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

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

Inosine triphosphate pyrophosphatase 94C> A polymorphism: clinical implications for patients with systemic lupus erythematosus treated with azathioprine

Importance of the field: Azathioprine (AZA) has immunosuppressive property and has been widely used in organ transplantation and in several autoimmune diseases including systemic lupus erythematosus. The use of AZA is limited by the occurrence of adverse drug reactions (ADRs) leading to treatment discontinuation. Under AZA therapy, inosine triphosphate pyrophosphatase (ITPA) deficiency presumably leads to accumulation of unusual thioinosine metabolites with the potential for ADRs. Japanese patients require lower doses of AZA compared with Caucasian patients to achieve the same concentration of active metabolites. This ethnic difference in part is probably due to genetic polymorphisms of ITPA.

Areas covered in this review: Relationships between ITPA genotype and enzyme activity, and efficacy and toxicity of AZA in both Caucasian and Asian populations are reviewed.

Take home message: Clinical studies using a dose of <?1.5 mg/kg/day in various autoimmune diseases have shown no association between ITPA genotype and ADRs. In studies using higher doses, ITPA deficiency appears to increase the risk for AZA toxicity. Genotyping of ITPA may be useful to achieve dose optimization. It is important to maintain the dose of AZA <?1.5 mg/kg/day for Asian patients with ITPA 94A allele, with careful monitoring of the therapeutic efficacy.     

硫嘌呤类药物遗传药理学研究进展

临床上硫嘌呤类药物应用广泛,但在此类药物的使用过程中,血液毒性、肝脏毒性、胰腺炎等不良反应的发生率较高。研究发现,硫嘌呤甲基转移酶(thiopurine S-methyltransferase,TPMT)的活性和遗传多态性,以及三磷酸肌苷焦磷酸酶(inosine triphosphate pyrophosphatase,ITPA)的遗传多态性与硫嘌呤类药物不良反应的发生密切相关。本文综述了TPMT活性和基因多态性,以及ITPA基因多态性的研究进展。     

Long-term outcome of using allopurinol co-therapy as a strategy for overcoming thiopurine hepatotoxicity in treating inflammatory bowel disease

Background  Hepatotoxicity results in the withdrawal of thiopurines drugs, azathioprine (AZA) and mercaptopurine (MP), in up to 10% of patients with inflammatory bowel disease. Our group previously demonstrated that allopurinol with AZA/ciclosporin/steroid 'triple therapy' improved renal graft survival.
Aim  To confirm the hypothesis that allopurinol may alleviate thiopurine hepatotoxicity by similar mechanisms as proposed in our renal study.
Methods  Unselected patients with acute thiopurine hepatotoxicity were offered allopurinol co-therapy with low-dose AZA or MP. The starting AZA/MP dose was determined by thiopurine methyltransferase (TPMT) activity (two patients were intermediate TPMT); then this dose was reduced to 25% for allopurinol co-therapy. Response to treatment was assessed by clinical severity indices, endoscopy and blood tests.
Results  Of 11 patients (three Crohn's disease, eight ulcerative colitis) treated, nine (82%) remain in long-term remission (median 42 months) with normal liver tests. One patient also successfully bypassed flu-like symptoms. Two stopped: one nausea, one abnormal liver function (steatosis on biopsy). Leucopenia occurred in two cases and resolved with minor dose reductions.
Conclusions  Allopurinol co-therapy with low-dose AZA/MP can alleviate thiopurine hepatotoxicity. It appears safe and effective for long-term use, but requires monitoring for myelotoxicity. Assessing the TPMT activity helps tailor the AZA/MP doses.     

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.     

Adverse drug reactions to azathioprine therapy are associated with polymorphism in the gene encoding inosine triphosphate pyrophosphatase (ITPase)

Adverse drug reactions to azathioprine (AZA), the pro-drug of 6-mercaptopurine (6-MP), occur in 15% to 28% of patients and the majority are not explained by thiopurine methyltransferase (TPMT) deficiency. Inosine triphosphate pyrophosphatase (ITPase) deficiency results in the benign accumulation of the inosine nucleotide ITP. 6-MP is activated through a 6-thio-IMP intermediate and, in ITPase deficient patients, potentially toxic 6-thio-ITP is predicted to accumulate. The association between polymorphism in the ITPA gene and adverse drug reactions to AZA therapy was studied in patients treated for inflammatory bowel disease. Sixty-two patients with inflammatory bowel disease suffering adverse drug reactions to AZA therapy were genotyped for ITPA 94C>A and IVS2 + 21A>C polymorphisms, and TPMT*3A, *3C, *2 polymorphisms. Genotype frequencies were compared to a consecutive series of 68 controls treated with AZA for a minimum of 3 months without adverse effect. The ITPA 94C>A deficiency-associated allele was significantly associated with adverse drug reactions [odds ratio (OR) 4.2, 95% confidence interval (CI) 1.6-11.5, P = 0.0034]. Significant associations were found for flu-like symptoms (OR 4.7, 95% CI 1.2-18.1, P = 0.0308), rash (OR 10.3, 95% CI 4.7-62.9, P = 0.0213) and pancreatitis (OR 6.2,CI 1.1-32.6, P = 0.0485). Overall, heterozygous TPMT genotypes did not predict adverse drug reactions but were significantly associated with a subgroup of patients experiencing nausea and vomiting as the predominant adverse reaction to AZA therapy (OR 5.5, 95% CI 1.4-21.3, P = 0.0206). Polymorphism in the ITPA gene predicts AZA intolerance. Alternative immunosuppressive drugs, particularly 6-thioguanine, should be considered for AZA-intolerant patients with ITPase deficiency.     

Pharmacogenetics of thiopurine therapy in paediatric IBD patients

BACKGROUND: Azathioprine is widely used in the treatment of children with inflammatory bowel disease. The occurrence and type of adverse events to azathioprine may be related to thiopurine S-methyltransferase (TPMT) enzyme activity and to inosine triphophate pyrophosphatase (ITPase) deficiency. AIM: Investigate frequencies of functional TPMT polymorphisms and ITPA polymorphisms and their association with the occurrence of adverse events during azathioprine therapy in a paediatric inflammatory bowel disease population. METHODS: Seventy-two azathioprine treated paediatric inflammatory bowel disease patients, 47% girls, mean age 12.5 years (range 6.5-17.5), were assessed for TPMT and ITPA polymorphisms and for adverse events. The relation between polymorphisms and adverse events is evaluated. RESULTS: Of all azathioprine treated patients, 11 experienced an adverse event for which azathioprine was stopped: pancreatitis (n = 4), leucopenia (n = 2) and 'general malaise' (n = 5). Of the 11 patients who stopped azathioprine because of adverse events, 10 had wild-type alleles for all investigated genotypes. Genotyping of ITPA 94C>A polymorphisms showed that two patients were homozygous, both tolerated azathioprine well. CONCLUSIONS: No association of functional ITPA and TPMT polymorphisms and the occurrence of azathioprine related adverse events could be detected. Pharmacogenetic assessment prior to thiopurine therapy does not seem warranted.     

Prospective evaluation of the pharmacogenetics of azathioprine in the treatment of inflammatory bowel disease

Background One‐third of patients with inflammatory bowel disease (IBD) receiving azathioprine (AZA) withdraw treatment due to side effects or lack of clinical response. Aim To investigate whether pharmacogenetic loci or metabolite concentrations explain clinical response or side effects to AZA. Methods Patients with IBD were given 2 mg/kg of AZA without dose escalation or adjustment. Serial clinical response, thiopurine methyl transferase (TPMT) activity and thioguanine nucleotide (TGN) concentrations were measured over 6 months. All patients were genotyped for inosine triphosphatase (ITPase) and TPMT. Clinical response and side effects were compared to these variables. Results Two hundred and seven patients were analysed. Thirty‐nine per cent withdrew due to adverse effects. Heterozygous TPMT genotype strongly predicted adverse effects (79% heterozygous vs. 35% wild‐type TPMT, P < 0.001). The ITPA 94C>A mutation was associated with withdrawal due to flu‐like symptoms (P = 0.014). A baseline TPMT activity below 35 pmol/h/mg/Hb was associated with a greater chance of clinical response compared with a TPMT above 35 pmol/h/mg/Hb (81% vs. 43% respectively, P < 0.001). Patients achieving a mean TGN level above 100 were significantly more likely to respond (P = 0.0017). Conclusions TPMT testing predicts adverse effects and reduced chance of clinical response (TPMT >35 pmol/h/mg/Hb). ITPase deficiency is a predictor of adverse effects and TGN concentrations above 100 correlate with clinical response.     

Pharmacogenomic studies of the anticancer and immunosuppressive thiopurines mercaptopurine and azathioprine

AIMS

To examine the allelic variation of three enzymes involved in 6-mercaptopurine/azathioprine (6-MP/AZA) metabolism and evaluate the influence of these polymorphisms on toxicity, haematological parameters and metabolite levels in patients with acute lymphoblastic leukaemia (ALL) or inflammatory bowel disease (IBD).

METHODS

Clinical data and blood samples were collected from 19 ALL paediatric patients and 35 IBD patients who were receiving 6-MP/AZA therapy. All patients were screened for seven genetic polymorphisms in three enzymes involved in mercaptopurine metabolism [xanthine oxidase, inosine triphosphatase (C94→A and IVS2+21A→C) and thiopurine methyltransferase]. Erythrocyte and plasma metabolite concentrations were also determined. The associations between the various genotypes and myelotoxicity, haematological parameters and metabolite concentrations were determined.

RESULTS

Thiopurine methyltransferase variant alleles were associated with a preferential metabolism away from 6-methylmercaptopurine nucleotides (P = 0.008 in ALL patients, P = 0.038 in IBD patients) favouring 6-thioguanine nucleotides (6-TGNs) (P = 0.021 in ALL patients). Interestingly, carriers of inosine triphosphatase IVS2+21A→C variants among ALL and IBD patients had significantly higher concentrations of the active cytotoxic metabolites, 6-TGNs (P = 0.008 in ALL patients, P = 0.047 in IBD patients). The study confirmed the association of thiopurine methyltransferaseheterozygosity with leucopenia and neutropenia in ALL patients and reported a significant association between inosine triphosphatase IVS2+21A→C variants with thrombocytopenia (P = 0.012).

CONCLUSIONS

Pharmacogenetic polymorphisms in the 6-MP pathway may help identify patients at risk for associated toxicities and may serve as a guide for dose individualization.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• 6-Mercaptopurine (6-MP) and azathioprine (AZA) are both inactive prodrugs that require intracellular activation into the active 6-thioguanine nucleotides (6-TGNs).
• This metabolic process undergoes three different competitive pathways that are catalysed by three different enzymes; xanthine oxidase (XO), thiopurine methyltransferase (TPMT) and inosine triphosphatase (ITPA), all of which exhibit genetic polymorphisms.
• Although the impact of genetic variation in the TPMT gene on treatment outcome and toxicity has been demonstrated, the role of other polymorphisms remains less well known.

WHAT THIS STUDY ADDS

• New information on the allelic variation of these three enzymes (XO, TPMT and ITPA) and their influence on 6-MP/AZA metabolism and toxicity.
• Confirmation of the association of TPMT polymorphism with haematological toxicity.
• Identified potential genetic characteristics that may contribute to higher risk of adverse events (such as ITPA IVS2+21A→C mutation).

     

The impact of thiopurine s-methyltransferase polymorphism on azathioprine-induced myelotoxicity in renal transplant recipients

Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. TPMT activity exhibits an interindividual variability, mainly as a result of genetic polymorphism. Patients with intermediate or deficient TMPT activity are at risk for toxicity after receiving standard doses of thiopurine drugs. It has previously been reported that 3 variant alleles: TPMT*2, *3A, and *3C are responsible for over 95% cases of low enzyme activity. The purpose of this study was to explore the association between these polymorphisms and the occurrence of azathioprine adverse effects in 112 renal transplant recipients undergoing triple immunosuppressive therapy including azathioprine, cyclosporine, and prednisone. TPMT genetic polymorphism was determined using PCR-RFLP and allele-specific PCR methods. Azathioprine dose, leukocyte, erythrocyte, and platelet counts, graft rejection episodes, as well as cyclosporine levels were analyzed throughout the first year after organ transplantation. We found the frequency of leukopenia episodes (WBC < 4.0 x 10(9)/L) significantly higher in heterozygous patients (53.8%) compared with those with TPMT wild-type genotype (23.5%). One patient, who was a compound homozygote (3A/*3C), experienced severe azathioprine-related myelotoxicity each time after receiving the standard drug dose. Our results suggest that polymorphisms in TPMT gene may be responsible for approximately 12.5% of all leukopenia episodes in renal transplant recipients treated with azathioprine. Genotyping for the major TPMT variant alleles may be a valuable tool in preventing AZA toxicity and optimization of immunosuppressive therapy.     

Low‐dose azathioprine or mercaptopurine in combination with allopurinol can bypass many adverse drug reactions in patients with inflammatory bowel disease

Aliment Pharmacol Ther 31 , 640–647

Summary

Background The thiopurine drugs, azathioprine and mercaptopurine (MP), are established treatments for IBD. However, therapeutic failure caused by adverse drug reactions occurs frequently. Aim To study combination of allopurinol with reduced‐dose thiopurine in an attempt to avoid adverse drug reactions in the treatment of IBD. Methods Patients with drug reactions to full‐dose thiopurines were recruited for combination therapy in two IBD centres in this retrospective study. Dosing was guided by measuring thiopurine methyltransferase (for UK patients) or thioguanine nucleotides and methyl‐6MP (Australian patients). Response was monitored by clinical activity indices. Results Of 41 patients, 25 had non‐hepatic and 16 had hepatitic reactions. Clinical remission was achieved in 32 patients (78%) with a median follow‐up of 41 weeks (range 0.5–400). Patients who did not respond to combination therapy tended to fail early with the same adverse reaction. The relative risk of having an adverse reaction with methyl‐6MP in the top interquartile range was 2.7 (1.3–28) times that with methyl‐6MP in the lower three quartiles (95% confidence interval). Conclusion The combined experience from our centres is the largest reported experience of this combination therapy strategy in IBD, and the first to provide evidence for benefit in thiopurine and allopurinol co‐therapy to avoid non‐hepatitic adverse drug reactions.     

Influence of xanthine oxidase on thiopurine metabolism in Crohn's disease

Background  The thiopurines, azathioprine (AZA) and mercaptopurine are extensively used in Crohn's disease (CD). Thiopurine bioactivation can be diverted by either thiopurine methyltransferase (TPMT), or by xanthine oxidase/dehydrogenase (XOD) which forms 6-thiouric acid (6TU).
Aim  To investigate whether chronic inflammation could influence small intestinal XOD activity using urinary excretion of 6TU as a surrogate marker of XOD activity.
Methods  6-Thiouric acid excretion was compared between 32 CD patients and nine dermatology patients (control group), on AZA. Six CD patients were interesting: five with low TPMT activity (one deficient, four intermediate), and one receiving AZA/allopurinol co-therapy.
Results  There was no statistical difference in 6TU excretion between the CD and control group. CD location, severity or surgery did not affect excretion. The TPMT-deficient patient excreted 89% of daily AZA dose as 6TU, but excretion by TPMT carriers was essentially normal. Concurrent 5-aminosalicylic acid therapy increased 6TU excretion significantly (median 32.9%), consistent with inhibiting TPMT. 6TU was undetectable in the patient on AZA/allopurinol co-therapy.
Conclusions  The results refuted our hypothesis, but fitted a model where most of an oral thiopurine dose effectively escapes first-pass metabolism by gut XOD, but is heavily catabolized by TPMT. Bioavailability of thiopurines may be competitively inhibited by dietary purines.     

<Emphasis Type="Italic">TPMT</Emphasis> but not <Emphasis Type="Italic">ITPA</Emphasis> gene polymorphism influences the risk of azathioprine intolerance in renal transplant recipients

Purpose  Thiopurine drugs have to be withdrawn in 10–30% of cases due to side effects, and it has been presented that genetic factors may be responsible for some of reported toxicity cases. Among polymorphic enzymes of thiopurines’ metabolic pathway, thiopurine S-methyltransferase (TPMT) has been studied most extensively, and some recent studies point to inosine triphosphate pyrophosphohydrolase (ITPA) polymorphism as an additional toxicity risk factor. Methods  The aim of the current study was to evaluate an association between TPMT and ITPA gene polymorphisms and drug intolerance in a cohort of 157 renal transplant recipients treated with azathioprine (AZA). Each subject was genotyped for the presence of variant TPMT (*2, *3A, *3B, and *3C) and ITPA (94C>A and IVS2+21A>C) alleles. Results  Mean AZA dose, mean white-blood-cell count, and platelet count in the course of treatment were lower in carriers of variant TPMT alleles compared to patients with TPMT wild-type genotype. Leukocyte numbers fell below 4.0 × 10⁹/L in 41.2% of TPMT heterozygous renal transplant recipients, compared to only 18.0% of wild-type patients (P < 0.01). In contrast, ITPA genotype did not influence AZA dose, hematological parameters, or leucopenia risk. Conclusions  Our results suggest that routine genotyping of renal transplant recipients for TPMT variants may be useful in reducing the risk of AZA-related myelotoxicity, but there is not enough evidence to introduce ITPA testing into clinical practice.     

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.     

Assessment of thiopurine methyltransferase and metabolite formation during thiopurine therapy: results from a large Swedish patient population

This study examined thiopurine methyltransferase (TPMT) and the relationship to thioguanine nucleotides (TGN) and methylthioinosine monophosphate (meTIMP) in a large Swedish patient population. The current hypothesis is that the cytotoxic effects of thiopurine drugs are mediated by the incorporation of TGN into DNA. The authors assayed the TPMT activity in red blood cells from 1151 subjects and the concentrations of TGN (n = 602) and meTIMP (n = 593) from patients treated with thiopurine drugs. The TPMT frequency distribution in both adults and children showed some differences from what had been found in unselected general populations. Children had lower median TPMT activity than adults (12.0 versus 12.9 U/mL RBC; P < 0.001). Relative differences in both TGN formation [medians: normal TPMT, 1.3; intermediate TPMT, 3.3; low TPMT, 47.9 pmol/8 x 10(8) RBC per mg azathioprine (AZA); P < 0.001] and meTIMP formation (medians: normal TPMT, 13; intermediate TPMT, 7.3; low TPMT, 0 pmol/8 x 10(8) RBC per mg AZA; P = 0.001) per 1 mg administered drug were noted among the 3 TPMT activity groups. Women formed higher concentrations of both TGN (1.5 versus 1.3 pmol/8 x 10(8) RBC per mg AZA; P = 0.01) and meTIMP (14.4 versus 10.7 pmol/8 x 10(8) RBC per mg AZA; P = 0.01) than men did. There was a significant correlation between the AZA dose and the meTIMP concentrations (r = 0.45; P < 0.001). Furthermore, dose alterations made in subjects with normal TPMT (n = 84) and intermediate TPMT (n = 22) activity resulted in more pronounced increases in TGN concentrations (170 versus 30 pmol/8 x 10(8) RBC; P < 0.001) in intermediate TPMT activity, whereas in normal TPMT activity changes in meTIMP concentrations were more pronounced (1.3 versus 0 nmol/8 x 10(8) RBC; P < 0.001). In normal TPMT activity both metabolites increased in a dose-dependent fashion, whereas in intermediate TPMT activity only TGN concentrations increased. The results of this study demonstrate the dynamic nature of thiopurine metabolism and its importance for thiopurine dosing.     

Meta-analysis: Inosine triphosphate pyrophosphatase polymorphisms and thiopurine toxicity in the treatment of inflammatory bowel disease

BACKGROUND: Thiopurines are widely used for the treatment of inflammatory bowel disease, but are associated with the development of side effects. It has been suggested that the enzyme inosine triphosphate pyrophosphatase (ITPA) plays a role in the digestion of thiopurines and that defective activity resulting from polymorphisms in the inosine triphosphate pyrophosphatase encoding genes may be associated with thiopurine-induced side effects. Current studies are controversial regarding this hypothesis. AIM: To perform a meta-analysis and gain more insight into a possible correlation between thiopurine-induced side effects and ITPA polymorphisms. METHODS: We explored Medline for articles on ITPA polymorphisms and thiopurine toxicity. Studies that compared ITPA polymorphism frequencies among thiopurine-tolerant and -intolerant adult inflammatory bowel disease patients were included in this meta-analysis. RESULTS: Nine published studies investigated associations between ITPA polymorphisms and thiopurine toxicity. Six studies (with 751 patients included) met our inclusion criteria and were processed in the meta-analysis. This analysis demonstrates that the ITPA 94C-->A polymorphism, is not significantly associated with any of the studied side effect parameters. CONCLUSIONS: This meta-analysis does not prove a correlation between the development of thiopurine toxicity and the ITPA 94C-->A polymorphism. This implies that there is no clinical relevance to determine ITPA polymorphisms in thiopurine-treated patients.     

Azathioprine therapy and adverse drug reactions in patients with inflammatory bowel disease: impact of thiopurine S-methyltransferase polymorphism

The efficacy of the immunosuppressants azathioprine and 6-mercaptopurine has been well established in the therapy of inflammatory bowel diseases (IBD). However, its use has been complicated by a high incidence of serious adverse drug reactions such as hematotoxicity, hepatotoxicity, pancreatitis and gastrointestinal disturbances. Whereas azathioprine-related pancytopenia has been clearly linked to thiopurine S-methyltransferase (TPMT) polymorphism limited data are available to explain gastrointestinal side effects. In a retrospective analysis of 93 adults with IBD and azathioprine therapy both phenotyping and genotyping was used to explore systematically the relationship between TPMT and azathioprine-related adverse reactions. At time of inclusion, 69 patients were still receiving azathioprine therapy and had never experienced side effects. Azathioprine had been withdrawn in 10 patients for non-medical reasons or lack of response and 14 patients (15%) had stopped medication or were on reduced dose due to severe azathioprine-related side effects. Nine of these 14 patients had developed gastrointestinal side effects (hepatotoxicity, n = 3; pancreatitis, n = 3; others, n = 3), but their normal red blood cell TPMT activities were in accordance to TPMT wild-type. TPMT deficiency in one patient had led to pancytopenia whereas only two of the remaining four patients with hematotoxicity displayed an intermediate phenotype of TPMT. This study demonstrates that azathioprine-related gastrointestinal side effects are independent of the TPMT polymorphism. Nevertheless pharmacogenetic testing for TPMT prior to commencing thiopurine therapy should become routine practice in order to avoid severe hematotoxicity in TPMT deficient patients and lowering the incidence of hematological side effects in individuals heterozygous for TPMT.     

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.     

Review article: the benefits of pharmacogenetics for improving thiopurine therapy in inflammatory bowel disease

Aliment Pharmacol Ther 2012; 35: 15–36

Summary

Background Thiopurines represent an effective and widely prescribed therapy in inflammatory bowel disease (IBD). Concerns about toxicity, mainly resulting from a wide inter‐individual variability in thiopurine metabolism, restrict their use. Optimal thiopurine dosing is challenging for preventing adverse drug reactions and improving clinical response. Aim To review efficacy and toxicity of thiopurines in IBD. To provide pharmacogenetic‐based therapeutic recommendations. Methods We conducted a query on PubMed database using ‘inflammatory bowel disease’, ‘thiopurine’, ‘azathioprine’, ‘6‐mercaptopurine’, ‘TPMT’, ‘pharmacogenetics’, ‘TDM’, and selected relevant articles, especially clinical studies. Results Thiopurine metabolism – key enzyme: thiopurine S‐methyltransferase (TPMT) – modulates clinical response, as it results in production of the pharmacologically active and toxic metabolites, the thioguanine nucleotides (6‐TGN). Adjusting dosage according to TPMT status and/or metabolite blood levels is recommended for optimising thiopurine therapy (e.g. improving response rate up to 30% or decreasing haematological adverse events of 25%). Other enzymes or transporters of interest, as inosine triphosphatase (ITPase), glutathione S‐transferase (GST), xanthine oxidase (XO), aldehyde oxidase (AOX), methylene tetrahydrofolate reductase (MTHFR) and ATP‐binding cassette sub‐family C member 4 (ABCC4) are reviewed and discussed for clinical relevance. Conclusions Based on the literature data, we provide a therapeutic algorithm for thiopurines therapy with starting dose recommendations depending on TPMT status and thereafter dose adjustments according to five metabolite profiles identified with therapeutic drug monitoring (TDM). This algorithm allows a dosage individualisation to optimise the management of patients under thiopurine. Furthermore, identification of new pharmacogenetic biomarkers is promising for ensuring maximal therapeutic response to thiopurines with a minimisation of the risk for adverse events.