Some scientists believe the drug, anastrozole, marketed by Zeneca as Arimidex, could challenge the current treatment, tamoxifen, which can stimulate womb cancer in some patients as well as problems with blood clots and hot flushes.

However, the Cancer Research Campaign (CRC) has warned that a lot more evidence is required before the drug can be considered as a viable alternative to tamoxifen.

The European Cancer Conference in Vienna will hear on Thursday that a large international study involving more than 1,000 patients has shown anastrozole is at least as effective and well-tolerated as tamoxifen.

Early findings from part of the study being carried out in the USA show that anastrozole is more effective.

The data will be submitted to regulatory authorities with the aim of allowing doctors to prescribe anastrozole as a first-line treatment for advanced breast cancer in postmenopausal women.

Anastrozole is currently only used as a treatment for women who have failed to respond to tamoxifen or other drugs.

Experts surprised

Tamoxifen is the current front-line treatment

The findings from the US study have taken experts in the field by surprise.

They also showed that women in the study treated with tamoxifen had a 44% higher risk of their disease progressing than those treated with anastrozole.

This suggests that anastrozole, which works in a different way to tamoxifen, could be a more effective treatment option for postmenopausal women with advanced disease.

One of the lead investigators, Professor Jean-Marc Nabholtz, from the University of Alberta School of Medicine and the Cross Cancer Institute in Edmonton, Canada, said: "We know that tamoxifen and anastrozole are both effective treatments for postmenopausal women with advanced breast cancer, but until now, the two have never been compared directly in a clinical trial.

"What interested us is that the two drugs work in very different ways - tamoxifen primarily by blocking oestrogen receptors in tumour cells and anastrozole by suppressing synthesis of oestrogen, so that there is less to bind with oestrogen receptors.

"Because they work differently, there are also possible differences between the two compounds in terms of their side-effect profiles."

The two compounds appeared to be similar in terms of the number of side-effects reported, although anastrozole produces less blood clots in the circulatory system.

The finding that anastrozole was more effective than tamoxifen were not replicated in the part of the study carried out in Europe.

This could be because more than 88% of the women in the US study had breast cancer which was hormone-sensitive, compared to just 43% in the European study.

Professor Jacques Bonneterre, from the Centre Oscar Lambret in Lille, France and one of the lead investigators in the European study, said:

"The North American study certainly suggests that anastrozole could be significantly more effective.

"Anastrozole may even have some tolerability advantages. These findings alone mean that clinicians and patients have the prospect of an increased choice of therapies available to them."

More data needed

Professor Gordon McVie, director general of the CRC, said the results were "interesting", but warned against any rush to make anastrozole the drug of choice.

He said: "We need to get lots more data in clinical trials before one can come up with a clear view that this drug should replace tamoxifen as a first-line treatment.

"We have 10 million women years of experience with tamoxifen, and know its safety profile inside out."