Monitoring of thiopurine methyltransferase and thiopurine metabolites to optimize azathioprine therapy in inflammatory bowel disease

Determination of the activity of thiopurine methyltransferase (TPMT) and of thiopurine metabolites (6-thioguanine and 6-methylmercaptopurine nucleotides) could be useful for individualized monitoring of azathioprine (AZA) and 6-mercaptopurine (6-MP) doses. TPMT activity in the general population follows a trimodal distribution, in which approximately 0.3% of the population is homozygotic for the low-activity allele. A notable correlation has been observed between the low TPMP activity genotype or phenotype and the risk of myelotoxicity. Patients with a high TPMT activity genotype or homozygous phenotype should receive immunosuppressive doses that have clearly been demonstrated to be effective. In contrast, in patients with a low TPMT activity genotype or homozygous phenotype, the use of AZA/6-MP should be contraindicated or only very small doses should be administered. Importantly, TPMP deficiency explains only some cases of myelotoxicity and consequently periodic laboratory testing should be performed in patients receiving AZA/6-MP, even though TPMP function may be normal. Currently, the utility of routine thiopurine metabolite determinations in patients undergoing AZA/6-MP therapy has not been established and this practice should be limited to specific situations such as lack of response to thiopurine therapy or the occurrence of thiopurine-related adverse effects. Randomized trials comparing the routine strategy of AZA/6-MP dosing (based exclusively on the patient's weight) versus individualized monitoring (based on quantification of TPMP activity and/or thiopurine metabolites) are required before definitive conclusions on the most effective alternative can be drawn.     

A skewed thiopurine metabolism is a common clinical phenomenon that can be successfully managed with a combination of low-dose azathioprine and allopurinol

Background and aimsA skewed thiopurine metabolism is a phenomenon associated with both poor treatment response and toxicity. Our aim was to evaluate the frequency of this phenomenon and the relationship to thiopurine methyltransferase (TPMT) function.MethodsAll thiopurine metabolite measurements in adult patients (n = 4033) between January 2006 and April 2012 were assessed to evaluate the occurrence of a skewed metabolism and the relationship to TPMT genotype and activity.ResultsA skewed metabolism was observed in 14% of all patients. It only developed in patients with a normal TPMT genotype, but was observed at all TPMT activity levels within the normal range (9.1–24.2 U/ml RBC). Two cases that illustrate typical clinical scenarios of a skewed metabolism and the effect of combination treatment with low-dose azathioprine and allopurinol are presented.ConclusionsA skewed metabolism is a common clinical phenomenon in patients with a normal TPMT function, which can develop at all TPMT activity levels within the normal range. We suggest that metabolite measurements should be considered in patients not responding to treatment and in those with hepatotoxicity or myelotoxicity in order to detect a skewed metabolism, since this phenomenon can be successfully managed by a combination of low-dose azathioprine and allopurinol.     

Monitoring of long-term thiopurine therapy among adults with inflammatory bowel disease

BACKGROUND: The immunosuppressive effects of thiopurine drugs are mainly mediated through their intracellular metabolism into active 6-thioguanine nucleotide (6-TGN) metabolites, which are incorporated into DNA. Erythrocyte 6-TGN (E-6TGN) levels have been proposed as an instrument for monitoring treatment. The aim of the study was to use erythrocyte E-6TGN, methylated mercaptopurine (MeMP) metabolites, and thiopurine methyltransferase (TPMT) measurements in a clinical setting to determine the clinical outcome in relation to thiopurine metabolism. METHODS: Fifty-five adult patients with inflammatory bowel disease were included in a prospective study and followed for 6 months. Metabolite levels were measured and correlated to outcome and AZA/6-MP dose. RESULTS: The E-6TGN level was significantly related to the TPMT genotype (P = 0.008). Patients in disease remission had higher E-6TGN levels than patients with disease activity both at baseline (P < 0.05) and after 6 months (P = 0.02). Active disease was more frequent among subjects with E-6TGN < or = 125 nmol/mmol Hb at baseline (P = 0.04), but not at 6 months. AZA/6-MP drug dose was positively correlated to E-MeMP levels (r = 0.48; P < 0.001) and E-MeMP/E-6TGN ratio (r = 0.41; P = 0.002). Dose changes were positively correlated with the changes in E-MeMP levels (P = 0.01) and E-MeMP/E-6TGN ratio (P = 0.03). CONCLUSIONS: E-6TGN level was the only factor in this study related to disease activity, while there was no relationship between AZA/6-MP dose and E-6TGN levels. This finding illustrates the clinical usefulness of E-6TGN monitoring in the evaluation of treatment intensity.     

Normal thiopurine methyltransferase phenotype testing in a Crohn disease patient with azathioprine induced myelosuppression

Severe cytopenias in patients with autoimmune conditions treated with azathioprine are well-recognized. Thiopurine methyltransferase (TPMT) enzymatic activity is subject to individual and ethnic variability. Patients with low TPMT activity (poor metabolizers) are at high risk of developing severe and potentially fatal haematopoietic toxicity. Studies have shown that essentially all TPMT-deficient patients will develop haematopoietic toxicity on administration of conventional thiopurine dosages (6-mercaptopurine, azathioprine). Therefore, screening for TPMT polymorphisms in patients before prescribing thiopurine drugs has been proposed. However, despite normal in vitro enzymatic activity, cytopenia may still occur in vivo . This is the case report of an Asian patient with Crohn disease harbouring a rare TPMT mutation on DNA sequencing, who developed neutropenic sepsis and anaemia after a flare of Crohn disease. The report illustrates the importance of monitoring for cytopenia in the setting of active inflammatory disease despite prior normal phenotyping, the role of predictive pharmacogenetics and the limitations of TPMT phenotype assays that may result in misclassification of at-risk patients.     

The pharmacokinetic effect of adalimumab on thiopurine metabolism in Crohn's disease patients

Background and aimsA drug interaction between infliximab and azathioprine has previously been reported in Crohn's disease patients: the concentration of the main active thiopurine metabolites, the 6-thioguanine nucleotides (6-TGN), increased 1–3 weeks after the first infliximab infusion by 50% compared to baseline.The aim of this prospective study was to determine the effect of adalimumab on thiopurine metabolism in Crohn's disease patients, evaluated by 6-TGN and 6-methylmercaptopurine ribonucleotides (6-MMPR) concentration measurement.MethodsCrohn's disease patients on azathioprine or mercaptopurine maintenance therapy starting with concomitant adalimumab treatment were included. 6-TGN and 6-MMPR concentrations were determined before initiation of adalimumab and after 2, 4, 6 and 12 weeks of combination therapy. The activity of three essential enzymes involving thiopurine metabolism, thiopurine S-methyltransferase (TPMT), hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and inosine-triphosphate pyrophosphatase (ITPase), was evaluated at baseline and week 4. Clinical outcome was evaluated by the Crohn's disease activity index and C-reactive protein concentrations at baseline, week 4 and week 12.ResultsTwelve Crohn's disease patients were analyzed. During the follow-up period of 12 weeks the median 6-TGN and 6-MMPR concentrations did not significantly change compared to baseline. TPMT, ITPase and HGPRT enzyme activity did not change either after 4 weeks. In two patients (17%) myelotoxicity was observed within 2–4 weeks, in whom both low therapeutic 6-TGN and 6-MMPR concentrations were found.ConclusionsIn this study in Crohn's disease patients no pharmacokinetic interaction was shown between adalimumab and the conventional thiopurines, azathioprine and mercaptopurine.     

Relevance of thiopurine methyltransferase status in rheumatology patients receiving azathioprine

Azathioprine (AZA) is widely used in the management of rheumatological diseases. Despite its efficacy, AZA can often cause bone marrow suppression, notably leucopenia, which has been recorded in up to 17% of patients taking AZA for rheumatoid arthritis, though this can be considered clinically significant in about 3% overall. Severe myelosuppression, associated with abnormal AZA metabolism, is linked to the thiopurine methyltransferase (TPMT) genetic polymorphism. TPMT status can be assessed prior to AZA treatment by measuring enzyme activity or genotyping techniques. Analysis of recent data suggests that by optimizing the AZA dose on the basis of TPMT status testing (with a substantial reduction in dose for patients homozygous for mutant TPMT alleles), a reduction in drug-induced morbidity and cost savings can be made by avoiding hospitalization and rescue therapy for leucopenic events. In this article we review the pharmacogenetic and clinical implications of the TPMT polymorphism, emphasizing its relevance to rheumatologists managing diseases with AZA.     

Analysis of thiopurine methyltransferase variant alleles in childhood acute lymphoblastic leukaemia

The role of 6-mercaptopurine (6MP) in the treatment of childhood acute lymphoblastic leukaemia (ALL) is well established. However, the efficacy of 6MP is significantly influenced by inactivation by the polymorphic enzyme thiopurine methyltransferase (TPMT). In the general population 89-94% have high TPMT activity, 6-11% have intermediate activity, and approximately 0.3% have low activity. Individuals with low-activity experience severe or fatal toxicity with standard 6MP doses. Prospective identification of this group of patients might prevent this problem. Recent identification of the molecular basis for low TPMT activity enabled rapid assessment of altered 6MP metabolism by PCR methods. This study evaluated the frequency of mutant TPMT alleles in 147 children with ALL. One patient was homozygous mutant (0.7%), and 16 patients were heterozygous for variant TPMT alleles (10.9%). The majority of mutant alleles were TPMT*3A. Both the allele frequency and the pattern of TPMT mutations were similar to that observed in an adult British population. The number of weeks when 6MP therapy was administered at full dose was determined in patients on MRC UKALL X and XI. The 94 patients spent a median 11% of the maintenance period receiving no therapy as a result of haematological toxicity. There was no significant difference in the number of weeks when no therapy could be administered among patients with a wild-type or heterozygous genotype. However, the one patient with a homozygous mutant genotype had severe haematological toxicity and no therapy could be administered for 53% of the maintenance period. This study demonstrates that 11.6% of the children had variant TPMT alleles. Prospective identification of TPMT genotype may be a promising tool for decreasing excessive haematological toxicity in individuals with low activity.     

Clinical relevance of thiopurine S-methyltransferase gene polymorphisms

The therapeutic response to thiopurines may result in either severe toxic or inadequate effect based on the interindividual genetic variability. Same drug doses of various anticancer drugs cause considerable interindividual differences in the therapeutic response. Genetic factors have a major impact on effectiveness of several anticancer drugs such as mercaptopurine, 5-fluorouracil, platinum agents, and cyclophosphamide. Heredity related differences in interindividual response to thiopurine therapy represent perhaps the most compelling evidence of pharmacogenomics' usefulness in identification of patients in risk for adverse drug reactions. A number of variations in the gene for thiopurine methyltransferase (TPMT) have been associated with the low activity of this enzyme. Patients with intermediate and low activity of TPMT have a greater incidence of thiopurine toxicity. This minireview summarizes results of studies assessing the role of genetic polymorphisms in the gene encoding TPMT and their relationship to the toxicity of thiopurines.     

A comparison of molecular and enzyme-based assays for the detection of thiopurine methyltransferase mutations

S-Methylation by thiopurine methyltransferase (TPMT) is an important route of metabolism for the thiopurine drugs. About one in 300 individuals are homozygous for a TPMT mutation associated with very low enzyme activity and severe myelosuppression if treated with standard doses of drug. To validate the use of molecular genetic techniques for the detection of TPMT deficiency, we have determined red blood cell TPMT activity in 240 adult blood donors and 55 normal children. Genotype was determined by restriction fragment length analysis of polymerase chain reaction products in a cohort of 79 of the blood donors and five cases of azathioprine-induced myelosupression, and this confirmed a close relationship between genotype and phenotype. In 17 of the 24 cases in which mutations were found, DNA was also available from remission bone marrow. In one of these cases, DNA from the remission marrow sample indicated the presence of a non-mutated allele that had not been seen in the blast DNA sample obtained at presentation. These results indicate that polymerase chain reaction-based assays give reliable and robust results for the detection of TPMT deficiency, but that caution should be exercised in relying exclusively on DNA obtained from lymphoblasts in childhood leukaemia.     

Polymorphic thiopurine methyltransferase in erythrocytes is indicative of activity in leukemic blasts from children with acute lymphoblastic leukemia

The activity of thiopurine methyltransferase (TPMT) exhibits genetic polymorphism, with approximately 1 in 300 individuals inheriting TPMT deficiency as an autosomal recessive trait, and about 11% having intermediate activity (ie, heterozygotes). Patients with TPMT deficiency accumulate excessive concentrations of 6-thioguanine nucleotides (TGNs) and develop severe toxicity when treated with standard dosages of mercaptopurine. High TPMT activity has been associated with lower concentrations of TGNs, yielding a higher risk of treatment failure in children with acute lymphoblastic leukemia (ALL). As the biochemical basis of these pharmacodynamic relationships has not been fully elucidated, we investigated the variability and relationship of TPMT activity in erythrocytes and lymphoblasts from children with ALL. A 58-fold range of erythrocyte TPMT activity was found among 119 patients receiving ALL chemotherapy (0.6 to 34.9 U/mL packed erythrocytes), but relatively low intrapatient variability (coefficient of variation, 13.5%) was observed over 1 year. A 27-fold range in TPMT activity was observed in leukemic blasts obtained from 42 patients at initial diagnosis (3.3 to 88.9 U/1 x 10(9) cells). TPMT activity in leukemic blasts at diagnosis was significantly correlated with TPMT in erythrocytes before therapy (rs = .75, P < .0001, N = 13). These data document extensive interpatient variability of TPMT activity in ALL blasts and establish its linkage to polymorphic TPMT activity in erythrocytes, providing a new mechanism by which erythrocytes serve as prognostic markers of mercaptopurine metabolism and TPMT activity in children with ALL.     

Identification of thiopurine methyltransferase (TPMT) polymorphisms cannot predict myelosuppression in systemic lupus erythematosus patients taking azathioprine.

OBJECTIVE: To determine whether the presence of polymorphisms associated with reduced or absent activity of thiopurine methyltransferase (TPMT), an enzyme involved in azathioprine metabolism, can predict side-effects, particularly myelosuppression, in patients taking this drug. METHODS: The TPMT genotype was determined in 120 patients with systemic lupus erythematosus (SLE) together with 15 patients with inflammatory bowel disease (IBD) and correlated with the effects of clinical exposure to azathioprine. RESULTS: TPMT polymorphisms were detected in eight patients. Severe marrow toxicity occurred in the single homozygote identified. Azathioprine was generally well tolerated, but 11 drug-associated neutropenias were detected. In only one of the 11 cases was a TPMT polymorphism identified. CONCLUSION: Homozygous TPMT deficiency was associated with severe marrow suppression. In the majority of cases, however, TPMT genotyping prior to azathioprine therapy would not have predicted myelosuppressive events and may augment, but not replace, regular blood monitoring.     

Azathioprine and diffuse alveolar haemorrhage: the pharmacogenetics of thiopurine methyltransferase.

Current guidelines support the use of corticosteroids and azathioprine as one possible treatment strategy for idiopathic pulmonary fibrosis (IPF). However, some patients with genetic polymorphisms of thiopurine methyltransferase (TPMT) are at risk of severe azathioprine myelotoxicity. The current authors present the case of an 85-yr-old Caucasian male with IPF who developed diffuse alveolar haemorrhage as a complication of azathioprine-induced myelosuppression. Leukocyte genetic TPMT testing revealed that the patient had homozygous polymorphisms associated with the absence of TPMT activity and severe azathioprine-induced myelotoxicity. Thiopurine methyltransferase deficiency should be considered in patients who develop leukopenia early in treatment with azathiopurine, or who present with severe marrow suppression at usual doses. For centres with equipped laboratories, a dosing suggestion is provided based on thiopurine methyltransferase testing. Even with screening strategies, frequent monitoring of complete blood count and liver biochemistry should remain the mainstay of surveillance for azathioprine toxicity.     

Azathioprine and 6-mercaptopurine pharmacogenetics and metabolite monitoring in inflammatory bowel disease

The thiopurine drugs azathioprine and 6-mercaptopurine (6-MP) are well-established in the treatment of inflammatory bowel disease (IBD). However, there is a wide inter- and intra-patient variation in the concentrations of active and toxic metabolites due to their complex metabolism and genetic polymorphisms in metabolizing enzymes. Serious drug toxicity leads to cessation of therapy in 9-25% of patients, and there is failure to achieve efficacy in approximately 15% of cases. Advances in the understanding of thiopurine drug metabolism have led to new genetic and metabolite tests to help clinicians optimize thiopurine use. Thiopurine methyltransferase (TPMT) enzyme activity can predict life-threatening myelotoxicity in the one in 300 patients who are TPMT-deficient. However, myelotoxicity can also occur in the presence of normal TPMT activity so blood count monitoring should remain standard practice. TPMT testing may also aid in dose individualization. 6-Thioguanine nucleotides (6-TGN) are thought to be the predominant active metabolites of the thiopurines. 6-thioguanine nucleotide concentration is correlated with bone marrow toxicity and may also correlate with efficacy in IBD. Measurement of 6-TGN and 6-methylmercaptopurine (6-MMP) concentration is most useful in determining why a patient is not responding to a standard dose of a thiopurine drug and may help in avoiding myelosuppression. The ratio of these metabolites can help distinguish non-compliance, under-dosing, thiopurine-resistant and thiopurine-refractory disease. Some of these investigations are entering routine clinical practice but more research is required to determine their optimal use in patients with IBD.     

Human thiopurine S-methyltransferase pharmacogenetics: variant allozyme misfolding and aggresome formation

Thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs. TPMT genetic polymorphisms represent a striking example of the potential clinical value of pharmacogenetics. Subjects homozygous for TPMT*3A, the most common variant allele for low activity, an allele that encodes a protein with two changes in amino acid sequence, are at greatly increased risk for life-threatening toxicity when treated with standard doses of thiopurines. These subjects have virtually undetectable levels of TPMT protein. In this study, we tested the hypothesis that TPMT*3A might result in protein misfolding and aggregation. We observed that TPMT*3A forms aggresomes in cultured cells and that it aggregates in vitro, functional mechanisms not previously described in pharmacogenetics. Furthermore, there was a correlation among TPMT half-life values in rabbit reticulocyte lysate, aggresome formation in COS-1 cells, and protein aggregation in vitro for the three variant allozymes encoded by alleles that include the two TPMT*3A single-nucleotide polymorphisms. These observations were compatible with a common structural explanation for all of these effects, a conclusion supported by size-exclusion chromatography and CD spectroscopy. The results of these experiments provide insight into a unique pharmacogenetic mechanism by which common polymorphisms affect TPMT protein function and, as a result, therapeutic response to thiopurine drugs.     

A single point mutation leading to loss of catalytic activity in human thiopurine S-methyltransferase.

Thiopurine S-methyltransferase (TPMT; S-adenosyl-L-methionine:thiopurine S-methyltransferase, EC 2.1.1.67) activity exhibits genetic polymorphism, with approximately 0.33% of Caucasians and African-Americans inheriting TPMT deficiency as an autosomal recessive trait. To determine the molecular genetic basis for this polymorphism, we cloned the TPMT cDNA from a TPMT-deficient patient who had developed severe hematopoietic toxicity during mercaptopurine therapy. Northern blot analysis of RNA isolated from leukocytes of the deficient patient demonstrated the presence of TPMT mRNAs of comparable size to that in subjects with high TPMT activity. Sequencing of the mutant TPMT cDNA revealed a single point mutation (G238-->C), leading to an amino acid substitution at codon 80 (Ala80-->Pro). When assessed in a yeast heterologous expression system, this mutation led to a 100-fold reduction in TPMT catalytic activity relative to the wild-type cDNA, despite a comparable level of mRNA expression. A mutation-specific PCR amplification method was developed and used to detect the G238-->C mutation in genomic DNA of the propositus and her mother. This inactivating mutation in the human TPMT gene provides insights into the genetic basis for this inherited polymorphism in drug metabolism.     

Genotyping should be considered the primary choice for pre-treatment evaluation of thiopurine methyltransferase function

Background and aimsA pre-treatment determination of the thiopurine S-methyltransferase (TPMT) genotype or phenotype can identify patients at risk of developing severe adverse reactions from thiopurine treatment. The risk of misclassifying a patient might be dependent on the method used. The aim of this study was to investigate the concordance between TPMT genotyping and phenotyping.MethodsThe data consist of 7195 unselected and consecutive TPMT genotype and phenotype determinations sent to the division of Clinical Pharmacology, Linköping, Sweden. TPMT activity was measured in red blood cells (RBC) and the genotype determined by pyrosequencing for the three most common TPMT variants (TPMT *2, *3A, *3C).ResultsTPMT genotyping identified 89% as TPMT wild type (*1/*1), 10% as TPMT heterozygous and 0.5% as TMPT defective. The overall concordance between genotyping and phenotyping was 95%, while it was 96% among IBD patients (n = 4024). Genotyping would have misclassified 8% of the TPMT defectives as heterozygous as compared to 11% if only TPMT activity had been measured. 11% of the heterozygous patients had a normal TPMT activity (> 8.9 U/ml RBC) and 3% of the TPMT wild-type patients had an intermediate TPMT activity (2.5–8.9 U/ml RBC).ConclusionsThere is a risk for TPMT misclassification when only genotyping or phenotyping is used, but it is not reasonable to check both in all patients. Since TPMT genotyping is the more reliable test, especially in TPMT heterozygotes, we suggest that genotyping should be considered the primary choice for the pre-treatment evaluation of TPMT function before initiation of thiopurine therapy.     

Normal thiopurine methyltransferase levels do not eliminate 6-mercaptopurine or azathioprine toxicity in children with inflammatory bowel disease

6-mercaptopurine (6-MP) and azathioprine (AZA) are used to treat inflammatory bowel disease (IBD). Side effects include infection, leukopenia, hepatitis, and pancreatitis. The level of thiopurine methyltransferase (TPMT), which metabolizes 6-MP to 6-methylmercaptopurine, may reflect the risk of side effects. We sought to evaluate the relationship between the side effects of these medications and the TPMT level of pediatric patients with IBD. The medical records of our patients who were diagnosed with IBD and who received 6-MP or AZA were reviewed for measured TPMT levels. All red blood cell (RBC) TPMT levels were determined at the Mayo Medical Laboratories, Rochester, MN. The occurrence of leukopenia, elevated aminotransferases, and pancreatitis was evaluated. Twenty-two patients, mean age 13.7 years, received 6-MP or AZA and had TPMT levels measured. The TPMT levels ranged 10.7-27.5 U/mL RBC with a mean of 17.2 +/- 3.2 U/mL RBC. Two children had levels below the accepted norm of 13.8 U/mL RBC. One of these patients (50%) developed both elevation of aminotransferases and leukopenia. Of all, 20 children had normal levels, 3 (15.0%) exhibited side effects: hepatitis (n = 2) and leukopenia (n = 1). We conclude that side effects of 6-MP or AZA occur despite normal TPMT levels.     

The effect of allopurinol and low-dose thiopurine combination therapy on the activity of three pivotal thiopurine metabolizing enzymes: Results from a prospective pharmacological study

IntroductionThiopurine therapy is often discontinued in inflammatory bowel disease (IBD) patients. The xanthine oxidase (XO) inhibitor allopurinol has previously shown to enhance thiopurine efficacy and to prevent adverse reactions, the mechanism of this beneficial interaction is not completely clarified. The aim of this study is to observe possible effects of allopurinol and low-dose thiopurine combination therapy on the activity of three pivotal thiopurine metabolizing enzymes.MethodsA prospective study of IBD patients failing thiopurine therapy due to a skewed thiopurine metabolism was performed. Patients were treated with allopurinol and azathioprine or mercaptopurine. Xanthine oxidase, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and thiopurine S-methyl transferase (TPMT) activities, and thiopurine metabolites concentrations were measured during thiopurine monotherapy, and after 4 and 12 weeks of combination therapy.ResultsOf fifteen IBD patients, XO activity decreased from 0.18 (IQR 0.08–0.3) during thiopurine monotherapy to 0.14 (IQR 0.06–0.2) and 0.11 (IQR 0.06–0.2; p = 0.008) mU/hour/ml at 4 and 12 weeks, respectively. HGPRT activity increased from 150 (IQR 114–176) to 180 (IQR 135–213) and 204 nmol/(h × mg protein) (IQR 173–213; p = 0.013). TPMT activity seemed not to be affected. 6-Thioguanine nucleotide concentrations increased from 138 (IQR 119–188) to 235 (223–304) and to 265 pmol/8 × 10^8 (IQR 188–344), whereas 6-methyl mercaptopurine ribonucleotides concentrations decreased from 13230 (IQR 7130–17420) to 690 (IQR 378–1325) and 540 (IQR 240–790) pmol/8 × 10^8 at 4 and 12 weeks of combination therapy (both p < 0.001).ConclusionAllopurinol and thiopurine combination-therapy seems to increase HGPRT and decrease XO activity in IBD patients, which at least in part may explain the observed changes in thiopurine metabolite concentrations.     

Genotypic analysis of thiopurine S-methyltransferase in patients with Crohn's disease and severe myelosuppression during azathioprine therapy

BACKGROUND & AIMS: Myelosuppression in patients with Crohn's disease (CD) treated with azathioprine has been attributed to low activity of thiopurine S-methyltransferase (TPMT). Allelic variants of the TPMT gene responsible for changes in the enzyme activity have been characterized. We investigated the distribution of mutant alleles associated with TPMT deficiency in patients with CD and myelosuppression during azathioprine/6-mercaptopurine therapy. METHODS: Forty-one patients with CD were included. They developed leukopenia or thrombocytopenia during azathioprine or 6-mercaptopurine treatment. Polymerase chain reaction-based methods were used to search for mutations associated with TPMT deficiency. RESULTS: Four patients (10%) had 2 mutant alleles associated with TPMT deficiency, 7 (17%) had 1 mutant allele, and 30 (73%) had no known TPMT mutation. The delay between administration of the drug and occurrence of bone marrow toxicity was less than 1.5 months in the 4 patients with 2 mutant alleles, and ranged from 1 to 18 months in patients with 1 mutant allele and from 0.5 to 87 months in patients with normal genotype. CONCLUSIONS: Twenty-seven percent of patients with CD and myelosuppression during azathioprine therapy had mutant alleles of the TPMT gene associated with enzyme deficiency. Myelosuppression is more often caused by other factors. Continued monitoring of blood cell counts remains mandatory in patients treated with azathioprine.     

Adverse reactions to azathioprine cannot be predicted by thiopurine S-methyltransferase genotype in Japanese patients with inflammatory bowel disease

Background and Aims:  Azathioprine (AZA) is associated with a high frequency of adverse reactions. We examined polymorphism of the thiopurine S-methyltransferase (TPMT) gene to determine whether the TPMT genotype would be a predictive marker for the development of adverse reactions to AZA.
Methods:  The frequency of TPMT mutations was investigated in 147 Japanese inflammatory bowel disease (IBD) patients retrospectively. In these subjects, the presence of four mutant alleles (TPMT*2, *3B, *3C and *8) was determined by direct sequencing. The incidence of adverse reactions among patients carrying wild-type TPMT was investigated. The blood level of 6-thioguanine nucleotide (6-TGN) was measured and analyzed in 47 patients with wild-type TPMT. The results were analyzed in relation to the concomitant use of aminosalicylates (ASA).
Results:  Of the 147 patients, 144 (98.0%) were wild-type for TPMT (TPMT*1/*1) and three (2.0%) carried a mutant TPMT allele (TPMT*1/*3C). The incidence of adverse reactions was 33.3% (38/114) in the wild-type group. Leukopenia (WBC ≤ 3000/µL) was seen in 15.8% of the patients with wild-type TPMT. 6-TGN levels varied among 47 patients with wild-type TPMT. The blood levels of 6-TGN were significantly higher in the patients receiving concomitant ASA treatment compared with those not receiving concomitant ASA treatment ( P  = 0.0033).
Conclusion:  The frequency of TPMT gene mutations is low among Japanese IBD patients. The incidence of adverse reactions to AZA was high, even in patients carrying wild-type TPMT. It is concluded that determination of TPMT genotype may not be useful in Japanese IBD patients to predict adverse reactions to AZA.