Breast cancer tissue under the microscope.

Drugs called aromatase inhibitors reduce the risk of breast cancer returning by compared with oestrogen receptor blocking drugs like tamoxifen, a new study part-funded by Cancer Research UK has found.

And, crucially, these drugs are just as effective in women aged under 35 , who have a higher risk of recurrence than older women.

Caroline Geraghty, specialist cancer information nurse at Cancer Research UK, said: “This finding has the potential to make a difference in the lives of thousands of women.”

Researchers say they need to follow up patients for longer to find out if the treatment can reduce deaths.

Blocking oestrogen

Many breast cancers are fuelled by the hormone oestrogen, so drugs that interfere with this hormone can be used to treat the disease, including to prevent recurrence.

One class of drugs, oestrogen-receptor blockers (which include widely used drug tamoxifen), stop breast cancer cells from taking up the hormone. Others, known as aromatase inhibitors, prevent the body from producing it in the first place.

Scientists funded by Cancer Research UK have helped define the use of both classes of drugs, but a long-standing issue has been which is better at preventing recurrence for women with early-stage breast cancer.

Reducing recurrence

Researchers based at Oxford Population Health, part of the University of Oxford, combined data from 4 large-scale randomised controlled trials, which involved more than 7,000 women from around the world with early-stage breast cancer.

Each patient, who had received an ovarian suppressing treatment – a procedure which permanently or temporarily stops the ovaries from producing oestrogen – was randomly allocated an aromatase inhibitor or tamoxifen for 3 or 5 years.

The researchers looked for differences between the 2 groups in breast cancer recurrence, along with deaths from breast cancer or any cause, publishing their findings in The Lancet Oncology.

Of the total study population, 12.6% had a breast cancer recurrence. But this was significantly reduced in the group who had received aromatase inhibitors instead of tamoxifen.

The main benefit was seen in the first 5 years, where the risk of recurrence was a third lower in the aromatase inhibitor group.

The probability that someone’s breast cancer would return was 6.9% in the aromatase inhibitor group compared with 10.1% in the tamoxifen group.

Benefits for pre-menopausal women

Lead author Rosie Bradley, from Oxford Population Health, said: “Our aim was to find out whether premenopausal women treated with ovarian suppression could benefit more from aromatase inhibitors than tamoxifen, and these findings conclusively show this is the case for preventing breast cancer recurrence.”

When used alone, aromatase inhibitors aren’t effective for premenopausal women with hormone receptive breast cancer, as people’s ovaries respond by increasing oestrogen production. However, the study found that combining aromatase inhibitors with either surgery or drugs to suppress ovarian function is an effective strategy to reduce recurrence.

Caroline Geraghty, specialist cancer information nurse at Cancer Research UK, said: “Breast cancer recurrence is especially problematic in younger women, so it’s great news to see that the use of aromatase inhibitors is just as effective in women under the age of 35.”

There was no apparent difference in the number of deaths from breast cancer or any cause. However, survival benefits from aromatase inhibitors may become clearer after a longer period of follow-up studies.

“Effects on quality of life need to be considered as well, and it is important that clinicians discuss with patients the potential benefits and risks for each treatment approach,” added Bradley.

Aromatase inhibitors can increase the risk of osteoporosis, which can cause bone fractures. In the analysis, a slightly higher proportion of women in the aromatase inhibitor group (6.4%) had a bone fracture over the follow-up period, compared with the tamoxifen group (5.1%). However, the frequency of bone fractures was low overall.

“Though there is potential for some side effects, aromatase inhibitors could increase the chance of these women remaining disease free – allowing them to return to normal life,” said Geraghty.

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