Assay of mercaptopurine in plasma using paired-ion high-performance liquid chromatography

A sensitive, quantitative, and specific high-performance liquid chromatographic method for mercaptopurine in plasma is described. The analysis, in which mercaptopurine and the internal standard, 6-methylthio-2-hydroxypurine, are chromatographed as ion-pairs with heptane-sulfonic acid, employs a simple and rapid sample preparation based on deproteination using 60% trichloroacetic acid. Quantitation of plasma samples to 0.2 microgram of mercaptopurine/ml is reported. The retention times of the major metabolites do not interfere.