Pharmacokinetics of 6-thioguanine in patients with inflammatory bowel disease

Key factors associated with 6-thioguanine and 6-methylmercaptopurine nucleotide concentrations in children treated by thiopurine for acute leukaemia and inflammatory bowel disease

AIMS

Azathioprine (AZA) and 6-mercaptopurine are prescribed in acute lymphoblastic leukaemia (ALL) and inflammatory bowel diseases (IBD). Metabolism to active 6-thioguanine (6TGN) and 6-methylmercaptopurine nucleotides (6MMPN) is variable but therapeutic drug monitoring (TDM) remains debatable. This study reports on factors impacting on red blood cell (RBC) metabolites concentrations in children to facilitate TDM interpretation.

METHODS

The first paediatric TDM samples received during year 2021 were analysed, whatever indication and thiopurine drug. Target concentration ranges were 200-500, <6000 pmol/8 × 108 RBC for 6TGN and 6MMPN.

RESULTS

Children (n = 492) had IBD (64.8%), ALL (22.6%) or another autoimmune disease (12.6%): mean ages at TDM were 7.5 in ALL and 13.7 years in IBD (P < .0001). ALL received 6-mercaptopurine (mean dose 1.7 mg/kg/d with methotrexate), IBD received AZA (1.9 mg/kg/d with anti-inflammatory drugs and/or monoclonal antibodies). Median 6TGN and 6MMPN concentrations were 213.7 [interquartile range: 142.5; 309.6] and 1144.6 [419.4; 3574.3] pmol/8 × 108 RBC, 38.8% of patients were in the recommended therapeutic range for both compounds. Aminotransferases and blood tests were abnormal in 57/260 patients: 8.1% patients had high alanine aminotransaminase, 3.4% of patients had abnormal blood count. Factors associated with increased 6TGN were age at TDM and thiopurine methyltransferase genotype in ALL and AZA dose in IBD. The impact of associated treatment in IBD patients was also significant.

CONCLUSION

TDM allowed identification of children who do not reach target levels or remain over treated. Including TDM in follow-up may help physicians to adjust dosage with the aim of reducing adverse effects and improve treatment outcome.

Implications of Tioguanine Dosing in IBD Patients with a TPMT Deficiency

Tioguanine is metabolised by fewer enzymatic steps compared to azathioprine and mercaptopurine, without generating 6-methylmercaptopurine ribonucleotides. However, thiopurine S-methyl transferase (TPMT) plays a role in early toxicity in all thiopurines. We aimed to describe the hazards and opportunities of tioguanine use in inflammatory bowel disease (IBD) patients with aberrant TPMT metabolism and propose preventative measures to safely prescribe tioguanine in these patients. In this retrospective cohort study, all determined TPMT genotypes (2016-2021) were evaluated for aberrant metabolism (i.e., intermediate and poor TPMT metabolisers). Subsequently, all IBD patients on tioguanine with aberrant TPMT genotypes were evaluated for tioguanine dosages, adverse drug events, lab abnormalities, treatment duration and effectiveness. TPMT genotypes were determined in 485 patients, of whom, 50 (10.3%) and 4 patients (0.8%) were intermediate and poor metabolisers, respectively. Of these patients, 12 intermediate and 4 poor TPMT metabolisers had been prescribed tioguanine in varying doses. In one poor TPMT metaboliser, tioguanine 10 mg/day induced delayed pancytopenia. In general, reduced tioguanine dosages of 5 mg/day for intermediate TPMT metabolisers, and 10 mg two-weekly for poor TPMT metabolisers, resulted in a safe, long-term treatment strategy. Diminished or absent TPMT enzyme activity was related with a pharmacokinetic shift of tioguanine metabolism which is associated with relatively late-occurring myelotoxicity in patients on standard tioguanine dose. However, in strongly reduced dose regimens with strict therapeutic drug and safety monitoring, tioguanine treatment remained a safe and effective option in IBD patients with dysfunctional TPMT.

Effectiveness and safety of thioguanine as a maintenance therapy of inflammatory bowel disease: Systematic review, meta-analysis and meta-regression

INTRODUCTION

Thiopurines are an important therapy for the maintenance of remission in inflammatory bowel disease (IBD). However, the use of thioguanine has been limited by concerns regarding its toxicity. We performed a systematic review to evaluate its effectiveness and safety in IBD.

METHODS

Electronic databases were searched to identify studies reporting clinical responses and/or adverse events of thioguanine therapy in IBD. We calculated the pooled clinical response and clinical remission rates of thioguanine in IBD. Subgroup analyses were done for the dosage of thioguanine and the type of studies (prospective or retrospective). Meta-Regression was used to analyze the impact of dose on clinical efficacy and occurrence of nodular regenerative hyperplasia.

RESULTS

A total of 32 studies were included. The pooled clinical response rate of thioguanine therapy in IBD was 0.66 (95% C.I. 0.62 - 0.70; I2 = 16%). The pooled clinical response rate with low-dose was similar to high-dose thioguanine therapy [0.65 (95% C.I. 0.59 - 0.70; I2 = 24%) and 0.68 (95% C.I. 0.61 - 0.75; I2 = 18%) respectively]. The pooled remission maintenance rate was 0.71 (95% C.I. 0.58 - 0.81; I2 = 86%). The pooled rates of occurrence of nodular regenerative hyperplasia, liver function tests abnormalities and cytopenia were 0.04 (95% C.I. 0.02 - 0.08; I2 = 75%), 0.11 (95% C.I. 0.08 - 0.16; I2 = 72%) and 0.06 (95% C.I. 0.04 - 0.09; I2 = 62%) respectively. Meta-regression suggested that the risk of nodular regenerative hyperplasia is related to the dose of thioguanine.

CONCLUSION

TG is an efficacious and well-tolerated drug in most patients with IBD. Nodular regenerative hyperplasia, cytopenias, and liver function abnormalities occur in a small subset. Future studies should look into TG as primary therapy in IBD.

Cytotoxicity of Thiopurine Drugs in Patients with Inflammatory Bowel Disease

The effectiveness of thiopurine drugs in inflammatory bowel disease (IBD) was confirmed more than a half-century ago. It was proven that these can be essential immunomodulatory medications. Since then, they have been used routinely to maintain remission of Crohn’s disease (CD) and ulcerative colitis (UC). The cytotoxic properties of thiopurines and the numerous adverse effects of the treatment are controversial. However, the research subject of their pharmacology, therapy monitoring, and the search for predictive markers are still very relevant. In this article, we provide an overview of the current knowledge and findings in the field of thiopurines in IBD, focusing on the aspect of their cytotoxicity. Due to thiopurines’ benefits in IBD therapy, it is expected that they will still constitute an essential part of the CD and UC treatment algorithm. More studies are still required on the modulation of the action of thiopurines in combination therapy and their interaction with the gut microbiota.

Association of ITPA gene polymorphisms with adverse effects of AZA/6-MP administration: a systematic review and meta-analysis

Azathioprine (AZA) and its metabolite, mercaptopurine (6-MP), are widely used immunosuppressant drugs. Polymorphisms in genes implicated in AZA/6-MP metabolism, reportedly, could account in part for their potential toxicity. In the present study we performed a systematic review and a meta-analysis, comprising 30 studies and 3582 individuals, to investigate the putative genetic association of two inosine triphosphatase (ITPA) polymorphisms with adverse effects in patients treated with AZA/6-MP. We found that rs1127354 is associated with neutropenia in general populations and in children (OR: 2.39, 95%CI: 1.97-2.90, and OR: 2.43, 95%CI: 2.12-2.79, respectively), and with all adverse effects tested herein in adult populations (OR: 2.12, 95%CI: 1.22-3.69). We also found that rs7270101 is associated with neutropenia and leucopenia in all-ages populations (OR: 2.93, 95%CI: 2.36-3.63, and OR: 2.82, 95%CI: 1.76-4.50, respectively) and with all adverse effects tested herein in children (OR: 1.74, 95%CI: 1.06-2.87). Stratification according to background disease, in combination with multiple comparisons corrections, verified neutropenia to be associated with both polymorphisms, in acute lymphoblastic leukemia (ALL) patients. These findings suggest that ITPA polymorphisms could be used as predictive biomarkers for adverse effects of thiopurine drugs to eliminate intolerance in ALL patients and clarify dosing in patients with different ITPA variants.

Current status and future perspectives on the use of therapeutic drug monitoring of thiopurine metabolites in patients with inflammatory bowel disease

INTRODUCTION

Despite new treatment options for inflammatory bowel disease (IBD), conventional thiopurines remain a common treatment option for maintaining remission, particularly in non-Westernized countries. Therapeutic drug monitoring (TDM) is advised in standard care for optimizing therapy strategies to improve effectiveness, reveal nonadherence and reduce toxicity. Still, the rationale of TDM is debated.

AREAS COVERED

Key insights on TDM of thiopurine metabolites are discussed. The pharmacology of thiopurines is described, emphasizing the interindividual differences in pharmacogenetics, pharmacokinetics and pharmacodynamics. Pharmacological differences between conventional thiopurines and tioguanine are outlined. Finally, several optimization strategies for thiopurine therapy in IBD are discussed.

EXPERT OPINION

TDM has been a useful, but limited, tool to individualize thiopurine therapy. Pharmacokinetic data on the active thiopurine metabolites, derived from measurements in erythrocytes, associated with clinical response only partially predict effectiveness and toxicity. An additional pharmacodynamic marker, such as Rac1/pSTAT3 expression in leukocytes, may improve applicability of TDM in the future.

Thiopurine pharmacogenomics and pregnancy in inflammatory bowel disease

The thiopurine drugs azathioprine and 6-mercaptopurine are widely used for the maintenance of clinical remission in steroid-dependent inflammatory bowel disease (IBD). Thiopurines are recommended to be continued throughout pregnancy in IBD patients, but conclusive safety data in pregnant patients remain still insufficient. On the other hand, a strong association between a genetic variant of nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15 p.Arg139Cys) and thiopurine-induced myelotoxicity has been identified. Pharmacokinetic studies have revealed that thiopurine metabolism is altered in pregnant IBD patients and suggested that the fetus may be exposed to the active-thiopurine metabolite, 6-thioguaninetriphosphate, in the uterus. A recent study using knock-in mice harboring the p.Arg138Cys mutation which corresponds to human p.Arg139Cys showed that oral administration of 6-MP at clinical dose induces a severe toxic effect on the fetus harboring the homozygous or heterozygous risk allele. This suggests that NUDT15 genotyping may be required in both women with IBD who are planning pregnancy (or pregnant) and their partner to avoid adverse outcomes for their infant. The risk to the fetus due to maternal thiopurine use is minimal but there are some concerns that are yet to be clarified. In particular, a pharmacogenomic approach to the fetus is considered necessary.

Relationship Between Thiopurine S-Methyltransferase Genotype/Phenotype and 6-Thioguanine Nucleotide Levels in 316 Patients With Inflammatory Bowel Disease on 6-Thioguanine

BACKGROUND

In inflammatory bowel disease (IBD), conventional thiopurine users cease treatment in 60% of cases within 5 years, mostly because of adverse events or nonresponse. In this study, the authors aimed to investigate the role of 6-thioguanine nucleotide (TGN) measurements, geno/phenotyping of thiopurine S-methyltransferase (TPMT), and their mutual relationship with TG therapy in IBD.

METHODS

An international retrospective, multicenter cohort study was performed at 4 centers in the Netherlands (Máxima Medical Centre) and the United Kingdom (Guy's and St. Thomas' Hospital, Queen Elizabeth Hospital, and East Surrey Hospital).

RESULTS

Overall, 526 6-TGN measurements were performed in 316 patients with IBD. The median daily dosage of TG was 20 mg/d (range 10-40 mg/d), and the median duration of TG use was 21.1 months (SD, 28.0). In total, 129 patients (40.8%) had a known TPMT status. In the variant-type and wild-type TPMT genotype metabolism groups, median 6-TGN values were 1126 [interquartile range (IQR) 948-1562] and 467.5 pmol/8 × 10E8 red blood cells (RBCs) (IQR 334-593). A significant difference was observed between the 2 groups (P = 0.0001, t test). For TPMT phenotypes, in the slow, fast, and normal metabolism groups, the median 6-TGN values were 772.0 (IQR 459-1724), 296.0 (IQR 200-705), and 774.5 pmol/8 × 10E8 RBCs (IQR 500.5-981.5), with a significant difference observed between groups (P < 0.001, analysis of variance).

CONCLUSIONS

Our findings indicated that TPMT measurements at TG initiation can be useful but are not necessary for daily practice. TPMT genotypes and phenotypes are both associated with significant differences in 6-TGN levels between metabolic groups. However, the advantage of TG remains that RBC 6-TGN measurements are not crucial to monitor treatments in patients with IBD because these measurements did not correlate with laboratory result abnormalities. This presents as a major advantage in countries where patients cannot access these diagnostic tests.

Advances in Thiopurine Drug Delivery: The Current State-of-the-Art

Thiopurines (mercaptopurine, azathioprine and thioguanine) are well-established maintenance treatments for a wide range of diseases such as leukemia, inflammatory bowel disease (IBD), systemic lupus erythematosus (SLE) and other inflammatory and autoimmune diseases in general. Worldwide, millions of patients are treated with thiopurines. The use of thiopurines has been limited because of off-target effects such as myelotoxicity and hepatotoxicity. Therefore, seeking methods to enhance target-based thiopurine-based treatment is relevant, combined with pharmacogenetic testing. Controlled-release formulations for thiopurines have been clinically tested and have shown promising outcomes in inflammatory bowel disease. Latest developments in nano-formulations for thiopurines have shown encouraging pre-clinical results, but further research and development are needed. This review provides an overview of novel drug delivery strategies for thiopurines, reviewing modified release formulations and with a focus on nano-based formulations.

Clinical Considerations Regarding the Use of Thiopurines in Older Patients with Inflammatory Bowel Disease

The number of older patients with inflammatory bowel disease (IBD) is increasing due to both improvements in the life expectancy of patients with long-lasting IBD and later onset of the disease. In spite of a less aggressive IBD phenotype, disease management in older patients is hampered by comorbidities and polypharmacy (which increase the risk of drug-related adverse events and errors in medication intake) and also by an increased risk of the infections and malignancies associated with the immunosuppressive drugs that are frequently used to treat IBD. Thiopurines are the most frequently used immunosuppressive drugs in IBD, though they are often discontinued due to adverse events. However, when tolerated, thiopurines are efficient in the maintenance of remission in ulcerative colitis and Crohn's disease. In fact, thiopurines still have a role to play in the treatment algorithm of older patients with IBD because anti-tumor necrosis factor agents do not provide clear advantages for this population in terms of their safety profile, while data on the new biological drugs are still scarce. In this article, we review the optimal use of thiopurines in older patients with IBD.

Advances in Management of Intestinal Behçet's Disease: A Perspective From Gastroenterologists

Intestinal Behçet's disease (intestinal BD) is a rare chronic inflammatory disorder of the intestine that is characterized by recurrent intestinal manifestations with other systemic features of BD. Intestinal BD is diagnosed when a typically shaped ulcer is observed in the gastrointestinal tract, and the clinical findings meet the diagnostic criteria for BD. Owing to the small number of patients, intestinal BD is easily underestimated. On the other hand, but it often requires surgical treatment because of severe complications, including intestinal perforations or massive bleeding. The same treatment strategies used for inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis, are used for intestinal BD. 5-Aminosalicylic acids, corticosteroids, and immunomodulators are considered conventional therapies, but a considerable number of patients eventually become unresponsive to these pharmaceutical treatments. Recently, biologic agents, such as anti-tumor necrosis factor-alpha inhibitors, have also been suggested as a new treatment option for intestinal BD. This article reviews the pathogenesis and diagnosis of intestinal BD and the current treatment strategies that are expected to be useful for rheumatologic specialists.

Severe pancytopenia and aspergillosis caused by thioguanine in a thiopurine S-methyltransferase deficient patient: a case report

Azathioprine and mercaptopurine are widely used in the treatment of inflammatory bowel disease. However, its use is limited by adverse drug event related to the relatively narrow therapeutic index of the active metabolites. Several patients discontinue treatment because of intolerable adverse events or toxicity such as leucopenia and hepatotoxicity. High 6-thioguanine nucleotides and 6-methylmercaptopurine ribonucleotides levels are associated with toxicity. Variations in the thiopurine S-methyltransferase (TPMT) gene can lead to diminished TPMT enzyme activity and to an increased incidence of myelotoxicity due to high 6-methylmercaptopurine ribonucleotides levels after treatment with azathioprine and mercaptopurine. Unlike azathioprine and mercaptopurine, thioguanine is more directly metabolized to the active metabolites without formation of the toxic 6-methylmercaptopurine ribonucleotides. Taking this into account, it seems likely that thioguanine is less associated with myelotoxicity due to TPMT deficiency. However, we report the case of a Crohn's disease patient with life-threatening complications on 6TG treatment due to TPMT deficiency. Our patient developed a severe pancytopenia on thioguanine therapy, with 6-thioguanine nucleotides levels more than 10 times higher than the upper limit of the therapeutic window and was found to be a TPMT poor metabolizer (TPMT *3A/*3A). This case strongly illustrates that knowledge of TPMT enzyme activity is very important in the use of all thiopurines, including thioguanine. In conclusion, clinicians should be aware of the impact of TPMT deficiency on the metabolism of thioguanine and should consider performing preemptive TPMT genotyping in combination with frequent blood test monitoring when using thiopurines in general.

Concerns and Side Effects of Azathioprine During Adalimumab Induction and Maintenance Therapy for Japanese Patients With Crohn's Disease: A Subanalysis of a Prospective Randomised Clinical Trial [DIAMOND Study]

BACKGROUND

Combining a thiopurine with the human anti-tumour necrosis factor-α monoclonal antibody adalimumab for Crohn's disease [CD] treatment is controversial with regard to efficacy and safety. By conducting a subanalysis of a multicentre, randomised, prospective, open-label trial [the DIAMOND study, UMIN registration number 000005146], we studied the risk of discontinuation of thiopurine in combination with adalimumab.

METHODS

In the preceding DIAMOND study, we analysed the: [i] timing and reasons for dropout in the monotherapy group and combination group; [ii] risk factors for dropout in the combination group.

RESULTS

There was no significant difference in the dropout rate up to Week 52 between the monotherapy group and combination group [p = 0.325]. The main reason for study dropout was active CD in the monotherapy group, whereas it was adverse effects in the combination group [Fisher's exact test, p <0.001]. Kaplan-Meier analyses revealed significantly earlier dropout in the combination group [log-rank test, p = 0.001]. Multivariable analysis revealed low body weight to be a risk for dropout due to adverse effects in the combination group.

CONCLUSIONS

Combination of azathioprine with adalimumab resulted in dropout in the early stage of the study due to side effects of azathioprine, in comparison with late dropout due to active CD in the adalimumab monotherapy group.

Thiopurine Therapy in Patients With Inflammatory Bowel Disease: A Focus on Metabolism and Pharmacogenetics

Thiopurines have been widely used for the maintenance of remission or steroid sparing in patients with inflammatory bowel disease. However, potential drug-related adverse events frequently interfere with their use. Indeed, drug withdrawals associated with adverse reactions have been reported in approximately 25% of patients. To balance the efficacy, safety, and tolerability of thiopurines, regular monitoring of biomarkers (complete blood cell count, liver function test, and metabolic profiles), steady dose escalation, and pretreatment thiopurine S-methyltransferase (TPMT) genotype screening have been routinely recommended. However, the complex thiopurine metabolic pathway and individual differences attributed to pharmacogenetic diversity limit the effectiveness of these strategies in the optimization of thiopurine therapy. Recently, in an effort to facilitate more accurate and personalized prediction of thiopurine response or toxicity, novel genetic markers including NUDT15 and FTO genes were discovered. These discoveries are remarkable because TPMT screening has minimal efficacy for predicting myelosuppression especially in Asian populations, despite the fact that thee populations have a higher frequency of myelosuppression than Western populations. This review focuses on the current understanding of the metabolic pathway and the pharmacogenetics of thiopurines and suggests a personalized preventive strategy against potential adverse drug reactions to optimize their therapeutic application.

Low 6-thioguanine nucleotide level: Effective in maintaining remission in Chinese patients with Crohn's disease

BACKGROUND AND AIM

Measuring 6-thioguanine nucleotide (6-TGN) level is useful in optimizing dose of azathioprine (AZA) and monitoring for toxicity. Lower dose of AZA was suggested for maintenance of clinical remission in Asian patients than Caucasian patients with Crohn's disease (CD). However, the optimal 6-TGN threshold required in Asian patients is undetermined. Therefore, the aim of the current study is to explore the optimal 6-TGN threshold required in Asian patients with CD for maintenance of clinical remission.

METHODS

A retrospective cohort study in a tertiary referral center recruited 252 CD patients. The primary endpoint was disease relapse. The levels of 6-TGN and AZA dose were compared in remission group and relapse group. Remission rate was compared across the increased 6-TGN level and dose range.

RESULTS

Patients with 6-TGN range of 0-180.94 pmol/8 × 10⁸ red blood cells (RBC) had lower remission rate compared with those with 180.94-255.50 pmol/8 × 10⁸ RBC (P = 0.020). Quartile analysis showed that increasing 6-TGN level beyond 180 pmol/8 × 10⁸ RBC produced negligible gain in rate of remission. Frequency of adverse events significantly increased in patients with 6-TGN level > 355 pmol/8 × 10⁸ RBC (8.0% with 6-TGN > 355 pmol/8 × 10⁸ RBC vs 2.7% with 6-TGN < 355 pmol/8 × 10⁸ RBC, P = 0.035).

CONCLUSION

Our study suggested that optimal 6-TGN threshold required to maintain clinical remission in Chinese patients was 180-355 pmol/8 × 10⁸ RBC.

Pharmacogenetics of thiopurines for inflammatory bowel disease in East Asia: prospects for clinical application of NUDT15 genotyping

The thiopurine drugs 6-mercaptopurine (6-MP) and azathiopurine (AZA) are widely used to treat inflammatory bowel disease. However, the incidence of adverse reactions is high, particularly in Asia, and the mechanisms of toxicity in Asian populations remain unclear. Thiopurine S-methyltransferase (TPMT) is a well-known enzyme that inactivates AZA or 6-MP through methylation and is one of the few pharmacogenetic predictors used in clinical settings in Western countries. Individuals carrying TPMT-deficient genetic variants require reduced drug doses, but this treatment modification is are not applicable to East Asian populations. Several genes code thiopurine-metabolizing enzymes, including TPMT, multidrug-resistance protein 4, and inosine triphosphatase. These genes have been studied as candidate pharmacogenetic markers; however, it remains unclear why Asian populations seem to be more intolerant than other ethnic groups to a full dose of thiopurines. A genome-wide association approach to identify Asian-specific pharmacogenetic markers in Korean patients with Crohn's disease revealed that a non-synonymous single nucelotide polymorphism in nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) which causes p.Arg139Cys was strongly associated with thiopurine-induced early leukopenia. Six common haplotypes of NUDT15 were reported, and five variants showed medium-to-low enzyme activities, compared with the wild haplotype. NUDT15 hydrolyzes the thiopurine active metabolites 6-thio-GTP and 6-thio-dGTP; variants of NUDT15 had lower enzyme activities, causing higher levels of thiopurine active metabolites, resulting in thiopurine-induced leukopenia. In clinical application, NUDT15 genotyping is a good candidate for predicting thiopurine toxicity in East Asian populations. However, the association of NUDT15 diplotypes with thiopurine toxicity remains unclear. Further analyses with large cohorts to confirm the clinical effects of each haplotype are planned.

Is there a role for thioguanine therapy in IBD in 2017 and beyond?

Conventional thiopurines are effective for the maintenance of remission of Crohn's disease and ulcerative colitis, however, up to half of patients are intolerant or unresponsive to these medications. Thioguanine is an alternative thiopurine that has shown efficacy in inflammatory bowel disease, and is particularly useful to circumvent certain side effects associated with conventional thiopurines, for example, pancreatitis. Its association with nodular regenerative hyperplasia of the liver has hindered its widespread use. Areas covered: We aim to outline the rational use of thioguanine, including safety monitoring, with particular regard to hepatotoxicity. A literature search was performed: PubMed was searched for full papers and abstracts published in English since January 2000 using the following terms, alone and in combination: 'azathioprine', 'thiopurine', 'Crohn's disease', 'inflammatory bowel disease', 'nodular regenerative hyperplasia', 'mercaptopurine', 'thioguanine', 'ulcerative colitis'. Further relevant papers were identified from the reference lists of selected papers. Expert commentary: Despite optimisation strategies such as metabolite measurements and the use of allopurinol, a significant proportion of patients will remain intolerant to thiopurines, especially those with allergic reactions, including pancreatitis. For this subgroup of patients we suggest that low dose thioguanine is an alternative to other therapies that are either parenteral or expensive.

Monitoring thiopurine metabolites in inflammatory bowel disease

Thiopurines (azathioprine and mercaptopurine) are one of the immunosuppressive mainstays for the treatment of inflammatory bowel disease. In spite of its widespread use, thiopurine metabolism is still not fully understood, and a significant proportion of patients suffer toxicity or lack of efficacy. Different enzymatic pathways with individual variations constitute a pharmacogenetic model that seems to be suitable for monitoring and therapeutic intervention. This review is focused on current concepts and recent research that may help clinicians to rationally optimise thiopurine treatment in patients with inflammatory bowel disease.

Thioguanine in inflammatory bowel disease: Long-term efficacy and safety

BACKGROUND

Thioguanine (TG) is efficacious in inflammatory bowel disease (IBD), but its toxicity, particularly nodular regenerative hyperplasia (NRH) of the liver, has limited its use. We assessed the long-term clinical outcomes and safety of TG in patients whom were intolerant or refractory to conventional immunomodulators.

METHODS

This is a retrospective, single-centre study of IBD patients treated with TG from 2001-2013. Response was defined as clinical remission (Harvey-Bradshaw Index < 5 for Crohn's disease (CD), Simple Clinical Colitis Activity Index < 4 for ulcerative colitis (UC)) without corticosteroids or, if receiving anti-tumour-necrosis-factor (anti-TNF) therapy, absence of dose escalation. We recorded TG failure, withdrawal and adverse events. Patients were monitored with biochemistry, liver biopsy and/or magnetic resonance imaging (MRI).

RESULTS

54 patients (47 CD and 7 UC) whom received TG (mean dose: 27 mg/d (range: 20-40 mg/d)) as monotherapy (n = 36) or concomitantly with anti-TNF (n = 18) for a median inter-quartile range of 16 (5-37) months (126 patient-years of follow-up). 32 (59%) patients responded to TG at 6 months and 23 (43%) at 12 months. Pancreatitis did not recur amongst the 19 patients with prior thiopurine-induced pancreatitis. 16 (30%) patients ceased TG due to intolerance or toxicity (four serious); NRH was not observed. 6-thioguanine nucleotide concentrations did not correlate with efficacy nor with toxicity.

CONCLUSIONS

TG was efficacious and well tolerated in one out of two patients who had previously failed conventional immunomodulators. NRH did not occur.

NUDT15 R139C-related thiopurine leukocytopenia is mediated by 6-thioguanine nucleotide-independent mechanism in Japanese patients with inflammatory bowel disease

BACKGROUND

NUDT15 R139C (rs116855232) is a recently identified genetic factor responsible for thiopurine-induced leukocytopenia and hair loss. In this study, we investigated the association of NUDT15 R139C with 6-thioguanine nucleotide (6-TGN) levels and thiopurine-induced leukocytopenia in Japanese patients with inflammatory bowel disease (IBD).

METHODS

Two hundred and sixty-four subjects (103 healthy volunteers and 161 IBD patients treated with thiopurines) were enrolled. Genotyping for NUDT15 R139C was performed using Custom TaqMan® SNP genotyping assays.

RESULTS

The NUDT15 C/C, C/T, and T/T genotypes were 80.7, 18.2, and 1.1 %, respectively. The allelic frequency was 10.2 %. Among 161 IBD patients, there was no significant difference in 6-TGN levels among the NUDT15 genotypes. Forty-five patients (27.9 %) developed leukocytopenia (WBC <3000/μl), and the C/T and T/T genotypes were significantly associated with the development of leukocytopenia (P = 1.7 × 10(-5)). In these patients, 6-TGN levels were not significantly different between NUDT15 genotypes. NUDT15 R139C was significantly associated with early (<8 weeks) (P = 1.03 × 10(-4)) and late (>8 weeks) leukocytopenia (P = 4.3 × 10(-4)). The decrease in WBC count at 2 and 4 weeks was significantly higher in patients with the C/T or T/T genotypes as compared to the patients with the C/C genotype. All patients with the T/T genotype (n = 2) developed early severe hair loss and severe leukocytopenia (<1000/μl). The logistic regression analysis revealed that NUDT15 R139C was the sole genetic factor responsible for the thiopurine-induced leukocytopenia (P = 0.001).

CONCLUSIONS

These results suggest that NUDT15 R139C-related thiopurine-induced leukocytopenia is mediated by a 6-TGN-independent mechanism.

Monitoring thiopurine metabolites in korean pediatric patients with inflammatory bowel disease

PURPOSE

This study aimed to assess the role of thiopurine S-methyltransferase (TPMT) and 6-thioguanine nucleotide (6-TGN) as predictors of clinical response and side effects to azathioprine (AZA), and estimate the optimal AZA dose in Korean pediatric inflammatory bowel disease (IBD) patients.

MATERIALS AND METHODS

One hundred and nine pediatric IBD patients in whom AZA treatment was required were enrolled. Thiopurine metabolites were monitored since September 2010. Among them, 83 patients who had prescribed AZA for at least 3 months prior to September 2010 were enrolled and followed until October 2011 to evaluate optimal AZA dose, adverse effects and disease activity before and after thiopurine metabolite monitoring.

RESULTS

The result of the TPMT genotype was that 102 patients were *1/*1 (wild type), four were *1/*3C, one was *1/*6, one was *1/*16 (heterozygote) and one was *3C/*3C (homozygote). Adverse effects happened in 31 patients pre-metabolite monitoring and in only nine patients post-metabolite monitoring. AZA dose was 1.4±0.31 mg/kg/day before monitoring and 1.1±0.46 mg/kg/day after monitoring (p<0.001). However, there were no statistical differences in disease activity during metabolite monitoring period (p=0.34). Adverse effects noticeably decreased although reduction of the AZA dose since monitoring.

CONCLUSION

TPMT genotype and thiopurine metabolite monitoring could be helpful to examine TPMT genotypes before administering AZA and to measure 6-TGN concentrations during prescribing AZA in IBD patients.

Screening for 392 polymorphisms in 141 pharmacogenes

Pharmacogenomics is the study of the association between inter-individual genetic differences and drug responses. Researches in pharmacogenomics have been performed in compliance with the use of several genotyping technologies. In this study, a total of 392 single-nucleotide polymorphisms (SNPs) located in 141 pharmacogenes, including 21 phase I, 13 phase II, 18 transporter and 5 modifier genes, were selected and genotyped in 150 subjects using the GoldenGate assay or the SNaPshot technique. These variants were in Hardy-Weinberg equilibrium (HWE) (P>0.05), except for 22 SNPs. Genotyping of the 392 SNPs revealed that the minor allele frequencies of 47 SNPs were <0.05, 105 SNPs were monomorphic and 22 variants were not in HWE. Also, based on previous studies, we predicted the association between the polymorphisms of certain pharmacogenes, such as cytochrome P450 2D6, cytochrome P450 2C9, vitamin K epoxide reductase complex, subunit 1, cytochrome P450 2C19, human leukocyte antigen, class I, B and thiopurine S-methyltransferase, and drug efficacy. In conclusion, our study demonstrated the allele distribution of SNPs in 141 pharmacogenes as determined by high-throughput screening. Our results may be helpful in developing personalized medicines by using pharmacogene polymorphisms.

Use of thiopurines in inflammatory bowel disease: Safety issues

Thiopurines are widely used for maintenance treatment of inflammatory bowel disease. Inter-individual variability in clinical response to thiopurines may be attributed to several factors including genetic polymorphisms, severity and chronicity of disease, comorbidities, duration of administration, compliance issues and use of concomitant medication, environmental factors and clinician and patient preferences. The purpose of this review is to summarise the current evidence on thiopurine safety and toxicity, to describe adverse drug events and emphasise the significance of drug interactions, and to discuss the relative safety of thiopurine use in adults, elderly patients, children and pregnant women. Thiopurines are safe to use and well tolerated, however dose adjustment or discontinuation of treatment must be considered in cases of non-response, poor compliance or toxicity. Drug safety, clinical response to treatment and short to long term risks and benefits must be balanced throughout treatment duration for different categories of patients. Treatment should be individualised and stratified according to patient requirements. Enzymatic testing prior to treatment commencement is advised. Surveillance with regular clinic follow-up and monitoring of laboratory markers is important. Data on long term efficacy, safety of thiopurine use and interaction with other disease modifying drugs are lacking, especially in paediatric inflammatory bowel disease. High quality, collaborative clinical research is required so as to inform clinical practice in the future.

New genetic biomarkers predicting azathioprine blood concentrations in combination therapy with 5-aminosalicylic acid

BACKGROUND AND AIMS

Azathioprine (AZA) is widely used for the treatment of inflammatory bowel disease (IBD) patients. AZA is catabolized by thiopurine S-methyltransferase (TPMT), which exhibits genetic polymorphisms. It has also been reported that 5-aminosalicylic acid (5-ASA) inhibits TPMT activity, and that increased 6-thioguanine nucleotide (6-TGN, a metabolite of AZA) blood concentrations result in an increased number of ADRs. In this study, single nucleotide polymorphisms (SNPs) related to differential gene expression affecting AZA drug metabolism in combination therapy with 5-ASA were examined.

METHODS

To identify genetic biomarkers for the prediction of 6-TGN blood concentration, ExpressGenotyping analysis was used. ExpressGenotyping analysis is able to detect critical pharmacogenetic SNPs by analyzing drug-induced expression allelic imbalance (EAI) of premature RNA in HapMap lymphocytes. We collected blood samples on 38 patients with inflammatory bowel disease treated with AZA and corroboration of the obtained SNPs was attempted in clinical samples.

RESULTS

A large number of SNPs with AZA/5-ASA-induced EAI within the investigated HapMap lymphocytes was identified by ExpressGenotyping analysis. The respective SNPs were analyzed in IBD patients' blood samples. Among these SNPs, several that have not yet been described to be induced by AZA/5-ASA were found. SNPs within SLC38A9 showed a particular correlation with patients' 6-TGN blood concentrations.

CONCLUSIONS

Based on these results, ExpressGenotyping analysis and genotyping of patients appears to be a useful way to identify inter-individual differences in drug responses and ADRs to AZA/5-ASA. This study provides helpful information on genetic biomarkers for optimized AZA/5-ASA treatment of IBD patients.

Monitoring and safety of azathioprine therapy in inflammatory bowel disease

Azathioprine is the most common drug used to maintain clinical remission in inflammatory bowel disease. This drug is also important as a steroid-sparing agent in steroid-dependent and chronically active inflammatory bowel disease. Nevertheless, many questions remain concerning the optimal treatment regimens of azathioprine. The dose of azathioprine has to be reduced or the therapy has to be discontinued frequently because of drug-induced toxicity. In this review, we discuss monitoring of thiopurines, adverse events, malignant complications and how to use azathioprine safely and usefully.

Validation of a genotyping method for analysis of TPMT polymorphisms

BACKGROUND

Thiopurine methyltransferase (TPMT) catalyzes the methylation of thiopurine drugs such as azathioprine and 6-mercaptopurine. Several mutations in the TPMT gene correlate with low enzyme activity and adverse effects such as myelotoxicity. Hence, genotyping TPMT makes it possible to identify patients at high risk for drug toxicity.

OBJECTIVE

The aim of this study was to validate a TPMT genotyping method by comparing it with a conventional polymerase chain reaction (PCR) approach.

METHODS

LightSNiP is a real-time PCR method for the detection of TPMT*2, *3B, and *3C without a sequencing step. We evaluated the frequencies of 3 TPMT alleles in 111 white adult patients by comparing genotyping by LightSNiP with conventional PCR (sequencing).

RESULTS

No differences were observed between conventional genotyping with sequencing and LightSNiP for *2, *3B, and *3C, suggesting the validity of this method.

CONCLUSIONS

Compared with the conventional PCR sequencing method, the data suggest that LightSNiP correctly detected the TPMT *2, *3B, and *3C in this select population.

Hypoxanthine guanine phosphoribosyltransferase activity is related to 6-thioguanine nucleotide concentrations and thiopurine-induced leukopenia in the treatment of inflammatory bowel disease

BACKGROUND

Thiopurine drugs are widely used in the treatment of inflammatory bowel disease (IBD). The polymorphic enzyme thiopurine S-methyltransferase (TPMT) is of importance for thiopurine metabolism and adverse events occurrence. The role of other thiopurine-metabolizing enzymes is less well known. This study investigated the effects of TPMT and hypoxanthine guanine phosphoribosyltransferase (HPRT) activities on 6-thioguanine nucleotides (6-TGNs) concentrations and thiopurine-induced leukopenia in patients with IBD.

METHODS

Clinical data and blood samples were collected from 120 IBD patients who were receiving azathioprine (AZA)/6-mercaptopurine (6-MP) therapy. Erythrocyte TPMT, HPRT activities and 6-TGNs concentrations were determined. HPRT activity and its correlation with TPMT activity, 6-TGNs level, and leukopenia were evaluated.

RESULTS

The HPRT activity of all patients ranged from 1.63-3.33 (2.31 ± 0.36) μmol/min per g Hb. HPRT activity was significantly higher in patients with leukopenia (27, 22.5%) than without (P < 0.001). A positive correlation between HPRT activity and 6-TGNs concentration was found in patients with leukopenia (r = 0.526, P = 0.005). Patients with HPRT activity > 2.70 μmol/min per g Hb could have an increased risk of developing leukopenia (odds ratio = 7.47, P < 0.001). No correlation was observed between TPMT activity and HPRT activity, 6-TGNs concentration, or leukopenia.

CONCLUSIONS

High levels of HPRT activity could be a predictor of leukopenia and unsafe 6-TGN concentrations in patients undergoing AZA/6-MP therapy. This could partly explain the therapeutic response or toxicity that could not be adequately explained by the polymorphisms of TPMT.

Biotransformation of 6-thioguanine in inflammatory bowel disease patients: a comparison of oral and intravenous administration of 6-thioguanine

BACKGROUND AND PURPOSE

Although 6-mercaptopurine and azathioprine are effective treatments in inflammatory bowel disease (IBD), many patients discontinue treatment because of side effects. 6-Thioguanine (6-TG) may be an alternative rescue therapy in these intolerant patients but the pharmacokinetics of 6-TG are not fully described. Here we have measured the pharmacokinetics of the biotransformation of 6-TG into the pharmacologically active metabolites, 6-thioguanine nucleotides (6-TGN), in IBD patients.

EXPERIMENTAL APPROACH

In 12 patients with IBD, levels of 6-TGN and activities of thiopurine S-methyltransferase, xanthine oxidase and hypoxanthine guanine-phosphoribosyl-transferase were measured in a two-stage (i.v. and p.o. administration of 0.3 mg·kg(-1) 6-TG), prospective study. Median exposure of 6-TGN in red blood cells (RBC) was expressed as the ratio of the area under the curve (AUC) per mg 6-TG after i.v. dosing and that after p.o. dosing.

KEY RESULTS

The median AUC per mg 6-TG was 1068 (p.o.) and 7184 (i.v.) pmol·h (8 × 10(8) RBC)(-1) . Median exposure of 6-TGN in RBC was 15% (9-28). Hypoxanthine guanine-phosphoribosyl-transferase activity correlated with peak 6-TGN and with AUC per mg (r= 0.7, P= 0.02 and r= 0.6, P= 0.03 respectively). Thiopurine S-methyltransferase activity was inversely related to AUC per mg (r=-0.8, P= 0.001), whereas that of xanthine oxidase was correlated with a lower peak 6-TGN (r=-0.7, P= 0.02).

CONCLUSIONS AND IMPLICATIONS

The great variability of the AUC per mg for 6-TG observed after p.o. and i.v. administration of 6-TG, was partly explained by variability in activities of metabolizing enzymes. Exposure of 6-TGN was low in all patients.

Accuracy of genotyping using the TaqMan PCR assay for single nucleotide polymorphisms responsible for thiopurine sensitivity in Japanese patients with inflammatory bowel disease

Thiopurine drugs are the most common drugs used to maintain clinical remission in inflammatory bowel disease (IBD). Three single-nucleotide polymorphisms (SNPs), TPMT A719G (rs1142345), inosine triphosphate pyrophosphatase (ITPase) C94A (rs1127354) and multidrug resistance protein 4 MRP4 G2269A (rs3765534), have been reported to account for heightened sensitivity to thiopurine drugs in the Japanese population. We investigated the usefulness of the TaqMan(®) PCR assay (Applied Biosystems) for the rapid detection of these SNPs to improve the safety of thiopurine therapy. We enrolled 44 healthy volunteers and 235 IBD patients. Genotyping of the SNPs was performed using Custom TaqMan SNP genotyping assays, direct sequencing and PCR-RFLP. Genotyping for MRP4 G2269A by the TaqMan PCR assay was successfully achieved in all samples. Comparison with our previous data using direct sequencing indicated one discordant result, and re-sequencing showed that the TaqMan PCR assay was correct. The overall accuracy of the TaqMan assay for MRP4 G2269A was 100%. The TaqMan PCR genotyping for TPMT A719G and ITPase C94A was successfully performed in all samples. The results of TPMT A719G by the TaqMan assay were identical with those of PCR-RFLP. In ITPase C94A, a comparison of the TaqMan assay and PCR-RFLP yielded 12 discordant results, and direct sequencing showed that the TaqMan PCR assay was correct. The allelic frequency determined by the TaqMan assay was 0.145 for MRP4 G2269A, 0.009 for TPMT A719G and 0.121 for ITPase C94A, respectively. In conclusion, the TaqMan(®) PCR assay is useful for genotyping of SNPs responsible for thiopurine sensitivity in Japanese IBD patients.

Immunomodulators and immunosuppressants for Japanese patients with ulcerative colitis

Ulcerative colitis (UC) is characterized by a long-standing chronic course with remissions and exacerbations. Previously, patients do not respond to 5-aminosalicylic acid compounds and corticosteroids are considered for colectomies, however, in recent years, alternative treatments emerged for steroid-refractory or steroid-dependent UC. In this review article, we focus on immunomodulators (such as azathioprine [AZA] and 6-mercaptopurine [6-MP]) and immunosuppressants (such as cyclosporine A [CSA] and tacrolimus [FK506]) for steroid-refractory or steroid-dependent ulcerative colitis. The characteristics, efficacy and adverse effects of these drugs are outlined. Although the randomized trial of FK506 is conducted in Japan, the clinical data of CSA in Japanese patients are limited. The short-, mid- and long-term follow-ups of CSA administration in Japanese patients are discussed. As for thipurine drugs, the clinical importance of multidrug-resistance protein 4 (MRP4) in Japanese patients is highlighted.

The multidrug-resistance protein 4 polymorphism is a new factor accounting for thiopurine sensitivity in Japanese patients with inflammatory bowel disease

BACKGROUND

Multidrug resistance protein 4 (MRP4) functions as an efflux pump of nucleoside monophosphate analogs, such as 6-mercaptopurine (6-MP) and 6-thioguanine nucleotide (6-TGN). A single-nucleotide polymorphism in human MRP4 (rs3765534) dramatically reduces MRP4 function and results in the intracellular accumulation of 6-TGN. In this study, we investigated the association between MRP4 G2269A polymorphism and thiopurine sensitivity in Japanese IBD patients.

METHODS

Direct sequencing of the MRP4 exon 18 was performed. The TPMT A719G and ITPase C94A polymorphisms were determined by polymerase-chain reaction-restriction fragment length polymorphism analyses.

RESULTS

Of the 279 samples analyzed (44 healthy volunteers and 235 IBD patients), 68 samples showed a heterozygote of MRP4 G2269A and 7 carried a homozygote. The allelic frequency of MRP4 G2269A was 14.7%. In 130 IBD patients treated with azathioprine/6-MP, the white blood cell count was significantly lower in patients with theMRP4 variant alone (n = 26) than in patients with a wild allelotype (n = 74) (P = 0.014) or in patients with the ITPase variant alone (n = 22) (P = 0.0095). The 6-TGN levels were significantly higher in patients with the MRP4 variant alone than in patients with the wild allelotype(P = 0.049). Of the 15 patients who experienced leucopenia (<3 x 10⁹/l), 7 patients carried the MRP4 variant.The odds ratio of carrying the MRP4 variant alone and having leukopenia was 3.30 (95% confidence interval 1.03–10.57, P = 0.036).

CONCLUSIONS

These results suggest that MRP4 G2269A might be a new factor accounting for thiopurine sensitivity in Japanese patients with IBD.

Tolerability and usefulness of mercaptopurine in azathioprine-intolerant Japanese patients with ulcerative colitis

BACKGROUND AND AIM

Azathioprine (AZA) and mercaptopurine (6-MP) are established as effective therapeutic drugs for the induction and maintenance of remission in patients with ulcerative colitis (UC). However, AZA is often intolerable due to adverse effects. Evidence regarding the approach of switching from AZA to 6-MP in patients of Asian ethnicity is lacking. We assessed the tolerability and usefulness of 6-MP in Japanese UC patients who had shown intolerance to AZA.

METHODS

One-hundred and ten UC patients who had been treated with AZA and/or 6-MP from January 1985 to October 2008 were examined retrospectively.

RESULTS

Among 110 patients, 107 were treated first with AZA; only three were treated first with 6-MP. Thirty-five (33%) of the 107 patients were intolerant of AZA, with adverse effects including myelosuppression (8/35, 23%), hepatotoxicity (8/35, 23%), and abdominal symptoms (6/35, 17%). Among 35 AZA-intolerant patients, 23 were switched to 6-MP treatment. The cumulative probability of colectomy was significantly higher in patients not treated with 6-MP than in patients treated with 6-MP (log-rank test, P =0.0002). Among the 26 patients (23 AZA-intolerant and three AZA-untreated) treated with 6-MP, 22 (85%) could tolerate the therapy. Adverse effects due to 6-MP were abdominal symptoms (2/4), myelosuppression (1/4), and rash (1/4). The median initial dose of 6-MP was 20 mg/day, and the median final dose was 30 mg/day.

CONCLUSIONS

 6-MP was tolerated in 83% of AZA-intolerant patients, and it was effective for maintenance therapy of UC patients. 6-MP treatment should be considered in AZA-intolerant patients.

Limited intra-individual variability in hypoxanthine-Guanine phosphoribosyl transferase, thiopurine S-methyl transferase, and xanthine oxidase activity in inflammatory bowel disease patients during 6-thioguanine therapy

6-Thioguanine (6-TG) may be indicated in case of intolerance of or resistance to conventional thiopurines in the treatment of inflammatory bowel diseases (IBD). The aim of our study was to evaluate the intrapatient variability in the 6-TG metabolizing enzymes: hypoxanthine-guanine phosphoribosyl transferase (HGPRT), thiopurine S-methyl transferase and xanthine oxidase. We performed a pharmacokinetic study of 6-TG after oral and intravenous administration in IBD patients in remission. The enzyme activities were determined at baseline and 1 week after the initiation of 6-TG in red blood cells, peripheral blood mononuclear cells (PBMC) or plasma. From the results we conclude that HGPRT activity in erythrocytes decreases following the initiation of 6-TG therapy, which may imply that HGPRT is a rate limiting enzyme in 6-TG metabolism. Moreover, little intrapatient variability in enzyme activities was observed except for HGPRT activity in PBMC. These data may have implications in regard of future therapeutic drug monitoring.

6-Thioguanine therapy in Crohn's disease--observational data in Swedish patients

BACKGROUND AND AIMS

Adverse events (AE) leading to discontinuation or dose-reduction of thiopurine therapy (TP) occur in 9-28% of patients with inflammatory bowel disease. 6-Thioguanine (6-TG) has been proposed as an alternative treatment in patients intolerant for azathioprine (AZA), but some concerns have been raised about drug safety.

METHODS

We evaluated in a prospective manner the tolerance and efficacy of 6-TG in 23 Crohn's disease (CD) patients (13 men, median age 41 (19-65) years) with prior intolerance (n=18) or resistance (n=5) to AZA and/or 6-mercaptopurine (6-MP). In addition, eight patients had tried mycophenolate mofetil. Seventeen patients (74%) had undergone intestinal resection, often several times.

RESULTS

Patients were treated with a median daily dose of 40 mg 6-TG (range 20-60) for 259 (15-2272) days. Seven of 13 patients (54%) with active disease went into remission after 8 (4-26) weeks. Sixteen patients (70%) experienced AE that lead to discontinuation (n=10) after 85 (15-451) days or dose reduction (n=6) after 78 (10-853) days. Ten of 18 patients (56%) with prior TP-intolerance discontinued 6-TG treatment due to AE compared to none of five patients with TP-resistance (p=0.046). Of 13 patients that tolerated 6-TG, eight discontinued the drug due to therapeutic failure (n=5) or safety concerns (n=3). Eight patients (35%) continued treatment beyond 12 months. There was no significant difference in maximum thioguanine nucleotide levels between patients with AE leading to discontinuation/dose reduction and patients without AE, 652 (99-2488) vs. 551 (392-1574) pmol/8 x 10(8) RBC; p=0.80.

CONCLUSIONS

In this cohort of CD patients with severe disease failing traditional thiopurine treatment, a small fraction (22%) had long-term benefit of 6-TG-treatment. 6-TG therapy seems to offer a limited therapeutic gain for patients intolerant to both AZA and 6-MP and other treatment options should be considered.

Monitoring 6-thioguanine nucleotide concentrations in Japanese patients with inflammatory bowel disease

BACKGROUND AND AIM

There have been no reports on 6-thioguanine nucleotide (6-TGN) concentrations in Japanese patients with inflammatory bowel disease (IBD) undergoing azathioprine (AZA) or 6-mercaptopurine (6-MP) therapy. The aim of this study was to assess 6-TGN concentrations in Japanese IBD patients.

METHODS

Eighty-three patients with Crohn's disease (n = 42) and ulcerative colitis (n = 41) were enrolled. In 69 patients, AZA was prescribed at 50 mg/day, and seven patients were given 75 (n = 5) or 100 mg/day (n = 2). 6-MP was administered at 30 mg/day (n = 7). The 6-TGN concentrations were then assayed by high-performance liquid chromatography.

RESULTS

The mean 6-TGN concentrations of the entire study population (n = 83) were 277.9 +/- 179.8 pmol/8 x 10(8) red blood cells (RBC). The mean 6-TGN concentrations in those patients with active disease (n = 38) and those in remission (n = 45) were 232.9 +/- 159.7(mean +/- SD) and 342.8 +/- 184.6 pmol/8 x 10(8) RBC, respectively (P < 0.05). The odds ratio of being in remission and having a 6-TGN value >235 pmol/8 x 10(8) RBC was 2.6 (95% CI 1.05-6.2). A significant inverse correlation was found between the white blood cell (WBC) counts and 6-TGN concentrations (r = -0.301, P < 0.05, n = 83); the mean WBC counts of the active patients (6780 +/- 2412) were significantly higher than the patients in clinical remission (5468 +/- 1920, P < 0.05). Three patients with severe leukopenia and 10 patients with high 6-TGN concentrations had no thiopurine S-methyl transferase mutations.

CONCLUSION

The 6-TGN concentrations in Japanese patients with IBD on low-dose AZA and 6-MP therapy were comparable to those reported from Western countries. The monitoring of 6-TGN concentrations may be helpful for developing a therapeutic strategy for Japanese IBD patients.

Analysis of thiopurine S-methyltransferase genotypes in Japanese patients with inflammatory bowel disease

BACKGROUND AND AIMS

Myelosuppression observed in patients with inflammatory bowel disease (IBD) treated with azathioprine (AZA) has been attributed to low thiopurine S-methyltransferase (TPMT) activity. TPMT activity is dependent on the genetic polymorphism of high-versus low-metabolizing alleles. We investigated the association between TPMT genotypes and myelosuppression in Japanese IBD patients.

METHODS

Forty-one healthy volunteers and 70 IBD patients (UC, n = 50; CD, n = 20) were recruited. All IBD patients were treated with AZA. The TPMT genotypes were determined by polymerase-chain reaction-restriction fragment length polymorphism (PCR-RFLP) analyses.

RESULTS

One healthy volunteer showed a heterozygous mutation of TPMT*1/*3C. All other volunteers and the 70 IBD patients were of the wild alleleotype (TPMT*1/*1). In the IBD patients, 7 patients developed leucopenia (<3,000/microL). One of them developed severe leucopenia (<1,000 microL) with agranulocytosis on day 14 after drug initiation.

CONCLUSION

TPMT mutations are not associated with myelosuppression in Japanese IBD patients. Even in IBD patients with a wild TPMT genotype, clinicians should pay attention for the possible development of myelosuppression.

Extended thiopurine metabolite assessment during 6-thioguanine therapy for immunomodulation in Crohn's disease

The proposed metabolic advantage of 6-thioguanine (6-TG) is the direct conversion into the pharmacologically active 6-thioguaninenucleotides (6-TGN). The authors assessed metabolic characteristics of 6-TG treatment in patients with Crohn's disease (N = 7) on therapy with 20 mg 6-TG. 6-thioguanine-monophosphate (6-TGMP), 6-thioguanine-diphosphate (6-TGDP), and 6-thioguanine-triphosphate (6-TGTP) were measured by high-performance liquid chromatography analysis in erythrocytes. Thiopurine S-methyltransferase activity and total 6-TGN levels were determined by standard methods. High interindividual variance in metabolite measurements was observed. Main metabolites were 6-TGTP (median = 531 pmol/8 x 10(8) red blood cells) and 6-TGDP (median = 199 pmol/8 x 10(8) red blood cells). Traces of 6-TGMP (median = 39 pmol/8 x 10(8) red blood cells) and 6-TG (2 patients) could be detected. 6-TGN levels correlated with 6-TGTP levels (r = 0.929, P = .003) and with the sum of separate nucleotides (r = 0.929, P = .003). No correlations were established between TPMT activity (median = 13 pmol/h/10(7)) and 6-TG metabolites. The 1-step metabolism of 6-TG still leads to high interindividual variance in metabolite concentrations. Total 6-TGN level monitoring may suffice for clinical practice.