Purine analogs, 6-mercaptopurine (6-MP) and the prodrug azathioprine (Aza) are used as immunosuppressants in the treatment of many diseases including cancer, autoimmune disorders and inflammatory diseases ofthe digestive tract. Treatment with thiopurines is complicated by the high variability in response observed in a patient population. The need to titrate treatment to adequate therapeutic levels is exacerbated by the cytotoxicity that can result from overdosing patients. In this comprehensive study, we evaluated the response of 946 individuals, with known thiopurine S-methyltransferase (TPMT) genotypes, to treatment with 6-MP and Aza read more..