The launch of new research into a class of breast cancer drugs that have been hailed as the greatest advance in the treatment of the disease for 25 years has been marred by a dispute about their long-term safety.

The launch of new research into a class of breast cancer drugs that have been hailed as the greatest advance in the treatment of the disease for 25 years has been marred by a dispute about their long-term safety.

Findings from an international trial of one of the drugs, called aromatase inhibitors, released yesterday show that breast cancer patients who switched from a different drug, tamoxifen, halfway through treatment to the new drug cut their risk of the disease returning by a third.

The researchers urged all women to consider switching to aromatase inhibitors, in discussion with their doctors. The benefit over treatment with tamoxifen alone was equivalent to preventing almost five extra cancers in every 100 women treated, they said.

That recommendation was criticised by others who said the long-term risks of aromatase inhibitors were unknown.

The international trial of the drug exemestane (brand name Aromasin) was co-ordinated by Cancer Research UK and involved 4,742 women from 37 countries. It was stopped early because the findings were so dramatic. They are published today in the New England Journal of Medicine.

Presenting the results in London yesterday, Professor Charles Coombes of Imperial College London, who led the trial, said: "Clinicians should consider switching treatment to exemestane between two to three years after the start of tamoxifen therapy for the 80 per cent of post-menopausal women whose cancers contain oestrogen receptors."

Unlike tamoxifen, which blocks the action of oestrogen on the breast, aromatase inhibitors halt the production of oestrogen in the body, reducing its level close to zero.

But Jeffrey Tobias, professor of cancer medicine at University College Hospital, who is a supporter of aromatase inhibitors, said the data from the trial was "not mature enough" to justify such a recommendation.

"It is a really important paper and I am sure it is a real effect. There is increasing evidence that switching from tamoxifen to aromatase inhibitors may be beneficial but it is very early days to be saying change now."

An editorial in the New England Journal criticised researchers in two earlier trials of aromatase inhibitors for ending them before evidence of the long-term safety of the drugs was clear. It warned that patients had only been followed for two and a half years and results of the trials, including the current one, did "not allow us to conduct a useful risk-benefit analysis".

The editorial, by Martine Piccart-Gebhart of the Jules Bordet Institute in Brussels, said: "In my opinion, women whose risk of recurrence is high are reasonable candidates for the inclusion of an aromatase inhibitor ... whereas women with a low risk of recurrence might give more weight to long-term safety and be better served by tamoxifen," .

A spokesman for CancerBacup, the UK breast cancer charity, said the findings of the latest study should be treated with caution. "While these results are very promising, more research is needed to assess any long-term risks, overall survival rates and quality of life issues before firm recommendations can be given to patients."

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