Thiopurine S-methyltransferase and inosine triphosphate pyrophosphohydrolase genes in Japanese patients with inflammatory bowel disease in whom adverse drug reactions were induced by azathioprine/6-mercaptopurine treatment

Background The main cause of azathioprine (AZA)/6-mercaptopurine (6MP)-induced adverse reactions is a reduction in the activities of the metabolizing enzymes thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphohydrolase (ITPA). Adverse reactions develop at a high frequency in Japanese patients at half the dose required for European and American patients; however, the association with TPMT and ITPA gene polymorphisms in Japanese has not been fully investigated.