Determination of azathioprine/6-mercaptopurine metabolites in dried blood spots: Correlation with RBC concentrations.

The analysis of 6-thioguanine (6-TG) and 6-methylmercaptopurine (6-mMP) in biological samples is not straight forward and requires pre-treatment of samples. There are no validated published methods for the analysis of azathioprine/6-mercaptopurine (AZA/6-MP) metabolites in dried blood spot (DBS) samples that study the correlation with red blood cells (RBC) concentrations. DBS was prepared by applying fifteen microliters of blood [spiked with analytes or samples obtained from patients] to a Guthrie card which was then dried at room temperature overnight. The sample treatment procedure used protein precipitation followed by a hydrolysis step in which, 6-mMP was converted into 4-amino-5-(methylthio)carbonyl imidazole (AMTCI) then analytes were transferred to a solid phase extraction cartridge. The extracted sample was chromatographed using a reversed phase system (C18) column preceded by a guard column of matching chemistry. The method gave a linear calibration over the range 0.5-15 μmol/L and 3.75-175 μmol/L for 6-TG and 6-mMP, respectively. The method has been applied successfully to the determination of 6-TG and 6-mMP concentrations in DBS finger-prick samples from 27 paediatric patients with IBD who were receiving (AZA/6-MP). Patient 6-TG and 6-mMP RBC concentrations, calculated from whole blood finger prick DBS samples and those measured in RBCs derived from matched venous samples (analyzed using conventional HPLC-UV technique) showed good agreement using the Bland-Altman test. This is the first published method for determining 6-TG and 6-mMP in DBS that studies their correlation with RBCs concentrations. It is applicable to a range of clinical studies such as adherence and pharmacokinetic studies involving AZA/6-MP.