Comparison of three methods for measuring thiopurine methyltransferase activity in red blood cells and human leukemia cells

Thiopurine efficacy is partly reflected by the genetic polymorphism of the thiopurine methyltransferase (TPMT) enzyme, which is responsible for variation in the metabolism, toxicity and therapeutic efficacy of the thiopurines azathioprine (AZA), 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG). Determination of TPMT activity before administration of thiopurines is thus crucial for individualized dosing in order to prevent toxicity in TPMT deficient individuals. These individuals must be treated with markedly lower (eg, 5–10% of the standard) doses of the prescribed medications. This paper describes a comparison of three different methods for the quantification of TPMT activity in red blood cells (RBC) and cultured human cell lines. We succeeded to perform the measurement of TPMT activity in a minimum amount of 1 × 106 cultured cells with an HPLC-UV system modified and optimized in our laboratory. The TPMT activity was linearly correlated with the cell concentration of the cultured cell line in a range of 1–10 × 106 cells. A significant correlation of determination of TPMT activity in RBC between radiometric detection by HPLC, classic radiochemical detection and UV detection by HPLC, was observed, correlation coefficient (r) were 0.72 and 0.73, respectively. The within-day and day-to-day coefficients of variation of the HPLC-UV-based method were 8% and 16%, respectively. The evaluation of the methods was demonstrated by studying the TPMT activity in RBC isolated from 198 patients, as well as in MOLT4 leukemic cell line and its sub-cell lines with acquired resistance to 6-MP and 6-TG.