Azathioprine in the therapy of paediatric inflammatory bowel disease – part II: pharmacodynamics, pharmacokinetics, and the possibilities of measuring its metabolites in clinical practice

Summary Background: Thiopurines (predominantly azathioprine and mercaptopurine) are widely used in paediatrics to maintain remission in the treatment of inflammatory bowel disease. After its absorption from the gastrointestinal tract, azathioprine is converted into 6 mercaptopurine in approximately 90%. Several enzymes (such as thiopurine methyltransferase, xanthine oxidase and hypoxanthine-guanine phosphoribosyl transferase) participate in its further metabolism, producing non-active methylated metabolites (6 methylmercaptopurine) and thiouric acid and active 6-thioguanine nucleotide. The concentration of these metabolites can be measured in red blood cells. Aim: To map the benefits and possibilities of thiopurine metabolites measurements in patients suffering from inflammatory bowel disease. Conclusion: The measurement of active and non-active metabolites can help evaluate the bioavailability of those drugs, identify some causes of adverse effects and reveal non-adherence. Keywords Crohn’s disease, merkaptopurin, pediatrie, thiopuriny, ulcerative colitis

Personalized Treatment of 6-Mercaptopurine in Thai Children with Acute Lymphoblastic Leukemia

Thanks to its ability to inhibit deoxyribonucleic acid synthesis, 6-mercaptopurine (6-MP), is one of the indispensable medications for acute lymphoblastic leukemia (ALL) patients. Nevertheless, some patients may succumb to myelotoxicity, leading to treatment disruption or even life-threatening events. Owing to the advances in pharmacogenomics, the genetic polymorphism of genes regulating purine synthesis has been identified and physicians can adjust the dose of 6-MP according to each polymorphism. Such polymorphisms genetically vary among ethnicities. In this article, 2 genetic polymorphisms, namely thiopurine methyltransferase and Nudix (nucleoside diphosphate linked moiety X) type motif 15, are clinically discussed, with a special focus on the clinical studies in Thai children with ALL.

Normal Ranges of Thiopurine Methyltransferase Activity Do Not Affect Thioguanine Nucleotide Concentrations With Azathioprine Therapy in Inflammatory Bowel Disease

Background Thiopurine methyltransferase (TPMT) activity influences azathioprine conversion into active metabolite 6-thioguanine nucleotide (6-TGN). Low TPMT activity correlates with high 6-TGN and risk for myelosuppression. Conversely, normal-to-high TPMT activity may be associated with low 6-TGN and drug resistance, the so-called hypermetabolizers. Our aim was to identify the effect of normal-to-high TPMT activity on 6-TGN concentrations in an inflammatory bowel disease population. Methods A retrospective chart review of patients aged ≥18 with inflammatory bowel disease, on azathioprine, with documented TPMT activity and 6-TGN concentration was performed. Correlations were evaluated via the Spearman rho correlation coefficient. Linear regression was used to determine the effect of TPMT activity on 6-TGN accounting for confounders. Relationships between TPMT activity, drug dose, and 6-TGN levels were defined via average causal mediation effects. Results One hundred patients were included. No correlation was observed between TPMT activity, azathioprine dosing, and metabolite concentrations. Overall, 39% of the cohort had a therapeutic 6-TGN level of >230 pmol/8 × 108 red blood cells (RBCs). No patient under 1 mg/kg achieved a therapeutic 6-TGN level, whereas 42% of patients taking 2.5 mg/kg did. The median 6-TGN concentration was higher for those in remission (254 pmol/8 × 108 RBCs, interquartile range: 174, 309) versus those not in remission (177 pmol/8 × 108 RBCs, interquartile range: 94.3, 287.8), though not significantly (P = 0.08). Smoking was the only clinical factor associated with 6-TGN level. On multivariate linear regression, only age, azathioprine dose, and obese body mass index were predictive of metabolite concentration. Conclusions Variations within the normal range of TPMT activity do not affect 6-TGN concentration.