Toxicity of low dose azathioprine and 6-mercaptopurine in rat hepatocytes. Roles of xanthine oxidase and mitochondrial injury

Response to azathioprine treatment in autoimmune hepatitis is dependent on glutathione transferase genotypes

BACKGROUND

Azathioprine (AZA) is part of the standard treatment for autoimmune hepatitis (AIH). The first step in the complex bioconversion of AZA to active metabolites is mediated by glutathione transferases (GSTs).

AIMS

Elucidate the association between GSTM1 and GSTT1 copy number variation (CNV), genetic variation in GSTA2, GSTP1, and inosine-triphosphate-pyrophosphatase, and the response to AZA in AIH.

METHODS

Genotyping was performed in AIH patients (n = 131) on AZA, and in a Swedish background population (n = 283). Thiopurine metabolites in blood erythrocytes were determined by high performance liquid chromatography.

RESULTS

GSTM1 and GSTT1 CNV were associated with treatment response to AZA. Gene deletion of GSTM1-but not of GSTT1-was associated with the liver transaminase levels. None of the studied genetic variants were associated with the thiopurine metabolite concentrations, suggesting non-enzymatic mechanisms of GSTM1 and GSTT1 in the context of AZA efficacy in AIH. The prevalence of GSTM1 and GSTT1 CNV genotypes was similar in AIH and in the background population.

CONCLUSION

This study shows the effects of GSTM1 and GSTT1 CNV on AZA efficacy in AIH, not previously described. It also elaborates on the impact of the definition of treatment response, on the importance of the various GSTs studied. Furthermore, the GSTM1 and GSTT1 CNV frequencies previously reported in European populations were confirmed.

Azathioprine promotes intestinal epithelial cell differentiation into Paneth cells and alleviates ileal Crohn's disease severity

Paneth cells (PCs), a subset of intestinal epithelial cells (IECs) found at the base of small intestinal crypts, play an essential role in maintaining intestinal homeostasis. Altered PCs function is associated with diverse intestinal pathologies, including ileal Crohn's disease (CD). CD patients with ileal involvement have been previously demonstrated to display impairment in PCs and decreased levels of anti-microbial peptides. Although the immunosuppressive drug Azathioprine (AZA) is widely used in CD therapy, the impact of AZA on IEC differentiation remains largely elusive. In the present study, we hypothesized that the orally administered drug AZA also exerts its effect through modulation of the intestinal epithelium and specifically via modulation of PC function. AZA-treated CD patients exhibited an ileal upregulation of AMPs on both mRNA and protein levels compared to non-AZA treated patients. Upon in vitro AZA stimulation, intestinal epithelial cell line MODE-K exhibited heightened expression levels of PC marker in concert with diminished cell proliferation but boosted mitochondrial OXPHOS activity. Moreover, differentiation of IECs, including PCs differentiation, was boosted in AZA-treated murine small intestinal organoids and was associated with decreased D-glucose consumption and decreased growth rates. Of note, AZA treatment strongly decreased Lgr5 mRNA expression as well as Ki67 positive cells. Further, AZA restored dysregulated PCs associated with mitochondrial dysfunction. AZA-dependent inhibition of IEC proliferation is accompanied by boosted mitochondria function and IEC differentiation into PC.

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

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

Prevalence of hepatotoxicity and myelosuppression with alternate day use of azathioprine and glucocorticoids for treatment of dermatological conditions in dogs

BACKGROUND

The use of azathioprine (AZA) in dogs is limited by the potential for hepatotoxicity and myelosuppression.

HYPOTHESIS/OBJECTIVES

To determine the prevalence of AZA-associated hepatotoxicity in dogs with dermatological conditions receiving alternate-day AZA. The hypothesis was that dogs receiving AZA every other day (EOD) would have a lower prevalence of hepatotoxicity compared to published data for dogs receiving daily AZA. A secondary aim was to determine the prevalence of AZA-associated myelosuppression over the same time period and population.

ANIMALS

Forty-one client-owned dogs with dermatological conditions treated with AZA EOD and glucocorticoids with clinical and haematological follow-up available for a minimum of two months of AZA therapy.

METHODS

Retrospective analysis of data from April 1994 to July 2020. Hepatotoxicity was defined as elevation of alanine aminotransferase (ALT) at least twofold above the reference range.

RESULTS

Azathioprine-associated hepatotoxicity was observed in two of 41 dogs (4.9%), with onset at 18 and 40 days, respectively. One dog receiving AZA at 1.9 mg/kg EOD had a fourfold increase in ALT. The other dog (AZA dose 2.3 mg/kg EOD) had a 30-fold increase in ALT. Azathioprine was not associated with thrombocytopenia, anaemia or neutropenia in any dogs. Lymphopenia developed in one dog (2.4%) with onset at 105 days.

CONCLUSIONS AND CLINICAL RELEVANCE

Alternate-day AZA administration with tapering glucocorticoids was well-tolerated in dogs with dermatological conditions.

Non-cytochrome P450 enzymes involved in the oxidative metabolism of xenobiotics: Focus on the regulation of gene expression and enzyme activity

Oxidative metabolism is one of the major biotransformation reactions that regulates the exposure of xenobiotics and their metabolites in the circulatory system and local tissues and organs, and influences their efficacy and toxicity. Although cytochrome (CY)P450s play critical roles in the oxidative reaction, extensive CYP450-independent oxidative metabolism also occurs in some xenobiotics, such as aldehyde oxidase, xanthine oxidoreductase, flavin-containing monooxygenase, monoamine oxidase, alcohol dehydrogenase, or aldehyde dehydrogenase-dependent oxidative metabolism. Drugs form a large portion of xenobiotics and are the primary target of this review. The common reaction mechanisms and roles of non-CYP450 enzymes in metabolism, factors affecting the expression and activity of non-CYP450 enzymes in terms of inhibition, induction, regulation, and species differences in pharmaceutical research and development have been summarized. These non-CYP450 enzymes are detoxifying enzymes, although sometimes they mediate severe toxicity. Synthetic or natural chemicals serve as inhibitors for these non-CYP450 enzymes. However, pharmacokinetic-based drug interactions through these inhibitors have rarely been reported in vivo. Although multiple mechanisms participate in the basal expression and regulation of non-CYP450 enzymes, only a limited number of inducers upregulate their expression. Therefore, these enzymes are considered non-inducible or less inducible. Overall, this review focuses on the potential xenobiotic factors that contribute to variations in gene expression levels and the activities of non-CYP450 enzymes.

Systematic review and meta-analysis of drug induced liver injury secondary to biologic medications in inflammatory bowel disease

Introduction: Drug-induced Hepatotoxicity and biologic drugs have historically been challenging in IBD. We aim to study the prevalence of hepatotoxicity in adult patients using biologic medications. Methodology: With the guidelines described by PRISMA-P, a detailed search strategy for each electronic database was developed based on PubMed, Medline, and Embase. We include RCTs that assessed the efficacy and hepatotoxicity of biologics in IBD patients. Hepatotoxicity was defined as AST and/or ALT >2x upper limit of normal or cholestasis. The Odds ratio (OR) was calculated with a 95% confidence interval (CI). Heterogeneity was assessed using the Chi2 test and the I2 statistic. Results: 862 records identified in total. After removing the duplicates 564 records were left for review. Four studies did not report on how participants were randomized to treatment groups or how allocation concealment was achieved, we rated these studies at unclear risk of bias for these domains. There was no presence of any heterogeneity among studies by (Chi2= 2.21, df = 6, p = 0.90, and I2 = 0%). Our meta-analysis was conducted on the fixed effects model, with the (0.770, 95% CI [-0.630, 0.957], and p = 0.02). Hepatotoxicity was not related to any TNF-α antagonist. Thiopurine induced liver injury occurred more frequently within the first months of treatment, 50% of cases within the first 3 months (11.4% vs. 2.3%, p < 0.05). Conclusion: When hepatotoxicity occurred, the treatment was withdrawn in thirty one percent of patients. This group of patients had a dose-dependent hepatotoxicity rather than an immunologic hepatitis.

Allopurinol in combination with thiopurine induces mucosal healing and improves clinical and metabolic outcomes in IBD

BACKGROUND

Thiopurines, azathioprine (AZA) and 6-mercaptopurine (6-MP) are common maintenance medications for inflammatory bowel disease (IBD). Excessive methylation via thiopurine methyltransferase (TPMT) frequently causes therapeutic failure. Allopurinol reduces excessive 6-methyl-mercaptopurine (6-MMP) while enhancing 6-thioguanine (6-TGN) levels. The aim of this study was to evaluate clinical, metabolic and endoscopic impact of allopurinol in combination with low-dose thiopurine in IBD.

METHODS

Retrospective review of consecutive cases treated with allopurinol. Metabolites and their ratios (6-MMP/6-TGN) were compared pre- and post-allopurinol. Clinical and endoscopic remission were assessed.

RESULTS

Allopurinol (n = 66) reduced mean dose of AZA by 70% (p < 0.01). Baseline levels (SD) 6-TGN, 6-MMP and 6-MMP/6-TGN were 165 (64), 9388 (5234) and 59.8 (30.3), respectively. These values improved on allopurinol to 297 (102), 896 (1031) and 3.4 (4.0), respectively (p < 0.0001). Therapeutic 6-TGN level (>235) was achieved in 49/58 cases on allopurinol combination therapy, versus 9/58 monotherapy (p = 0.0001). Among the thiopurine failure group (40 patients), clinical remission or response was observed in 65% and 22% of patients, respectively. In the asymptomatic group with excessive 6-MMP, 11/14 achieved sustained remission on allopurinol. Repeat colonoscopy (n = 28) showed mostly endoscopic remission (67.9%) or improvement (17.8%). Few had unimproved lesions (14.3%). Importantly, 46% of cases had complete mucosal healing. Two patients had cancer on combination therapy (de novo pancreatic cancer and fatal recurrence of metastatic testicular cancer). Elevated transaminases were reduced on allopurinol (48.2 versus 6.9%) (p < 0.001); no change in leukopenic or infectious events occurred.

CONCLUSION

Allopurinol in combination with low-dose thiopurine corrected excessive 6-MMP levels, resulting in clinical remission and mucosal healing in the majority of cases. The potential cancer risk of allopurinol and thiopurine combination therapy needs further research.

New and cost effective cell-based assay for Dialyzed Leukocyte Extract (DLE)-induced Jurkat cells proliferation under azathioprine treatment

The human Dialyzed Leukocyte Extract (DLE) is a heterogeneous mix of oligopeptides of <10kDa, extracted from leukocytes of healthy donors. There is significant clinical evidence of improvement using DLE during treatment of allergies, cancer,immunodeficiencies, and in mycotic and viral infections. Nevertheless, the DLE exact nature and mechanism of action have been elusive for more than 50 years. DLE biological activity testing is necessary in DLE production and quality control. Both in vitro and in vivo assays exist: E-rosette test, induction of delayed type hypersensitivity in mice, leukocyte migration and IFN-γ secretion. The animal-origin materials and in vivo assays convey a considerable logistic, ethic and economic burden, meanwhile the available in vitro assays have been reported with limited reproducibility and sometimes contradictory results. Here we are reporting a new DLE biological activity cell-based assay. The A20 and Jurkat cell lines were treated with (+Aza) or without (-Aza) azathioprine, DLE (+DLE) or both (+Aza/+DLE). After 72h, the cell proliferation was analyzed by the MTT or BrdU incorporation assays. In +Aza/+DLE treated cells, we observed a significant higher proliferation, when compared with +Aza/-DLE. In the absence of Aza, cells did not present any proliferation difference between -DLE or +DLE treatments. Both assays, MTT and BrdU showed similar results, being the MTT test more cost effective and we select it for validation as DLE biological assay using Jurkat cells only. We tested three different lyophilized DLE batches and we found consistent results with acceptable assay reproducibility and linearity. The DLE capacity for rescuing Jurkat cell proliferation during +Aza treatment was consistent using different liquid and lyophilized DLE batches, presenting also consistent chromatographic profiles. Finally, DLE treatment in Jurkat cells did not result into significant IL-2 of IFN-γ secretion, and known lymphocyte proliferative drugs failed to rescue Jurkat cells viability in presence of +Aza, as +DLE treatment did in our MTT assay. In conclusion, our new cell-based MTT assay has excellent DLE biological activity consistency, robustness and is cost effective, presenting important advantages over previous DLE activity in vitro and in vivo assays.

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.

Evaluation of a New Molecular Entity as a Victim of Metabolic Drug-Drug Interactions-an Industry Perspective

Under the guidance of the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ), scientists from 20 pharmaceutical companies formed a Victim Drug-Drug Interactions Working Group. This working group has conducted a review of the literature and the practices of each company on the approaches to clearance pathway identification (fCL), estimation of fractional contribution of metabolizing enzyme toward metabolism (fm), along with modeling and simulation-aided strategy in predicting the victim drug-drug interaction (DDI) liability due to modulation of drug metabolizing enzymes. Presented in this perspective are the recommendations from this working group on: 1) strategic and experimental approaches to identify fCL and fm, 2) whether those assessments may be quantitative for certain enzymes (e.g., cytochrome P450, P450, and limited uridine diphosphoglucuronosyltransferase, UGT enzymes) or qualitative (for most of other drug metabolism enzymes), and the impact due to the lack of quantitative information on the latter. Multiple decision trees are presented with stepwise approaches to identify specific enzymes that are involved in the metabolism of a given drug and to aid the prediction and risk assessment of drug as a victim in DDI. Modeling and simulation approaches are also discussed to better predict DDI risk in humans. Variability and parameter sensitivity analysis were emphasized when applying modeling and simulation to capture the differences within the population used and to characterize the parameters that have the most influence on the prediction outcome.

Long-term Safety and Efficacy of Low-dose Azathioprine and Allopurinol Cotherapy in Inflammatory Bowel Disease: A Large Observational Study

BACKGROUND

Low-dose azathioprine with allopurinol (LDAA) has been proposed as a potent therapy in inflammatory bowel disease (IBD) with the benefit of overcoming side effects regularly associated with thiopurine monotherapy and poor responses. Concerns regarding safety remain, while a layer of complexity has been added by the trend toward treatment directed by red cell thioguanine nucleotide (TGN) profiling. We report on the clinical efficacy and safety of LDAA use in IBD undirected by metabolite profiling.

METHODS

Observational study of clinical practice from a single IBD center. Patient outcomes were defined clinically based on established activity scores and corticosteroid withdrawal. Red cell TGN was monitored only for suspected nonadherence.

RESULTS

Overall, 113/164 (69%) patients with Crohn's disease and 83/136 (61%) patients with ulcerative/unclassified colitis had a clinical response by the end of follow-up (median 19 months), while 85 (52%) patients with Crohn's disease and 74 (54%) patients with ulcerative/unclassified colitis were in clinical remission. Clinical response was seen in 45/57 (79%) patients with Crohn's disease and 34/53 (64%) patients with ulcerative/unclassified colitis who were thiopurine naive, had active IBD, and received LDAA as the first line immunomodulator, while in 35 (61%) and 28 (53%), respectively, remission was achieved. LDAA was stopped in 20/300 (7%) patients because of side effects, all of which resolved on drug cessation.

CONCLUSIONS

This is the largest cohort supporting the favorable safety profile and high efficacy of LDAA in IBD. It presents 2 advances in therapy: prescribing LDAA for thiopurine-naive patients, and bypassing TGN monitoring in favor of clinical monitoring (blood counts, etc.), which will make it more accessible for clinics without access to TGN assays.

Effects of azathioprine, 6-mercaptopurine, and 6-thioguanine on canine primary hepatocytes

OBJECTIVE

To investigate the cytotoxic effects of azathioprine, 6-mercaptopurine, and 6-thioguanine on canine hepatocytes.

SAMPLE

Commercially available cryopreserved canine primary hepatocytes.

PROCEDURES

The study consisted of 2 trials. In trial 1, hepatocytes were incubated with azathioprine, 6-mercaptopurine, or 6-thioguanine at 1 of 6 concentrations (0.468, 0.937, 1.875, 3.750, 7.500, or 15.000 μmol/L) for 24, 48, or 72 hours. At each time, cell viability and lactate dehydrogenase (LDH) activity were determined for each thiopurine-concentration combination, and alanine aminotransferase (ALT) activity was determined for cells incubated with each thiopurine at a concentration of 15 μmol/L. In trial 2, hepatocytes were incubated with azathioprine, 6-mercaptopurine, or 6-thioguanine at 1 of 3 concentrations (18.75, 37.50, or 75.00 μmol/L) for 24 hours, after which the free glutathione concentration was determined for each thiopurine-concentration combination and compared with that for hepatocytes incubated without a thiopurine (control).

RESULTS

Incubation of hepatocytes with each of the 3 thiopurines adversely affected cell viability in a time- and concentration-dependent manner; however, this decrease in cell viability was not accompanied by a concurrent increase in LDH or ALT activity. Likewise, free glutathione concentration for hepatocytes incubated for 24 hours with supratherapeutic thiopurine concentrations (> 18.75 μmol/L) did not differ significantly from that of control cells.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that thiopurines adversely affected the viability of canine hepatocytes in a time- and concentration-dependent manner but had a nonsignificant effect on the LDH and ALT activities and free glutathione depletion of those hepatocytes.

Role of oxidative stress mediated by glutathione-s-transferase in thiopurines' toxic effects

Azathioprine (AZA), 6-mercaptopurine (6-MP), and 6-thioguanine (6-TG) are antimetabolite drugs, widely used as immunosuppressants and anticancer agents. Despite their proven efficacy, a high incidence of toxic effects in patients during standard-dose therapy is recorded. The aim of this study is to explain, from a mechanistic point of view, the clinical evidence showing a significant role of glutathione-S-transferase (GST)-M1 genotype on AZA toxicity in inflammatory bowel disease patients. To this aim, the human nontumor IHH and HCEC cell lines were chosen as predictive models of the hepatic and intestinal tissues, respectively. AZA, but not 6-MP and 6-TG, induced a concentration-dependent superoxide anion production that seemed dependent on GSH depletion. N-Acetylcysteine reduced the AZA antiproliferative effect in both cell lines, and GST-M1 overexpression increased both superoxide anion production and cytotoxicity, especially in transfected HCEC cells. In this study, an in vitro model to study thiopurines' metabolism has been set up and helped us to demonstrate, for the first time, a clear role of GST-M1 in modulating AZA cytotoxicity, with a close dependency on superoxide anion production. These results provide the molecular basis to shed light on the clinical evidence suggesting a role of GST-M1 genotype in influencing the toxic effects of AZA treatment.

Allopurinol and 5-aminosalicylic acid influence thiopurine-induced hepatotoxicity in vitro

INTRODUCTION

The use of thiopurines is frequently accompanied by hepatotoxicity. Studies on hepatocyte cultures showed a time- and dose-dependent increase of thiopurine toxicity. 5-Aminosalicylic acid (5-ASA) and allopurinol can influence thiopurine metabolism; however, it is unknown whether this affects in vitro cytotoxicity.

METHODS

Human hepatoma cells (Huh7, HepG2 and HepaRG) were incubated with increasing concentrations of thiopurines, 5-ASA or allopurinol. Water-soluble tetrazolium salt-1 (WST-1) cytotoxicity assays were used to calculate cell survival curves and half maximal inhibitory concentrations (IC50). Combination experiments with thiopurines with a fixed dose of 200 μM 5-ASA or 100 μM allopurinol were conducted in HepaRG cells. Caspase-3/7 activation was evaluated, and single cell electrophoresis analysis was performed.

RESULTS

A time- and dose-related cytotoxic effect was seen with azathioprine (AZA) in all hepatoma cells, whereas Huh7 and HepG2 cells did not show toxicity to 6-mercaptopurine (6-MP). HepaRG cells expressed the highest levels of drug metabolising enzymes, and therefore, combination experiments were conducted in HepaRG cells. Addition of a non-toxic dose of allopurinol resulted in a twofold to threefold increased cytotoxicity of all thiopurines, which seemed to be mediated by apoptosis/DNA damage.

CONCLUSION

The addition of allopurinol to thiopurines leads to a two-threefold increased cytotoxicity in HepaRG cells.

Thiopurines induce oxidative stress in T-lymphocytes: a proteomic approach

Thiopurines are extensively used immunosuppressants for the treatment of inflammatory bowel disease (IBD). The polymorphism of thiopurine S-methyltransferase (TPMT) influences thiopurine metabolism and therapy outcome. We used a TPMT knockdown (kd) model of human Jurkat T-lymphocytes cells to study the effects of treatment with 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) on proteome and phosphoproteome. We identified thirteen proteins with altered expression and nine proteins with altered phosphorylation signals. Three proteins (THIO, TXD17, and GSTM3) with putative functions in cellular oxidative stress responses were altered by 6-TG treatment and another protein PRDX3 was differentially phosphorylated in TPMT kd cells. Furthermore, reactive oxygen species (ROS) assay results were consistent with a significant induction of oxidative stress by both TPMT knockdown and thiopurine treatments. Immunoblot analyses showed treatment altered expression of key antioxidant enzymes (i.e., SOD2 and catalase) in both wt and kd groups, while SOD1 was downregulated by 6-TG treatment and TPMT knockdown. Collectively, increased oxidative stress might be a mechanism involved in thiopurine induced cytotoxicity and adverse effects (i.e., hepatotoxicity) and an antioxidant cotherapy might help to combat this. Results highlight the significance of oxidative stress in thiopurines' actions and could have important implications for the treatment of IBD patients.

Incidence, timing, and risk factors of azathioprine hepatotoxicosis in dogs

Background

The use of azathioprine (AZA) in dogs is limited by the development of hepatotoxicosis and cytopenias.

Hypothesis and Objectives

To characterize the observed incidence, timing, and risk factors for AZA hepatotoxicosis in dogs treated clinically, and to determine the relationship between the development of hepatotoxicosis and cytopenias.

Animals

Fifty‐two dogs treated with AZA with clinical and biochemical follow‐up, with a subset of 34 dogs available for determination of changes in liver enzyme activities in serum.

Methods

Retrospective medical record review, from January 2009 through December 2013.

Results

Hepatotoxicosis (as defined by a >2‐fold increase in serum ALT) was observed in 5 of 34 dogs (15%) within a median onset of 14 days (range, 13–22 days). Dogs had a median 9‐fold increase in ALT and 8‐fold increase in ALP, which stabilized or resolved with drug discontinuation or dose reduction. German shepherds were significantly over‐represented (3 of 5 dogs with hepatotoxicosis; P = .0017). Thrombocytopenia or neutropenia were seen in 4 of 48 dogs with CBC follow‐up (8% of dogs), but occurred significantly later in treatment (median onset, 53 days; range 45–196 days) compared to hepatotoxicosis (P = .016).

Conclusions and Clinical Importance

These results support the routine monitoring of liver enzymes during the first 1–4 weeks of AZA treatment in dogs, with continued monitoring of the CBC. Additional studies are warranted to characterize the apparently higher risk of AZA hepatotoxicosis in German shepherds.

Age-related differences in reporting of drug-associated liver injury: data-mining of WHO Safety Report Database

BACKGROUND/AIMS

Age-differences in the frequency and manifestations of drug-induced liver injury are not fully characterized. Data-mining analyses were performed to assess the impact of age on liver event reporting frequency with different phenotypes and agents.

METHODS

236 drugs associated with hepatotoxicity were evaluated using the Empirical Bayes Geometric Mean (EBGM) of the relative reporting ratio with 90% confidence interval (EB05 and EB95) calculated for the age groups: 0-17, 18-64, and⩾65years (or elderly), for overall, serious (acute liver failure), hepatocellular, and cholestatic liver injury, using the WHO Safety Report Database.

RESULTS

Overall, cases of age 0-17, 18-64, and 65years or older comprised 6%, 62%, and 32% of liver event reports. Acute liver failure and hepatocellular injury were more frequently reported among children compared to adults and the elderly while reports with cholestatic injury were more frequent among the elderly (p<0.00001). A potential to cause mitochondrial dysfunction was more prevalent among the drugs with increased pediatric reporting frequency while high lipophilicity and biliary excretion were more common among the drugs associated with higher reporting frequency in the elderly.

CONCLUSION

Age-specific phenotypes and potential drug properties associated with age-specific hepatotoxicity were identified in reported liver events; further analyses are warranted.

Evaluation of azathioprine-induced cytotoxicity in an in vitro rat hepatocyte system

Azathioprine (AZA) is widely used in clinical practice for preventing graft rejection in organ transplantations and various autoimmune and dermatological diseases with documented unpredictable hepatotoxicity. The potential molecular cytotoxic mechanisms of AZA towards isolated rat hepatocytes were investigated in this study using “Accelerated Cytotoxicity Mechanism Screening” techniques. The concentration of AZA required to cause 50% cytotoxicity in 2 hrs at 37°C was found to be 400 μM. A significant increase in AZA-induced cytotoxicity and reactive oxygen species (ROS) formation was observed when glutathione- (GSH-) depleted hepatocytes were used. The addition of N-acetylcysteine decreased cytotoxicity and ROS formation. Xanthine oxidase inhibition by allopurinol decreased AZA-induced cytotoxicity, ROS, and hydrogen peroxide (H₂O₂) formation and increased % mitochondrial membrane potential (MMP). Addition of N-acetylcysteine and allopurinol together caused nearly complete cytoprotection against AZA-induced hepatocyte death. TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl), a known ROS scavenger and a superoxide dismutase mimic, and antioxidants, like DPPD (N,N′-diphenyl-p-phenylenediamine), Trolox (a water soluble vitamin E analogue), and mesna (2-mercaptoethanesulfonate), also decreased hepatocyte death and ROS formation. Results from this study suggest that AZA-induced cytotoxicity in isolated rat hepatocytes may be partly due to ROS formation and GSH depletion that resulted in oxidative stress and mitochondrial injury.

Pharmacogenetics of azathioprine in inflammatory bowel disease: a role for glutathione-S-transferase?

Azathioprine is a purine antimetabolite drug commonly used to treat inflammatory bowel disease (IBD). In vivo it is active after reaction with reduced glutathione (GSH) and conversion to mercaptopurine. Although this reaction may occur spontaneously, the presence of isoforms M and A of the enzyme glutathione-S-transferase (GST) may increase its speed. Indeed, in pediatric patients with IBD, deletion of GST-M1, which determines reduced enzymatic activity, was recently associated with reduced sensitivity to azathioprine and reduced production of azathioprine active metabolites. In addition to increase the activation of azathioprine to mercaptopurine, GSTs may contribute to azathioprine effects even by modulating GSH consumption, oxidative stress and apoptosis. Therefore, genetic polymorphisms in genes for GSTs may be useful to predict response to azathioprine even if more in vitro and clinical validation studies are needed.

Using aggregated, de-identified electronic health record data for multivariate pharmacosurveillance: a case study of azathioprine

OBJECTIVE

To demonstrate the use of aggregated and de-identified electronic health record (EHR) data for multivariate post-marketing pharmacosurveillance in a case study of azathioprine (AZA).

METHODS

Using aggregated, standardized, normalized, and de-identified, population-level data from the Explore platform (Explorys, Inc.) we searched over 10 million individuals, of which 14,580 were prescribed AZA based on RxNorm drug orders. Based on logical observation identifiers names and codes (LOINC) and vital sign data, we examined the following side effects: anemia, cell lysis, fever, hepatotoxicity, hypertension, nephrotoxicity, neutropenia, and neutrophilia. Patients prescribed AZA were compared to patients prescribed one of 11 other anti-rheumatologic drugs to determine the relative risk of side effect pairs.

RESULTS

Compared to AZA case report trends, hepatotoxicity (marked by elevated transaminases or elevated bilirubin) did not occur as an isolated event more frequently in patients prescribed AZA than other anti-rheumatic agents. While neutropenia occurred in 24% of patients (RR 1.15, 95% CI 1.07-1.23), neutrophilia was also frequent (45%) and increased in patients prescribed AZA (RR 1.28, 95% CI 1.22-1.34). After constructing a pairwise side effect network, neutropenia had no dependencies. A reduced risk of neutropenia was found in patients with co-existing elevations in total bilirubin or liver transaminases, supporting classic clinical knowledge that agranulocytosis is a largely unpredictable phenomenon. Rounding errors propagated in the statistically de-identified datasets for cohorts as small as 40 patients only contributed marginally to the calculated risk.

CONCLUSION

Our work demonstrates that aggregated, standardized, normalized and de-identified population level EHR data can provide both sufficient insight and statistical power to detect potential patterns of medication side effect associations, serving as a multivariate and generalizable approach to post-marketing drug surveillance.

Does active Crohn's disease have decreased intestinal antioxidant capacity?

BACKGROUND AND AIMS

Oxidative stress is presumed to play an important role in Crohn's disease (CD) pathogenesis. Nevertheless, the evaluation of the intestinal antioxidant capacity through the analysis of glutathione peroxidase activity in CD remains to be determined.

METHODS

20 CD outpatients and 16 volunteers going through colonic cancer screening were enrolled. Colonoscopy with biopsies was performed in all individuals. Samples from inflamed and non-inflamed mucosa were taken when there was CD endoscopic activity. Spectrophotometric assays were performed to measure tissue glutathione peroxidase (GPx) activity, and total (GSHT) and oxidized (GSSG) glutathione in all samples. Demographics and clinical characteristics were collected from clinical charts.

RESULTS

Inflamed CD mucosa presented reduced GPx activity compared to non-inflamed CD mucosa (42.94mU/mg protein vs 79.62mU/mg protein, P<0.05) and control mucosa (42.94mU/mg protein vs 95.08mU/mg protein, P<0.001). GSHT concentration was reduced in inflamed mucosa when compared to non-inflamed CD mucosa (0.78μmol/g vs 1.98μmol/g, P<0.01) and the control group (0.78μmol/g vs 2.11μmol/g, P<0.001). A significant correlation was detected between GPx activity and GSSG (r=-0.599), disease duration (r=0.546), and thiopurine treatment (r=-0.480) in non-inflamed CD mucosa.

CONCLUSION

Our findings suggest that reduced GPx activity is present in inflamed CD mucosa. In addition, endoscopic activity, disease duration and thiopurine therapy could be associated with mucosal decreased antioxidant activity.

Oxidation-mediated DNA cross-linking contributes to the toxicity of 6-thioguanine in human cells

The thiopurines azathioprine and 6-mercaptopurine have been extensively prescribed as immunosuppressant and anticancer agents for several decades. A third member of the thiopurine family, 6-thioguanine (6-TG), has been used less widely. Although known to be partly dependent on DNA mismatch repair (MMR), the cytotoxicity of 6-TG remains incompletely understood. Here, we describe a novel MMR-independent pathway of 6-TG toxicity. Cell killing depended on two properties of 6-TG: its incorporation into DNA and its ability to act as a source of reactive oxygen species (ROS). ROS targeted DNA 6-TG to generate potentially lethal replication-arresting DNA lesions including interstrand cross-links. These triggered processing by the Fanconi anemia and homologous recombination DNA repair pathways. Allopurinol protected against 6-TG toxicity by acting as a ROS scavenger and preventing DNA damage. Together, our findings provide mechanistic evidence to support the proposed use of thiopurines to treat HR-defective tumors and for the coadministration of 6-TG and allopurinol as an immunomodulation strategy in inflammatory disorders.

Gene polymorphisms involved in manifestation of leucopenia, digestive intolerance, and pancreatitis in azathioprine-treated patients

BACKGROUND

Approximately 10-28 % of patients experience adverse drug reactions related to treatment with thiopurines. The most serious reaction is myelosuppression, typically manifested as leucopenia, which occurs in approximately 2-5 % of patients. Other adverse drug reactions that often accompany thiopurine therapy are pancreatitis, hepatotoxicity, allergic reactions, digestive intolerance, arthralgia, febrile conditions, and rash.

OBJECTIVE

The objective of this study was to assess the relationship between variant alleles of thiopurine S-methyltransferase (SNPs 238G > C, 460G > A and 719A > G), inosine triphosphate diphosphatase (SNPs 94C > A and IVS2 + 21A > C), and xanthine dehydrogenase (837C > T) and the occurrence of adverse drug reactions to azathioprine therapy.

METHODS

Genotype was determined for 188 Caucasians diagnosed with inflammatory bowel disease treated with a standard dose of azathioprine (1.4-2.0 mg/kg/day). Allelic variants were determined by PCR-REA and real-time PCR methods. Results were statistically evaluated by use of Fisher's test and by odds ratio calculation.

RESULTS

Variant genotype thiopurine S-methyltransferase predisposes to development of leucopenia (P = 0.003, OR = 5, CI 95 %, 1.8058-13.8444). Although not statistically significant, we observed a trend that suggested correlation between the occurrence of digestive intolerance and the variant genotype inosine triphosphate diphosphatase (P = 0.1102; OR 15.63, CI 95 %, 1.162-210.1094), and between the occurrence of pancreatitis and the variant allele xanthine dehydrogenase 837T (P = 0.1124; OR 12,1, CI 95 %, 1.15-126.37).

CONCLUSION

The variant genotype thiopurine S-methyltransferase has been associated with the occurrence of leucopenia. The involvement of polymorphisms in inosine triphosphate diphosphatase and xanthine dehydrogenase genes in the development of digestive intolerance and pancreatitis will require further verification.

A Histopathologic Pattern of Centrilobular Hepatocyte Injury Suggests 6-Mercaptopurine-Induced Hepatotoxicity in Patients With Inflammatory Bowel Disease

Context.—Hepatotoxicity is an important side effect of thiopurine analog treatment for inflammatory bowel disease. A variety of histopathologic findings have been observed in patients with inflammatory bowel disease with thiopurine-induced hepatotoxicity, including nodular regenerative hyperplasia, vascular injury, and cholestasis.

Objective.—To describe the histologic features shared by 3 cases of thiopurine-induced hepatotoxicity in patients with inflammatory bowel disease.

Design.—We identified 3 patients with inflammatory bowel disease who developed hepatotoxicity due to 6-mercaptopurine from the educational files of the Department of Pathology at Massachusetts General Hospital (Boston). Histology slides (stained with hematoxylin-eosin, trichrome, periodic-acid Schiff with diastase digestion, and iron stains) and patients' medical records were reviewed retrospectively.

Results.—All 3 patients were receiving 6-mercaptopurine monotherapy at therapeutic doses, had normal thiopurine metabolite levels, and presented with elevated aminotransferase levels. Biopsies from all 3 cases exhibited a pattern of centrilobular hepatocyte injury characterized by ceroid-laden macrophages, hepatocyte anisonucleosis, and increased lipofuscin pigment, as well as centrilobular steatosis. Aminotransferase levels trended downward and either normalized or remained at borderline elevated levels after 6-mercaptopurine dose was reduced (in 1 patient) or discontinued (in 2 patients).

Conclusions.—Recognition of a pattern of centrilobular injury enables pathologists to suggest thiopurine-induced liver injury as the cause of elevated aminotransferases in patients with inflammatory bowel disease.

The evaluation of drug rechallenge: the casopitant Phase III program

Drug rechallenge (or reinitiation), following an event of drug-induced liver injury, is associated with 13% mortality in prospective series. Rechallenge generally results in much more rapid injury than the initial liver event. The neurokinin-1 antagonist casopitant or its placebo was administered cyclically with ondansetron and dexamethasone in two randomized chemotherapy-induced nausea and vomiting clinical trials in nearly 3000 subjects. Grade 3 ALT elevations were observed in up to 2% of subjects receiving casopitant or placebo treatment. Similar rates of positive rechallenge were observed in the casopitant 8/29 (28%) and placebo groups 2/8 (25%), with no Grade 4 ALT elevations, hypersensitivity or liver-related serious adverse events. Publishing available rechallenge data (positive and negative) will advance our clinical understanding. Rechallenge should only be considered when the potential drug benefit exceeds the risk.

Building a tiered approach to in vitro predictive toxicity screening: a focus on assays with in vivo relevance

One of the greatest challenges facing the pharmaceutical industry today is the failure of promising new drug candidates due to unanticipated adverse effects discovered during preclinical animal safety studies and clinical trials. Late stage attrition increases the time required to bring a new drug to market, inflates development costs, and represents a major source of inefficiency in the drug discovery/development process. It is generally recognized that early evaluation of new drug candidates is necessary to improve the process. Building in vitro data sets that can accurately predict adverse effects in vivo would allow compounds with high risk profiles to be deprioritized, while those that possess the requisite drug attributes and a lower risk profile are brought forward. In vitro cytotoxicity assays have been used for decades as a tool to understand hypotheses driven questions regarding mechanisms of toxicity. However, when used in a prospective manner, they have not been highly predictive of in vivo toxicity. Therefore, the issue may not be how to collect in vitro toxicity data, but rather how to translate in vitro toxicity data into meaningful in vivo effects. This review will focus on the development of an in vitro toxicity screening strategy that is based on a tiered approach to data collection combined with data interpretation.

Differences among allelic variants of human glutathione transferase A2-2 in the activation of azathioprine

Azathioprine has been clinically used for decades in connection with organ transplantation, autoimmune disease, and treatment of cancer. Toxic side-reactions are common and have been linked to the liberation of excessively high concentrations of 6-mercaptopurine and corresponding toxic metabolites. An allelic variant of thiopurine methyltransferase with low activity is associated with elevated concentrations of 6-mercaptopurine. However, other genetic markers remain to be identified in order to fully account for adverse reactions and efficacy failure. In the present study, we studied the five known allelic variants of human glutathione transferase A2-2 (GST A2-2) (EC 2.5.1.18), abundantly expressed in liver and efficiently catalyzing the bioactivation of azathioprine to release 6-mercaptopurine. All five variants exhibited high activity with azathioprine, but allelic variant E of GST A2-2 displayed a 3-4-fold elevated catalytic efficiency compared to the other variants. High GST activity can lead to overproduction of 6-mercaptopurine, and the nature of the multiple forms of GSTs in a patient will obviously affect the metabolism of azathioprine. In addition to GST A2-2, the polymorphic GST M1-1 is also highly active with azathioprine. Considering our findings, it appears that the genotypic and phenotypic variations in the GST complement may influence the presentation of adverse reactions in patients treated with azathioprine. Clinical trials will be required to clarify the impact of the GST expression in comparison with the established biomarker thiopurine methyltransferase as predictors of adverse reactions.

Hepatic perturbations provoked by azathioprine: a paradigm to rationalize the cytoprotective potential of Ficus hispida Linn

The present study was embarked upon in an endeavor to ascertain whether Ficus hispida leaf extract (FHLE) modulates azathioprine-induced hepatic damage. Azathioprine treated rats displayed a plethora of pathological events, which include loss of hepatocellular membrane integrity, mitochondrial dysfunction, and nuclear damage; whilst FHLE pretreated rats significantly precluded these abnormalities. These data were in harmony with the transmission electron microscopic studies. Observations from this investigation directed us to propose the plausible mechanisms through which FHLE thwarts the repercussions of azathioprine-induced hepatocellular necrosis: upholding of thiol homeostasis, curtailing the membrane effects, and perpetuation of adenine nucleotide status. These data offer credence to the notion that FHLE might be a beneficial intervention in the prevention of hepatotoxicity in azathioprine therapy.

[Hepatotoxicity induced by new immunosuppressants]

Immunosuppressants are among the pharmacological agents with the greatest potential to cause adverse reactions, although induction of hepatotoxicity is paradoxical from the pathogenic point of view, since the response of the innate and acquired immune system is a key element in the chain of events leading to chemical liver damage. Hepatotoxicity induced by immunosuppressants is difficult to evaluate since these drugs are sometimes used to treat liver diseases, or in combination with other drugs that can also cause hepatotoxicity, or in the context of liver transplantation, in which rejection or biliary complications can act as confounding factors. In addition, immunosuppressant therapy can favor the development of infections, which by themselves can cause liver damage, or reactivate latent chronic viral hepatitis. Corticosteroids and calcineurin inhibitors only exceptionally cause hepatotoxicity. Methotrexate at high doses and in patients with risk factors can induce advanced fibrosis and cirrhosis. Thiopurine agents can cause a spectrum of hepatic lesions, including hepatocellular of cholestatic lesions, and hepatic vascular alterations. Leflunomide has high hepatotoxic potential, especially when combined with methotrexate. Anti-tumor necrosis factor-alpha agents have rarely been associated with hepatotoxicity, often with detectable autoantibodies, and most of the reactions - some severe - have been linked to infliximab, especially when used in patients with rheumatological diseases.

Novel pharmacogenetic markers for treatment outcome in azathioprine-treated inflammatory bowel disease

BACKGROUND

Azathioprine (AZA) pharmacogenetics are complex and much studied. Genetic polymorphism in TPMT is known to influence treatment outcome. Xanthine oxidase/dehydrogenase (XDH) and aldehyde oxidase (AO) compete with TPMT to inactivate AZA.

AIM

To assess whether genetic polymorphism in AOX1, XDH and MOCOS (the product of which activates the essential cofactor for AO and XDH) is associated with AZA treatment outcome in IBD.

METHODS

Real-time PCR was conducted for a panel of single nucleotide polymorphism (SNPs) in AOX1, XDH and MOCOS using TaqMan SNP genotyping assays in a prospective cohort of 192 patients receiving AZA for IBD.

RESULTS

Single nucleotide polymorphism AOX1 c.3404A > G (Asn1135Ser, rs55754655) predicted lack of AZA response (P = 0.035, OR 2.54, 95%CI 1.06-6.13) and when combined with TPMT activity, this information allowed stratification of a patient's chance of AZA response, ranging from 86% in patients where both markers were favourable to 33% where they were unfavourable (P < 0.0001). We also demonstrated a weak protective effect against adverse drug reactions (ADRs) from SNPs XDH c.837C > T (P = 0.048, OR 0.23, 95% CI 0.05-1.05) and MOCOS c.2107A > C, (P = 0.058 in recessive model, OR 0.64, 95%CI 0.36-1.15), which was stronger where they coincided (P = 0.019).

CONCLUSION

These findings have important implications for clinical practice and our understanding of AZA metabolism.

Nelfinavir/ritonavir reduces acinar injury but not inflammation during mouse caerulein pancreatitis

There is no clinical treatment that reduces acinar injury during pancreatitis. Human immunodeficiency virus (HIV) protease inhibitors (PI), including nelfinavir (NFV) and ritonavir (RTV), may reduce the rate of pancreatitis in HIV-infected patients. Since permeability transition pore (PTPC)-mediated mitochondrial dysfunction occurs during pancreatitis, and we have shown that PI prevents PTPC opening, we studied its effects in a model of pancreatitis. The effect of NFV plus RTV (NFV/RTV) or vehicle on caerulein-induced pancreatitis in mice was compared by measuring changes in mitochondrial membrane potential in vitro and cytochrome c leakage in vivo. Histological and inflammatory makers were also compared. NFV/RTV improved DiOC6 retention in acini exposed to caerulein in vitro. In vivo NFV prevented cytosolic leakage of cytochrome c and reduced pancreatic acinar injury, active caspase-3 staining, TUNEL-positive acinar cells, and serum amylase (P < 0.05). Conversely, trypsin activity, serum cytokine levels, and pancreatic and lung inflammation were unaffected. NFV/RTV reduces pancreatic injury and acinar cell death in experimental mouse caerulein-induced pancreatitis but does not impact inflammation.

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.

The haemotoxicity of azathioprine in repeat dose studies in the female CD-1 mouse

Azathioprine (AZA) is a cytotoxic immunosuppressive drug used in the prevention of rejection in organ transplants and the treatment of auto-immune diseases. However, AZA is haemotoxic causing significant bone marrow depression. The present studies were to characterize the haemotoxicity of AZA in the female CD-1 mouse. In Experiment 1, a dose-ranging study, with AZA gavaged daily for 10 days, clinical evidence of toxicity was evident at 125 mg/kg and above. Experiment 2 was a dose-response study with AZA gavaged daily for 10 days at 40-120 mg/kg. At day 1 after the final dose, AZA induced a dose-related pancytopaenia, reduced femoral marrow cellularity, increases in serum levels of the cytokine fms-like tyrosine kinase 3 ligand, reduction in granulocyte-monocyte colony-forming units and erythroid colonies, and increased bone marrow apoptosis. Histology demonstrated hepatocyte hypertrophy, thymic atrophy, reduced splenic extramedullary haemopoiesis, and reduced cellularity of sternal bone marrow. In Experiment 3, AZA was dosed for 10 days at 100 mg/kg with autopsies at 1, 3, 9, 22, 29, 43 and 57 days postdosing. At 1, 3 and 9 days, haematological parameters reflected changes in Experiment 2. At 22/29 days, many blood parameters were returning towards normal; at 43/57 days, most parameters compared with controls. However, there was some evidence of a persistent (i.e. residual/late-stage) mild reduction in RBC and erythroid progenitor cell counts at day 43/57. We conclude that the CD-1 mouse provides an acceptable model for the haemotoxicity of AZA in man.

Mitochondrial involvement in non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is an increasing recognized condition that may progress to end-stage liver disease. There are consistent evidences that mitochondrial dysfunction plays a central role in NASH whatever its origin. Mitochondria are the key controller of fatty acids removal and this is part of an intensive gene program that modifies hepatocytes to counteract the excessive fat storage. Mitochondrial dysfunction participates at different levels in NASH pathogenesis since it impairs fatty liver homeostasis and induces overproduction of ROS that in turn trigger lipid peroxidation, cytokines release and cell death. In this review we briefly recall the role of mitochondria in fat metabolism and energy homeostasis and focus on the role of mitochondrial impairment and uncoupling proteins in the pathophysiology of NASH progression. We suggest that mitochondrial respiratory chain, UCP2 and redox balance cooperate in a common pathway that permits to set down the mitochondrial redox pressure, limits the risk of oxidative damage, and allows the maximal rate of fat removal. When the environmental conditions change and high energy supply occurs, hepatocytes are unable to replace their ATP store and steatosis progress to NASH and cirrhosis. The beneficial effects of some drugs on mitochondrial function are also discussed.

Thiopurine treatment in inflammatory bowel disease: clinical pharmacology and implication of pharmacogenetically guided dosing

This review summarises clinical pharmacological aspects of thiopurines in the treatment of chronic inflammatory bowel disease (IBD). Current knowledge of pharmacogenetically guided dosing is discussed for individualisation of thiopurine therapy, particularly to avoid severe adverse effects. Both azathioprine and mercaptopurine are pro-drugs that undergo extensive metabolism. The catabolic enzyme thiopurine S-methyltransferase (TPMT) is polymorphically expressed, and currently 23 genetic variants have been described. On the basis of an excellent phenotype-genotype correlation for TPMT, genotyping has become a safe and reliable tool for determination of a patient's individual phenotype. Thiopurine-related adverse drug reactions are frequent, ranging from 5% up to 40%, in both a dose-dependent and -independent manner. IBD patients with low TPMT activity are at high risk of developing severe haematotoxicity if pharmacogenetically guided dosing is not performed. Based on several cost-benefit analyses, assessment of TPMT activity is recommended prior to thiopurine therapy in patients with IBD. The underlying mechanisms of azathioprine/mercaptopurine-related hepatotoxicity, pancreatitis and azathioprine intolerance are still unknown. Although the therapeutic response appears to be related to 6-thioguanine nucleotide (6-TGN) concentrations above a threshold of 230-260 pmol per 8 x 10(8) red blood cells, at present therapeutic drug monitoring of 6-TGN can be recommended only to estimate patients' compliance.Drug-drug interactions between azathioprine/mercaptopurine and aminosalicylates, diuretics, NSAIDs, warfarin and infliximab are discussed. The concomitant use of allopurinol without dosage adjustment of azathioprine/mercaptopurine leads to clinically relevant severe haematotoxicity due to elevated thiopurine levels. Several studies indicate that thiopurine therapy in IBD during pregnancy is safe. Thus, azathioprine/mercaptopurine should not be withdrawn in strictly indicated cases of pregnant IBD patients. However, breastfeeding is contraindicated during azathioprine/mercaptopurine therapy. Use of azathioprine/mercaptopurine for induction and maintenance of remission in corticosteroid-dependent or corticosteroid-refractory IBD, particularly Crohn's disease, is evidence based. To improve response rates in thiopurine therapy of IBD, comprehensive analyses including metabolic patterns and genome-wide profiling in patients with azathioprine/mercaptopurine treatment are required to identify novel candidate genes.

Mechanism of teratogenesis: Electron transfer, reactive oxygen species, and antioxidants

Teratogenesis has been a topic of increasing interest and concern in recent years, generating controversy in association with danger to humans and other living things. A veritable host of chemicals is known to be involved, encompassing a wide variety of classes, both organic and inorganic. Contact with these chemicals is virtually unavoidable due to contamination of air, water, ground, food, beverages, and household items, as well as exposure to medicinals. The resulting adverse effects on reproduction are numerous. There is uncertainty regarding the mode of action of these chemicals, although various theories have been advanced, e.g., disruption of the central nervous system (CNS), DNA attack, enzyme inhibition, interference with hormonal action, and insult to membranes, proteins, and mitochondria. This review provides extensive evidence for involvement of oxidative stress (OS) and electron transfer (ET) as a unifying theme. Successful application of the mechanistic approach is made to all of the main classes of toxins, in addition to large numbers of miscellaneous types. We believe it is not coincidental that the vast majority of these substances incorporate ET functionalities (quinone, metal complex, ArNO2, or conjugated iminium) either per se or in metabolites, potentially giving rise to reactive oxygen species (ROS) by redox cycling. Some categories, e.g., peroxides and radiation, appear to generate ROS by non-ET routes. Other mechanisms are briefly addressed; a multifaceted approach to mode of action appears to be the most logical. Our framework should increase understanding and contribute to preventative measures, such as use of antioxidants.

Glutathione-S-transferase genotypes and the adverse effects of azathioprine in young patients with inflammatory bowel disease

BACKGROUND

Adverse drug reactions to azathioprine, the prodrug of 6-mercaptopurine, occur in 15%-38% of patients and the majority are not explained by thiopurine-S-methyltransferase (TPMT) deficiency. Azathioprine is known to induce glutathione depletion and consumption of glutathione is greater in cells with high glutathione-S-transferase (GST) activity compared with those with low activity; moreover, some reports indicate that GST might play a direct role in the reaction of glutathione with azathioprine. The association between polymorphisms of GST-M1, GST-P1, GST-T1, and TPMT genes and the adverse effects of azathioprine was therefore investigated.

METHODS

Seventy patients with inflammatory bowel disease (IBD), treated with azathioprine, were enrolled and clinical data were retrospectively determined. TPMT and GST genotyping were performed by polymerase chain reaction (PCR) assays on DNA extracted from blood samples.

RESULTS

Fifteen patients developed adverse effects (21.4%); there was a significant underrepresentation of the GST-M1 null genotype among patients developing adverse drug reactions to azathioprine (odds ratio [OR] = 0.18, 95% confidence interval [CI] = 0.037-0.72, P = 0.0072) compared with patients who did not develop adverse effects. Patients heterozygous for TPMT mutations presented a marginally significant increased probability of developing adverse effects (OR = 6.38, 95% CI = 0.66-84.1, P = 0.062). Moreover, among the 55 patients who did not develop adverse effects, there was a significant underrepresentation of the GST-M1 null genotype among patients who displayed lymphopenia as compared with those that did not display this effect of azathioprine (OR = 0.15, 95% CI = 0.013-1.08, P = 0.032).

CONCLUSION

Patients with IBD with a wildtype GST-M1 genotype present increased probability of developing adverse effects and increased incidence of lymphopenia during azathioprine treatment.

Azathioprine hepatotoxicity and the protective effect of liquorice and glycyrrhizic acid

This study aimed to evaluate the responses of human hepatocytes to azathioprine hepatotoxicity in comparison with the well-studied azathioprine hepatotoxicity in rat hepatocytes and the effects of protective agents to suppress azathioprine hepatotoxicity. Azathioprine presented its hepatotoxicity at clinically relevant concentrations (lower than 10 microm) in primary rat hepatocytes after 48 h of treatment as shown by a severe decrease in cell viability as well as intracellular GSH depletion. However, primary human hepatocytes exhibited only significant intracellular GSH depletion after treatment with azathioprine at these clinically relevant concentrations, while a reduction in cell viability by 29% was only evidenced after 48 h of treatment with azathioprine at the high concentration of 50 microm. In addition, a monolayer culture of primary rat hepatocytes was used as an in vitro model to examine the protective effects of antihepatotoxic drugs including glutathione (GSH), N-acetylcysteine (NAC, a GSH precursor), liquorice and glycyrrhizic acid (GA), a major bioactive component of liquorice, against hepatotoxicity of 1 microm azathioprine. It was found that both liquorice and GA showed substantial protection according to assays of cell viability and intracellular GSH, while neither GSH nor NAC had such a protective function. Similarly, GA protected human hepatocytes from intracellular GSH depletion on exposure to 1 microm azathioprine. These results implied that GA or liquorice could be considered as potent protection agents against azathioprine hepatotoxicity.

Mitochondrial dysfunction in NASH: causes, consequences and possible means to prevent it

Calorie-enriched diet and lack of exercise are causing a worldwide surge of obesity, insulin resistance and lipid accretion in liver (i.e. hepatic steatosis), which can lead to steatohepatitis. Steatosis and nonalcoholic steatohepatitis (NASH) can also be induced by drugs such as amiodarone, tamoxifen and some antiretroviral drugs, including stavudine and zidovudine. There is accumulating evidence that mitochondrial dysfunction (more particularly respiratory chain deficiency) plays a key role in the physiopathology of NASH whatever its initial cause. In contrast, the mitochondrial beta-oxidation of fatty acids can be either increased (as in insulin resistance-associated NASH) or decreased (as in drug-induced NASH). However, in both circumstances, generation of reactive oxygen species (ROS) by the damaged respiratory chain can be augmented. ROS generation in an environment enriched in lipids in turn induces lipid peroxidation which releases highly reactive aldehydic derivatives (e.g. malondialdehyde) that have diverse detrimental effects on hepatocytes and other hepatic cells. In hepatocytes, ROS, reactive nitrogen species and lipid peroxidation products further impair the respiratory chain, either directly or indirectly through oxidative damage to the mitochondrial genome. This consequently leads to the generation of more ROS and a vicious cycle occurs. Mitochondrial dysfunction can also lead to apoptosis or necrosis depending on the energy status of the cell. ROS and lipid peroxidation products also increase the generation of several cytokines (TNF-alpha, TGF-beta, Fas ligand) playing a key role in cell death, inflammation and fibrosis. Recent investigations have shown that some genetic polymorphisms can significantly increase the risk of steatohepatitis and that several drugs can prevent or even reverse NASH. Interestingly, most of these drugs could exert their beneficial effects by improving directly or indirectly mitochondrial function in liver. Finding a drug, which could fully prevent oxidative stress and mitochondrial dysfunction in NASH is a major challenge for the next decade.

Thiopurine therapies: problems, complexities, and progress with monitoring thioguanine nucleotides

Metabolism of thiopurine drugs--azathioprine, 6-mercaptopurine, and 6-thioguanine--has provided a powerful pharmacogenetic model incorporating polymorphism of the enzyme thiopurine methyltransferase (TPMT) and the primary active metabolite, thioguanine nucleotide (TGN). However, a sense of uncertainty about the usefulness of TGNs and other thiopurine metabolites has appeared. This review critically appraises the basis of thiopurine metabolism and reveals the problems and complexities in TGN research. Erythrocyte TGN is used in transplantation medicine and in chronic inflammatory conditions such as Crohn's disease, as a "surrogate" pharmacokinetic parameter for TGN in the target cells: leukocytes or bone marrow. It is not generally appreciated that erythrocytes do not express the enzyme IMP dehydrogenase and cannot convert mercaptopurine to TGN, which explains some of the confusion in interpretation of erythrocyte TGN measurements. TGN routinely measured in erythrocytes derives from hepatic metabolism. Another concern is that TGN are not generally assayed directly: most methods assay the thiopurine bases. Ion-exchange HPLC and enzymatic conversion of TGNs to nucleosides have been used to overcome this, and may reveal undisclosed roles for an unusual cytotoxic nucleotide, thio-inosine triphosphate, and methylated thiopurines. There appear to be additional interactions between xanthine oxidase and TPMT, and folate and TPMT, that could predict leukopenia. Difficult questions remain to be answered, which may be assisted by technological advances. Prospective TGN studies, long overdue, are at last revealing clearer results.

Incidence, risk factors and clinical course of thiopurine-induced liver injury in patients with inflammatory bowel disease

BACKGROUND

The incidence of thiopurine-induced hepatotoxicity in patients with inflammatory bowel disease varies in different studies.

AIMS

To assess the rate of thiopurine-induced liver toxicity in patients with inflammatory bowel disease; to determine the predictive factors and to characterize its clinical course and management.

METHODS

A cohort of 161 patients was prospectively followed for a median of 271 days. Hepatotoxicity was established when alanine transaminase or alkaline phosphatase plasma levels were greater than twice the upper normal limit.

RESULTS

Abnormal liver function was detected in 21 patients (13%; 95% CI: 7-18). Hepatotoxicity occurred in 16 patients (10%; 95% CI: 6-16) after a median of 85 days. In five cases, treatment was withdrawn due to hepatotoxicity. Use of corticosteroids was associated with hepatotoxicity (OR: 4.94; 95% CI: 1.01-23.98) with antitumour necrosis factor concomitant therapy showing a protective role (OR: 0.3; 95% CI: 0.1-3.1). gamma-Glutamyl transferase plasma levels at the onset of hepatotoxicity showed the best predictive value for treatment withdrawal (area under the receiver operating characteristic curve: 0.95).

CONCLUSIONS

The incidence of hepatotoxicity in inflammatory bowel disease patients receiving thiopurines is relevant, mainly in patients co-treated with corticosteroids. gamma-Glutamyl transferase plasma level is a useful biomarker in therapy withdrawal prediction.

Promising treatment of autoimmune hepatitis with 6-thioguanine after adverse events on azathioprine

The use of corticosteroids in autoimmune hepatitis is an established therapy. To avoid the possible serious side effects of corticosteroids, immunosuppression with azathioprine is often warranted. Azathioprine, a purine analogue, is frequently used to taper or replace corticosteroids. However, approximately 10% of the patients are intolerant to azathioprine. Alternative therapies using mycophenolate, tacrolimus, budesonide, cyclosporine and 6-mercaptopurine have been studied, with variable results. The use of 6-thioguanine, an agent more directly leading to the down-stream active metabolites of azathioprine (6-thioguanine nucleotides) in inflammatory bowel disease patients intolerant to azathioprine or 6-mercaptopurine showed conflicting results. We report three patients with autoimmune hepatitis who could not tolerate azathioprine but tolerated 6-thioguanine 0.3 milligram per kilogram daily well. All three patients improved clinically. Therapeutic drug monitoring was performed. The prospective evaluation of 6-thioguanine as a possible immunosuppressive drug in autoimmune hepatitis patients is warranted.