The role of xanthine oxidase in thiopurine metabolism: a case report

Carvedilol alleviates the detrimental effects of azathioprine on hepatic tissues in experimental rats: Focusing on redox system, inflammatory and apoptosis pathways

PURPOSE

Drug-induced liver injury is becoming an increasingly important topic in drug research and clinical practice. Due to a lack of experimental animal models, predicting drug-induced liver injury in humans is challenging. Azathioprine (AZA) is a classical immunosuppressant with hepatotoxic adverse effects. The present study aimed to address the hepatoprotective effect of carvedilol (CAR) against AZA-induced hepatocellular injury via assessing redox-sensitive signals.

METHOD

To achieve this purpose, rats were allocated into four groups: control, CAR only, AZA only, and CAR plus AZA groups. The induction of hepatic injury was induced by a single intraperitoneal injection of AZA at a dose of 50 mg/kg on the 6th day of the experiment. Each experimental protocol was approved and supervised by the Ethics Committee for Animal Experiments.

RESULTS

The results of the present study revealed that CAR administration significantly diminished AZA-induced hepatic dysfunction, as evidenced by relief of hepatic function biomarkers and histopathological aberration induced by AZA injection. Besides, CAR restored oxidant/antioxidant balance as well as NRF2 expression. In addition, CAR suppressed inflammatory response induced by AZA challenge as evidenced by downregulation of TLR4, TNF-α, MPO, and eNOS/iNOS levels in hepatic tissue. Moreover, CAR recovered apoptotic/anti-apoptotic status by modulation of caspase-3/Bcl2 expression.

CONCLUSION

Taken together, CAR protects against AZA-induced hepatic injury via antioxidant, anti-inflammatory, and anti-apoptotic activities. These findings revealed that CAR could be a good candidate for hepatic injury protection and can be added to AZA therapeutic regimen to reduce their adverse effect.

Exposure of Azathioprine Metabolites and Clinical Outcome in Indian Patients with Crohn’s Disease

Aim:

The aim of this study was to determine the concentration of 6-thioguanine nucleotide (6-TGN) and 6-methylmercaptopurine (6-MMP), the interpatient variability, and the relationship with disease activity in patients with Chron’s disease on long-term stable doses of azathioprine (AZA).

Methods:

This is a prospective, tertiary care single-center hospital study in adult Chron’s disease patients treated with AZA. The quantification of phenotypic thiopurine methyltransferase enzyme activity in red blood cells and the estimation of the concentration of 6-TGN and 6-MMP in whole blood was performed using the HPLC-UV detector method. A clinical response was categorized as remission (Harvey-Bradshaw Index [HBI] < 5) or improvement (drop from baseline of at least three points of HBI) based on HBI. Exposure to metabolite concentrations and the clinical response to AZA treatment was observed.

Results:

Study analysis included 30 patients who were initiated on AZA, and they were followed up with an estimation of metabolite concentrations to determine their clinical outcome. At six months, 93% of ( n = 28) patients continued to be on AZA and had clinical improvement. All the patients achieved remission of Chron’s disease. Only two patients developed adverse effects such as joint pain and thrombocytopenia.

Conclusion:

AZA is a safe and effective therapy in managing Chron’s disease when administered after determining thiopurine methyltransferase phenotype and with dose optimization performed using therapeutic drug monitoring of 6-TGN and 6-MMP.

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.

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.

Xanthine oxidase activity in thiopurine curative Chinese inflammatory bowel disease patients

Xanthine oxidase (XO) competes with thiopurine S‐methyltransferase (TPMT) and hypoxanthine guanine phosphoribosyltransferase (HPRT) to metabolize azathioprine (AZA)/6‐mercaptopurine (6‐MP) in vivo. A retrospective investigation was performed to detect the activity of XO in thiopurine curative Chinese inflammatory bowel disease (IBD) patients. We also evaluated whether a relationship between XO activity and incidence of thiopurine‐induced adverse effects (AEs) existed. Clinical data and blood samples were collected from 140 IBD patients before receiving AZA/6‐MP therapy, and the erythrocyte XO activity was measured. The XO activities of all patients were 20.29 ± 4.43 U/g Hb. No sex difference in XO activity was observed (p = .728), and the XO activity showed no difference between the UC and CD patients (p = .082). AEs were observed in 41 (29.3%) patients including leukopenia (26, 18.57%), gastrointestinal intolerance (11, 7.86%), flu‐like symptom (5, 3.57%), alopecia (5, 3.57%), and hepatotoxicity (1, 0.71%). XO activity was significantly lower in the patients with AEs than in those without AEs (18.40 ± 3.73 vs. 21.07 ± 4.48 U/g Hb, p = .001), especially in the patients with leukopenia (18.29 ± 3.68 vs. 21.07 ± 4.48 U/g Hb, p = .004). However, no significant difference in XO activity was found between patients with and without other AEs. Decreased XO activity was observed in the patients who developed flu‐like symptoms (17.58 ± 3.50 U/g Hb) and alopecia (18.67 ± 2.91 U/g Hb) compared to those who did not, although the differences did not reach statistical significance. These findings suggested that patients with low XO expression might have a high risk of thiopurine‐induced toxicity.

Possible Pathways of Hepatotoxicity Caused by Chemical Agents

BACKGROUND

Liver injury induced by drugs has become a primary reason for acute liver disease and therefore posed a potential regulatory and clinical challenge over the past few decades and has gained much attention. It also remains the most common cause of failure of drugs during clinical trials. In 50% of all acute liver failure cases, drug-induced hepatoxicity is the primary factor and 5% of all hospital admissions.

METHODS

The various hepatotoxins used to induce hepatotoxicity in experimental animals include paracetamol, CCl4, isoniazid, thioacetamide, erythromycin, diclofenac, alcohol, etc. Among the various models used to induce hepatotoxicity in rats, every hepatotoxin causes toxicity by different mechanisms.

RESULTS

The drug-induced hepatotoxicity caused by paracetamol accounts for 39% of the cases and 13% hepatotoxicity is triggered by other hepatotoxic inducing agents.

CONCLUSION

Research carried out and the published papers revealed that hepatotoxins such as paracetamol and carbon- tetrachloride are widely used for experimental induction of hepatotoxicity in rats.

Key insights from therapeutic drug monitoring in Crohn's disease patients

INTRODUCTION

The incidence and prevalence of Crohn's disease are increasing causing a significant disease burden. Therapeutic drug monitoring (TDM) is advocated as a promising tool for personalized or individual-tailored therapy strategies and has been welcomed as a new means to improve current therapy strategies. Nevertheless, pharmacokinetic-based TDM has limitations, and straightforward target concentrations for most therapies are lacking. Areas covered: In the following concise review of literature, key insights of TDM in thiopurine, methotrexate, anti-TNF, vedolizumab and ustekinumab therapy for Crohn's disease are being described. Expert opinion: Therapeutic drug monitoring may, up till now, be helpful to adjust thiopurine and infliximab therapy, primarily in a reactive setting, in case of inefficacy and of occurrence of adverse event. With this restricted application, the goal of individualized therapy based on TDM has not yet been achieved.

Analytical Pitfalls of Therapeutic Drug Monitoring of Thiopurines in Patients With Inflammatory Bowel Disease

The use of thiopurines in the treatment of inflammatory bowel disease (IBD) can be optimized by the application of therapeutic drug monitoring. In this procedure, 6-thioguanine nucleotides (6-TGN) and 6-methylmercaptopurine (6-MMP) metabolites are monitored and related to therapeutic response and adverse events, respectively. Therapeutic drug monitoring of thiopurines, however, is hampered by several analytical limitations resulting in an impaired translation of metabolite levels to clinical outcome in IBD. Thiopurine metabolism is cell specific and requires nucleated cells and particular enzymes for 6-TGN formation. In the current therapeutic drug monitoring, metabolite levels are assessed in erythrocytes, whereas leukocytes are considered the main target cells of these drugs. Furthermore, currently used methods do not distinguish between active nucleotides and their unwanted residual products. Last, there is a lack of a standardized laboratorial procedure for metabolite assessment regarding the substantial instability of erythrocyte 6-TGN. To improve thiopurine therapy in patients with IBD, it is necessary to understand these limitations and recognize the general misconceptions in this procedure.

Genetic risk variants as therapeutic targets for Crohn's disease

The pathogenesis of Inflammatory bowel diseases (IBD) is multifactorial, with interactions between genetic and environmental factors. Despite the existence of genetic factors being largely demonstrated by epidemiological data and several genetic studies, only a few findings have been useful in term of disease prediction, disease progression and targeting therapy. Areas covered: This review summarizes the results of genome-wide association studies in Crohn's disease, the role of epigenetics and the recent discovery by genetic studies of new pathogenetic pathways. Furthermore, it focuses on the importance of applying genetic data to clinical practice, and more specifically how to better target therapy and predict potential drug-related toxicity. Expert opinion: Some genetic markers identified in Crohn`s disease have allowed investigators to hypothesize about, and in some cases, prove the usefulness of new specific therapeutic agents. However, the heterogeneity and complexity of this disease has so far limited the daily clinical use of genetic information. Finally, the study of the implications of genetics on therapy, either to predict efficacy or avoid toxicity, is considered still to be in its infancy.

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.

Role of Allopurinol in Optimizing Thiopurine Therapy in Patients with Autoimmune Hepatitis: A Review

Autoimmune hepatitis (AIH) is a chronic immune mediated liver disease characterized by elevated transaminases, hyper gammaglobulinemia, presence of autoantibodies and interface hepatitis in the absence of a known etiology of liver disease. Thiopurines (azathioprine [AZA]/6-mercaptopurine [6MP]) and steroids remain the first line of treatment of AIH in both children and adults. However, a small proportion of AIH patients are either non-responders or develop side effects with AZA. The metabolism of AZA is complex and mediated by multiple enzymes. After absorption and getting converted to 6MP, it is converted to 6-thiouric acid, 6-methyl mercaptopurine (6MMP) and 6-thioguanine (6TG) by different enzymes. Elevated 6MMP levels are associated with hepatotoxicity and also poor efficacy due to simultaneous lower levels of 6TG, which is the active drug metabolite related to both efficacy and myelosuppression. Allopurinol, a xanthine oxidase inhibitor shifts the metabolism of AZA away from 6MMP toward 6TG. This combination of allopurinol with reduced dose of AZA is an alternative to more expensive and toxic second line therapy to induce remission in patients with AIH. This article discusses the mechanism of action of allopurinol in inducing response to AZA, reviews the published literature on this combination therapy and gives guidelines on the use of allopurinol in patients with AIH.

The genetic burden of inflammatory bowel diseases: implications for the clinic?

Inflammatory bowel diseases (IBD), which include Crohn's disease (CD) and ulcerative colitis (UC), are characterized by chronic intestinal inflammation. Their etiology is multifactorial, with complex interactions between genetic and environmental factors, which are still largely unclear. Areas covered: The influence of genetics is clearly demonstrated by important epidemiological data, including familial aggregation and concordance in twins. In 2001, the first genetic susceptibility gene for IBD, the NOD2 gene, was identified. Currently, thanks to genetic wide association studies, over 200 susceptibility genetic markers are know. Expert commentary: However, clinically highly relevant gene associations are still very limited and the usefulness of these information in the current clinical strategies for treatment and surveillance of IBD is weak. Nevertheless, the recent identification of some genetic risk variants has clarified some newbiological pathways of these diseases thus paving the way for the discoveries in the near future of new targeted therapies.

Review article: The pharmacokinetics and pharmacodynamics of drugs used in inflammatory bowel disease treatment

BACKGROUND

The following review is a compilation of the recent advances and knowledge on the behaviour of the most frequently used compounds to treat inflammatory bowel disease in an organism.

RESULTS

It considers clinical aspects of each entity and the pharmacokinetic/pharmacodynamic relationship supported by the use of plasma monitoring, tissue concentrations, and certain aspects derived from pharmacogenetics.

Glutathione transferases in the bioactivation of azathioprine

The prodrug azathioprine is primarily used for maintaining remission in inflammatory bowel disease, but approximately 30% of the patients suffer adverse side effects. The prodrug is activated by glutathione conjugation and release of 6-mercaptopurine, a reaction most efficiently catalyzed by glutathione transferase (GST) A2-2. Among five genotypes of GST A2-2, the variant A2*E has threefold-fourfold higher catalytic efficiency with azathioprine, suggesting that the expression of A2*E could boost 6-mercaptopurine release and adverse side effects in treated patients. Structure-activity studies of the GST A2-2 variants and homologous alpha class GSTs were made to delineate the determinants of high catalytic efficiency compared to other alpha class GSTs. Engineered chimeras identified GST peptide segments of importance, and replacing the corresponding regions in low-activity GSTs by these short segments produced chimeras with higher azathioprine activity. By contrast, H-site mutagenesis led to decreased azathioprine activity when active-site positions 208 and 213 in these favored segments were mutagenized. Alternative substitutions indicated that hydrophobic residues were favored. A pertinent question is whether variant A2*E represents the highest azathioprine activity achievable within the GST structural framework. This issue was addressed by mutagenesis of H-site residues assumed to interact with the substrate based on molecular modeling. The mutants with notably enhanced activities had small or polar residues in the mutated positions. The most active mutant L107G/L108D/F222H displayed a 70-fold enhanced catalytic efficiency with azathioprine. The determination of its structure by X-ray crystallography showed an expanded H-site, suggesting improved accommodation of the transition state for catalysis.

How will insights from genetics translate to clinical practice in inflammatory bowel disease?

Inflammatory bowel disease, consisting of Crohn's disease and ulcerative colitis, is a chronic inflammatory disease of the gut, which arises through an excessive immune response to the normal gut flora in a genetically susceptible host. The disease affects predominantly young adults and due to its chronic and relapsing nature gives rise to a high disease burden both financially, physically and psychologically. Current therapy still cannot prevent the need for surgical intervention in more than half of IBD patients. Consequently, advances in IBD therapy are of high importance. Recently, several new forms of targeted therapy have been introduced, which should improve surgery-free prognosis of IBD patients. Recent identification of genetic risk variants for IBD has led to new insights into the biological mechanisms of the disease, which will, in the future, lead to new targeted therapy. In the meantime repositioning of drugs from biologically similar diseases towards IBD might lead to new IBD therapies.

Involvement of oxidative stress and immune- and inflammation-related factors in azathioprine-induced liver injury

Drug-induced liver injury (DILI) is a growing concern in the fields of drug development and clinical drug therapy because numerous drugs have been linked to hepatotoxicity. However, it is difficult to predict DILI in humans due to the lack of experimental animal models. Although azathioprine (AZA), which is a widely used immunosuppressive drug, is generally well tolerated, a small number of patients prescribed AZA develop severe hepatitis. However, the mechanism underlying this process has not yet been elucidated. In this study, we developed a mouse model of AZA-induced liver injury and investigated the mechanisms responsible for the hepatotoxicity of AZA. Female BALB/c mice were orally administered AZA. After AZA administration, the plasma levels of alanine aminotransferase and aspartate aminotransferase were increased, and liver damage was confirmed through a histological evaluation. In addition, the hepatic glutathione levels and superoxide dismutase activity were significantly decreased. The plasma levels of reactive oxygen species were significantly increased during the early phase of AZA-induced liver injury, and the hepatic mRNA levels of immune- and inflammation-related factors were also significantly changed. In conclusion, oxidative stress and the subsequently activated immune- and inflammation-related factors are involved in AZA-induced liver injury.

Current relevance of pharmacogenetics in immunomodulation treatment for Crohn's disease

No drug therapy is completely risk free, and the costs associated with non-response and adverse effects can exceed the cost of the therapy. The ultimate goal of pharmacogenetic research is to find robust genetic predictors of drug response that enable the development of prospective genetic tests to reliably identify patients at risk of non-response or of developing an adverse effect prior to the drug being prescribed. Currently, thiopurine S-methyltransferase (TPMT) deficiency is the only pharmacogenetic factor that is prospectively assessed before azathioprine or 6-mercaptopurine immunomodulation is commenced in patients with Crohn's disease (CD). As yet no other inherited determinant of drug response has made the transition from bench to bedside for the management of this disease. In this review we summarize what is known about TPMT deficiency and explore whether there is evidence to support a role of other genetic polymorphisms in predicting the response of CD patients to thiopurine drugs, methotrexate, and anti-tumor necrosis factor α (TNFα) therapy.

Therapeutic drug monitoring of thiopurine metabolites in adult thiopurine tolerant IBD patients on maintenance therapy

BACKGROUND AND AIMS

Therapeutic drug monitoring of active metabolites of thiopurines, azathioprine and 6-mercaptopurine, is relatively new. The proposed therapeutic threshold level of the active 6-thioguanine nucleotides (6-TGN) is ≥235 pmol/8×10(8) erythrocytes. The aim of this prospective cross-sectional study was to compare 6-TGN levels in adult thiopurine tolerant IBD patients with an exacerbation with those in remission, and to determine the therapeutic 6-TGN cut-off level.

METHODS

Hundred IBD patients were included. Outcome measures were thiopurine metabolite levels, calculated therapeutic 6-TGN cut-off level, CDAI/CAI scores, thiopurine dose and TPMT enzyme activity.

RESULTS

Forty-one patients had an exacerbation, 59 patients were in remission. In 17% of all patients 6-TGN levels were compatible with non-compliance. The median 6-TGN levels were not significantly different between the exacerbation and remission group (227 versus 263 pmol/8×10(8) erythrocytes, p=0.29). The previous reported therapeutic 6-TGN cut-off level of 235 pmol/8×10(8) erythrocytes was confirmed in this study. Twenty-six of the 41 patients (63%) with active disease had 6-TGN levels below this threshold and 24 of 59 IBD patients (41%) in clinical remission (p=0.04).

CONCLUSIONS

Thiopurine non-compliance occurs frequently both in active and quiescent disease. 6-TGN levels below or above the therapeutic threshold are associated with a significant higher chance of IBD exacerbation and remission, respectively. These data support the role of therapeutic drug monitoring in thiopurine maintenance therapy in IBD to reveal non-compliance or underdosing, and can be used as a practical tool to optimize thiopurine therapy, especially in case of thiopurine non-response.

Pharmacogenetic determinants of mercaptopurine disposition in children with acute lymphoblastic leukemia

BACKGROUND

The backbone of drug therapy used in acute lymphoblastic leukemia (ALL) in children includes 6-mercaptopurine (6-MP). Intracellular metabolism of this prodrug is a key component of the therapeutic response. Many metabolizing enzymes are involved in 6-MP disposition and active 6-MP metabolites are represented by 6-thioguanine nucleotides (6-TGN) and methylated metabolites primarily methylated by the thiopurine S-methyltransferase enzyme (TPMT). The genetic polymorphism affecting TPMT activity displays an important inter-subject variability in metabolites pharmacokinetics and influences the balance between 6-MP efficacy and toxicity: patients with high 6-TGN levels are at risk of myelosuppression while patients with high levels of methylated derivates are at hepatotoxic risk. However, the genetic TPMT polymorphism does not explain all 6-MP adverse events and some severe toxicities leading to life-threatening conditions remain unexplained. Additional single nucleotide polymorphisms (SNPs) in genes encoding enzymes involved in 6-MP metabolism and 6-MP transporters may also be responsible for this inter-individual 6-MP response variability.

AIM

This review presents the pharmacogenetic aspects of 6-MP metabolism in great detail. We have focused on published data on ALL treatment supporting the great potential of 6-MP pharmacogenetics to improve efficacy, tolerance, and event-free survival rates in children with ALL.

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.

Optimizing thiopurine therapy in inflammatory bowel disease

Despite recent advances, the therapeutic armamentarium for inflammatory bowel disease (IBD) is still limited. In addition, a step-up approach is recommended for most IBD patients. Thus, optimizing each medical therapy before switching to another drug class is the rule in clinical practice. Conventional therapies for IBD have not received the same amount of attention as biologic therapies over the last decade. However, due to their efficacy, safety, and low cost the thiopurine drugs azathioprine and 6-mercaptopurine remain the backbone of therapy for IBD. Pharmacogenomic advances and increased knowledge of their metabolism are allowing dosage optimization. Herein, after describing the pharmacogenetics and pharmacokinetics of thiopurines, we will discuss how to optimize thiopurine therapy. We will then underscore the need to take into account safety issues when optimizing thiopurine treatment.

Allopurinol to Febuxostat: How far have we come?

For decades allopurinol has been used as a xanthine oxidase inhibitor for treatment of hyperuricemia and gout. Although effective in many patients, some experience sensitivity to the drug. In some cases, this sensitivity may lead to allopurinol hypersensitivity disorder, which if untreated can be fatal. Recently the Food and Drug Administration has approved the use of febuxostat as an alternative therapy for hyperuricemia and gout. Febuxostat is a new xanthine oxidase inhibitor, but is not purine based and therefore decreases adverse reactions due to patient sensitivity. This review is a comprehensive look at the background of hyperuricemia and gout treatment with allopurinol compared to recent clinical studies with febuxostat. Each clinical study is evaluated and summarized, identifying the advances in treatment that have been made as well as the concerns that still exist with either treatment.

Thiopurine metabolism monitoring: implications in inflammatory bowel diseases

BACKGROUND

Thiopurines (TP) are widely used in the management of inflammatory bowel diseases. Side effects and inefficacy are a major concern as they lead to withdrawal of the drug.

MATERIALS AND METHODS

Tools investigating TP metabolism are useful to avoid inadequate cessation of TP therapy.

RESULTS

TP metabolism is complex and many enzymes are involved. Among them, Thiopurine methyl transferase is the only one routinely measured by pheno- or genotyping. A decreased TPMT activity results in a potential overdosing of TP drugs leading to myelotoxicity, whereas an ultra-high activity leads to TP ineffectiveness and overproduction of methylated compounds responsible for hepatotoxicity. TPMT determination prior to TP treatment results in an individual adapted dose. Xanthine oxidase/dehydrogenase (XOD), inosine triphosphate pyrophosphatase (ITPA) and glutathion-S-transferase (GST) are other promising enzyme targets that might help to explain TP efficacy or toxicity. ITPA and GST polymorphisms might potentially be related to some TP side effects, while a XOD inhibition by allopurinol could avoid TP-related hepatotoxicity.

CONCLUSIONS

Utilization of thiopurine metabolites, 6-thioguanine nucleotides and 6-methylmercaptopurine, is discussed, specifically, in case of thiopurine failure and recommendations are given about their interpretation and potential dose optimization. These enzymes and metabolites tests are complementary to the regular monitoring of blood cells count and liver tests which remains mandatory.

Predicting enzyme targets for cancer drugs by profiling human metabolic reactions in NCI-60 cell lines

BACKGROUND

Drugs can influence the whole metabolic system by targeting enzymes which catalyze metabolic reactions. The existence of interactions between drugs and metabolic reactions suggests a potential way to discover drug targets.

RESULTS

In this paper, we present a computational method to predict new targets for approved anti-cancer drugs by exploring drug-reaction interactions. We construct a Drug-Reaction Network to provide a global view of drug-reaction interactions and drug-pathway interactions. The recent reconstruction of the human metabolic network and development of flux analysis approaches make it possible to predict each metabolic reaction's cell line-specific flux state based on the cell line-specific gene expressions. We first profile each reaction by its flux states in NCI-60 cancer cell lines, and then propose a kernel k-nearest neighbor model to predict related metabolic reactions and enzyme targets for approved cancer drugs. We also integrate the target structure data with reaction flux profiles to predict drug targets and the area under curves can reach 0.92.

CONCLUSIONS

The cross validations using the methods with and without metabolic network indicate that the former method is significantly better than the latter. Further experiments show the synergism of reaction flux profiles and target structure for drug target prediction. It also implies the significant contribution of metabolic network to predict drug targets. Finally, we apply our method to predict new reactions and possible enzyme targets for cancer drugs.

Functional characterization of genetic polymorphisms identified in the promoter region of the xanthine oxidase gene

Xanthine oxidase (XO) catalyzes the oxidation of endogenous and exogenous purines and pyrimidines. In the present study, we investigated polymorphisms in the promoter region of the XO gene. Sequence variations in the 5'-flanking region were screened using denaturing high-performance liquid chromatography (DHPLC) on DNA samples from 196 unrelated Japanese individuals. Thirteen polymorphisms were identified and 13 haplotypes were classified by haplotype analysis. The promoter activities of these polymorphisms were measured by luciferase assay in the human hepatoma cell lines HepG2 and Huh-7. Transcriptional activity was significantly lower in cell lines transfected with the reporter construct containing 5-kb upstream fragments with -1756T than in those with wild-type -1756C. Our results indicate that genetic variation in the promoter region of XO may determine interindividual differences in XO gene expression.

Long term efficacy and safety of allopurinol and azathioprine or 6-mercaptopurine in patients with inflammatory bowel disease

BACKGROUND AND AIMS

We previously reported that IBD patients who are non-responders to thiopurines with preferential shunting of metabolites to hepatotoxic 6-methylmercaptopurine ribonucleotides compared to 6-thioguanine nucleotides can reverse the ratio of 6-MMP/6-TGN and respond to thiopurines with the addition of allopurinol. The objective of this study is to report long term efficacy and safety, along with results for an additional 11 patients.

METHODS

Retrospective chart review of patients at the University of Chicago IBD Center treated with allopurinol in addition to thiopurines.

RESULTS

Twenty five patients with Crohn's disease or ulcerative colitis were enrolled. Within the first month of therapy 6-TGN metabolite levels increased from a mean of 186.5±17.4 (SE) to 352.8±37.8 pmol/8×10(8) (p=0.0001). Over the same period 6-MMP levels decreased from a mean of 11,966±1697 to 2004±536 pmol/8×10(8) (p<0.0001). The mean daily dosage of prednisone decreased from 19.8±3.8 mg to 5.3±2.7 mg (p=0.03). Thirteen patients have a minimum of one year follow-up. Nine of these thirteen patients have continued on therapy for at least 2 years. All thirteen of these patients continue to be in clinical remission at the last follow-up visit. No patients have had evidence of sustained thrombocytopenia or abnormal liver enzymes.

CONCLUSIONS

In AZA/6-MP non-responders with increased 6-MMP/6-TGN ratios, addition of allopurinol continues to demonstrate safety and efficacy for long-term maintenance and steroid-sparing in IBD.

Thiopurine hepatotoxicity in inflammatory bowel disease: the role for adding allopurinol

BACKGROUND

Immunomodulator therapy with the thiopurine analogues azathioprine or 6-mercaptopurine is commonly prescribed for the treatment of inflammatory bowel disease (IBD). Drug adverse effects and the lack of efficacy, however, commonly require withdrawal of therapy. Allopurinol, a xanthine oxidase inhibitor, was recently evaluated in its role in modifying thiopurine metabolism and improving drug efficacy in IBD.

OBJECTIVE

This article reviews the role and safety of allopurinol co-therapy in the setting of thiopurine hepatotoxicity and/or non-responsiveness in IBD.

METHODS

Published articles on thiopurines in the treatment of IBD were examined.

CONCLUSION

The addition of low dose allopurinol to dose-reduced thiopurine analogue seems safe but careful monitoring for adverse effects and profiling of thiopurine metabolites is essential. There is evidence of improved immunomodulator efficacy and reduced hepatotoxicity clinically but further confirmatory studies are required before more definitive treatment recommendations can be given.

Beyond TPMT: genetic influences on thiopurine drug responses in inflammatory bowel disease

Azathioprine and 6-mercaptopurine are widely used in the management of inflammatory bowel disease (IBD). However, approximately 25% of IBD patients experience toxicity, and up to 10% show resistance to these thiopurine drugs. The importance of genetic variability in determining thiopurine toxicity was first recognized over 25 years ago with the discovery of the thiopurine S-methyltransferase (TPMT) polymorphism and the occurrence of azathioprine-induced myelosuppression in TPMT-deficient patients. In the intervening period, TPMT has become the foremost example of pharmacogenetics, and TPMT deficiency represents one of the few pharmacogenetic phenomena that have successfully made the transition from the research laboratory to diagnostics. While TPMT activity predicts some cases of myelosuppression, deficiency in this enzyme is neither predictive of other adverse drug reactions, nor resistance to thiopurine therapy. As myelosuppression only accounts for approximately 2.5% of adverse reactions in IBD patients, researchers are increasingly turning their attention to other enzymes involved in thiopurine metabolism to find molecular explanations for intolerance and resistance to azathioprine and 6-mercaptopurine. In this review, we summarize the current state of knowledge with regards to TPMT, and also explore genetic variability, beyond TPMT, that may contribute to thiopurine response in IBD patients.