Limited relevance and progression of histological alterations in the liver during thioguanine therapy in inflammatory bowel disease patients.

Background: Thioguanine is associated with liver toxicity, especially nodular regenerative hyperplasia (NRH). We assessed if liver histology alters during long-term maintenance treatment with thioguanine in patients with inflammatory bowel disease (IBD). Methods: Liver specimens of thioguanine treated IBD patients with at least two liver biopsies were revised by two independent liver pathologists, blinded to clinical characteristics. Alterations in histopathological findings between first and sequential liver specimen were evaluated and associated clinical data, including laboratory parameters and abdominal imaging reports, were collected. Results: Twenty-five IBD patients underwent sequential liver biopsies prior to, at time of, or after cessation of thioguanine treatment. The median time between the first and second biopsy was 25 months (range: 14-54). Except for one normal liver specimen, any degree of irregularities including inflammation, steatosis, fibrosis and some vascular disturbances were observed in the biopsies. The rates of perisinusoidal fibrosis (91%), sinusoidal dilatation (68%) and nodularity (18%) were the same in the first and second liver biopsies. A trend towards statistical significance was observed for phlebosclerosis (36% of the first vs. 68% of the second biopsies, p = .092). Presence of histopathological liver abnormalities was not associated with clinical outcomes. Furthermore, two patients in this cohort had portal hypertension in presence of phlebosclerosis. In another two patients, nodularity of the liver resolved upon thioguanine withdrawal. Conclusion: Vascular abnormalities of the liver were commonly observed in thioguanine treated IBD patients, although these were not progressive and remained of limited clinical relevance over time.

Exploring the role of oxidative stress and the effect of N-acetylcysteine in thiopurine-induced liver injury in inflammatory bowel disease: A randomized crossover pilot study.

Thiopurine treatment is regularly complicated by drug-induced liver injury. It has been suggested that oxidative stress may play a synergistic role. To assess whether thiopurine-induced liver injury coincides with increased oxidative stress and whether co-administration with N-acetylcysteine is protective, we performed a randomized open label crossover pilot study in inflammatory bowel disease patients with thiopurine-induced increased serum liver tests. The study comprised four stages of 4 weeks. Patients received no additional therapy followed by N-acetylcysteine 1200 mg twice a day, or the other way around, alongside ongoing thiopurine treatment. The third and fourth stages comprised a washout period and thiopurine reintroduction period. Nine patients completed the study, and the addition of N-acetylcysteine decreased myeloperoxidase concentrations (33.6-24.5 pmol/L, p = 0.038). The other biomarkers remained unchanged, including thiopurine metabolites, xanthine oxidase activity, thiopurine S-methyltransferase activity and serum liver enzyme activity tests. Reintroduction of thiopurines led to an increase of F2-isoprostanes (101-157 ng/mmol, p = 0.038), but not of serum liver enzyme activity tests. Results suggests that thiopurines may increase oxidative stress and although the addition of N-acetylcysteine led to a decrease in plasma myeloperoxidase concentrations, it does not protect from thiopurine-induced increase of serum liver tests.

Therapeutic drug monitoring of methotrexate in patients with Crohn's disease.

BACKGROUND: Therapeutic drug monitoring (TDM) has the potential to improve efficacy and diminish side effects. Measuring methotrexate-polyglutamate (MTX-PG) in erythrocytes might enable TDM for methotrexate in patients with Crohn's disease (CD). AIM: To investigate the relationship between MTX-PGs and methotrexate drug survival, efficacy and toxicity METHODS: In a multicentre prospective cohort study, patients with CD starting subcutaneous methotrexate without biologics were included and followed for 12 months. Primary outcome was subcutaneous methotrexate discontinuation or requirement for step-up therapy. Secondary outcomes included faecal calprotectin (FCP), Harvey Bradshaw Index (HBI), hepatotoxicity and gastrointestinal intolerance. Erythrocyte MTX-PGs were analysed at weeks 8, 12, 24 and 52 or upon treatment discontinuation. RESULTS: We included 80 patients with CD (mean age 55 ± 13y, 35% male) with a median FCP of 268 µg/g (IQR 73-480). After the 12-month visit, 21 patients (26%) were still on subcutaneous methotrexate monotherapy. Twenty-one patients stopped because of disease activity, 29 because of toxicity, and four for both reasons. Five patients ended study participation or stopped methotrexate for another reason. A higher MTX-PG3 concentration was associated with a higher rate of methotrexate drug survival (HR 0.86, 95% CI 0.75-0.99), lower FCP (ß -3.7, SE 1.3, p < 0.01) and with biochemical response (FCP ≤250 if baseline >250 µg/g; OR 1.1, 95% CI 1.0-1.3). Higher MTX-PGs were associated with less gastrointestinal intolerance. There was no robust association between MTX-PGs and HBI or hepatotoxicity. CONCLUSIONS: Higher MTX-PG3 concentrations are related to better methotrexate drug survival and decreased FCP levels. Therefore, MTX-PG3 could be used for TDM if a target concentration can be established.

Exposure to Thioguanine During 117 Pregnancies in Women With Inflammatory Bowel Disease.

BACKGROUND: Safety of thioguanine in pregnant patients with inflammatory bowel disease [IBD] is sparsely recorded. This study was aimed to document the safety of thioguanine during pregnancy and birth. METHODS: In this multicentre case series, IBD patients treated with thioguanine during pregnancy were included. Data regarding disease and medication history, pregnancy course, obstetric complications, and neonatal outcomes were collected. RESULTS: Data on 117 thioguanine-exposed pregnancies in 99 women were collected. Most [78%] had Crohn's disease and the mean age at delivery was 31 years. In 18 pregnancies [15%], IBD flared. Obstetric and infectious complications were seen in 15% [n = 17] and 7% [n = 8] of pregnancies, respectively. Ten pregnancies [8.5%] resulted in a first trimester miscarriage, one in a stillbirth at 22 weeks of gestational age and one in an induced abortion due to trisomy 21. In total, 109 neonates were born from 101 singleton pregnancies and four twin pregnancies. One child was born with a congenital abnormality [cleft palate]. In the singleton pregnancies, 10 children were born prematurely and 10 were born small for gestational age. Screening for myelosuppresion was performed in 16 neonates [14.7%]; two had anaemia in umbilical cord blood. All outcomes were comparable to either the general Dutch population or to data from three Dutch cohort studies on the use of conventional thiopurines in pregnant IBD patients. CONCLUSION: In this large case series, the use of thioguanine during pregnancy is not associated in excess with adverse maternal or neonatal outcomes.

H1N1 vaccines in a large observational cohort of patients with inflammatory bowel disease treated with immunomodulators and biological therapy.

BACKGROUND: Safety data are lacking on influenza vaccination in general and on A (H1N1)v vaccination in particular in patients with inflammatory bowel disease (IBD) receiving immmunomodulators and/or biological therapy. AIMS AND METHODS: The authors conducted a multicentre observational cohort study to evaluate symptoms associated with influenza H1N1 adjuvanted (Pandemrix, Focetria, FluvalP) and non-adjuvanted (Celvapan) vaccines and to assess the risk of flare of IBD after vaccination. Patients with stable IBD treated with immunomodulators and/or biological therapy were recruited from November 2009 until March 2010 in 12 European countries. Harvey-Bradshaw Index and Partial Mayo Score were used to assess disease activity before and 4 weeks after vaccination in Crohn's disease (CD) and ulcerative colitis (UC). Vaccination-related events up to 7 days after vaccination were recorded. RESULTS: Of 575 patients enrolled (407 CD, 159 UC and nine indeterminate colitis; 53.9% female; mean age 40.3 years, SD 13.9), local and systemic symptoms were reported by 34.6% and 15.5% of patients, respectively. The most common local and systemic reactions were pain in 32.8% and fatigue in 6.1% of subjects. Local symptoms were more common with adjuvanted (39.3%) than non-adjuvanted (3.9%) vaccines (p < 0.0001), whereas rates of systemic symptoms were similar with both types (15.0% vs 18.4%, p = 0.44). Among the adjuvanted group, Pandemrix more often induced local reactions than FluvalP and Focetria (51.2% vs 27.6% and 15.4%, p < 0.0001). Solicited adverse events were not associated with any patient characteristics, specific immunomodulatory treatment, or biological therapy. Four weeks after vaccination, absence of flare was observed in 377 patients with CD (96.7%) and 151 with UC (95.6%). CONCLUSION: Influenza A (H1N1)v vaccines are well tolerated in patients with IBD. Non-adjuvanted vaccines are associated with fewer local reactions. The risk of IBD flare is probably not increased after H1N1 vaccination.

Validation and update of a prediction model for risk of relapse after cessation of anti-TNF treatment in Crohn's disease.

BACKGROUND: Anti-tumor necrosis factor (TNF) therapy is effective for the treatment of Crohn's disease. Cessation may be considered in patients with a low risk of relapse. We aimed to externally validate and update our previously developed prediction model to estimate the risk of relapse after cessation of anti-TNF therapy. METHODS: We performed a retrospective cohort study in 17 Dutch hospitals. Crohn's disease patients in clinical, biochemical or endoscopic remission were included after anti-TNF cessation. Primary outcome was a relapse necessitating treatment. Discrimination and calibration of the previously developed model were assessed. After external validation, the model was updated. The performance of the updated prediction model was assessed in internal-external validation and by using decision curve analysis. RESULTS: 486 patients were included with a median follow-up of 1.7 years. Relapse rates were 35 and 54% after 1 and 2 years. At external validation, the discriminative ability of the prediction model was equal to that found at the development of the model [c-statistic 0.58 (95% confidence interval (CI) 0.54-0.62)], though the model was not well-calibrated on our cohort [calibration slope: 0.52 (0.28-0.76)]. After an update, a c-statistic of 0.60 (0.58-0.63) and calibration slope of 0.89 (0.69-1.09) were reported in internal-external validation. CONCLUSION: Our previously developed and updated prediction model for the risk of relapse after cessation of anti-TNF in Crohn's disease shows reasonable performance. The use of the model may support clinical decision-making to optimize patient selection in whom anti-TNF can be withdrawn. Clinical validation is ongoing in a prospective randomized trial.

Systematic Review of Development and Content Validity of Patient-reported Outcome Measures in Inflammatory Bowel Disease: Do We Measure What We Measure?

BACKGROUND AND AIMS: Patient-reported outcome measures are increasingly important in daily care and research in inflammatory bowel disease [IBD]. This study provides an overview of the content and content validity of IBD-specific patient-reported outcome measures on three selected constructs. METHODS: Databases were searched up to May 2019 for development and/or content validity studies on IBD-specific self-report measures on health-related quality of life, disability, and self-report disease activity in adults. Evidence was synthesised on content validity in three aspects: relevance, comprehensiveness, and comprehensibility following the COnsensus-based Standards for the selection of health Measurement INstruments methodology. Questionnaire items were organised in themes to provide an overview of important aspects of these constructs. RESULTS: For 14/44 instruments, 25 content validity studies were identified and 25/44 measures had sufficient content validity, the strongest evidence being of moderate quality, though most evidence is of low or very low quality. The Crohn's Life Impact Questionnaire and IBD questionnaire-32 on quality of life, the IBD-Control on disease activity, and the IBD Disability Index Self-Report and its 8-item version on disability, have the strongest evidence of sufficient relevance, comprehensiveness, and comprehensibility, ranging from moderate to very low quality. A fair number of recurring items themes, possibly important for the selected constructs, was identified. CONCLUSIONS: The body of evidence for content validity of IBD-specific health-related quality of life, self-report disease activity, and disability self-report measures is limited. More content validity studies should be performed after reaching consensus on the constructs of interest for IBD, and studies should involve patients.

Leukopenia due to parvovirus B19 in a Crohn's disease patient using azathioprine.

Thiopurines such as azathioprine (AZA) and 6-mercaptopurine are frequently used for the treatment of inflammatory bowel diseases. Patients with low or absent thiopurine S-methyltransferase (TPMT) activity, resulting in high 6-thioguanine nucleotide levels, have an increased risk of developing leukopenia. Alternatively, certain viral infections could induce leukopenia. We present the case of an adult Crohn's disease patient with a parvovirus B19 infection and leukopenia during long-term AZA therapy. The uncomplicated long-term use of adequately-dosed AZA and stable non-toxic metabolite levels could not acknowledge TPMT deficiency as a primary cause of the leukopenia. parvovirus B19 was assumed to induce the leukopenia by restraining myeloid proliferation. In addition, AZA probably potentiated susceptibility to this viral infection and may have inhibited adequate immunological defense. Leukopenia during thiopurine therapy not explained by TPMT deficiency could be induced by parvovirus B19 infection and compels temporal but not permanent cessation of thiopurine therapy.

Nodular Regenerative Hyperplasia of the Liver in Patients with IBD Treated with Allopurinol-Thiopurine Combination Therapy.

BACKGROUND: Thiopurine therapy, particularly thioguanine, has been associated with nodular regenerative hyperplasia (NRH) of the liver. Combination therapy of allopurinol and an adapted low-dose thiopurine leads to a pharmacokinetic profile that has similarities to that of thioguanine. Therefore, allopurinol-thiopurine combination therapy may also be associated with NRH of the liver. We assessed the prevalence of NRH in patients with inflammatory bowel disease (IBD) treated with allopurinol-thiopurine combination therapy by liver biopsy specimen examination. METHODS: An observational, cross-sectional study was conducted in a Dutch IBD-referral center. Adult patients with IBD, treated for at least 1 year with allopurinol-thiopurine combination therapy were eligible. All patients underwent a liver biopsy, after standard laboratory and thiopurine metabolite concentration assessments. Histopathology was assessed by an experienced liver pathologist. RESULTS: Twenty-two patients with IBD were included. The mean duration of combination therapy at the time of the liver biopsy was 24.7 months (SD 5.7). NRH was observed in one of the biopsies (4.8%), any grade of nodularity was observed in 3 biopsy specimens (14%). Other findings included phlebosclerosis (24%), perisinusoidal fibrosis (81%), sinusoidal dilatation (43%), perivenular fibrosis (14%), and periportal fibrosis (29%). Around the time of biopsy, the median 6-thioguanine nucleotide and 6-methylmercaptopurine ribonucleotide concentrations were 705 pmol × 10 red blood cells (RBC) (interquartile range 498-915) and 355 pmol × 10 RBC (interquartile range 225-670). CONCLUSIONS: The prevalence of histologically assessed NRH in patients with IBD, who were treated with allopurinol-thiopurine combination therapy, was 5%. This percentage is in line with thiopurine-naive and thioguanine-using patients with IBD. None of the included patients had clinical symptoms or signs suggestive of (noncirrhotic) portal hypertension.

The Prevalence of Nodular Regenerative Hyperplasia in Inflammatory Bowel Disease Patients Treated with Thioguanine Is Not Associated with Clinically Significant Liver Disease.

BACKGROUND: Nodular regenerative hyperplasia (NRH) of the liver is associated with inflammatory-mediated diseases and certain drugs. There is conflicting data on the prevalence of NRH and its clinical implications in inflammatory bowel disease (IBD) patients treated with thioguanine. METHODS: A retrospective cohort study involving 7 Dutch centers comprised all IBD patients who were being treated with thioguanine and underwent a liver biopsy as part of the standard toxicity screening. Liver biopsy specimens were reviewed by 2 experienced liver pathologists. Clinical data as well as liver chemistry, blood counts, and abdominal imaging were collected. RESULTS: One hundred eleven IBD patients who submitted to liver biopsy were treated with thioguanine in a daily dose of 0.3 mg/kg for a median duration of 20 (4-64) months. NRH was detected in 6% of patients (7; 95% confidence interval, 3-14 patients). Older age (P = 0.02), elevated gamma-glutamyl transferase (P = 0.01) and alkaline phosphatase (P = 0.01) levels, a higher mean corpuscular volume (P = 0.02), and a lower platelet or leukocyte count (P < 0.01 and P = 0.02, respectively) were associated with NRH. Three of the 7 patients with NRH did not have any associated clinical symptoms or signs. The other 4 had minor biochemical abnormalities only. Ultrasonography revealed splenomegaly in 3 of the 78 patients (4%; 95% confidence interval, 0%-9%), only one of whom had NRH. There was no clinically overt portal hypertension. CONCLUSIONS: The prevalence of NRH was 6% in liver biopsies obtained from IBD patients treated with thioguanine. Histopathological irregularities including NRH were not associated with clinically significant findings over the period of observation.

Diagnosing Nodular Regenerative Hyperplasia of the Liver Is Thwarted by Low Interobserver Agreement.

BACKGROUND AND AIMS: Nodular regenerative hyperplasia (NRH) of the liver is associated with several diseases and drugs. Clinical symptoms of NRH may vary from absence of symptoms to full-blown (non-cirrhotic) portal hypertension. However, diagnosing NRH is challenging. The objective of this study was to determine inter- and intraobserver agreement on the histopathologic diagnosis of NRH. METHODS: Liver specimens (n=48) previously diagnosed as NRH, were reviewed for the presence of NRH by seven pathologists without prior knowledge of the original diagnosis or clinical background. The majority of the liver specimens were from thiopurine using inflammatory bowel disease patients. Histopathologic features contributing to NRH were also assessed. Criteria for NRH were modified by consensus and subsequently validated. Interobserver agreement was evaluated by using the standard kappa index. RESULTS: After review, definite NRH, inconclusive NRH and no NRH were found in 35% (23-40%), 21% (13-27%) and 44% (38-56%), respectively (median, IQR). The median interobserver agreement for NRH was poor (κ = 0.20, IQR 0.14-0.28). The intraobserver variability on NRH ranged between 14% and 71%. After modification of the criteria and exclusion of biopsies with technical shortcomings, the interobserver agreement on the diagnosis NRH was fair (κ = 0.45). CONCLUSIONS: The interobserver agreement on the histopathologic diagnosis of NRH was poor, even when assessed by well-experienced liver pathologists. Modification of the criteria of NRH based on consensus effort and exclusion of biopsies of poor quality led to a fairly increased interobserver agreement. The main conclusion of this study is that NRH is a clinicopathologic diagnosis that cannot reliably be based on histopathology alone.

Hepatotoxicity associated with 6-methyl mercaptopurine formation during azathioprine and 6-mercaptopurine therapy does not occur on the short-term during 6-thioguanine therapy in IBD treatment.

BACKGROUND AND AIMS: High concentrations of methylated thiopurine metabolites, such as 6-methyl mercaptopurine, are associated with hepatotoxicity during administration of the conventional thiopurines azathioprine or 6-mercaptopurine in IBD patients. Metabolization of the non-conventional thiopurine 6-thioguanine does not generate 6-methyl mercaptopurine. Hence, the aim of our study was to evaluate hepatotoxicity during 6-thioguanine in IBD patients who previously failed conventional thiopurines due to 6-methyl mercaptopurine associated hepatotoxicity. METHODS: A retrospective single center intercept cohort study was performed of IBD patients using 6-thioguanine between January 2006 and July 2010 after failing conventional thiopurine therapy due to 6-methyl mercaptopurine associated hepatotoxicity. The primary outcome was the occurrence of 6-thioguanine induced hepatotoxicity, scaled according to the Common Terminology Criteria for Adverse Events. RESULTS: Nineteen patients were included. Median duration of 6-thioguanine therapy (median daily dosage 21 mg (9-24)) was 23 weeks (6-96). Hepatotoxicity did not reoccur in 15 out of 19, whereas grade 1 toxicity persisted in 4 patients (p<0.001). Median aspartate aminotransferase and alanine aminotransferase concentrations decreased from 34 U/l (20-59) and 64 U/l (15-175) to 23 U/l (18-40; p=0.003) and 20 U/l (14-48; p=0.019), respectively. CONCLUSION: Hepatotoxicity does not reoccur during 6-thioguanine treatment in most IBD patients who failed conventional thiopurines due to 6-methyl mercaptopurine associated hepatotoxicity. Hence, at least at short-term, 6-thioguanine appears a justifiable alternative thiopurine for these IBD patients.

Allopurinol enhances the activity of hypoxanthine-guanine phosphoribosyltransferase in inflammatory bowel disease patients during low-dose thiopurine therapy: preliminary data of an ongoing series.

Thiopurines are crucial in the treatment of inflammatory bowel disease. The phenotype of pivotal metabolic enzymes determines whether thioguanine nucleotides (6-TGN) are generated in clinically sufficiently high levels. The first step in activation of thiopurine prodrugs to 6-TGN is catalysis by hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Often, patients exhibit a clinically unfavorable metabolism, leading to discontinuation of conventional thiopurine therapy. The combination of allopurinol and low-dose thiopurine therapy may optimize this variant metabolism, presumably by affecting enzyme activities. We performed a prospective pharmacodynamic study to determine the effect of combination therapy on the activity of HGPRT. The activity of HGPRT and 6-TGN concentrations was measured in red blood cells during thiopurine monotherapy and after 4 weeks of combination therapy. The activity of HGPRT was also measured after 12 weeks of combination therapy. From the results, we conclude that combination therapy increases the activity of HGPRT and subsequently 6-TGN concentrations.

Difficulties and possibilities with thiopurine therapy in inflammatory bowel disease--proceedings of the first Thiopurine Task Force meeting.

BACKGROUND: Thiopurines, such as azathioprine and mercaptopurine, are of pivotal importance in the treatment of inflammatory bowel disease. Although these drugs have been used for several decades, still many questions remain unanswered. AIM: To provide an overview of clinically and scientifically challenging topics concerning thiopurine therapy in inflammatory bowel disease treatment. METHODS: The first meeting of the Thiopurine Task Force Interest Group was held during the 2009 United European Gastroenterology Week in London (GASTRO2009). The topics of this meeting were of particular clinical and scientific interest. Additional literature was identified by performing a Pubmed search using the search terms 'inflammatory bowel disease', 'azathioprine', '6-mercaptopurine' and 'thioguanine'. RESULTS: The following topics were discussed: therapeutic drug monitoring; the synergy of thiopurines with aminosalicylates and allopurinol; serious adverse events such as opportunistic infections, hepatotoxicity, carcinogenicity and pancreatitis; prolongation of thiopurines during clinical remission; indications for thiopurines in the postoperative setting; and the potential use of thioguanine. Specific interesting and clinically relevant topics for potential future research are provided. CONCLUSIONS: Thiopurines remain central to inflammatory bowel disease treatment, although future studies are required to substantiate a more personalised medicine approach to their use.

Prolonged thioguanine therapy is well tolerated and safe in the treatment of ulcerative colitis.

BACKGROUND: Thioguanine has been used for the treatment of inflammatory bowel disease, in particular for patients who failed conventional thiopurine therapy. To date, thioguanine has been infrequently studied in ulcerative colitis. AIM: To evaluate the tolerability, safety and efficacy of thioguanine in the treatment of ulcerative colitis. METHODS: A database analysis was performed on inflammatory bowel disease patients who had failed conventional thiopurine therapy and were treated with thioguanine. Rates and reasons for treatment failure were assessed. Laboratory values, abdominal ultrasonography, liver biopsy and endoscopic remission rates were evaluated. RESULTS: Forty-six patients were included and median treatment duration was 22 months (range 0.3-72.0). Nine patients failed thioguanine therapy: six due to adverse events, three due to therapy resistance. Concomitant treatment with aminosalicylates protected against thioguanine failure (hazard ratio (HR) 0.11, 95% CI 0.03-0.48). When performed, ultrasonography (n = 21) revealed no suspected therapy-related pathology in all but one patient, in whom hepatomegaly was observed. Liver histology (n = 12) predominantly revealed no abnormalities (n = 4) or non-specific regeneration (n = 4); none showed nodular regenerative hyperplasia. At follow-up, 40% of colonoscopies revealed endoscopic remission as compared with 10% at baseline (P = 0.180). CONCLUSIONS: Long-term use of thioguanine appears to be well tolerated and relatively safe in ulcerative colitis patients who failed conventional thiopurine therapy.

Dosing 6-thioguanine in inflammatory bowel disease: expert-based guidelines for daily practice.

Conventional thiopurines are considered to be effective and safe in the treatment of inflammatory bowel disease (IBD) patients; unfortunately more than 50% of patients discontinue thiopurine therapy, mainly due to the development of intractable adverse events. In recent years, the use of 6-thioguanine has been proposed as an alternative thiopurine in IBD patients failing to tolerate or to respond to conventional thiopurine therapy. In this clinical review, we describe the rationale for 6-thioguanine therapy and discuss the reported hepatotoxicity of 6-thioguanine (especially nodular regenerative hyperplasia). We propose expert-based guidelines for balanced treatment.

On therapeutic drug monitoring of thiopurines in inflammatory bowel disease; pharmacology, pharmacogenomics, drug intolerance and clinical relevance.

Thiopurines such as azathioprine, 6-mercaptopurine and 6-thioguanine are antimetabolites that have been used for several decades in the treatment of several diseases including inflammatory bowel diseases. Additional anti-inflammatory properties of these thiopurines have been discovered in recent years. Thiopurine metabolism is complex due to the involvement of multiple enzymes, of which the activities are genetically determined and cell type dependent. Single nucleotide polymorphisms in the genes encoding these enzymes have been correlated with altered activities and drug intolerance. Detailed implications of these will be reviewed. Over the years several methods of therapeutic drug monitoring have been developed in an attempt to relate thiopurine drug availability with efficacy and intolerance. In this respect, monitoring pharmacologically active 6-thioguanine nucleotide concentrations is most widely used. So far, however, the clinical usefulness of these methods is hampered by methodological limitations. Some drug interactions may optimize the metabolization of thiopurines and consequently increase its efficacy and decrease drug intolerance. This review focuses on the clinical relevance and usefulness of therapeutic drug monitoring of thiopurines and provides suggestions to optimize thiopurine therapy in the treatment of inflammatory bowel diseases.