The role of xanthine oxidase in thiopurine metabolism: a case report.

Azathioprine (AZA) is widely used in the treatment of autoimmune inflammatory diseases. AZA is normally rapidly and almost completely converted to 6-mercaptopurine (6-MP) in the liver, which is further metabolized into a variety of pharmacologic active thiopurine metabolites. 6-MP is catabolized by xanthine oxidase (XO) to the inactive metabolite 6-thiouric acid.