole to their recommended drugs for women who are predisposed to developing breast cancer."

Kate Law, Cancer Research UK's director of clinical research, says:

"We now need accurate tests that will predict which women will most benefit from anastrozole and those who will have the fewest side effects."

The charity has been funding the IBIS II trial for over 10 years.

Prof. Cuzick was also on a panel that published a review about the future of breast cancer prevention in Lancet Oncology in March 2011. The panel agreed that all women with a higher than 4% above average risk of getting breast cancer in the next 10 years should be offered preventive measures and close monitoring.