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6-mercaptopurine (6-MP) metabolite monitoring and TPMT testing in the treatment of inflammatory bowel disease with 6-MP or azathioprine

Richard P MacDermott, MD
Section Editor
Paul Rutgeerts, MD, PhD, FRCP
Deputy Editor
Kristen M Robson, MD, MBA, FACG


The therapeutic efficacy, bone marrow toxicity, and liver toxicity of azathioprine (AZA) and 6-mercaptopurine (6-MP) may be related to their metabolites: 6-thioguanine (6-TG) and 6-methylmercaptopurine (6-MMP). AZA is a prodrug that is metabolized to 6-MP, which is then further metabolized along an anabolic pathway to several metabolites including 6-TG and 6-MMP. Two enzymes are responsible for catalyzing these reactions: thiopurine methyltransferase (TPMT) and hypoxanthine phosphoribosyl transferase (figure 1). 6-TG levels between 230 and 400 may correlate with response and remission of inflammatory bowel disease. Bone marrow suppression may correlate with elevated levels of 6-TG greater than 400, while elevated levels of 6-MMP levels greater than 5700 may correlate with liver toxicity, manifested as increased liver enzymes.


Thiopurine methyltransferase (TPMT) enzyme activity is a major factor determining azathioprine (AZA) and 6-MP metabolism, and therefore 6-thioguanine (6-TG) and 6-methylmercaptopurine (6-MMP) levels. Approximately 89 percent of the population has wild type TPMT, which is associated with normal or "high" TPMT enzyme activity, while 11 percent are heterozygous and have corresponding low TPMT enzyme activity [1,2]. Most importantly, 0.3 percent (1 in 300) of the population is homozygous for mutations of TPMT and thus have negligible activity, which causes 6-MP to be preferentially metabolized to produce high levels of 6-TG, which then leads to bone marrow suppression. Intermediate and normal metabolizers can have up to a threefold difference in initial target doses of AZA and 6-MP to achieve therapeutic 6-TG concentrations [3].

TPMT genotyping — Genetic polymorphism of TPMT causes some individuals to be particularly vulnerable to side effects and makes empiric dose-adjustments of AZA and 6-MP riskier. Measurement of TPMT genotypes and/or TPMT enzyme activity before instituting AZA or 6-MP may help prevent toxicity by identifying individuals with low or absent TPMT enzyme activity, which may lead to increased risk of myelosuppression [1,2,4-6]. (See "Overview of pharmacogenomics".)

Dosing strategies involving such testing may be cost-effective [5-7]. Multiple studies have described the use of TPMT genotyping or measurement of activity in predicting toxicity. As a general rule, these have demonstrated that prediction of toxicity is possible in some patients but is not consistently reliable. An illustrative report included 67 patients receiving AZA for rheumatic disease, six of whom were heterozygous for mutant TPMT alleles [4]. Five of these patients had to discontinue therapy within one month because of leukopenia, while the sixth did not adhere to therapy. In comparison, the median duration of therapy was 39 weeks in those with wild type alleles.

However, the majority of patients who develop myelosuppression while taking AZA do not have detectable TPMT gene mutations. This was illustrated in a study that included 41 patients who developed myelosuppression during treatment with AZA and in whom only 27 percent had mutant alleles of the TPMT gene associated with enzyme deficiency [8]. Thus, even though TPMT testing is performed, a complete blood count (CBC), and also liver function tests, must still be obtained.


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Literature review current through: Feb 2017. | This topic last updated: Wed Apr 29 00:00:00 GMT 2015.
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  1. Lennard L, Gibson BE, Nicole T, Lilleyman JS. Congenital thiopurine methyltransferase deficiency and 6-mercaptopurine toxicity during treatment for acute lymphoblastic leukaemia. Arch Dis Child 1993; 69:577.
  2. Lennard L, Van Loon JA, Weinshilboum RM. Pharmacogenetics of acute azathioprine toxicity: relationship to thiopurine methyltransferase genetic polymorphism. Clin Pharmacol Ther 1989; 46:149.
  3. Gardiner SJ, Gearry RB, Begg EJ, et al. Thiopurine dose in intermediate and normal metabolizers of thiopurine methyltransferase may differ three-fold. Clin Gastroenterol Hepatol 2008; 6:654.
  4. Black AJ, McLeod HL, Capell HA, et al. Thiopurine methyltransferase genotype predicts therapy-limiting severe toxicity from azathioprine. Ann Intern Med 1998; 129:716.
  5. Cuffari C, Dassopoulos T, Turnbough L, et al. Thiopurine methyltransferase activity influences clinical response to azathioprine in inflammatory bowel disease. Clin Gastroenterol Hepatol 2004; 2:410.
  6. Dubinsky MC, Reyes E, Ofman J, et al. A cost-effectiveness analysis of alternative disease management strategies in patients with Crohn's disease treated with azathioprine or 6-mercaptopurine. Am J Gastroenterol 2005; 100:2239.
  7. Winter J, Walker A, Shapiro D, et al. Cost-effectiveness of thiopurine methyltransferase genotype screening in patients about to commence azathioprine therapy for treatment of inflammatory bowel disease. Aliment Pharmacol Ther 2004; 20:593.
  8. Colombel JF, Ferrari N, Debuysere H, et al. Genotypic analysis of thiopurine S-methyltransferase in patients with Crohn's disease and severe myelosuppression during azathioprine therapy. Gastroenterology 2000; 118:1025.
  9. Lichtenstein GR. Use of laboratory testing to guide 6-mercaptopurine/azathioprine therapy. Gastroenterology 2004; 127:1558.
  10. Relling MV, Gardner EE, Sandborn WJ, et al. Clinical Pharmacogenetics Implementation Consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing. Clin Pharmacol Ther 2011; 89:387.
  11. Relling MV, Gardner EE, Sandborn WJ, et al. Clinical pharmacogenetics implementation consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing: 2013 update. Clin Pharmacol Ther 2013; 93:324.
  12. Sandborn WJ, Tremaine WJ, Wolf DC, et al. Lack of effect of intravenous administration on time to respond to azathioprine for steroid-treated Crohn's disease. North American Azathioprine Study Group. Gastroenterology 1999; 117:527.
  13. Gisbert JP, Niño P, Rodrigo L, et al. Thiopurine methyltransferase (TPMT) activity and adverse effects of azathioprine in inflammatory bowel disease: long-term follow-up study of 394 patients. Am J Gastroenterol 2006; 101:2769.
  14. Szumlanski CL, Weinshilboum RM. Sulphasalazine inhibition of thiopurine methyltransferase: possible mechanism for interaction with 6-mercaptopurine and azathioprine. Br J Clin Pharmacol 1995; 39:456.
  15. Dewit O, Vanheuverzwyn R, Desager JP, Horsmans Y. Interaction between azathioprine and aminosalicylates: an in vivo study in patients with Crohn's disease. Aliment Pharmacol Ther 2002; 16:79.
  16. Gilissen LP, Bierau J, Derijks LJ, et al. The pharmacokinetic effect of discontinuation of mesalazine on mercaptopurine metabolite levels in inflammatory bowel disease patients. Aliment Pharmacol Ther 2005; 22:605.
  17. de Boer NK, Wong DR, Jharap B, et al. Dose-dependent influence of 5-aminosalicylates on thiopurine metabolism. Am J Gastroenterol 2007; 102:2747.
  18. Lichtenstein GR, Abreu MT, Cohen R, et al. American Gastroenterological Association Institute medical position statement on corticosteroids, immunomodulators, and infliximab in inflammatory bowel disease. Gastroenterology 2006; 130:935.
  19. Osterman MT, Kundu R, Lichtenstein GR, Lewis JD. Association of 6-thioguanine nucleotide levels and inflammatory bowel disease activity: a meta-analysis. Gastroenterology 2006; 130:1047.
  20. Dubinsky MC, Lamothe S, Yang HY, et al. Pharmacogenomics and metabolite measurement for 6-mercaptopurine therapy in inflammatory bowel disease. Gastroenterology 2000; 118:705.
  21. Cuffari C, Théorêt Y, Latour S, Seidman G. 6-Mercaptopurine metabolism in Crohn's disease: correlation with efficacy and toxicity. Gut 1996; 39:401.
  22. Gupta P, Gokhale R, Kirschner BS. 6-mercaptopurine metabolite levels in children with inflammatory bowel disease. J Pediatr Gastroenterol Nutr 2001; 33:450.
  23. Mardini HE, Arnold GL. Utility of measuring 6-methylmercaptopurine and 6-thioguanine nucleotide levels in managing inflammatory bowel disease patients treated with 6-mercaptopurine in a clinical practice setting. J Clin Gastroenterol 2003; 36:390.
  24. Wright S, Sanders DS, Lobo AJ, Lennard L. Clinical significance of azathioprine active metabolite concentrations in inflammatory bowel disease. Gut 2004; 53:1123.
  25. Goldenberg BA, Rawsthorne P, Bernstein CN. The utility of 6-thioguanine metabolite levels in managing patients with inflammatory bowel disease. Am J Gastroenterol 2004; 99:1744.
  26. Belaiche J, Desager JP, Horsmans Y, Louis E. Therapeutic drug monitoring of azathioprine and 6-mercaptopurine metabolites in Crohn disease. Scand J Gastroenterol 2001; 36:71.
  27. Roblin X, Serre-Debeauvais F, Phelip JM, et al. 6-tioguanine monitoring in steroid-dependent patients with inflammatory bowel diseases receiving azathioprine. Aliment Pharmacol Ther 2005; 21:829.
  28. Moreau AC, Paul S, Del Tedesco E, et al. Association between 6-thioguanine nucleotides levels and clinical remission in inflammatory disease: a meta-analysis. Inflamm Bowel Dis 2014; 20:464.
  29. Haines ML, Ajlouni Y, Irving PM, et al. Clinical usefulness of therapeutic drug monitoring of thiopurines in patients with inadequately controlled inflammatory bowel disease. Inflamm Bowel Dis 2011; 17:1301.
  30. Roblin X, Peyrin-Biroulet L, Phelip JM, et al. A 6-thioguanine nucleotide threshold level of 400 pmol/8 x 10(8) erythrocytes predicts azathioprine refractoriness in patients with inflammatory bowel disease and normal TPMT activity. Am J Gastroenterol 2008; 103:3115.
  31. Shaye OA, Yadegari M, Abreu MT, et al. Hepatotoxicity of 6-mercaptopurine (6-MP) and Azathioprine (AZA) in adult IBD patients. Am J Gastroenterol 2007; 102:2488.
  32. Dubinsky MC, Yang H, Hassard PV, et al. 6-MP metabolite profiles provide a biochemical explanation for 6-MP resistance in patients with inflammatory bowel disease. Gastroenterology 2002; 122:904.
  33. Sparrow MP, Hande SA, Friedman S, et al. Allopurinol safely and effectively optimizes tioguanine metabolites in inflammatory bowel disease patients not responding to azathioprine and mercaptopurine. Aliment Pharmacol Ther 2005; 22:441.
  34. Sparrow MP, Hande SA, Friedman S, et al. Effect of allopurinol on clinical outcomes in inflammatory bowel disease nonresponders to azathioprine or 6-mercaptopurine. Clin Gastroenterol Hepatol 2007; 5:209.
  35. Rahhal RM, Bishop WP. Initial clinical experience with allopurinol-thiopurine combination therapy in pediatric inflammatory bowel disease. Inflamm Bowel Dis 2008; 14:1678.