The inflammatory bowel disease committee of NASPGHAN has published a review and recommendations for the use of thiopurine metabolite testing (JPGN 2013; 56: 333-40).
The effectiveness of thiopurines is reviewed with a few key points:
- Cochrane reviews have shown that azathioprine and 6-mercaptopurine are effective in inducing remission in Crohn’s disease. For active disease the overall response rate has been reported as 54% compared with 33% for placebo.
- For ulcerative colitis, a Cochrane analysis deemed the methodologic quality of 4 of the 6 studies as unsatisfactory and that all studies were small. “The reviewers concluded that azathioprine may be effective treatment for patients with ulcerative colitis.”
The review covers the potential adverse effects of the thiopurines: myelosuppression, pancreatitis, elevated transaminases, susceptibility to infection, and malignancy. The review suggests that the risk of non-Hodgkin lymphoma is probably increased up to 4-fold. Though, it is difficult to determine how much of the risk is truly due to the usage of thiopurines or other confounding factors. The studies about the risk of non-melenoma skin cancer are not discussed.
Other covered topics: advantages of thiopurine metabolite testing (6-thioguanine [6-TGN] and 6-methylmercaptopurine[ 6-MMP]), potential disadvantages of metabolite testing (e.g.. cost), use of genotype versus phenotype (phenotypic testing is generally preferred), thiopurine metabolism, devising target levels, use of allopurinol, and misinterpretation of metabolite testing.
Specific Consensus Recommendations:
- Obtain thiopurine methyltransferase (TPMT) testing prior to initiation of thiopurines.
- Avoid use of thiopurines in individuals with extremely low TPMT activity or who are homozygous recessive for TPMT activity
- TPMT testing does not predict all cases of leukopenia. All individuals receiving thiopurines should have routine monitoring with CBCs. “Most adverse effects from thiopurines are not directly related to 6-TGN or 6-MMP levels.”
- Metabolite testing can help determine adherence to therapy
- Metabolite testing may be helpful in guiding dose adjustment and/or adding allopurinol treatment
- Routine and repetitive testing of metabolites is not recommended in patients who are doing well on an acceptable thiopurine dosage.
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