Estren is a synthetic version of the female hormone oestrogen which appears to have no effect on breast cancer cells in a test tube.

There are concerns that HRT including oestrogen or oestrogen-like compounds might promote the growth of breast cancer.

Even though there is no proof that it can cause a breast cancer to develop in a healthy woman, it is possible that it speeds the growth of cancer cells once a tumour exists.

Although the extra risk is relatively small - perhaps eight extra cases in 10,000 women taking HRT per year - it has still prompted scientists to search for a safer alternative.

Their drug would have to show all the advantages, such as the protection against colon cancer and bone loss - and obviously the relief of menopausal symptoms

Dr John Stevenson, Women's Health Concern

The new compound not only does not have any effect on breast cancer cells, but still manages to prevent bone loss - one of the key advantages of taking HRT.

In mice, Estren actually improved bone density and strength.

No sex effect

However, while oestrogen in its natural form had a noticeable effect in maintaining the weight and size of the animals' wombs, in those given estren, the wombs reduced in size.

This suggests that the drug has no powerful effect on the sex organs - perhaps reducing the chance of uterine cancer.

Dr Stavros Manolagas, who led the study at the University of Arkansas in the US, said that he felt that the new compound might also be able to offer protection to blood vessels and nerve cells.

Balancing act

Dr John Stevenson, from the charity Women's Health Concern, told BBC News Online that any new form of HRT would have to strike a delicate balance between reducing health risks while hanging on to the benefits.

He told BBC News Online: "Their drug would have to show all the advantages, such as the protection against colon cancer and bone loss - and obviously the relief of menopausal symptoms.

"The increased number of breast cancer cases among women currently taking HRT is very small, and there is no increased death rate among these women."

A recent trial in the US suggested that oestrogen and progestogen HRT might actually increase the risk of heart disease very slightly.

A similar UK trial was abandoned this week after it became clear that its results would be irrelevant by the time it issued its conclusions.

The research was published in the journal Science.