A great many more women are likely to be candidates for Herceptin in the future - any progress in this area would be most welcome

Kefah Mokbel, St George's Hospital, London

Dr Kuo-Fen Lee from Salk Institute in California created laboratory mouse embryos lacking the protein - and their heart and brain development was so stunted that they could not even be born.

His new experiments also knocked out the gene which create the protein, but only after birth and in the heart itself.

This allowed him to mimic the effects of treating a more developed animal with the drug.

Hearts taken from the mice at between one and six months of age showed clear signs of heart problems, including a thickening and weakening of the heart muscle.

Heart muscle cells from the mice appeared to be more vulnerable from damage from other chemotherapies used in concert with Herceptin.

Drug help

Dr Lee said that it was possible that the heart could be protected during Herceptin treatment.

He said: "If we could develop agents that stimulate the heart...then those might allow you to use Herceptin aggressively while protecting the heart."

However, he said that he hoped that a Herceptin-like drug could be engineered to interfere with breast cancer cell growth without impacting on the heart.

Mr Kefah Mokbel, a breast cancer surgeon and Herceptin researcher from St George's Hospital in south London, said that most doctors now avoided giving Herceptin in combination with anthracyclines.

"The potential danger far outweighs the potential benefits from the drug combination," he said.

"This is a very interesting study, as a great many more women are likely to be candidates for Herceptin in the future - any progress in this area would be most welcome."

The government has ruled, after taking advice from its drugs watchdog the National Institute for Clinical Excellence, that Herceptin should be widely available on the NHS.