First-pass metabolism of cyclosporin by the gut.

Cyclosporin is thought to be exclusively metabolised in the liver. We instilled cyclosporin into the small bowel of 2 patients during the anhepatic phase of liver transplantation; cyclosporin metabolites were readily detected in portal venous blood. Our findings indicate that the small intestine is a major site of cyclosporin breakdown: such intestinal metabolism might help to explain the poor oral bioavailability and drug interactions of cyclosporin.