Catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine. Defects in TPMT are the cause of thiopurine S-methyltransferase deficiency (TPMT deficiency).TPMT is an enzyme involved in the normal metabolic inactivation of thiopurine drugs. These drugs are generally used as immunosupressants or cytotoxic drugs and are prescribed for a variety of clinical conditions including leukemia, autoimmune disease and organ transplantation. Patients with intermediate or no TPMT activity are at risk of toxicity after receiving standard doses of thiopurine drugs and it is shown that inter-individual differences in response to these drugs are largely determined by genetic variation at the TPMT locus.
