Azathioprine Metabolites in Erythrocytes and DNA for Therapy Monitoring in Very Early Onset Inflammatory Bowel Disease Pediatric Patients

Azathioprine is used for inflammatory bowel disease (IBD) therapy. Patients under 6 years of age (very early onset, VEO-IBD) showed distinctive clinical characteristics, such as increased activity of thiopurine-methyltransferase (TPMT), a crucial enzyme for thiopurine metabolism. TPMT and PACSIN2 polymorphisms were associated with azathioprine efficacy. This study investigated the role of age in thiopurine active metabolites and disease activity. Also, the effects of age, TPMT and PACSIN2 polymorphisms on azathioprine metabolites and disease activity were evaluated. Erythrocytes thioguanine nucleotides (TGN) were measured by HPLC, and leukocytes incorporated deoxythioguanosine (DNA-TG) byLC-MS/MS in 12 VEO-IBD patients (median age 4.13 ± 0.98, 7 females, 6 Crohn’s disease (CD)), 11 IBD children (median age 9.36 ± 1.52, 8 females, 1 CD ), and 73 IBD adolescents (median age 14.92 ± 1.81, 33 females, 37 CD). VEO-IBD subjects required a higher azathioprine dose (p-value = 0.048) and showed a lower DNA-TG/azathioprine dose ratio (p-value = 0.049) and TGN/azathioprine dose ratio (p-value = 0.013). DNA-TG was positively correlated with TGN (p-value = 4.15 × 10-5) and disease score (p-value = 1.54 × 10-4). TPMT rs1142345 (76 wild type, 10 heterozygous) was associated with increased concentrations of DNA-TG and TGN (p-value = 0.024 and 0.00038, respectively), whereas PACSIN2 rs2413739 (37 wild types, 34 heterozygous, 16 homozygous variants) was associated with the disease score (p-value = 0.04). Together, these data confirmed that VEO-IBD patients show enhanced azathioprine metabolism, which can be accurately reflected by both TGN and DNA-TG levels, highlighting their ability to be good biomarkers of azathioprine metabolism.