ITPA genotyping is not predictive for the development of side effects in AZA treated inflammatory bowel disease patients

The level of thiopurine methyltransferase (TPMT) activity is determined by a common genetic polymorphism. 1 It was shown that low TPMT activity is linked to a higher relative risk of development of myelosuppression after AZA treatment. 2 Testing for TPMT genotype before the start of AZA treatment is of limited clinical value as myelosuppression resulting from TPMT mutations occurs in less then one third of patients with myelosuppression. 3 Polymorphisms in genes encoding inosine triphosphate pyrophosphatase (ITPase), another enzyme involved in metabolism of AZA, have also been suggested to be associated with the development of side effects in AZA treatment. 4 Colombel et al show that there was no difference in the frequency of ITPA polymorphisms in 41 patients who developed AZA related myelosuppression in comparison with a previously published control population. 5 Unfortunately, this still leaves the question of other side effects such as flu-like symptoms, rash, and pancreatitis unanswered.