Development and Validation of a Sensitive LC–MS/MS Method for Determination of Deoxythioguanosine in Human DNA and its Application in Thiopurine-Treated Inflammatory Bowel Disease Patients

Deoxythioguanosine (dTG), incorporated into DNA as DNA-TG, is considered a more relevant thiopurine metabolite than erythrocyte thioguanine nucleotides, as erythrocytes are not the drug target. We established a simple and sensitive liquid chromatography–tandem mass spectrometry method to detect dTG in human blood nucleated cell DNA and validated its clinical applicability in Chinese patients with inflammatory bowel disease to improve individualized thiopurine treatment. dTG was released from 1 μg DNA in a single hydrolysis step, separated by the X-Terra RP18 column and quantified by tandem mass spectrometry with 2′- Deoxyguanosine-13C,15N2 as the internal standard. The calibration curve covered eight concentration levels (0.125–25 ng/mL) with the correlation coefficients r2 ≥ 0.99. The intra- and inter-assay variability was achieved less than 9.9%. This method was employed in a study where a total of 125 inflammatory bowel disease patients with a median DNA-TG level of 279.7 fmol/μg DNA (interquartile range 183.5 ~ 410.6 fmol/μg DNA). Nucleoside diphosphate-linked moiety X-type motif 15 variant carriers (*2, *3, *5, *6) showed significantly higher DNA-TG/dose and DNA-TG/erythrocyte thioguanine nucleotides ratios than wild-type patients (P = 0.0023, P = 0.0001). In summary, this method was simple and stable to detect dTG in DNA, which may act as a promising biomarker predicting the toxicity and efficacy in patients treated with thiopurine.