Frequency of serious adverse events of thiopurine treatment in normal thiopurine S-methyltransferase genotype children with inflammatory bowel disease

Introduction: Genetic polymorphism of thiopurine S-methyltransferase, the key enzyme in metabolism of thiopurines (azathioprine and 6-mercaptopurine) used in the treatment of inflammatory bowel disease, results in different enzyme activities. Decreased enzyme activity causes myelosuppression whereas abnormally high activity results in hepatotoxicity at standard thiopurine doses. Four allele variants (TMPT*2, TMPT*3A, TPMT*3B and TPMT*3C) account for decreased activity in more than 95% of cases. Aim: To evaluate the frequency of severe side effects, such as myelosuppression and hepatotoxicity, at standard or decreased azathioprine doses in children with inflammatory bowel disease who do not exhibit any of the four most common variant alleles. Method: Retrospective analysis of children with inflammatory bowel disease treated with azathioprine at a single tertiary referral center. Results: 51 patients were identified (44: Crohn's disease, 7: ulcerative colitis; male ratio: 28/51; mean age at diagnosis: 12.4 years). Two patients discontinued azathioprine arbitrarily whereas in one patient it was stopped due to serious pancreatitis and in another one because of severe flu-like symptoms. None of the remaining 47 patients exhibited hepatotoxicity suggesting abnormally high thiopurine S-methyltransferase activity. Four patients (8.5%) had profound myelosuppression on less than 1 mg/kg/day azathioprine requiring discontinuation of the drug, and all of them showed complete bone marrow recovery subsequently. No myelosuppression occurred in the remaining 43 patients on 2.17 +/- 0.31 mg/kg/day (mean +/- SD) azathioprine treatment. Conclusions: Regular blood tests are necessary on thiopurine therapy despite normal thiopurine 5-methyltransferase genotype because of the risk of myelosuppression. The four most common variant alleles were identified in routine genotyping only, therefore most likely rare variant allele(s) or polymorphism of other enzymes involved in thiopurine metabolism account for the aforementioned four cases with profound bone marrow suppression. Thiopurine metabolite monitoring is the key for individualized treatment when optimal dosing can be achieved with the least side effects.