Rapamycin is just coming on to the market - it's a very promising drug

Mr Geoff Koffman, British Transplantation Society

Some studies have suggested patients receiving conventional immunosuppressive drugs have a 20 to 500 fold higher incidence of certain types of cancer.

Skin cancer, and lymphoma, cancer of the lymph glands, are much more likely in transplant patients than the general population

It is not yet understood why transplant patients have this increased risk.

Blocking growth

Scientists at the University of Regensburg, Germany tested rapamycin and cyclosporine.

They tested the drugs on mice which had secondary tumours in their livers which had metastasised from colon cancer.

Those mice which had been given rapamycin had less liver tumour tissue.

But those which had been given cyclosporine had an increased amount of liver cancer.

The scientists believe that rapamycin prevents the development of tumours by blocking angiogenesis - the formation of new blood cells.

Mr Geoff Koffman, a transplant surgeon at Guy's and St Thomas's Hospital in London and a spokesman for the British Transplantation Society told BBC News Online: "Rapamycin is just coming on to the market. It's a very promising drug.

"Whatever the cause of cancers in immunosuppressed patients, this may have a beneficial effect.

"But it beggars the question, how about using this as an anti-tumour agent in non-transplant patients."

He added: "The real crunch is where we can stop these tumours from forming, not just where we can stop them from spreading".

The research is published in the journal Nature Medicine.