Cyclosporine, a drug that suppresses the immune system and so inhibits the body's natural mechanism for tissue rejection, directly alters cancer cells to make them highly dangerous, the study shows.
The findings have been called "provocative" by experts, some of whom remain unconvinced that cyclosporine can directly instigate a cancer. The sceptics believe immune suppression in general creates the conditions for the creation of cancers.
Cyclosporine is given to most of the 2,500 patients who undergo organ transplants each year in Britain and experts are concerned that the research may frighten people off a drug that has saved many lives.
However, a team of Japanese scientists, led by Minoru Hojo of Teikyo University, and now at Cornell University in New York, found that cyclosporine causes cancer cells in a test tube to become mobile and "invasive". This would make a cancer able to spread around the body.
In a research paper published in the journal Nature they produce evidence to show the effects are also seen in laboratory animals, and that the observed changes can be reversed if cyclosporine is inhibited by a blocking agent - further implicating the drug as a cancer-causing agent.
"Our findings suggest that immunosuppressants like cyclosporine can promote cancer progression by a direct cellular effect that is independent of its effect on the host's immune cells," the researchers conclude.Reuse content