Thiopurine Methyltransferase Polymorphisms Research Articles

Outcome in acute lymphoblastic leukemia: Influence of thiopurine methyltransferase genetic polymorphisms

Current treatment protocols for childhood acute lymphoblastic leukemia always include thiopurine drugs such as 6-mercaptopurine (6-MP), which is administrated daily during maintenance therapy. Once 6-MP is an inactive prodrug, it needs to be activated to thioguanine nucleotides (TGNs) to exert cytotoxicity. However, 6-MP can also be methylated by the polymorphic enzyme thiopurine S-methyltransferase (TPMT), thus reducing TGNs formation. In order to assess the influence of TPMT polymorphism in ALL, we have used PCR/HCSGE (Horizontal Conformation-Sensitive Gel Electrophoresis) based methods to characterise molecularly a sample of 110 children with ALL. Four distinct alleles associated with TPMT deficiency (TPMT*3A, *3C, *2 and *8) were found in heterozygous individuals representing 11.8% of the sample. We have also compared several parameters related with ALL outcome between subsamples of children homozygous or heterozygous for TPMT. In the heterozygous group, 6-MP dosages were lower and the number of whole interruptions during treatment or interruptions due to toxicity as well as the number of relapses was higher compared to the other group. Despite our results do not reach statistical significance, this study emphasizes the importance to introduce the prospective analysis of TPMT genotype, prior to any ALL treatment.

Impracticalities of thiopurine S‐methyltransferase determination in daily inflammatory bowel disease practice

Sirs, We greatly enjoyed the discussion concerning the cost-effectiveness of prior-to-treat genotyping of thiopurine S-methyltransferase (TPMT) in inflammatory bowel disease patients (IBD).1, 2 Thiopurines (azathioprine and mercaptopurine) are frequently used immunosuppressive agents in the treatment of Crohn's disease and ulcerative colitis. The metabolism of thiopurines is complex as multiple enzymatic steps are needed to ultimately generate the pharmacologically active 6-thioguanine nucleotides (6-TGN).3 The activity of the methylating enzyme TPMT plays a key role in the bioavailability of 6-TGN. Less efficient methylation by TPMT leads to relative high levels of 6-TGN, which are considered to be a causative factor in the development of myelotoxicity.4 High levels of methylated thiopurine metabolites [e.g. 6-methylmercaptopurine (6-MMP)] will be generated as a consequence of high-TPMT activity. Elevated 6-MMP levels have been associated with the development of hepatotoxicity, but also with refractoriness to thiopurine therapy.5 For this reason, prior-to-treat determination of the TPMT status by genotyping (or phenotyping) has been proposed to avoid toxicity by adjusting the dosage to the individual TPMT status. Winter et al.1 conclude that pre-treatment screening for TPMT polymorphisms in IBD patients commencing thiopurine therapy represents good value for money. In order to evaluate whether prior-to-treat determination of the TPMT status is currently utilized in daily IBD practice, a questionnaire regarding this topic was sent to all Dutch registered gastroenterologists (n = 250) per e-mail. A total of 133 questionnaires (53%) was returned. The results of the questions concerning the currently used and preferred toxicity screening techniques are depicted in Table 1. Five percent of gastroenterologists currently determined the TPMT status and 36% wants to determine the TPMT status prior to initiation of thiopurine therapy (χ2-test, P = 0.001). The aforementioned discrepancy may be explained by the lack of proper laboratory facilities in the Netherlands. Therefore, it is likely that gastroenterologists will expand their toxicity screening by determining the TPMT status when facilities become generally available. Nevertheless, the greater part of physicians (64%) does not consider TPMT determination as an additional or valuable toxicity screening tool. Possible explanations, as stated by several respondents, are that knowledge of the TPMT status may result in a decreased possibility of induction of toxicity because of thiopurine treatment; however, frequent determination of laboratory parameters remains mandatory for optimal care.6 Knowledge of the patients’ TPMT status seems, for that reason, not needed. In addition, genotyping of TPMT alleles is no guarantee for not developing a leucopenia as the majority of reported cases is reported to be in IBD patients without one or two mutant TPMT alleles.3, 4 Finally, a logistic problem arises when adapting to this novel pharmacogenetic strategy as the determination of TPMT status may take some time and patients in need of thiopurines have to await the results before thiopurine therapy can be initiated. We conclude that in spite of possible clinical and pharmaco-economic advantages, TPMT determination in daily Dutch IBD practice is considered to be of limited value and surplus to requirements. Its implementation seems hampered by logistical difficulties and by the lack of convincing evidence.

Azathioprine dosed by thiopurine methyltransferase activity for moderate-to-severe atopic eczema: a double-blind, randomised controlled trial

Atopic eczema affects 1-2% of adults, and can cause considerable morbidity. We aimed to assess the safety and efficacy of azathioprine as systemic monotherapy for moderate-to-severe atopic eczema, and the therapeutic importance of the thiopurine methyltransferase (TPMT) polymorphism (a key determinant of azathioprine-induced myelotoxicity) by using TPMT enzyme activity to establish azathioprine dose. We did a parallel-group, double-blind, placebo-controlled trial in an outpatient setting. Minimisation was used to assign 63 patients with active disease despite optimum topical therapy to treatment with azathioprine (n=42) or placebo (n=21) for 12 weeks. As maintenance treatment, patients with heterozygous range TPMT activity received azathioprine 1.0 mg/kg daily, compared with 2.5 mg/kg daily in patients with normal TPMT activity. For the first 4 weeks, all participants received lower azathioprine doses (0.5 and 1.0 mg/kg daily, respectively) to reduce gastrointestinal side-effects. The primary measure of clinical response was disease activity assessed by the SASSAD (six area six sign atopic dermatitis) score. Analysis was by intention-to-treat. This study is registered as an International Standard Randomised Controlled Trial, number ISRCTN58943280. 54 (86%) participants completed the study; two (3%) withdrew from the placebo group and seven (11%) from the azathioprine group. At week 12, there was a 37% (12.0 unit) improvement in mean disease activity with azathioprine compared with a 20% (6.6 unit) improvement with placebo (17% [5.4 unit] difference, 95% CI 4.3-29%). This finding was accompanied by significant improvements in patient-reported itch, area of involvement, global assessment, and quality of life. Between participants there was a wide variation in response to the drug. Generally, azathioprine was well tolerated, although two individuals developed drug hypersensitivity. Participants with heterozygous range TPMT activity responded to azathioprine in similar proportions to other participants, but none developed bone-marrow toxicity. TPMT-based dosing seemed to reduce predicted toxicity, and drug efficacy was maintained. Treatment with azathioprine as systemic monotherapy produces clinically relevant improvement in moderate-to-severe atopic eczema that remains active despite optimum therapy with topical corticosteriods. We believe the study of azathioprine as systemic monotherapy for atopic eczema has major advantages, which should allow clarification of the relation between azathioprine effectiveness and metabolite profiles in other inflammatory diseases.

Introducing a fast and simple PCR‐RFLP analysis for the detection of mutant thiopurine S‐methyltransferase alleles TPMT*3A and TPMT*3C

Azathioprine, in combination with corticosteroids, is the first-line therapy of severe forms of pemphigus vulgaris. Patients with an impaired thiopurine S-methyltransferase (TPMT) activity are at risk of developing severe myelo-suppression upon treatment with thiopurines such as azathioprine. Analysis of the TPMT status prior to drug administration is therefore highly recommended. However, because of the limited availability of TPMT testing outside of specialized centres, pre-emptive TPMT testing is not widespread. To avoid laborious biochemical and sequencing assays, we evaluated a new restriction fragment length polymorphism (RFLP) analysis. We designed a rapid genetic polymerase chain reaction (PCR)-RFLP screen for the most prevalent mutant TPMT*3A and TPMT*3C alleles that are known to result in reduced TPMT enzyme activity. Validating our fast system on 871 Caucasian DNA samples, we observed that 8.61% of our probands carried the TPMT*3A allele and 0.23% were heterozygous for the TPMT*3C allele, which is in concordance with previously reported allele frequencies. This simple and low-cost PCR-RFLP TPMT polymorphism testing approach can be performed in a standard laboratory. It should be applied to all patients prior to receiving thiopurine drug therapy to avoid the severe, but predictable, haematopoietic side-effects of thiopurine drug administration.

Genotyping of thiopurine methyltransferase in patients with acute leukemia using LightCycler PCR

Patients with acute lymphoblastic leukemia (ALL) are treated with thiopurines. Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurines and is subject to genetic polymorphism. The TPMT genotype was determined in 55 patients with ALL. Three patients were heterozygous for allelic variants of TPMT. We describe an assay for the genotyping of the TPMT polymorphism using real-time fluorescence polymerase chain reaction (PCR) to facilitate rapid processing of samples. This strategy is superior to standard PCR-restriction fragment length polymorphism genotyping methods and provides informative data on TPMT polymorphisms in patients prior to treatment with thiopurines.

Inosine Triphosphate Pyrophosphatase and Thiopurine S-Methyltransferase Genotypes Relationship to Azathioprine-Induced Myelosuppression

The use of azathioprine (AZA) in inflammatory bowel disease (IBD) patients is limited by toxicity, which occurs in up to 20% of treated patients. Mutations in the thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) genes have been associated with the occurrence of AZA-related toxicity. The aim of our study was to determine the relative contribution of ITPA and TPMT mutations to the development of toxicity induced by AZA treatment in IBD patients. ITPA(94C>A, IVS2+21A>C) and TPMT (238G>C, 460G>A, and 719A>G) genotypes were assessed in 262 IBD patients (159 females, 103 males; 67 patients with ulcerative colitis, 195 patients with Crohn's disease) treated with AZA and were correlated with the development of leukopenia and hepatotoxicity. Leukopenia (leukocyte count, <3.0 x 10(9)/L) was observed in 4.6% of treated patients. The frequencies of mutant ITPA 94C>A and TPMT alleles were significantly higher in the leukopenic population compared with patients without leukopenia (16.7% and 5.4%, respectively, for ITPA 94C>A, and 20.8% and 4%, respectively, for TPMT). Moreover, the ITPA 94C>A and TPMT mutations predicted leukopenia: ITPA 94C>A odds ratio, 3.504; 95% confidence interval, 1.119-10.971 (P = .046); TPMT odds ratio, 6.316; 95% confidence interval, 2.141-18.634 (P = .004). Neither TPMT nor ITPA genotype predicted hepatotoxicity. ITPA 94C>A and TPMT polymorphisms are associated with AZA-related leukopenia in IBD patients.

Genetic Polymorphism of Thiopurine S-Methyltransferase in the Russian Population.

Thiopurine S-methyltransferase (TPMT) is a key enzyme in the catabolism of 6-mercaptopurine (6MP), a key component of therapy for childhood acute lymphoblastic and promyelocytic leukemias and lymphoblastic non-Hodgkin lymphoma. TPMT mutations reduce TPMT enzymatic activity; therefore, TPMT-deficient patients develop severe hematopoietic toxicity when treated with standard doses of 6MP (50–75 mg/m2/day). The frequency of TPMT alleles has not previously been evaluated in the Russian Federation. We determined the allele frequency and the pattern of mutant TPMT alleles in Russian children with hematologic malignancies (leukemia/lymphoma) and other diseases, and in healthy adults, in different areas of the Russian Federation. A microchip was designed to detect the main mutant alleles of human TPMT (TPMT*2, *3A, *3B, *3D, *7, and *8) and was used to assay 982 samples. Only 3 inactivating mutations of the TPMT gene were identified: 45 subjects (4.6%) had TPMT genotype *1/*3A, 8 (0.8%) had *1/*3C, and 2 (0.2%) had *1/*2. No homozygous genotypes and no mutant *3B, *3D, *7, or *8 alleles were found. The estimated frequency of the wild-type allele was 97.1% (95% CI, 93.8%–100%) and that of variant alleles was 2.9% (95% CI, 0%–6%). Most individuals with the *1/*3A genotype were of Slavic background, and genotype *1/*3C was found in a buryat (Mongolian race) and in residents of the North Caucasian and the Urals regions. These findings indicate ethnic differences in the frequency of nonfunctional TPMT alleles. The overall frequency of mutant TPMT genotypes in the Russian Federation is 5.6%, which is similar to that observed in the white populations of the United States and Europe.

Genetic determinants of cancer drug efficacy and toxicity: practical considerations and perspectives

Drug-metabolizing enzymes are responsible for the activation or detoxification of cytotoxic drugs. Allelic variants are present with a variable frequency in different populations around the world and have an important role in the therapeutic index of such drugs. It is known that polymorphisms in thiopurine methyltransferase and dihydropyrimidine dehydrogenase have been associated with altered drug metabolism and increased risk of severe toxicity from 6-mercaptopurine and 5-fluorouracil, respectively. Additionally, a variant number of dinucleotide-repeat sequences in the promotor for uridine 5'-diphosphate glucuronosyltransferase 1A1 influences the glucuronidation of SN-38, the active metabolite of irinotecan, which is associated with severe toxicity, including diarrhea and neutropenia. In the same way, polymorphisms in thymidylate synthase have been associated with pyrimidine-associated toxicity and also with response to chemotherapy. The examples shown in this review demonstrate the usefulness of pre-screening patients for well-characterized polymorphism to identify the best-tolerated and most-effective treatment.

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.

Comprehensive Screening of the Thiopurine Methyltransferase Polymorphisms by Denaturing High-Performance Liquid Chromatography

The drug-metabolizing enzyme thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation of thiopurines such as 6-mercaptopurine, 6-thioguanine, and azathiopurine, which are used as immunosuppressants and in the treatment of acute lymphoblastic leukemia and rheumatoid arthritis. TPMT enzymatic activity is a polymorphic trait, and poor metabolizers may develop life-threatening bone marrow failure. To avoid such adverse effects, the TPMT enzymatic activity in patients' red blood cells (RBCs) is routinely measured prior to thiopurine administration in a limited number of oncology clinics. In the present study, we took advantage of a highly sensitive and specific automated denaturing high-performance liquid chromatography (dHPLC) technique that not only detects known polymorphic alleles, but also identifies previously uncharacterized sequence variants. We developed a dHPLC-based protocol to analyze the entire coding region and validated the protocol to detect all 16 previously described variant alleles. We further analyzed the entire coding region of the TPMT gene in 288 control samples collected worldwide and identified two novel amino acid substitutions Arg163Cys (487C>T) and Arg226Gln (677G>A) within exons 7 and 10, respectively. The clinical application of this comprehensive screening system for examining the entire TPMT gene would help to identify patients at risk for bone marrow failure prior to 6-mercaptopurine therapy.

The Clinical Impact of Thiopurine Methyltransferase Polymorphisms on Thiopurine Treatment

Acute lymphoblastic leukaemia (ALL) is the most common malignancy of childhood. Although current treatment results in long term survival in over 70% of cases there is evidence that as many as 50% could have been cured using a less complex regimen with a lower incidence of long term side effects. In previous studies it has been found that thiopurines given as part of continuing therapy are key agents in preventing relapse. However, optimal administration during continuing therapy is often not achieved. Variation in the level of thiopurine methyltransferase (TPMT) activity appears to be a major molecular determinant of the extent of thiopurine metabolism. TPMT activity shows a trimodal distribution pattern. A lack of activity is found in approximately one in 300 Caucasians; approximately 11% have intermediate activity and the remaining 89% high activity. Congenital loss of activity is associated with grossly elevated levels of active drug and profound myelosuppression on exposure to thiopurines. This loss o...

Thiopurine Methyltransferase Genotype and Phenotype Status in Japanese Patients with Systemic Lupus Erythematosus

We investigated the genotypic status of thiopurine methyltransferase (TPMT) polymorphism to evaluate the possible risk of the toxicity of azathioprine (AZA) in 68 patients with systemic lupus erythematosus (SLE). The allele frequency of TPMT mutation in the SLE group (2.9%) was higher than that in 174 Japanese healthy volunteers (1.1%), although it did not reach statistically significant difference (p=0.23). The mean value of TPMT activities in 51 subjects with TPMT*1/*1 was 40% higher than that of 4 subjects with TPMT*1/*3C in SLE group (18.1+/-6.1 nmol/h/ml packed red blood cells (pRBC) versus 13.2+/-3.2 nmol/h/ml pRBC; p=0.11). Two out of 4 SLE patients with TPMT*1/*3C had been treated with AZA, and one patient showed a leucopenia. The TPMT genotyping before AZA treatment is recommended for Japanese SLE patient group to avoid the AZA-induced adverse events, although detection of the patient with low TPMT activity by genotyping is still imperfect.

Genetic Testing in Crohn Disease

Inflammatory bowel disease (IBD), with its two subforms of Crohn disease and ulcerative colitis, is a polygenic disease that manifests due to environmental trigger factors on the background of a complex genetic predisposition. The first risk gene underlying susceptibility to Crohn disease has been identified as CARD15 (located on chromosome 16q12, encoding NOD2). Three single nucleotide polymorphisms in the leucine rich region (LRR) of this gene are strongly and independently associated with Crohn disease susceptibility and explain up to 20% of the total genetic predisposition for Crohn disease. These variants have been consistently replicated as associated with a particular sub-phenotype characterized by small bowel (ileum) involvement and early age at onset. Presently, genetic testing for the CARD15 variants has only a modest relevance in clinical practice. The most attractive use of genetic testing is for the prediction of response to therapy. Most therapies only show efficacy in subgroups of patients and no clinical parameters are available to distinguish, prior to therapy, whether the patients will be responders or non-responders, or if the patients will experience adverse effects. The pharmacogenetic basis of toxicity is well known for azathioprine: several thiopurine methyltransferase (TPMT) polymorphisms that are associated with reduced activity of this thiopurine drug metabolizing enzyme result in cytotoxic and immunosuppressive adverse effects of azathioprine. Genetic screening, which has found its way into routine clinical diagnostics, allows the identification of the patients who will not tolerate a standard dose of the drug. The extensive search for genetic predictors of response to the anti-tumor necrosis factor treatment with infliximab, which results in a remission rate of 30-40%, has, however, failed to identify a variation associated with a differential response.

The Clinical Impact of Thiopurine Methyltransferase Polymorphisms on Thiopurine Treatment

Acute lymphoblastic leukaemia (ALL) is the most common malignancy of childhood. Although current treatment results in long term survival in over 70% of cases there is evidence that as many as 50% could have been cured using a less complex regimen with a lower incidence of long term side effects. In previous studies it has been found that thiopurines given as part of continuing therapy are key agents in preventing relapse. However, optimal administration during continuing therapy is often not achieved. Variation in the level of thiopurine methyltransferase (TPMT) activity appears to be a major molecular determinant of the extent of thiopurine metabolism. TPMT activity shows a trimodal distribution pattern. A lack of activity is found in approximately one in 300 Caucasians; approximately 11% have intermediate activity and the remaining 89% high activity. Congenital loss of activity is associated with grossly elevated levels of active drug and profound myelosuppression on exposure to thiopurines. This loss of activity has been attributed to single nucleotide polymorphisms (SNPs) within the TPMT gene. The frequency of SNPs is related to ethnicity, with the most common in Caucasians being TPMT*3A which is characterized by a G to A transition at position 460 with a substitution of alanine for tyrosine at amino acid 154 (A154Y) and a transition of A to G at nucleotide 719 resulting in a change of tyrosine to cysteine at position 240 (Y240C). Polymorphisms have also been identified within the 5′ flanking promoter region of the TPMT gene due to a variable number of tandem repeats (VNTR*3–*8). An overview of the polymorphisms identified to date, their implication on the metabolism of the thiopurine drugs and therapeutic importance will be discussed.

THIOPURINE METHYLTRANSFERASE ACTIVITY IS CORRELATED WITH AZATHIOPRINE METABOLITE LEVELS IN INFLAMMATORY BOWEL DISEASE PATIENTS IN CLINICAL GASTROENTEROLOGY PRACTICE

Purpose: Genetic polymorphisms in thiopurine methyltransferase (TPMT) influence the metabolism of azathioprine (AZA) and 6-mercaptopurine (6-MP) in patients with inflammatory bowel disease (IBD). It has been suggested in studies using weight-based dosing that higher TPMT activity is associated with lower levels of 6-thioguanine (6-TG), the metabolite thought to provide therapeutic benefit, and higher levels of 6-methylmercaptopurine (6-MMP), the metabolite associated with hepatotoxicity. Some studies suggest that higher TPMT activity correlates with lower rates of clinical response. The aim of this study is to assess the relationship between TPMT activity and 6-TG and 6-MMP levels in patients with IBD in clinical practice. Methods: We analyzed the TPMT activity, the 6-TG level, and the 6-MMP level in all patients who had at least one TPMT assessment and at least two metabolite profiles sent from a gastroenterologist's office to Prometheus Laboratories (San Diego, CA) from June 2000 to February 2004. The use of AZA or 6-MP and the dosing of the drug was at the discretion of the patient's gastroenterologist. Linear regression was used to analyze the relationships and a Pearson correlation was calculated. Results: A total of 1,020 patients were identified. Linear regression showed a significant inverse relationship between TPMT activity and 6-TG level with a Pearson correlation of-0.162 (p <0.01), a significant direct relationship between TPMT activity and 6-MMP level with a Pearson correlation of 0.172 (p <0.01), and a significant direct relationship between TPMT activity and 6-MMP/6-TG ratio with a Pearson correlation of 0.141 (p <0.01). Conclusions: Higher TPMT activity is associated with lower 6-TG levels and higher 6-MMP levels in IBD patients treated with AZA or 6-MP in clinical practice with drug dosing at the discretion of the primary gastroenterologist. This suggests a possible role for TPMT activity assays in patients being considered for treatment with these agents.

The impact of thiopurine S-methyltransferase polymorphisms on azathioprine dose 1 year after renal transplantation

The impact of thiopurine S-methyltransferase polymorphisms on azathioprine dose 1 year after renal transplantation

The impact of thiopurine S-methyltransferase polymorphisms on azathioprine dose 1 year after renal transplantation.

Azathioprine metabolism is influenced by activity of the enzyme thiopurine S-methyltransferase (TPMT), which varies markedly between individuals. In this study we examined the influence of TPMT gene polymorphisms on azathioprine dose 1 year after renal transplantation. TPMT coding and promoter genotypes were determined using PCR-based assays. Azathioprine dose, white cell count, and intercurrent events throughout the first year after renal transplantation were ascertained from contemporaneous clinical notes. All patients analysed ( n=172) received an initial azathioprine dose of 1.5 mg/kg per day. Twelve individuals with one variant TPMT coding allele were detected (*3A n=11, *3C n=1). Of these, 58% required azathioprine dose reduction because of leucopenia, compared to only 30% of homozygous wild-type patients ( P=0.04). A significant correlation between the presence of >/=11 variable number tandem repeats (VNTRs) in the TPMT promoter and reduction in azathioprine dose was also identified ( P=0.001). We concluded that when azathioprine is administered at an initial dose of 1.5 mg/kg per day, both coding and promoter TPMT polymorphisms influence the dose tolerated.

Cost-effectiveness of thiopurine methyltransferase genotype screening in patients about to commence azathioprine therapy for treatment of inflammatory bowel disease.

Azathioprine is a useful agent in the management of inflammatory bowel disease. Its use is limited by its side-effect profile. Marrow toxicity occurs in approximately 3.2% of patients and is known to be associated with diminished thiopurine methyltransferase enzyme activity resulting from genetic polymorphisms. To evaluate the cost-effectiveness of screening for thiopurine methyltransferase gene polymorphisms prior to initiation of azathioprine therapy. Analysis of the literature was undertaken to calculate the expected frequency of leucopenia and its relationship with thiopurine methyltransferase polymorphisms in a model of theoretical inflammatory bowel disease patients. Decision analysis was then applied to assess the cost of a pre-treatment genotyping strategy, taking account of direct costs and cost per life-year saved. In 1000 inflammatory bowel disease patients treated with azathioprine, 32 will develop myelosuppression and one will die because of this. Of those who develop myelosuppression during azathioprine therapy, 32% are attributable to lower thiopurine methyltransferase activity. Pre-treatment genotyping costs pound 347 per life-year saved for a 30 year old and pound 817 per life-year saved for a 60 year old. This compares favourably with other health care technologies. The use of pre-treatment screening for thiopurine methyltransferase polymorphisms in inflammatory bowel disease patients commencing azathioprine therapy represents good value for money.

Distribution of ITPA P32T alleles in multiple world populations.

Dose-limiting toxicity from azathioprine treatment affects up to 37% of patients. Screening for thiopurine methyltransferase (TPMT) polymorphisms will prospectively identify approximately 10% of patients. Recently, a polymorphism in the inosine triphosphate pyrophosphatase gene (ITPA) has been associated with severe azathioprine toxicity. We demonstrate here that this proline to threonine substitution at codon 32 in the ITPA gene is found at low frequency in Central/South American populations (1-2%), at a constant frequency across Caucasian and African populations (6-7%), and is highest in Asian populations (14-19%). This data is consistent with previously described allele frequencies in other Caucasian (7%), African (5%), and Asian (11-15%) populations. This data provides a foundation on which prospective screening studies can be planned to identify patients at risk for severe toxicity from azathioprine therapy.

Pharmacogenetics of thiopurine S-methyltransferase and thiopurine therapy.

Most medications exhibit wide interpatient variability in their efficacy and toxicity. For many medications, these interindividual differences result in part from polymorphisms in genes encoding drug-metabolizing enzymes, drug transporters, and/or drug targets (eg, receptors, enzymes). Pharmacogenomics is a burgeoning field aimed at elucidating the genetic basis of differences in drug efficacy and toxicity, using genome-wide approaches to identify the network of genes that govern an individual's response to drug therapy. For some genetic polymorphisms, such as thiopurine S-methyltransferase (TPMT), monogenic traits have a marked effect on the pharmacokinetics of medications, such that individuals who inherit an enzyme deficiency must be treated with markedly different doses of the affected medications (eg, 5-10% of the standard thiopurine dose). This review uses the TPMT polymorphism and thiopurine therapy (eg, azathioprine, mercaptopurine) to illustrate the potential of pharmacogenomics to elucidate genetic determinants of drug response, and optimize the selection of drug therapy for individual patients.