Abstract
Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the metabolism of thiopurine medications that are used in the treatment of multiple malignant and nonmalignant immunologic conditions. Polymorphisms in the TPMT gene associated with low enzyme activity can produce pronounced pharmacologic effects during therapy. The determination of TPMT erythrocyte activity is a valuable adjunct test to genotyping for the assignment of TPMT phenotype, especially in the presence of indeterminate genotypes. We present a method for the determination TPMT activity in human erythrocytes whereby we utilize LC-MS/MS to measure the amount of 6-methylmercaptopurine formed in red blood cell (RBC) lysates using 6-mercaptopurine as substrate and S-adenosylmethionine (SAM) as methyl donor. This method has been successfully implemented and proven to be reliable for use in human specimens.
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