SMALL BOWEL AND NUTRITION Frontline Gastroenterol: first published as 10.1136/flgastro-2016-100738 on 29 August 2016. Downloaded from REVIEW A practical guide to thiopurine prescribing and monitoring in IBD Ben Warner,1 Emma Johnston,2 Monica Arenas-Hernandez,3 Anthony Marinaki,3 Peter Irving,2 Jeremy Sanderson2 11st Floor College House, St ABSTRACT year.3 Thiopurines are commenced ’ Thomas Hospital, London, UK Thiopurines are often the mainstay of treatment earlier in CD, where aminosalicylates 2Department of Gastroenterology, Guy’s and St for many patients with inflammatory bowel play little or no role, and have been Thomas’ NHS Foundation Trust, disease. As such, a general understanding of the demonstrated to reduce the need for London, UK evidence behind their use and of their surgery by 40%.4 The SONIC (Study of 3Purine Laboratory, Viapath, St metabolism is extremely useful in clinical practice. Biologic and Immunomodulator Naive Thomas’ Hospital, London, UK This review gives a practical overview of Patients in CD) trial study showed the Correspondence to thiopurine metabolism, the importance of benefit of thiopurines in addition to anti- Dr Ben Warner, 1st Floor thiopurine S-methyltransferase testing prior to tumour necrosis factor (TNF) agents in College House, St Thomas’ Hospital, Westminster Bridge the start of therapy and the monitoring of the treatment of CD, most likely through Road, London, SE1 7EH, UK; thioguanine nucleotide levels while on reduced formation of antidrug antibodies [email protected] treatment, guiding a personalised approach to against anti-TNF agents.5 optimising thiopurine therapy. Up to a third of patients have to stop Received 27 July 2016 Revised 5 August 2016 thiopurines due to side effects, the main Accepted 8 August 2016 concerns being leucopenia (1.3–12.6%), INTRODUCTION hepatotoxicity (4%), pancreatitis (3%) Inflammatory bowel disease (IBD) is a and gastric intolerance (1.3–6%).67 chronic inflammatory disorder of the gut, Thiopurine S-methyltransferase (TPMT) divided into two main diseases, ulcerative activity influences the incidence of colitis (UC) and Crohn’s disease (CD). adverse effects, particularly bone marrow Over the last decade there has been a toxicity, however, there is no association shift in paradigm of the treatment of IBD with hepatotoxicity or pancreatitis.8 with a ‘top down’ or ‘rapid step-up’ http://fg.bmj.com/ approach aimed at altering the natural history early in the course of the disease. THIOPURINE METABOLISM Newer drugs, especially biologics, are being The prodrug AZA is converted used for the treatment of IBD, and existing non-enzymatically by biogenic thiols, drugs such as thiopurines are being used including glutathione, to MP with the more effectively by guiding individual release of methyl-4-nitro-5-imidazole. on September 29, 2021 by guest. Protected copyright. dosing according to pharmacogenetic data Following uptake by transporters, MP and monitoring of drug metabolite levels. undergoes metabolism by three compet- Sixty per cent of patients with IBD ing pathways (see figure 1) to form the receive thiopurines (azathioprine (AZA), active metabolite, thioguanine nucleo- mercaptopurine (MP) and to a lesser tides (TGNs) which function as rogue extent tioguanine) with proven efficacy in nucleic acids, disrupting the DNA replica- maintaining steroid-free remission.1 tion of the most rapidly dividing cells Although the evidence for the use of such as activated T cell lymphocytes thiopurines in UC is not as strong as in where TGN concentrations have been patients with CD, it has been demon- found to be higher and where genes To cite: Warner B, involved in T cell immunity have been Johnston E, Arenas- strated that in patients with steroid- 9 Hernandez M, et al. Frontline dependent UC, 53% achieved steroid-free shown to be downregulated by AZA. Gastroenterology Published remission on AZA compared with 21% Online First: [please include 2 Day Month Year] on aminosalicylates. Therefore, thiopur- THIOPURINE S-METHYLTRANSFERASE doi:10.1136/flgastro-2016- ines are recommended once a patient has Methylation of MP by TPMT is a critical 100738 required two courses of steroids within a step in thiopurine metabolism. It was first Warner B, et al. Frontline Gastroenterology 2016;0:1–6. doi:10.1136/flgastro-2016-100738 1 SMALL BOWEL AND NUTRITION Frontline Gastroenterol: first published as 10.1136/flgastro-2016-100738 on 29 August 2016. Downloaded from Figure 1 Azathioprine metabolism. AO, aldehyde oxidase; AZA, azathioprine; HPRT, hypoxanthine phosphoribosyltransferase; IBD, inflammatory bowel disease; IMPDH, inosine monophosphate dehydrogenase; ITPase, inosine triphosphatase; MeMP, methylmercaptopurine; MeTIMP, methylthioinosine monophosphate; MP, mercaptopurine; TGMP, thioguanine monophosphate; TGN, thioguanine nucleotide; TGTP, thioguanine triphosphate; TIMP, thioinosine monophosphate; TITP, thioinosine triphosphate; TPMT, thiopurine S-methyltransferase; TUA, thiouric acid; XO, xanthine oxidase. The metabolism of MP involves three competing pathways: the first being degradation to TUA which is then excreted, the second is through methylation by TPMT into MeMP, and the third is the breakdown of MP into TIMP catalysed by HPRT. TIMP is then further metabolised via IMPDH into TGMP. Kinases convert this into the TGNs. Approximately 15–20% of patients with IBD demonstrate hypermethylation when treated with thiopurines. This means that during thiopurine metabolism, methylated thiopurine metabolites are preferentially produced instead of TGNs. noted in the 1980s that differences in TPMT activity completely deficient patients, irrespective of whether help account for the variability in tolerance to thiopur- the patient has a rare or common variant TPMT geno- ines. Rare and common genetic polymorphisms influ- type. Other factors influencing TPMT activity, for ence enzyme function resulting in a trimodal example red cell age, also result in discordance population distribution of activity. In Caucasians, com- between genotype and phenotype, particularly in the plete TPMT deficiency occurs in 1 in 300 individuals. high carrier range.15 Carriers of a deficiency-associated allele (heterozygotes) have around 50% enzyme activity and occur at an THIOGUANINE NUCLEOTIDES approximate frequency of 1 in 10 of the population.10 Although important, TPMT polymorphisms account The majority of cases of TPMT deficiency (∼95%) are for only 10% of overall thiopurine toxicity.16 Indeed, associated with three alleles, TPMT*2, TPMT*3A and 50–75% of all patients developing leucopenia have a TPMT*3C.11 The frequency of these variants depends normal TPMT and only 3% of hypermethylators have 17 on ethnicity with TPMT*3A being more common in ultrahigh TPMT. Measuring TGNs provides a http://fg.bmj.com/ Caucasian populations.12 summary of epigenetic and genetic factors influencing Individuals with complete TPMT deficiency who thiopurine metabolism and, together with measure- receive standard doses of AZA or MP are highly likely ment of MeMP, offers a means for therapeutic drug to develop severe and potentially fatal myelosuppres- monitoring. sion. There is a small case series of patients with IBD Measurement of TGNs and MeMP in red blood treated with around 10% of standard doses, all of cells (RBCs) has been shown to be clinically useful on September 29, 2021 by guest. Protected copyright. whom tolerated this well.13 Heterozygotes are also at after steady state is reached at 4–6 weeks.18 risk of toxicity at standard doses but this is prevented Meta-analyses suggest that a therapeutic range of by initiation at 50% of the conventional dose. Hence, TGN between 235 pmol/8×108 and 450 pmol/8×108 recommended practice is to check TPMT levels prior RBCs correlates best with a good clinical response.19 20 to starting therapy and adjusting the dose according In addition, there is evidence that TGN monitoring in to TPMT status. IBD is associated with improved outcomes with a Ultrahigh TPMT (>40 pmol/mg Hb/h) is associated negative correlation between RBC TGNs and disease with a skewed drug metabolism in a selection of activity.21 A treatment strategy using TGNs to deter- patients where MP is preferentially metabolised to mine optimal dosing resulted in improved outcomes methylmercaptopurine (MeMP) resulting in lower in 90% of patients compared with 33% in those not TGNs which in turn are associated with a poorer guided by TGNs.18 clinical response and side effects. This preferential metabolism to MeMP or ‘shunting’ is known as HYPERMETHYLATION hypermethylation.14 Approximately 15–20% of patients with IBD demon- TPMT phenotyping by enzyme assay is generally strate hypermethylation when treated with thiopur- preferred to genotyping, as the assay will detect all ines.16 The usual definition of hypermethylation is a 2 Warner B, et al. Frontline Gastroenterology 2016;0:1–6. doi:10.1136/flgastro-2016-100738 SMALL BOWEL AND NUTRITION Frontline Gastroenterol: first published as 10.1136/flgastro-2016-100738 on 29 August 2016. Downloaded from ratio of MeMP to TGN of >11. In this setting, sub- disorders with EBV infection being a further risk therapeutic TGNs risk a poor response to therapy. factor.28 The risk remains small and as yet, avoidance Moreover, MeMP >5700 pmol/8×108 RBCs results of these drugs in seronegative individuals is not in a higher risk of hepatotoxicity.20 advised. European guidelines suggests considering Allopurinol is a xanthine oxidase inhibitor that pre- anti-TNF monotherapy as opposed to combination vents the breakdown of thiopurines into thiouric acid therapy with thiopurines.25 (TUA). It was designed to increase the bioavailability of MP by preventing degradation to TUA. The com- OPTIMISING CLINICAL RESPONSE bination of low dose thiopurine and 100 mg of allo- Measuring TGNs has been proven to improve clinical purinol (LDTA) corrects hypermethylation in patients outcomes.18 We check TGNs in all patients who have who have failed therapy due to hepatotoxicity or who symptoms or active disease. We also check TGNs have had a poor response to treatment in association 4 weeks after starting thiopurines or following a with subtherapeutic TGNs.14 22 When using the com- change in the dose (see figure 2).