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. <i>TPMT</i> and <i>PACSIN2</i> 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, <i>TPMT</i> and <i>PACSIN2</i> 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 (<i>p</i>-value = 0.048) and showed a lower DNA-TG/azathioprine dose ratio (<i>p</i>-value = 0.049) and TGN/azathioprine dose ratio (<i>p</i>-value = 0.013). DNA-TG was positively correlated with TGN (<i>p</i>-value = 4.15 × 10^–5) and disease score (<i>p</i>-value = 1.54 × 10^–4). <i>TPMT</i> rs1142345 (76 wild type, 10 heterozygous) was associated with increased concentrations of DNA-TG and TGN (<i>p</i>-value = 0.024 and 0.00038, respectively), whereas <i>PACSIN2</i> rs2413739 (37 wild types, 34 heterozygous, 16 homozygous variants) was associated with the disease score (<i>p</i>-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.