Cyclosporine has always been linked to an increased risk of cancer, but it was previously thought that this was because of its role in suppressing the immune system.

US and Japanese scientists have now found that the drug may itself cause cancer.

The finding gives an insight into how the drug induces cancer and may help scientists find ways to treat it, such as developing new immunosuppressive drugs.

Cell division

Cyclosporine suppresses the immune system by altering the way genes that control immunity work.

In experiments on mice, the scientists found that cancer cells were more likely to divide, move and spread when exposed to cyclosporine.

When the scientists injected different types of tumour cells into mice with immune problems they found that more secondary tumours developed in those who had been exposed to cyclosporine.

The discovery sheds new light on the way cyclosporine works.

One explanation is that cyclosporine stimulates the production of the immunosuppressive protein, transforming growth factor-beta.

The protein is known to cause liver, skin and kidney damage in high quantities.

Known risks

In a comment piece in Nature, Gary Nabel of the University of Michigan says the findings are not an argument for abandonning cyclosporine.

He says it is not yet clear if the drug converts normal cells into cancerous cells or what effect it has on pre-cancerous cells.

He adds that it may also play a positive role in treating some forms of heart disease.

Experiments on mice have shown that it may be effective in treating enlarged hearts.

And he says it is already known that using cyclosporine increases the risk of developing cancer.

"This complication is a well-known side effect of cyclosporine therapy," he writes. "One that is currently balanced against the need to treat life-threatening disease.