Identification of factors regulating thiopurine methyltransferase activity in a Norwegian population.

Red blood cell (RBC) thiopurine methyltransferase (TPMT), an inactivating pathway of 6-mercaptopurine, is controlled by genetic polymorphism and is subject to ethnic variation. RBC TPMT is a good predictor of clinical outcome in children with acute lymphoblastic leukemia. RBC TPMT activity was determined in 226 patients, 176 of them living in northern Norway (of which 123 were Saami (Lapps)). Demographic variables, use of drugs and presence of chronic diseases were evaluated as possible predictors of RBC TPMT activity by a multiple regression model. Men had higher RBC TPMT activity compared to women. Living in the northernmost county of Norway was associated with increased RBC TPMT activity irrespective of ethnicity. The use of diuretics was associated with increased RBC TPMT activity. The gender difference in RBC TPMT activity may indicate a need to treat male subjects more aggressively with thiopurine drugs compared to female subjects.