Quantification of Thioguanine in DNA Using Liquid Chromatography-Tandem Mass Spectrometry for Routine Thiopurine Drug Monitoring in Patients With Pediatric Acute Lymphoblastic Leukemia.

Background: We developed an assay to measure DNA-incorporated 6-thioguanine (DNA-TG) and validated its clinical applicability in Korean pediatric patients with acute lymphoblastic leukemia (ALL) in order to improve individualized thiopurine treatment and reduce the life-threatening cytotoxicity.

Methods: The DNA-TG assay was developed based on liquid chromatography-tandem mass spectrometry, with isotope-labeled TG-d3 and guanine-d3 as internal standards. This method was applied to 257 samples of pediatric ALL patients. The DNA-TG level was compared with erythrocyte TG nucleotide (RBC-TGN) level in relation to the TPMT and NUDT15 genotypes, which affect thiopurine metabolism, using Spearman's rank test and repeated measure ANOVA.

Results: For DNA-TG quantification, a linearity range of 10.0-5,000.0 fmol TG/μg DNA; bias for accuracy of -10.4% -3.5%; coefficient of variation for intra- and inter-day precision of 3.4% and 5.8% at 80 fmol TG/μg DNA and of 4.9% and 5.3% at 800 fmol TG/μg DNA, respectively; and recovery of 85.7%-116.2% were achieved without matrix effects or carry-over. The median DNA-TG level in the 257 samples was 106.0 fmol TG/μg DNA (interquartile range, 75.8-150.9). There was a strong correlation between DNA-TG and RBC-TGN levels (ρ=0.68, P <0.0001). The DNA-TG/RBC-TGN ratio was significantly higher in NUDT15 intermediate metabolizers ( *1/*2 and *1/*3 ) than in patients with wild-type alleles ( P <0.0001).

Conclusions: This simple and sensitive method for measuring DNA-TG level can improve therapeutic drug monitoring for thiopurine treatment.