The role of inosine-5'-monophosphate dehydrogenase in thiopurine metabolism in patients with inflammatory bowel disease

Measurement of the intracellular active metabolites of thiopurine drugs to evaluate the enzymatic activity of nudix hydrolase 15 in human blood samples

Thiopurine is metabolized to 6-thio-(deoxy) guanosine triphosphate (6-thio-(d) GTP), which is then incorporated into DNA or RNA and causes cytotoxicity. Nudix hydrolase 15 (NUDT15) reduces the cytotoxic effects of thiopurine by converting 6-thio-(d) GTP to 6-thio-(d) guanosine monophosphate (6-thio-(d) GMP). NUDT15 polymorphisms like the Arg139Cys variant are strongly linked to thiopurine-induced severe leukocytopenia and alopecia. Therefore, measurement of NUDT15 enzymatic activity in individual patients can help predict thiopurine tolerability and adjust the dosage. We aimed to develop a quantitative assay for NUDT15 enzymatic activity in human blood samples. Blood samples were collected from donors whose NUDT15 genetic status was determined. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to assess the 6-thio-GTP metabolic activity in cell extracts. Because 6-thio-guanosine diphosphate (6-thio-GDP) and 6-thio-GMP were generated upon incubation of 6-thio-GTP with human blood cell extracts, the method detecting 6-thio-GTP, 6-thio-GDP, and 6-thio-GMP was validated. All three metabolites were linearly detected, and the lower limit of quantification (LLOQ) of 6-thio-GTP, 6-thio-GDP, and 6-thio-GMP were 5 μM, 1 μM, and 2 μM, respectively. Matrix effects of human blood cell extracts to detect 6-thio-GTP, 6-thio-GDP, and 6-thio-GMP were 99.0 %, 100.5 %, and 101.4 %, respectively, relative to the signals in the absence of blood cell extracts. The accuracy and precision of the method and the stability of the samples were also assessed. Using this established method, the genotype-dependent differences in NUDT15 activities were successfully determined using cell extracts derived from human blood cells with NUDT15 wild-type (WT) or Arg139Cys variant and 6-thio-GTP (100 μM) as a substrate (18.1, 14.9, and 6.43 μM/h/106 cells for WT, Arg139Cys heterozygous, and homozygous variant, respectively). We developed a method for quantifying intracellular NUDT15 activity in peripheral blood mononuclear cells (PBMCs), which we defined as the conversion of 6-thio-GTP to 6-thio-GMP. Although PBMCs preparation takes some time, its reproducibility in experiments makes it a promising candidate for clinical application. This method can tell the difference between WT and Arg139Cys homozygous blood samples. Even in patients with WT NUDT15, WT samples showed variations in NUDT15 activity, which may correlate with variations in thiopurine dosage.

Controlling taxa abundance improves metatranscriptomics differential analysis

BACKGROUND

A common task in analyzing metatranscriptomics data is to identify microbial metabolic pathways with differential RNA abundances across multiple sample groups. With information from paired metagenomics data, some differential methods control for either DNA or taxa abundances to address their strong correlation with RNA abundance. However, it remains unknown if both factors need to be controlled for simultaneously.

RESULTS

We discovered that when either DNA or taxa abundance is controlled for, RNA abundance still has a strong partial correlation with the other factor. In both simulation studies and a real data analysis, we demonstrated that controlling for both DNA and taxa abundances leads to superior performance compared to only controlling for one factor.

CONCLUSIONS

To fully address the confounding effects in analyzing metatranscriptomics data, both DNA and taxa abundances need to be controlled for in the differential analysis.

Liquid chromatographic methods in the determination of inosine monophosphate dehydrogenase enzyme activity: a review

Inosine monophosphate dehydrogenase (IMPDH) is a crucial enzyme involved in the de novo synthesis of purine nucleotides. IMPDH activity is used to evaluate the pharmacodynamics/pharmacokinetics of immunosuppressant drugs such as mycophenolic acid and thiopurines. These drugs are often used to prevent organ transplant rejection and as steroid-sparing agents in autoinflammatory diseases such as inflammatory bowel disease and rheumatoid arthritis. Numerous analytical techniques have been employed to evaluate IMPDH activity in biological matrices. However, hyphenated LC techniques were most widely used in the literature. This review focuses on hyphenated LC methods used to measure IMPDH activity and provides detailed insight into the sample preparation techniques, chromatographic conditions, enzymatic assay conditions, detectors and normalization factors employed in those methods.

Gut Microbiota Metabolism of Azathioprine: A New Hallmark for Personalized Drug-Targeted Therapy of Chronic Inflammatory Bowel Disease

Despite the growing number of new drugs approved for the treatment of inflammatory bowel disease (IBD), the long-term clinical use of thiopurine therapy and the well-known properties of conventional drugs including azathioprine have made their place in IBD therapy extremely valuable. Despite the fact that thiopurine S-methyltransferase (TPMT) polymorphism has been recognized as a major cause of the interindividual variability in the azathioprine response, recent evidence suggests that there might be some yet unknown causes which complicate dosing strategies causing either failure of therapy or toxicity. Increasing evidence suggests that gut microbiota, with its ability to release microbial enzymes, affects the pharmacokinetics of numerous drugs and subsequently drastically alters clinical effectiveness. Azathioprine, as an orally administered drug which has a complex metabolic pathway, is the prime illustrative candidate for such microbial metabolism of drugs. Comprehensive databases on microbial drug-metabolizing enzymes have not yet been generated. This study provides insights into the current evidence on microbiota-mediated metabolism of azathioprine and systematically accumulates findings of bacteria that possess enzymes required for the azathioprine biotransformation. Additionally, it proposes concepts for the identification of gut bacteria species responsible for the metabolism of azathioprine that could aid in the prediction of dose-response effects, complementing pharmacogenetic approaches already applied in the optimization of thiopurine therapy of IBD. It would be of great importance to elucidate to what extent microbiota-mediated metabolism of azathioprine contributes to the drug outcomes in IBD patients which could facilitate the clinical implementation of novel tools for personalized thiopurine treatment of 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.

Pharmacogenetics of Drug Therapies in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic systemic inflammatory disorder that can lead to severe joint damage and is often associated with a high morbidity and disability. Disease-modifying anti-rheumatic drugs (DMARDs) are the mainstay of treatment in RA. DMARDs not only relieve the clinical signs and symptoms of RA but also inhibit the radiographic progression of disease and reduce the effects of chronic systemic inflammation. Since the introduction of biologic DMARDs in the late 1990s, the therapeutic range of options for the management of RA has significantly expanded. However, patients' response to these agents is not uniform with considerable variability in both efficacy and toxicity. There are no reliable means of predicting an individual patient's response to a given DMARD prior to initiation of therapy. In this chapter, the current published literature on the pharmacogenetics of traditional DMARDS and the newer biologic DMARDs in RA is highlighted. Pharmacogenetics may help individualize drug therapy in patients with RA by providing reliable biomarkers to predict medication toxicity and efficacy.

Distinct Fecal and Plasma Metabolites in Children with Autism Spectrum Disorders and Their Modulation after Microbiota Transfer Therapy

Despite the prevalence of autism and its extensive impact on our society, no U.S. Food and Drug Administration-approved treatment is available for this complex neurobiological disorder. Based on mounting evidences that support a link between autism and the gut microbiome, we previously performed a pioneering open-label clinical trial using intensive fecal microbiota transplant. The therapy significantly improved gastrointestinal and behavioral symptoms. Comprehensive metabolomic measurements in this study showed that children with autism spectrum disorder (ASD) had different levels of many plasma metabolites at baseline compared to those in typically developing children. Microbiota transfer therapy (MTT) had a systemic effect, resulting in substantial changes in plasma metabolites, driving a number of metabolites to be more similar to those from typically developing children. Our results provide evidence that changes in metabolites are one mechanism of the gut-brain connection mediated by the gut microbiota and offer plausible clinical evidence for a promising autism treatment and biomarkers.

Accumulating evidence has strengthened a link between dysbiotic gut microbiota and autism. Fecal microbiota transplant (FMT) is a promising therapy to repair dysbiotic gut microbiota. We previously performed intensive FMT called microbiota transfer therapy (MTT) for children with autism spectrum disorders (ASD) and observed a substantial improvement of gastrointestinal and behavioral symptoms. We also reported modulation of the gut microbiome toward a healthy one. In this study, we report comprehensive metabolite profiles from plasma and fecal samples of the children who participated in the MTT trial. With 619 plasma metabolites detected, we found that the autism group had distinctive metabolic profiles at baseline. Eight metabolites (nicotinamide riboside, IMP, iminodiacetate, methylsuccinate, galactonate, valylglycine, sarcosine, and leucylglycine) were significantly lower in the ASD group at baseline, while caprylate and heptanoate were significantly higher in the ASD group. MTT drove global shifts in plasma profiles across various metabolic features, including nicotinate/nicotinamide and purine metabolism. In contrast, for 669 fecal metabolites detected, when correcting for multiple hypotheses, no metabolite was significantly different at baseline. Although not statistically significant, p-cresol sulfate was relatively higher in the ASD group at baseline, and after MTT, the levels decreased and were similar to levels in typically developing (TD) controls. p-Cresol sulfate levels were inversely correlated with Desulfovibrio, suggesting a potential role of Desulfovibrio on p-cresol sulfate modulation. Further studies of metabolites in a larger ASD cohort, before and after MTT, are warranted, as well as clinical trials of other therapies to address the metabolic changes which MTT was not able to correct.

IMPORTANCE Despite the prevalence of autism and its extensive impact on our society, no U.S. Food and Drug Administration-approved treatment is available for this complex neurobiological disorder. Based on mounting evidences that support a link between autism and the gut microbiome, we previously performed a pioneering open-label clinical trial using intensive fecal microbiota transplant. The therapy significantly improved gastrointestinal and behavioral symptoms. Comprehensive metabolomic measurements in this study showed that children with autism spectrum disorder (ASD) had different levels of many plasma metabolites at baseline compared to those in typically developing children. Microbiota transfer therapy (MTT) had a systemic effect, resulting in substantial changes in plasma metabolites, driving a number of metabolites to be more similar to those from typically developing children. Our results provide evidence that changes in metabolites are one mechanism of the gut-brain connection mediated by the gut microbiota and offer plausible clinical evidence for a promising autism treatment and biomarkers.

Review article: opportunities to improve and expand thiopurine therapy for autoimmune hepatitis

BACKGROUND

Thiopurines in combination with glucocorticoids are used as first-line, second-line and maintenance therapies in autoimmune hepatitis and opportunities exist to improve and expand their use.

AIMS

To describe the metabolic pathways and key factors implicated in the efficacy and toxicity of the thiopurine drugs and to indicate the opportunities to improve outcomes by monitoring and manipulating metabolic pathways, individualising dosage and strengthening the response.

METHODS

English abstracts were identified in PubMed by multiple search terms. Full-length articles were selected for review, and secondary and tertiary bibliographies were developed.

RESULTS

Thiopurine methyltransferase activity and 6-tioguanine (6-thioguanine) nucleotide levels influence drug efficacy and safety, and they can be manipulated to improve treatment response and prevent myelosuppression. Methylated thiopurine metabolites are associated with hepatotoxicity, drug intolerance and nonresponse and their production can be reduced or bypassed. Universal pre-treatment assessment of thiopurine methyltransferase activity and individualisation of dosage to manipulate metabolite thresholds could improve outcomes. Early detection of thiopurine resistance by metabolite testing, accurate estimations of drug onset and strength by surrogate markers and adjunctive use of allopurinol could improve the management of refractory disease. Dose-restricted tioguanine (thioguanine) could expand treatment options by reducing methylated metabolites, increasing the bioavailability of 6-tioguanine nucleotides and ameliorating thiopurine intolerance or resistance.

CONCLUSIONS

The efficacy and safety of thiopurines in autoimmune hepatitis can be improved by investigational efforts that establish monitoring strategies that allow individualisation of dosage and prediction of outcome, increase bioavailability of the active metabolites and demonstrate superiority to alternative agents.

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.

Red Blood cell IMPDH activity in adults and children with or without azathioprine: Relationship between thiopurine metabolites, ITPA and TPMT activities

Inosine monophosphate dehydrogenase (IMPDH) is considered as the limiting enzyme of thiopurine metabolism for the formation of 6-thioguanine nucleotides (6-TGN). No data are available on the influence of RBC IMPDH activity on the metabolism of thiopurine drugs in individuals with IBD. The aims of this study were as follows: (a) to carry out a phenotypic study of RBC IMPDH activity in adults and children treated or not with azathioprine (AZA) for autoimmune diseases, and (b) to investigate the relationship between the activities of IMPDH, thiopurine metabolites, inosine triphosphatase (ITPA) and thiopurine methyltransferase (TPMT). IMPDH activity was determined in 97 adults and 67 children treated or not with AZA. 6-Thioguanine nucleotides (6-TGN), 6-methylmercaptopurine nucleotide (6-MeMPN) levels, and ITPA as well as TPMT activities were measured in RBCs by HPLC. Using the Gaussian mixture model, distribution of IMPDH activity was evaluated. Influence of age, sex and AZA treatment on IMPDH activity was also assessed. A bimodal distribution in IMPDH activity was found with 87% of patients exhibiting normal activity and 13% of patients with high activity. No influence of age, sex and AZA therapy was found. There is no relationship between TPMT, ITPA and IMPDH activities. A negative correlation between IMPDH activity and 6-MeMPN was shown in adults and children (rs = -0.335 P = 0.014 and rs = -0.383 P = 0.012, respectively). Our results suggest that AZA-treated patients exhibiting lower IMPDH activity could have higher Me-6MPN levels with higher risk of hepatotoxicity. We demonstrated that RBC matrix could be an interesting alternative to lymphocyte matrix to monitor thiopurine metabolites and enzyme activity.

Revisiting the Role of Thiopurines in Inflammatory Bowel Disease Through Pharmacogenomics and Use of Novel Methods for Therapeutic Drug Monitoring

Azathioprine and 6-mercaptopurine, often referred to as thiopurine compounds, are commonly used in the management of inflammatory bowel disease. However, patients receiving these drugs are prone to developing adverse drug reactions or therapeutic resistance. Achieving predefined levels of two major thiopurine metabolites, 6-thioguanine nucleotides and 6-methylmercaptopurine, is a long-standing clinical practice in ensuring therapeutic efficacy; however, their correlation with treatment response is sometimes unclear. Various genetic markers have also been used to aid the identification of patients who are thiopurine-sensitive or refractory. The recent discovery of novel Asian-specific DNA variants, namely those in the NUDT15 gene, and their link to thiopurine toxicity, have led clinicians and scientists to revisit the utility of Caucasian biomarkers for Asian individuals with inflammatory bowel disease. In this review, we explore the limitations associated with the current methods used for therapeutic monitoring of thiopurine metabolites and how the recent discovery of ethnicity-specific genetic markers can complement thiopurine metabolites measurement in formulating a strategy for more accurate prediction of thiopurine response. We also discuss the challenges in thiopurine therapy, alongside the current strategies used in patients with reduced thiopurine response. The review is concluded with suggestions for future work aiming at using a more comprehensive approach to optimize the efficacy of thiopurine compounds in inflammatory bowel disease.

Combination treatment with 6-mercaptopurine and allopurinol in HepG2 and HEK293 cells - Effects on gene expression levels and thiopurine metabolism

Combination treatment with low-dose thiopurine and allopurinol (AP) has successfully been used in patients with inflammatory bowel disease with a so called skewed thiopurine metabolite profile. In red blood cells in vivo, it reduces the concentration of methylated metabolites and increases the concentration of the phosphorylated ones, which is associated with improved therapeutic efficacy. This study aimed to investigate the largely unknown mechanism of AP on thiopurine metabolism in cells with an active thiopurine metabolic pathway using HepG2 and HEK293 cells. Cells were treated with 6-mercaptopurine (6MP) and AP or its metabolite oxypurinol. The expression of genes known to be associated with thiopurine metabolism, and the concentration of thiopurine metabolites were analyzed. Gene expression levels were only affected by AP in the presence of 6MP. The addition of AP to 6MP affected the expression of in total 19 genes in the two cell lines. In both cell lines the expression of the transporter SLC29A2 was reduced by the combined treatment. Six regulated genes in HepG2 cells and 8 regulated genes in HEK293 cells were connected to networks with 18 and 35 genes, respectively, present at known susceptibility loci for inflammatory bowel disease, when analyzed using a protein-protein interaction database. The genes identified as regulated as well as the disease associated interacting genes represent new candidates for further investigation in the context of combination therapy with thiopurines and AP. However, no differences in absolute metabolite concentrations were observed between 6MP+AP or 6MP+oxypurinol vs. 6MP alone in either of the two cell lines. In conclusion; the effect of AP on gene expression levels requires the presence of 6MP, at least in vitro. Previously described AP-effects on metabolite concentrations observed in red blood cells in vivo could not be reproduced in our cell lines in vitro. AP’s effects in relation to thiopurine metabolism are complex. The network-identified susceptibility genes represented biological processes mainly associated with purine nucleotide biosynthetic processes, lymphocyte proliferation, NF-KB activation, JAK-STAT signaling, and apoptotic signaling at oxidative stress.

Determination of inosine 5'-monophosphate dehydrogenase activity in red blood cells of thiopurine-treated patients using HPLC

Thiopurine drugs are commonly used in immune diseases and to a lesser extent, in transplant rejection prophylaxis: however interindividual variability in drug response and in the occurrence of adverse events is observed. Genetic variation in thiopurine S-methyltransferase (TPMT) doesn't completely explain the occurrence of all adverse events and drug response variability. The potential implication of other enzymes involved in thiopurine metabolism, such as ITPA, has been investigated over the last decade but little data is available on inosine 5'-monophosphate dehydrogenase (IMPDH) in patients treated with thiopurine drugs. The authors reported a HPLC method to determine IMPDH activity in the red blood cells (RBCs) of thiopurine-treated patients. IMPDH activity was evaluated by enzymatic conversion of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP). The XMP formed was analyzed on a Luna^® NH₂ stationary phase, a weak anion exchange phase that exhibits both ionic and hydrophobic properties. XMP was eluted below 15min. Intra-assay and inter-assay precisions were below 9% for RBCs supplemented with 2, 40 and 80μmol/L of XMP. IMPDH activity was measured in adults without thiopurine treatment as well as in adult and paediatric patients treated with thiopurines. A wide interindividual variability in IMPDH activity in RBCs was observed. No difference in IMPDH activity was found between untreated subjects and adult and paediatric patients on thiopurine therapy (median value 11.8, 7.9 and 7.7nmol XPM/g Hb/h respectively). The method described is useful in the determination of IMPDH phenotype from patients on thiopurine therapy and in the investigation of the potential relationship between IMPDH activity in RBCs and the occurrence of adverse events and drug response variability.

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.

Liquid chromatography-mass spectrometry for measuring deoxythioguanosine in DNA from thiopurine-treated patients

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Thiopurines are effective immunosuppressant drugs.

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Monitoring of thiopurines is needed for research and clinical use.

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A sensitive assay of DNA-incorporated deoxythioguanosine is described.

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This method assays thiopurine nucleotides in DNA from nucleated blood cells.

Adverse reactions and non-response are common in patients treated with thiopurine drugs. Current monitoring of drug metabolite levels for guiding treatment are limited to analysis of thioguanine nucleotides (TGNs) in erythrocytes after chemical derivatisation. Erythrocytes are not the target tissue and TGN levels show poor correlations with clinical response. We have developed a sensitive assay to quantify deoxythioguanosine (dTG) without derivatisation in the DNA of nucleated blood cells. Using liquid chromatography and detection by tandem mass spectrometry, an intra- and inter-assay variability below 7.8% and 17.0% respectively were achieved. The assay had a detection limit of 0.0003125 ng (1.1 femtomoles) dTG and was quantified in DNA samples relative to endogenous deoxyadenosine (dA) in a small group of 20 patients with inflammatory bowel disease, all of whom had been established on azathioprine (AZA) therapy for more than 25 weeks. These patients had dTG levels of 20–1360 mol dTG/10⁶ mol dA; three patients who had not started therapy had no detectable dTG. This method, comparable to previous methods in sensitivity, enables the direct detection of a cytotoxic thiopurine metabolite without derivatisation in an easily obtainable, stable sample and will facilitate a better understanding of the mechanisms of action of these inexpensive yet effective drugs.

Update 2014: advances to optimize 6-mercaptopurine and azathioprine to reduce toxicity and improve efficacy in the management of IBD

BACKGROUND

The thiopurine drugs, 6-mercaptopurine (6-MP) and azathioprine (AZA), remain as a mainstay therapy in inflammatory bowel disease (IBD). Differences in metabolism of these drugs lead to individual variation in thiopurine metabolite levels that can determine its therapeutic efficacy and development of adverse reactions. In this update, we will review thiopurine metabolic pathway along with the up-to-date approaches in administering thiopurine medications based on the current literature.

METHODS

A search of the PubMed database by 2 independent reviewers identifying 98 articles evaluating thiopurine metabolism and IBD management.

RESULTS

Monitoring thiopurine metabolites can assist physicians in optimizing 6-MP and AZA therapy in treating patients with IBD. Of the dosing strategies reviewed, we found evidence for monitoring thiopurine metabolite level, use of allopurinol with thiopurine, use of mesalamine with thiopurine, combination therapy with thiopurine and anti-tumor necrosis factor agents, and split dosing of AZA or 6-MP to optimize thiopurine therapy and minimize adverse effects in IBD.

CONCLUSIONS

Based on the currently available literature, various dosing strategies to improve therapeutic response and reduce adverse reactions can be considered, including use of allopurinol with thiopurine, use of mesalamine with thiopurine, combination therapy with thiopurine and anti-tumor necrosis factor agents, and split dosing of thiopurine.

Decrease in factor V activity in patients treated with azathioprine or 6-mercaptopurine

BACKGROUND AND OBJECTIVE

A decrease in factor V activity has been reported in some patients treated with azathioprine or 6-mercaptopurine. This may lead to unnecessary treatment discontinuation in otherwise asymptomatic patients. Our aim was to review spontaneously reported cases of decreased factor V activity associated with both drugs and to identify the possible impact on patient care.

METHODS

Cases of decrease in prothrombin (PT) or factor V activity involving purine analogs were extracted from the French pharmacovigilance database. Reports with evidence of disseminated intravascular coagulation, signs of acute hepatocellular failure, liver cirrhosis or concomitant vitamin K antagonist treatment were excluded.

RESULTS

Twenty-four cases (azathioprine: 13 and 6-mercaptopurine: 11) were retained. Therapeutic indications were inflammatory bowel diseases in 11 patients, acute leukemia in eight, and other autoimmune diseases in five. PT activity before treatment was normal in all nine tested patients. The decrease in PT or factor V activity occurs after a median of 10 weeks of treatment and all patients were asymptomatic. The median PT and factor V activities values were 51.5% and 36.4%, respectively. Other coagulation factors were inconsistently decreased. Full recovery was observed within 3-60 days following purine analogs discontinuation. In four patients, drug rechallenge was associated with recurrence of the coagulation disorders.

CONCLUSIONS

Although the mechanism remains unknown, this series that includes cases with positive drug reintroduction strongly suggests the causative role of these drugs. As all patients remained asymptomatic, treatment discontinuation should be carefully considered in patients who clearly benefits from this treatment.

The effect of allopurinol and low-dose thiopurine combination therapy on the activity of three pivotal thiopurine metabolizing enzymes: results from a prospective pharmacological study

INTRODUCTION

Thiopurine therapy is often discontinued in inflammatory bowel disease (IBD) patients. The xanthine oxidase (XO) inhibitor allopurinol has previously shown to enhance thiopurine efficacy and to prevent adverse reactions, the mechanism of this beneficial interaction is not completely clarified. The aim of this study is to observe possible effects of allopurinol and low-dose thiopurine combination therapy on the activity of three pivotal thiopurine metabolizing enzymes.

METHODS

A prospective study of IBD patients failing thiopurine therapy due to a skewed thiopurine metabolism was performed. Patients were treated with allopurinol and azathioprine or mercaptopurine. Xanthine oxidase, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and thiopurine S-methyl transferase (TPMT) activities, and thiopurine metabolites concentrations were measured during thiopurine monotherapy, and after 4 and 12 weeks of combination therapy.

RESULTS

Of fifteen IBD patients, XO activity decreased from 0.18 (IQR 0.08-0.3) during thiopurine monotherapy to 0.14 (IQR 0.06-0.2) and 0.11 (IQR 0.06-0.2; p=0.008) mU/hour/ml at 4 and 12 weeks, respectively. HGPRT activity increased from 150 (IQR 114-176) to 180 (IQR 135-213) and 204 nmol/(h×mg protein) (IQR 173-213; p=0.013). TPMT activity seemed not to be affected. 6-Thioguanine nucleotide concentrations increased from 138 (IQR 119-188) to 235 (223-304) and to 265 pmol/8×10^8 (IQR 188-344), whereas 6-methyl mercaptopurine ribonucleotides concentrations decreased from 13230 (IQR 7130-17420) to 690 (IQR 378-1325) and 540 (IQR 240-790) pmol/8×10^8 at 4 and 12 weeks of combination therapy (both p<0.001).

CONCLUSION

Allopurinol and thiopurine combination-therapy seems to increase HGPRT and decrease XO activity in IBD patients, which at least in part may explain the observed changes in thiopurine metabolite concentrations.

Differential role of thiopurine methyltransferase in the cytotoxic effects of 6-mercaptopurine and 6-thioguanine on human leukemia cells

The thiopurine antimetabolites, 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) are inactive pro-drugs that require intracellular metabolism for activation to cytotoxic metabolites. Thiopurine methyltransferase (TPMT) is one of the most important enzymes in this process metabolizing both 6-MP and 6-TG to different methylated metabolites including methylthioinosine monophosphate (meTIMP) and methylthioguanosine monophosphate (meTGMP), respectively, with different suggested pharmacological and cytotoxic properties. While meTIMP is a potent inhibitor of de novo purine synthesis (DNPS) and significantly contributes to the cytotoxic effects of 6-MP, meTGMP, does not add much to the effects of 6-TG, and the cytotoxicity of 6-TG seems to be more dependent on incorporation of thioguanine nucleotides (TGNs) into DNA rather than inhibition of DNPS. In order to investigate the role of TPMT in metabolism and thus, cytotoxic effects of 6-MP and 6-TG, we knocked down the expression of the gene encoding the TPMT enzyme using specifically designed small interference RNA (siRNA) in human MOLT4 leukemia cells. The knock-down was confirmed at RNA, protein, and enzyme function levels. Apoptosis was determined using annexin V and propidium iodide staining and FACS analysis. The results showed a 34% increase in sensitivity of MOLT4 cells to 1μM 6-TG after treatment with TPMT-targeting siRNA, as compared to cells transfected with non-targeting siRNA, while the sensitivity of the cells toward 6-MP was not affected significantly by down-regulation of the TPMT gene. This differential contribution of the enzyme TPMT to the cytotoxicity of the two thiopurines is probably due to its role in formation of the meTIMP, the cytotoxic methylated metabolite of 6-MP, while in case of 6-TG methylation by TPMT substantially deactivates the drug.

Gene expression and thiopurine metabolite profiling in inflammatory bowel disease - novel clues to drug targets and disease mechanisms?

BACKGROUND AND AIMS

Thiopurines are effective to induce and maintain remission in inflammatory bowel disease (IBD). The methyl thioinosine monophosphate (meTIMP)/6-thioguanine nucleotide (6-TGN) concentration ratio has been associated with drug efficacy. Here we explored the molecular basis of differences in metabolite profiles and in relation to disease activity.

METHODS

Transcriptional profiles in blood samples from an exploratory IBD-patient cohort (n = 21) with a normal thiopurine S-methyltransferase phenotype and meTIMP/6-TGN ratios >20, 10.0-14.0 and ≤4, respectively, were assessed by hybridization to microarrays. Results were further evaluated with RT qPCR in an expanded patient cohort (n = 54). Additionally, 30 purine/thiopurine related genes were analysed separately.

RESULTS

Among 17 genes identified by microarray-screening, there were none with a known relationship to pathways of purines/thiopurines. For nine of them a correlation between expression level and the concentration of meTIMP, 6-TGN and/or the meTIMP/6-TGN ratio was confirmed in the expanded cohort. Nine of the purine/thiopurine related genes were identified in the expanded cohort to correlate with meTIMP, 6-TGN and/or the meTIMP/6-TGN ratio. However, only small differences in gene expression levels were noticed over the three different metabolite profiles. The expression levels of four genes identified by microarray screening (PLCB2, HVCN1, CTSS, and DEF8) and one purine/thiopurine related gene (NME6) correlated significantly with the clinical activity of Crohn's disease. Additionally, 16 of the genes from the expanded patient cohort interacted in networks with candidate IBD susceptibility genes.

CONCLUSIONS

Seventeen of the 18 genes which correlated with thiopurine metabolite levels also correlated with disease activity or participated in networks with candidate IBD susceptibility genes involved in processes such as purine metabolism, cytokine signaling, and functioning of invariant natural killer T cells, T cells and B cells. Therefore, we conclude that the identified genes to a large extent are related to drug targets and disease mechanisms of IBD.

Safety and effectiveness of long-term allopurinol-thiopurine maintenance treatment in inflammatory bowel disease

BACKGROUND

Thiopurines are the mainstay of conventional maintenance therapy in inflammatory bowel disease (IBD). Unfortunately, up to 50% of patients discontinue immunosuppressive therapy within 2 years due to intolerance or lack of efficacy. Allopurinol with low-dose thiopurine can optimize thiopurine metabolism for IBD patients with preferential shunting toward 6-methyl mercaptopurine (6-MMP) formation. The aim of this study was to assess long-term maintenance effectiveness and tolerability of allopurinol-thiopurine therapy in a larger multicenter cohort of IBD patients.

METHODS

Enrolled patients who failed monotherapy with thiopurines due to a skewed metabolism were subsequently treated with a combination therapy of allopurinol and low-dose thiopurine. Adverse events were monitored and therapeutic adherence was assessed. Seventy-seven IBD patients were enrolled with a mean follow-up of 19 months.

RESULTS

The median 6-thioguanine nucleotide concentration increased from 145 during monotherapy to 271 pmol/8 × 10(8) red blood cell (RBC) after at least 8 weeks of combination therapy while reducing the thiopurine dosage (P < 0.001). In contrast, median 6-MMP concentrations decreased from 10,110 to 265 pmol/8 × 10(8) RBC (P < 0.001). Leukopenia occurred in 12 patients (16%), requiring dose adaptation. Liver test abnormalities normalized in 81% of patients after the addition of allopurinol. Sixteen (21%) patients had to discontinue combination therapy. The percentage of patients still using combination therapy at 6, 12, 24, and 60 months was 87%, 85%, 76%, and 65%, respectively.

CONCLUSIONS

Long-term combination therapy with allopurinol and low-dose thiopurines is an effective and well-tolerated treatment in IBD patients with a skewed thiopurine metabolism.

Institutional Profile: Pharmacogenomics research at Newcastle University

Newcastle University has been active in the field of pharmacogenomics/pharmacogenetics research since 1988. Research activity is based at the Faculty of Medical Sciences and is led by four professors within two separate research institutes. This article describes the various ongoing research projects and the teams involved together with our teaching activities in the subject.