In the treatment of diseases such as acute childhood leukaemia (ALL) and inflammatory bowel disease (IBD), the thiopurines azathioprine, 6-mercaptopurine, and 6-thioguanine are used. Thiopurines are antimetabolites and immunomodulators used to maintain remission in patients. They are all prodrugs and must be converted into the competing antimetabolites thioguanosine triphosphate and deoxythioguanosine triphosphate for final incorporation into RNA or DNA. The current therapeutic drug monitoring (TDM) method measures the sum of the formed metabolites in the sample, after acidic hydrolysis at high temperature. In this work, the goal is to measure these drugs closer to their pharmacological endpoints, once incorporated into DNA. After extracting DNA from whole blood, followed by DNA hydrolysis, 2′-deoxythioguanosine (dTG) and the complementary natural nucleobase 2′-deoxycytidine (dC) were measured. Chromatographic separation on a HSS T3 column followed by mass spectrometric detection was performed in multi-reaction monitoring (MRM) mode on a Xevo TQ-XS with ESI in positive mode, within 5 min. The concentration range for dTG was 0.04-5 nmol/L, and for dC, 0.1-12.5 µmol/L. The lower limit of detection was determined to a concentration of 0.003 nmol/L for dTG and 0.019 µmol/L for dC. The intra- and inter-assay imprecision for the quality controls ranged between 3.0 and 5.1% and between 8.4 and 10.9%, respectively. Sample stability for up to 4 years is shown. In summary, a sensitive method to quantify the thiopurines incorporated into DNA as dTG has been developed and will be used in further clinical studies for a better understanding of the mode of action of the thiopurines and the use of this method in TDM.
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