Azathioprine is a pro-drug that is converted into 6-mercaptopurine (6-MP)  and is subsequently metabolized by one of three different enzymes to form either active or inactive metabolites .

There is substantial inter-individual variation in azathioprine metabolism so an understanding of these mechanisms is essential to safe and effective administration of thiopurine therapy. Metabolism of 6-MP by xanthine oxidase produces 6-thiouric acid, which is inactive.  Metabolism by hypoxanthine phosphoribosyltransferase produces the active 6-thioguanine (6-TG) nucleotides which are associated with improved efficacy in Crohn’s disease, but also an increased risk of leukopenia. Lastly, metabolism by thiopurine methyltransferase (TPMT)  produces an inactive metabolite, 6- methylmercaptopurine (6-MMP), which can be hepatotoxic at higher concentrations. The active metabolite, 6-TG, is a purine analogue that incorporates into cellular DNA, inhibits proliferation of lymphocytes, and stimulates apoptosis of T cells in the lamina propria. There are genetic polymorphisms in TPMT enzyme activity that profoundly impact the metabolism of azathioprine.  It is advised to check TPMT activity prior to initiating thiopurines in order to select optimal dosing and avoid toxicity. One in 300 patients has very low or absent TPMT activity, which can lead to severe leukopenia even with small doses of azathioprine and another 11 % have intermediate enzyme activity and may require dose reduction .  For patients with a high 6-MMP to 6-TG ratio, a xanthine oxidase inhibitor, allopurinol, can be added in conjunction with thiopurine dose reduction in order to favorably alter the 6-TG and 6-MMP metabolite profile. The addition of allopurinol in these patients has been shown to improve disease activity scores,  reduce daily prednisone dosage, and decrease aminotransferase levels .

However, this strategy carries a significant risk of leukopenia and requires close follow-up and careful monitoring.

The initial report of successful azathioprine use for IBD was in 1969 in a case series of six patients with severe Crohn’s disease .

Subsequently a 2-year randomized controlled trial in Crohn’s disease showed patients who received 6-MP were more likely to experience clinical improvement (67 % versus 8 %), closure of fistulas, and reduction in steroid dosing than those who received placebo. The response to azathioprine took between 3 and 6 months to see full effect. A second randomized controlled trial in 1995 evaluated patients concurrently treated with a 12-week tapering course of prednisolone in addition to azathioprine 2.5 mg/kg versus placebo. This landmark trial showed a significant difference in steroid-free remission at 15 months, 39 but no difference at 12 weeks, again suggesting that thiopurines are effective in maintaining remission in Crohn’s disease,  but may not be effective for short-term induction therapy.

Despite these early studies showing significant benefit for thiopurines, enthusiasm for this class of medication has been tempered by recent prospective studies with larger patient enrollment demonstrating a more modest treatment benefit. A 2013 open-label trial of adults recently diagnosed with Crohn’s disease (within 6 months) evaluated the early use of azathioprine compared with conventional management (defined as azathioprine only in case of steroid dependency, frequent flares,  severe perianal disease,  or poor steroid response). There was no difference in time spent in steroid and anti-TNF free remission during the first 3 years when comparing the two groups, although there was a lower rate of perianal surgery in the early azathioprine group.

By then end of the study 61 % of the patients in the conventional therapy group had been started on azathioprine. A similar double blind prospective study of azathioprine versus placebo within 8 weeks of disease diagnosis (only other medication allowed was corticosteroids) showed no difference in corticosteroid free remission at 76 weeks, although azathioprine did reduce the rate of moderate-severe relapse .  These studies suggest there is limited benefit to monotherapy with thiopurines early in the course of disease, for the endpoint of steroid-free remission.

Long-term cohort studies have shown that early azathioprine use within 3 years of initial diagnosis was associated with a reduced risk of initial abdominal surgery (HR 0.45), recurrent abdominal surgery (HR 0.44), and perianal surgery (HR 0.30). Overall, the risk of first major abdominal surgery in this cohort was 17.5 % at 1 year, 28.4 %  at 5 years,  and 39.5 %  at 10 years.

Similarly,  a recently published large meta- analysis of 17 observational studies and 21,632 patients showed a 40 % reduced risk of surgical resection in patients who received thiopurines.  These studies suggest that thiopurine therapy may have important long-term protective effects,  perhaps unseen in shorter efficacy studies.

Side effects with thiopurines are common and are an important consideration when treating patients with Crohn’s disease.  In a long-term prospectively maintained database of Spanish IBD patients on thiopurines over a median follow-up of 44 months, 17 % of patients discontinued treatment due to adverse events .  The most frequent side effects were nausea (8 %), hepatotoxicity (4 %), myelotoxicity (4 %), and pancreatitis (4 %). Other potential adverse events associated with azathioprine include allergic reactions,  rash,  fever,  headache,  fatigue, anorexia,  cholestatic hepatitis,  and arthralgias.

Approximately 50 %  of patients who discontinue azathioprine due to adverse events can subsequently tolerate 6-MP, but this should not be prescribed to patients who experienced bone marrow suppression or pancreatitis.

As many as 1.8 % of patients experience a serious infection with long-term azathioprine use.

Interestingly, in a 5-year follow-up analysis of the TREAT registry (a large prospective observational research program to evaluate the long- term safety of medications for Crohn’s), thiopurine immunosuppressives were not independently associated with a significantly increased risk of serious infection or mortality.

Studies have demonstrated an approximately three to sevenfold increased risk of non- Hodgkin’s lymphoma in patients treated with thiopurines, although the absolute risk is still very low.

This risk increases with age, and also increases significantly after 1- 2 years of thiopurine exposure, but returns to baseline after thiopurines are stopped. Cases of hepatosplenic T cell lymphoma have been reported in IBD patients taking thiopurines.  This is a rare,  aggressive, almost universally fatal extranodal lymphoma that primarily affects males younger than 35.

Among reported cases, the vast majority had at least 2 years of thiopurine exposure,  with a median duration of 5 years. Thiopurines are also associated with an increased risk of nonmelanoma skin cancer with a pooled adjusted hazard ratio of 2.28. Interestingly, the statistical significance was lost when studies with shorter follow-up (less than 6 years)  were excluded, suggesting a possible surveillance bias.

Regardless, patients receiving thiopurines should be counseled on the importance of sun protection and regular skin exams. 

### R. A. Fausel , MD
T. L. Zisman , MD, MPH
Division of Gastroenterology, University of Washington Medical Center , 1959 NE Pacific Street, Box 356424 , Seattle , WA 98195 , USA

References

1. Sands BE, Siegel CA. Chapter 111: Crohn's disease. In: Sleisenger MH, Feldman M, Friedman LS, Brandt LJ, editors. Sleisenger and Fordtran's gastrointestinal and liver disease pathophysiology, diagnosis, management. 9th ed. Philadelphia: Saunders/Elsevier; 2010. p. 1 online resource (2 volumes (xxv, 2299, xc pages)).
2. Summers RW, Switz DM, Sessions Jr JT, Becktel JM, Best WR, Kern Jr F, et al. National Cooperative Crohn's Disease Study: results of drug treatment. Gastroenterology. 1979;77(4 Pt 2):847- 69.
3. Singleton JW, Hanauer SB, Gitnick GL, Peppercorn MA, Robinson MG, Wruble LD, et al. Mesalamine capsules for the treatment of active Crohn's disease: results of a 16-week trial. Pentasa Crohn's Disease Study Group. Gastroenterology. 1993; 104(5):1293- 301.
4. Hanauer SB, Stromberg U. Oral Pentasa in the treatment of active Crohn's disease: a meta-analysis of double-blind, placebo-controlled trials. Clin Gastroenterol Hepatol. 2004;2(5):379- 88.
5. Lim WC, Hanauer S. Aminosalicylates for induction of remission or response in Crohn's disease. Cochrane Database System Rev. 2010(12):CD008870.
6. Ford AC, Kane SV, Khan KJ, Achkar JP, Talley NJ, Marshall JK, et al. Efficacy of 5-aminosalicylates in Crohn's disease: systematic review and meta- analysis. Am J Gastroenterol. 2011;106(4):617- 29.
7. Box SA, Pullar T. Sulphasalazine in the treatment of rheumatoid arthritis. Br J Rheumatol. 1997;36(3): 382- 6.
8. Navarro F, Hanauer SB. Treatment of inflammatory bowel disease: safety and tolerability issues. Am J Gastroenterol. 2003;98(12 Suppl):S18- 23.
9. Muller AF, Stevens PE, McIntyre AS, Ellison H, Logan RF. Experience of 5-aminosalicylate nephrotoxicity in the United Kingdom. Aliment Pharmacol Ther. 2005;21(10):1217- 24.
10. Sutherland L, Singleton J, Sessions J, Hanauer S, Krawitt E, Rankin G, et al. Double blind, placebo controlled trial of metronidazole in Crohn's disease. Gut. 1991;32(9):1071- 5.
11. Prantera C, Zannoni F, Scribano ML, Berto E, Andreoli A, Kohn A, et al. An antibiotic regimen for the treatment of active Crohn's disease: a randomized, controlled clinical trial of metronidazole plus ciprofioxacin. Am J Gastroenterol. 1996;91(2):328- 32.
12. Rutgeerts P, Hiele M, Geboes K, Peeters M, Penninckx F, Aerts R, et al. Controlled trial of metronidazole treatment for prevention of Crohn's recurrence after ileal resection. Gastroenterology. 1995;108(6):1617- 21.
13. Bernstein LH, Frank MS, Brandt LJ, Boley SJ. Healing of perineal Crohn's disease with metronidazole. Gastroenterology. 1980;79(2):357- 65.
14. Brandt LJ, Bernstein LH, Boley SJ, Frank MS. Metronidazole therapy for perineal Crohn's disease: a follow-up study. Gastroenterology. 1982;83(2):383- 7.
15. Thia KT, Mahadevan U, Feagan BG, Wong C, Cockeram A, Bitton A, et al. Ciprofioxacin or metronidazole for the treatment of perianal fistulas in patients with Crohn's disease: a randomized, double-blind, placebo-controlled pilot study. Inflamm Bowel Dis. 2009;15(1):17- 24.

16. Dewint P, Hansen BE, Verhey E, Oldenburg B, Hommes DW, Pierik M, et al. Adalimumab combined with ciprofioxacin is superior to adalimumab monotherapy in perianal fistula closure in Crohn's disease: a randomised, double-blind, placebo controlled trial (ADAFI). Gut. 2014;63(2):292- 9.
17. West RL, van der Woude CJ, Hansen BE, Felt- Bersma RJ, van Tilburg AJ, Drapers JA, et al. Clinical and endosonographic effect of ciprofioxacin on the treatment of perianal fistulae in Crohn's disease with infiiximab: a double-blind placebo- controlled study. Aliment Pharmacol Ther. 2004;20(11- 12):1329- 36.
18. Jigaranu AO, Nedelciuc O, Blaj A, Badea M, Mihai C, Diculescu M, et al. Is rifaximin effective in maintaining remission in Crohn's disease? Dig Dis. 2014;32(4):378- 83.
19. Prantera C, Lochs H, Campieri M, Scribano ML, Sturniolo GC, Castiglione F, et al. Antibiotic treatment of Crohn's disease: results of a multicentre, double blind, randomized, placebo-controlled trial with rifaximin. Aliment Pharmacol Ther. 2006;23(8):1117- 25.
20. Prantera C, Lochs H, Grimaldi M, Danese S, Scribano ML, Gionchetti P. Rifaximin-extended intestinal release induces remission in patients with moderately active Crohn's disease. Gastroenterology. 2012;142(3):473- 81.e4.
21. Khan KJ, Ullman TA, Ford AC, Abreu MT, Abadir A, Marshall JK, et al. Antibiotic therapy in inflammatory bowel disease: a systematic review and meta- analysis. Am J Gastroenterol. 2011;106(4):661- 73.
22. Malchow H, Ewe K, Brandes JW, Goebell H, Ehms H, Sommer H, et al. European Cooperative Crohn's Disease Study (ECCDS): results of drug treatment. Gastroenterology. 1984;86(2):249- 66.
23. Hayashi R, Wada H, Ito K, Adcock IM. Effects of glucocorticoids on gene transcription. Eur J Pharmacol. 2004;500(1- 3):51- 62.
24. Faubion Jr WA, Loftus Jr EV, Harmsen WS, Zinsmeister AR, Sandborn WJ. The natural history of corticosteroid therapy for infiammatory bowel disease: a population-based study. Gastroenterology. 2001;121(2):255- 60.
25. Benchimol EI, Seow CH, Steinhart AH, Griffiths AM. Traditional corticosteroids for induction of remission in Crohn's disease. Cochrane Database System Rev. 2008(2):CD006792.
26. Ford AC, Bernstein CN, Khan KJ, Abreu MT, Marshall JK, Talley NJ, et al. Glucocorticosteroid therapy in inflammatory bowel disease: systematic review and meta-analysis. Am J Gastroenterol. 2011;106(4):590- 9. quiz 600.
27. Lichtenstein GR, Feagan BG, Cohen RD, Salzberg BA, Diamond RH, Price S, et al. Serious infection and mortality in patients with Crohn's disease: more than 5 years of follow-up in the TREAT registry. Am J Gastroenterol. 2012;107(9):1409- 22.
28. Curkovic I, Egbring M, Kullak-Ublick GA. Risks of infiammatory bowel disease treatment with glucocorticosteroids and aminosalicylates. Dig Dis. 2013;31(3- 4):368- 73.