A Ansari

Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency.

Abstract. Inosine triphosphate pyrophosphohydrolase (ITPase) deficiency is a common inherited condition characterized by the abnormal accumulation of inosine triphosphate (ITP) in erythrocytes. The genetic basis and pathological consequences of ITPase deficiency are unknown. We have characterized the genomic structure of the ITPA gene, showing that it has eight exons. Five single nucleotide polymorphisms were identified, three silent (138G→A, 561G→A, 708G→A) and two associated with ITPase deficiency (94C→A, IVS2+21A→C). Homozygotes for the 94C→A missense mutation (Pro32 to Thr) had zero erythrocyte ITPase activity, whereas 94C→A heterozygotes averaged 22.5% of the control mean, a level of activity consistent with impaired subunit association of a dimeric enzyme. ITPase activity of IVS2+21A→C homozygotes averaged 60% of the control mean. In order to explore further the relationship between mutations and enzyme activity, we examined the association between genotype and ITPase activity in 100 healthy controls. Ten subjects were heterozygous for 94C→A (allele frequency: 0.06), 24 were heterozygotes for IVS2+21A→C (allele frequency: 0.13) and two were compound heterozygous for these mutations. The activities of IVS2+21A→C heterozygotes and 94C→A/IVS2+21A→C compound heterozygotes were 60% and 10%, respectively, of the normal control mean, suggesting that the intron mutation affects enzyme activity. In all cases when ITPase activity was below the normal range, one or both mutations were found. The ITPA genotype did not correspond to any identifiable red cell phenotype. A possible relationship between ITPase deficiency and increased drug toxicity of purine analogue drugs is proposed.

Thiopurine methyltransferase activity and the use of azathioprine in inflammatory bowel disease

Background : Azathioprine therapy is discontinued in one‐third of patients with inflammatory bowel disease because of toxicity or a lack of clinical response. Patients with thiopurine methyltransferase (TPMT) deficiency are intolerant to azathioprine, whilst carriers are at increased risk of side‐effects.

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.

Thiopurine methyltransferase: should it be measured before commencing thiopurine drug therapy?

Thiopurines [azathioprine (AZA), 6-mercaptopurine (6-MP) and thioguanine (6-TG)] have a well-established role as immunosuppressive agents in a variety of chronic inflammatory conditions, haematological neoplasia and in transplant rejection. Despite good overall clinical response rates, particularly when used as steroid sparing agents, adverse effects are a limiting problem leading to withdrawal in up to a quarter of patients. Severe myelosuppression is the most serious toxicity occurring early or occasionally later during treatment. An understanding of the competing pathways involved in the metabolism of thiopurines has important implications for predicting some of the more severe toxicity seen with these drugs. Thiopurine methyl transferase (TPMT) is an enzyme catalysing the methylation of 6-MP, competing with xanthine oxidase (XO) and hypoxanthine guanine phosphoribosyl transferase (HGPRT) to determine the amount of 6-MP metabolised to cytotoxic thioguanine nucleotides. Allelic polymorphisms in the TPMT gene predict the activity of the enzyme such that 1 in 10 of the population are heterozygous and have approximately 50% of normal activity, whilst 1 in 300 are completely deficient. As a result, these individuals are at high risk of severe myelosuppression. Conversely, individuals with very high levels of TPMT activity are hyper-methylators in whom clinical response is less likely. Prior knowledge of TPMT status avoids exposure of individuals with zero TPMT to potentially fatal treatment with AZA or 6-MP and provides one of the best examples of predictive pharmacogenetics in therapeutics. This article reviews literature on the role of TPMT measurement prior to treatment with thiopurines and provides some guidance to the use of TPMT as a guide to tailoring thiopurine therapy.

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.

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.

Experience with the use of low-dose methotrexate for inflammatory bowel disease.

Background Thiopurine drugs (azathioprine and 6-mercaptopurine) are well established in the treatment of patients with inflammatory bowel disease. However, some patients are intolerant or resistant to thiopurine drugs and their management remains a challenge. Several studies have suggested methotrexate is effective for the induction and maintenance of remission in Crohns disease. Objective This study was conducted because the overall data on clinical efficacy of methotrexate in inflammatory bowel disease remain limited and there are no data regarding fistulating Crohns disease or concomitant use of methotrexate with thiopurine drugs in inflammatory bowel disease. Methods This study was a retrospective review of medical notes. Clinical response was defined as sustained withdrawal of oral steroids or fistula improvement. New episodes of steroid therapy, infliximab or surgery during the first 6 months were considered as failure to achieve clinical response. Results Seventy-two patients were studied (66 Crohns disease and six ulcerative colitis). The mean dose of methotrexate used was 18.2 mg/week. Clinical response was achieved in 22 of 54 patients (40.7%) who completed 6 months of methotrexate treatment. For patients with Crohns disease, fistula improvement was achieved in eight of 18 (44.4%) patients compared with 11 of 30 (36.7%) receiving methotrexate for steroid withdrawal. Clinical response was achieved in six of 15 patients (40%) treated with methotrexate and azathioprine at the same time compared with 16 of 39 patients (41%) treated with methotrexate alone. Conclusions Methotrexate is reasonably effective in clinical practice as a steroid sparing agent in inflammatory bowel disease. The efficacy in fistulating Crohns disease justifies its use as an immunomodulator in these patients. Combined azathioprine and methotrexate treatment appears to offer no advantage over methotrexate alone.

Further experience with the use of 6-thioguanine in patients with Crohn's disease

Background: 6‐Thioguanine (6‐TG) is efficacious in patients with Crohns Disease (CD) failing conventional immunosuppression but there are reports of hepatotoxicity. We report our experience of the safety and efficacy of 6‐TG in a series of patients with CD. Methods: A retrospective study of patients with CD who failed thiopurines ± methotrexate between 2001 and 2006 was performed. Indications for 6‐TG were; active disease, to allow infliximab withdrawal, steroid sparing, or fistula closure. Patients underwent regular review and those treated longer than 1 year were advised to have liver magnetic resonance imaging (MRI) and liver biopsy. Results: All 30 patients treated with 6‐TG during the period were included. The median dose and duration of 6‐TG was 40 mg daily and 21.5 months, respectively. Initial clinical response was achieved in 18/30 (60%). Eleven of 29 (38%) (1 unrelated death) remained in remission at a median 44 months follow‐up. Seven of 30 (23%) discontinued 6‐TG due to adverse effects; 7/30 (23%) patients developed abnormal liver function tests (LFTs) during treatment, mostly transient and mild. One patient developed a portal hypertensive syndrome resolving on cessation of 6‐TG. Of 11 liver biopsies, none showed nodular regenerative hyperplasia (NRH). The median red blood cell 6‐thioguanine nucleotide (6‐TGN) level was 807 pmol/108. Conclusions: 6‐TG has good clinical efficacy for third‐line immunosuppression in CD but hepatotoxicity remains a concern. However, previous reports of NRH in 6‐TG‐treated inflammatory bowel disease patients have not been substantiated by this cohort.

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).

Splitting a Therapeutic Dose of Thioguanine May Avoid Liver Toxicity and Be an Efficacious Treatment for Severe Inflammatory Bowel Disease: A 2-Center Observational Cohort Study

Background:Thioguanine (TG) is a treatment for inflammatory bowel disease, but association with nodular regenerative hyperplasia has restricted its use. We conjectured that splitting a therapeutic daily dose of TG would be efficacious and should avoid liver toxicity. Methods:We report on 62 patients with severe inflammatory bowel disease not responding to prednisolone, conventional thiopurines, biologics, or calcineurin inhibitors. Patients were prescribed oral split-daily TG to avoid individual doses >0.3 mg/kg. Data on concomitant medication, clinical efficacy measured by Harvey–Bradshaw Index for Crohns, or Simple Clinical Colitis Score for ulcerative/indeterminate colitis (UC), and some paired endoscopies were available. Safety was followed clinically and with bloods at 2 centers. All patients at the U.K. center had a liver biopsy or magnetic resonance imaging after 6 months. Twenty-one patients had serial ultrasounds at the Australian center. Results:At 6 months, 19/21 of patients with Crohns disease and 27/38 with ulcerative colitis had improved clinical activity. At study end, 53% of patients maintained improved clinical activity of steroids. Median duration of TG was 8 (0.3–45) months, median dose was 0.6 (0.3–1) mg/kg per day. Previous thiopurine-related adverse reactions were not encountered. Twenty-nine patients withdrew because of loss to follow-up, medical adverse events, or surgery. Possible early nodular regenerative hyperplasia was found on liver biopsy in 1 patient who was heterozygote deficient for thiopurine methyltransferase; the TG dose was lowered. TG was discontinued in a patient with nodular regenerative hyperplasia and concomitant antiphospholipid syndrome. There was 1 successful term pregnancy; cord blood and breast milk TG were low. Conclusions:Split-dose TG seemed well tolerated and efficacious in this retrospective study of patients with difficult inflammatory bowel disease.