A drug that halves the risk of women getting breast cancer works seven years after they stop taking, scientists have found.
Anastrozole – branded as Arimidex – blocks oestrogen, which fuels most tumours. It only works in post-menopausal women.
Previous research has shown the drug can halve the risk of breast cancer in women who take the drug for five years.
But now, a study of almost 4,000 women around the world found it offers protection for a further seven years after, totalling 12 years.
Anastrozole, a 4p-a-day pill, works the same as well-known wonder drug tamoxifen – but has been shown to have longer-lasting effects.
It is already available on the NHS, but researchers at the Queen Mary University of London said the uptake by women at risk of breast cancer was low.
Anastrozole, prescribed under the brand name Arimidex, halves a woman’s risk of breast cancer for seven years after she stops taking it, scientists have found
Lead author Professor Jack Cuzick said: ‘Previous research confirmed anastrozole is very effective while women are still taking the drug.
‘But this is the only trial looking at whether it offers long term protection for women at high risk of developing breast cancer.
‘After a detailed analysis… we have concluded it is highly effective in reducing breast cancer occurrence for at least 12 years.’
The study, published in medical journal The Lancet, involved 3,864 women whose health was recorded over the course of 12 years. They took anastrozole for five years.
Forty-nine per cent fewer breast cancers were diagnosed in the half who were given anastrozole – even in women who stopped taking it after the first five years.
This is better than the 28 per cent decrease recorded for tamoxifen.
Dr Ivana Sestak, an expert in medical statistics at Queen Mary, said: ‘The findings mean that for every 29 women taking anastrozole for five years, one case of breast cancer will be prevented during a 12 year period.
‘Around 49 women would need to take tamoxifen for five years to prevent one breast cancer case during the same period.’
There have been concerns around the side-effects of Arimidex, such as joint stiffness, bone thinning, hot flushes and vaginal dryness.
But 75 per cent of the women taking the drug were able to stick with it, compared with 77 per cent taking a placebo.
This indicates even during treatment that side-effects were not severe enough to significantly deter women from taking the drug.
The researchers said anastrozole or Arimidex, manufactured by AstraZeneca, should be the ‘drug-of-choice’ for post-menopausal women at increased risk of developing the disease.
Professor Cuzick, who is co-chairman of International Breast Cancer Intervention Studies (IBIS), presented the findings at the San Antonio Breast Cancer Symposium in Texas.
He said: ‘This is an exciting finding which makes a strong case for anastrozole being the drug of choice for post-menopausal women at high risk of developing breast cancer.
‘Tamoxifen could be offered to the relatively few women who experience serious side-effects from anastrozole.’
Longer follow-up studies will be needed to evaluate whether anastrozole can reduce deaths from the disease.
The latest report also confirms that anastrozole does not have the same type of long term side-effects associated with tamoxifen – including endometrial cancer which begins in the lining of the womb.
However, only tamoxifen can be given to younger women.
Aromatase inhibitors (AIs) like anastrozole are not given to premenopausal women because their ovaries still produce oestrogen. AIs will not stop the ovaries from making the oestrogen that feeds the tumour.
Professor Tony Howell, of the University of Manchester, who was involved in the study said: ‘Importantly, this study shows there are no significant long term side-effects in the five years after completion of anastrozole, particularly no increase in fractures or heart disease.’
The first phase of the study published in 2013, called IBIS-1, reported breast cancer occurrence among women taking the drug fell by 53 per cent during five years.
Post-menopausal women at high risk of developing breast cancer, due to family history and other risk factors, have been recommended to take the drug since 2017 in the UK and 2019 in the US.
However, the drug is not currently offered to all the women who could benefit – possibly because some physicians are unsure of the evidence base for long term impact.
Professor Cuzick told the BBC: ‘Uptake has really been quite low. Currently it’s about 10 per cent of these women and we think it should be substantially higher.’
Experts believe tamoxifen could help prevent cancer in around 500,000 women in the UK but say the fear of side effects is putting them off.
Just 15 per cent of high-risk women in a survey said they were taking the drug. Nearly three quarters were worried about possible side-effects.
Prof Charles Swanton, Cancer Research UK’s chief clinician, said: ‘Both tamoxifen and anastrozole can be given to women who are at higher risk of breast cancer.
‘Up until now we only knew tamoxifen has long lasting benefits, so it’s reassuring that this study looking specifically at anastrozole, which has fewer long term side-effects, gives better protection to women years after they stopped taking the drug.
‘Doctors may still decide that tamoxifen is more appropriate for some women, but it’s great there are options.
‘Anyone with questions about these findings, or about their family history and risk of breast cancer, should speak to their doctor, who can help work out which medication is best for them.’
Breast cancer is the most common cancer in women and affects over 55,000 UK women each year, killing more than 11,000.
It is estimated more than two million women across the world developed the disease last year – with more than 600,000 deaths.