Transplant drug increases cancer risk.

After life-saving organ transplants, people must take drugs to suppress their immune system and prevent rejection of the transplanted organ. Unfortunately, a suppressed immune system also allows cancer cells to grow more freely, and researchers have assumed that this explains why transplant recipients face an increased risk of cancer.

Now, a study suggests that cyclosporine, a common immune-suppressing drug, may directly trigger the growth of cancer cells. In test tubes, normally slow-growing lung cancer cells that were exposed to cyclosporine developed hallmarks typical of more aggressive cancer cells. They grew without being anchored to a surface and increasingly piled on top of one another, Minoru Hojo of Teikyo University School of Medicine in Kawasaki, Japan, reports in the Feb. 11 Nature.

Hojo also reported evidence of cyclosporine's cancer-causing effects among mice bred to lack immune systems. Among 21 such mice injected with kidney cancer cells, the cancer spread to an average of 241 locations in the lungs of each mouse. Another 18 mice injected with the same type of cancer cells and cyclosporine each developed an average of 338 cancers in their lungs. Because the mice don't have an immune system that cyclosporine can suppress, the results appear to be due to the drug itself, Hojo says.

This study doesn't indicate that giving transplant patients cyclosporine will cause normal cells to become cancerous, but the data do suggest that the drug might exacerbate tumor growth in patients with existing malignancies, says Gary J. Nabel of the University of Michigan in Ann Arbor. Although the new findings help explain why organ transplants are associated with an increased risk of developing cancer, this already well-known risk must be balanced against the need to treat life-threatening diseases, Nabel said.