Shortcoming in the diagnosis of TPMT deficiency in a patient with Crohn's disease using phenotyping only

Circulating Biomarkers Instead of Genotyping to Establish Metabolizer Phenotypes

Pharmacogenomics (PGx) enables personalized treatment for the prediction of drug response and to avoid adverse drug reactions. Currently, PGx mainly relies on the genetic information of absorption, distribution, metabolism, and excretion (ADME) targets such as drug-metabolizing enzymes or transporters to predict differences in the patient's phenotype. However, there is evidence that the phenotype-genotype concordance is limited. Thus, we discuss different phenotyping strategies using exogenous xenobiotics (e.g., drug cocktails) or endogenous compounds for phenotype prediction. In particular, minimally invasive approaches focusing on liquid biopsies offer great potential to preemptively determine metabolic and transport capacities. Early studies indicate that ADME phenotyping using exosomes released from the liver is reliable. In addition, pharmacometric modeling and artificial intelligence improve phenotype prediction. However, further prospective studies are needed to demonstrate the clinical utility of individualized treatment based on phenotyping strategies, not only relying on genetics. The present review summarizes current knowledge and limitations.

Molecular Profiling of Inflammatory Bowel Disease: Is It Ready for Use in Clinical Decision-Making?

Inflammatory bowel disease (IBD) is a heterogeneous disorder in terms of age at onset, clinical phenotypes, severity, disease course, and response to therapy. This underlines the need for predictive and precision medicine that can optimize diagnosis and disease management, provide more cost-effective strategies, and minimize the risk of adverse events. Ideally, we can leverage molecular profiling to predict the risk to develop IBD and disease progression. Despite substantial successes of genome-wide association studies in the identification of genetic variants affecting IBD susceptibility, molecular profiling of disease onset and progression as well as of treatment responses has lagged behind. Still, thanks to technological advances and good study designs, predicting phenotypes using genomics and transcriptomics in IBD has been rapidly evolving. In this review, we summarize the current status of prediction of disease risk, clinical course, and response to therapy based on clinical case presentations. We also discuss the potential and limitations of the currently used approaches.

Pharmacogenetics of immunosuppressants: State of the art and clinical implementation - recommendations from the French National Network of Pharmacogenetics (RNPGx)

Therapeutic drug monitoring is already widely used for immunosuppressive drugs due to their narrow therapeutic index. This article summarizes evidence reported in the literature regarding the pharmacogenetics of (i) immunosuppressive drugs used in transplantation and (ii) azathioprine used in chronic inflammatory bowel disease. The conditions of use of currently available major pharmacogenetic tests are detailed and recommendations are provided based on a scale established by the RNPGx scoring tests as "essential", "advisable" and "potentially useful". Other applications for which the level of evidence is still debated are also discussed.

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Azathioprine, 6-mercaptopurine, and 6-thioguanine are immunosuppressive drugs indicated in the prevention of graft rejection, and treatment of auto-immune disease or inflammatory bowel disease. Their anti-nucleotidic properties are also used for the treatment of acute leukaemia. Their metabolism involves thiopurine methyl transferase, which activity varies according to genetic polymorphisms. In inflammatory bowel disease patients, there is no recommended therapeutic range of intra-erythrocyte 6-thioguanine nucleotide concentration, the active metabolite. Therapeutic drug monitoring of 6-thioguanine nucleotide concentrations is however proposed in the following clinical situations: to check the observance, to try to explain therapeutic failure, to manage patients with limited thiopurine methyl transferase activity or patients treated with associated drugs that can modify thiopurine methyl transferase activity. The literature review shows that high concentrations of 6-thioguanine nucleotides and methylated metabolites are associated with an increased risk of bone marrow toxicity. In addition, high concentrations of methylated metabolite might increase the risk of hepatic toxicity. These major side-effects can be prevented by the use of pre-treatment screening for thiopurine methyl transferase activity or genotype in inflammatory bowel disease patients in order to propose an adapted dosing.

Review article: recent advances in pharmacogenetics and pharmacokinetics for safe and effective thiopurine therapy in inflammatory bowel disease

BACKGROUND

Azathioprine and mercaptopurine have a pivotal role in the treatment of inflammatory bowel disease (IBD). However, because of their complex metabolism and potential toxicities, optimal use of biomarkers to predict adverse effects and therapeutic response is paramount.

AIM

To provide a comprehensive review focused on pharmacogenetics and pharmacokinetics for safe and effective thiopurine therapy in IBD.

METHODS

A literature search up to July 2015 was performed in PubMed using a combination of relevant MeSH terms.

RESULTS

Pre-treatment thiopurine S-methyltransferase typing plus measurement of 6-tioguanine nucleotides and 6-methylmercaptopurine ribonucleotides levels during treatment have emerged with key roles in facilitating safe and effective thiopurine therapy. Optimal use of these tools has been shown to reduce the risk of adverse effects by 3-7%, and to improve efficacy by 15-30%. For the introduction of aldehyde oxidase (AOX) into clinical practice, the association between AOX activity and AZA dose requirements should be positively confirmed. Inosine triphosphatase assessment associated with adverse effects also shows promise. Nucleoside diphosphate-linked moiety X-type motif 15 variants have been shown to predict myelotoxicity on thiopurines in East Asian patients. However, the impact of assessments of xanthine oxidase, glutathione S-transferase, hypoxanthine guanine phosphoribosyltransferase and inosine monophosphate dehydrogenase appears too low to favour incorporation into clinical practice.

CONCLUSIONS

Measurement of thiopurine-related enzymes and metabolites reduces the risk of adverse effects and improves efficacy, and should be considered part of standard management. However, this approach will not predict or avoid all adverse effects, and careful clinical and laboratory monitoring of patients receiving thiopurines remains essential.

Prevalence of TPMT polymorphism in Indian patients requiring immunomodulator therapy and its clinical significance

BACKGROUND

Thiopurine methyltransferase (TPMT) enzyme plays a key role in the metabolism of azathioprine/6-mercaptopurine (6-MP). Mutations in the enzyme lead to generation of excess thioguanine, which causes suppression of various cell lineages, especially neutrophils. Data on the prevalence of TPMT polymorphism are available from Western and some Asian countries; such data from India are sparse.

AIMS

The aim of this research is to study the prevalence of TPMT mutation in Indian patients requiring immunomodulator therapy and its relation to the development of neutropenia on azathioprine therapy.

METHODS

In this retrospective study, data of all patients who underwent TPMT genotyping by PCR-RFLP and allele-specific PCR prior to immunomodulator therapy were analyzed. The frequency of on-treatment development of neutropenia (total neutrophil count <1,500 per cubic millimeters) was noted.

RESULTS

Data were available on 126 patients (mean age, 42 [SD 13.6] years; 73 men and 53 women). The disease indications included ulcerative colitis (61), Crohn's disease (43), indeterminate colitis (1), autoimmune hepatitis (16), and others (5). TPMT genotype was wild in 120 patients (95.23 %) and heterozygous in 6 patients (4.77 %); no patient had homozygous mutation. Seven of 87 patients (6.8 %) who received azathioprine developed neutropenia; blood counts normalized on cessation of the drug in all. The incidence of neutropenia in patients with wild type was 6/84 (7.14 %) and with heterozygous type 1/3 (33 %) (p = 0.5764).

CONCLUSION

Nearly 5 % of this population of patients requiring immunomodulator therapy was heterozygous carriers of the TPMT gene. Neutropenia was equally common in patients without and with the mutation.

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

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.

Difficulties and possibilities with thiopurine therapy in inflammatory bowel disease--proceedings of the first Thiopurine Task Force meeting

BACKGROUND

Thiopurines, such as azathioprine and mercaptopurine, are of pivotal importance in the treatment of inflammatory bowel disease. Although these drugs have been used for several decades, still many questions remain unanswered.

AIM

To provide an overview of clinically and scientifically challenging topics concerning thiopurine therapy in inflammatory bowel disease treatment.

METHODS

The first meeting of the Thiopurine Task Force Interest Group was held during the 2009 United European Gastroenterology Week in London (GASTRO2009). The topics of this meeting were of particular clinical and scientific interest. Additional literature was identified by performing a Pubmed search using the search terms 'inflammatory bowel disease', 'azathioprine', '6-mercaptopurine' and 'thioguanine'.

RESULTS

The following topics were discussed: therapeutic drug monitoring; the synergy of thiopurines with aminosalicylates and allopurinol; serious adverse events such as opportunistic infections, hepatotoxicity, carcinogenicity and pancreatitis; prolongation of thiopurines during clinical remission; indications for thiopurines in the postoperative setting; and the potential use of thioguanine. Specific interesting and clinically relevant topics for potential future research are provided.

CONCLUSIONS

Thiopurines remain central to inflammatory bowel disease treatment, although future studies are required to substantiate a more personalised medicine approach to their use.

Individualized Therapy: Role of Thiopurine S-Methyltransferase Protein and Genetic Variants

Thiopurine S-methyltransferase (TPMT: EC 2.1.1.67) is an enzyme that metabolizes immunosuppressive thiopurine medications, used in the treatment of autoimmune diseases, cancer and in transplantation medicine. In some individuals, TPMT enzyme activity is significantly increased or decreased compared to the normal TPMT activity level. Structural and biochemical analyses of the TPMT protein revealed the existence of certain protein variants with altered activity. It has been shown that certain TPMT gene polymorphisms exist, that define different TPMT allozymes. Decreased TPMT enzyme activity can also be a consequence of lower protein synthesis, which depends on the promoter transcription activity. Promoter polymorphisms, such as variable number of tandem repeats (VNTR), can modulate the transcription. Administering thiopurine drugs in patients with certain genetic TPMT variants leads to severe hematologic toxicity. To avoid toxicity, therapy is being modified according to the TPMT genotype (pharmacogenetics). We investigated the polymorphisms in exons and regulatory elements (promoter) of the TPMT gene which affect TPMT enzyme activity in the Serbian population. We used PCR-based methodology and sequencing in the detection of genetic variants on TPMT gene. We showed that genetic variants in exons account for 7.5% of all TPMT variants with decreased enzyme activity. The therapy for patients with these pharmacogenetic markers was modified, which contributed to the efficiency of treatment. Functional assaysin vitroshowed that the TPMT promoter activity and, therefore, the quantity of TPMT protein synthesized, depended on the architecture of VNTRs (i.e. number and type) in the promoter. Promoter of the TPMT gene specifically responds to mercaptopurine treatment of K562 cells in a VNTR-dependent manner. Study of DNA-protein interactions revealed that Sp1 and Sp3 transcription factors interact with VNTRs. Our research pointed out that the VNTR promoter region of the TPMT gene could become a new pharmacogenetic marker, clinically significant for the individualization of thiopurine therapy.

Association between inosine triphosphate pyrophosphohydrolase deficiency and azathioprine-related adverse drug reactions in the Chinese kidney transplant recipients

Azathioprine (AZA) is a thiopurine prodrug commonly used in patients with kidney transplantation. The aim of this study is to explore in patients with kidney transplantation whether AZA-related side effects can be explained by the inosine triphophate pyrophosphatase (ITPA) or thiopurine S-methyltransferase (TPMT) polymorphisms using both pheno-and genotyping. Erythrocyte ITPA and TPMT activity of 155 patients with kidney transplantation and AZA therapy was determined by HPLC. The frequencies of ITPA and TPMT polymorphisms were detected. Among 155 patients, three cases with zero activity were homozygote for 94C>A. The allele frequency of the 94C>A polymorphism was 0.12. Allele for the IVS2+21A>C mutation in the patients of this study was not found. Thirty-five cases had stopped azathioprine medication or were on reduced dose due to AZA-related side effects, including hematotoxicity (n = 12), hepatotoxicity (n = 18), gastrointestinal toxicity (n = 5, one patient developed hepatotoxicity simultaneously) and flu-like symptoms (n = 1). No statistical significant associations between ITPA 94C>A phenotype or genotype and AZA-related hematotoxicity or hepatotoxicity could be detected. However, five patients who developed gastrointestinal disturbance, two patients were homozygote for 94C>A and other three patients had 94C>A heterozygous allele. The patient who experienced flu-like symptoms were the remaining homozygote for 94C>A. This study demonstrates that ITPA activity reduced in patients with 94C>A mutation (P < 0.01). Patients with ITPA 94C>A homozygous allele are at high risk to develop AZA-related gastrointestinal toxicity and flu-like symptoms (P < 0.01). TPMT wild-type/ITPA variant (homozygote) is closely related to the AZA-induced side effects (P < 0.01).

The clinical role of genetic polymorphisms in drug-metabolizing enzymes

For most drug-metabolizing enzymes (DMEs), the functional consequences of genetic polymorphisms have been examined. Variants leading to reduced or increased enzymatic activity as compared to the wild-type alleles have been identified. This review tries to define potential fields in the therapy of major medical conditions where genotyping (or phenotyping) of genetically polymorphic DMEs might be beneficial for drug safety or therapeutic outcome. The possible application of genotyping is discussed for depression, cardiovascular diseases and thromboembolic disorders, gastric ulcer, malignant diseases and tuberculosis. Some drugs used for relief of these ailments are metabolized with participation of genetically polymorphic DMEs including CYP2D6, CYP2C9, CYP2C19, thiopurine-S-methyltransferase, dihydropyrimidine dehydrogenase, uridine diphosphate glucuronosyltransferase and N-acetyltransferase type 2. Current evidence suggests that taking genetically determined metabolic capacities of DMEs into account has the potential to improve individual risk/benefit relationship. However, more prospective studies with clinical endpoints are needed before the paradigm of 'personalized medicine' based on DME variants can be established.

Assessment of thiopurine methyltransferase enzyme activity is superior to genotype in predicting myelosuppression following azathioprine therapy in patients with inflammatory bowel disease

BACKGROUND

Myelosuppression occurs in 2-7% of inflammatory bowel disease (IBD) patients treated with azathioprine, and can be associated with reduced activity of thiopurine methyltransferase (TPMT) in some patients. It has been proposed that pretreatment assessment of TPMT status reduces the incidence of toxicity and is cost-effective.

AIMS

To determine if screening for TPMT status predicts side-effects to azathioprine in patients with IBD and to ascertain whether screening by TPMT enzyme activity or genotype is superior.

METHODS

Sequential IBD patients were identified and azathioprine tolerance recorded. Blood was collected for measurement of TPMT activity and TPMT*3C, TPMT*3A and TPMT*2 genotypes.

RESULTS

Of 130 patients, 25% stopped azathioprine because of toxicity. Four patients experienced severe myelosuppression (WCC < 2). Eleven of 17 patients with reduced TPMT activity were heterozygotes, including one patient with marked TPMT deficiency who experienced severe myelosuppression. There was no association between intermediate TPMT deficiency and any side-effect.

CONCLUSIONS

Moderate reduction of TPMT activity in heterozygotes was not associated with toxicity, but very low TPMT activity caused severe myelosuppression in one patient. This would have been predicted by measuring TPMT activity but not by genotyping. Measurement of TPMT activity may therefore be superior to genotype in predicting severe myelosuppression.

[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.

Monitoring of Thiopurine Methyltransferase Activity in Postsurgical Patients With Crohn's Disease During 1 Year of Treatment With Azathioprine or Mesalazine

Thiopurine methyltransferase (TPMT) activity determines biotransformation of azathioprine and, thereby, drug efficacy and safety. Evaluation of a possible long-term effect of mesalazine or azathioprine on TPMT activity is of particular clinical importance because both drugs can to be given for several years in inflammatory bowel disease. Monitoring of TPMT activity and three thiopurine metabolites was performed prospectively during a 1 year postoperative period in 21 patients with Crohn's disease randomly assigned to azathioprine (2.0-2.5 mg/kg per day) or mesalazine (4 g/day). TPMT activity did not change significantly within each treatment group during 52 weeks. At any study visit, TPMT activity was not different between 13 patients on azathioprine and eight patients on mesalazine. Concentrations of 6-thioguanine nucleotides (6-TGN, active moiety of azathioprine) and 6-methyl-mercaptopurine ribonucleotides (6-MMPR) did not alter significantly during the observation period, except for a slight decrease in 6-TGN levels when comparing the first with the last visit. In this first report of serial monitoring of 6-methyl-thioguanine nucleotides (6-MTGN) in patients with inflammatory bowel disease taking azathioprine, high levels of 6-TGN were correlated with high levels of 6-MTGN, with the mean 6-TGN:6-MTGN ratio being 2.4. In a well-standardized clinical setting of inflammatory bowel disease, neither mesalazine nor azathioprine significantly affected TPMT activity during a whole year of treatment.

Review article: thiopurines in inflammatory bowel disease

BACKGROUND

In the past 10-20 years, knowledge of both thiopurine pharmacology and -pharmacogenetics has been extended dramatically and used to develop new strategies to improve efficacy and reduce toxicity.

AIM

To review thiopurine efficacy, toxicity, pharmacology, pharmacogenetics, interactions in patients with inflammatory bowel disease. Special attention was paid to new strategies for optimization of pharmacotherapy.

METHODS

To collect relevant scientific articles, a Pubmed search was performed from 1966 through January 2006 with the following key words (MeSH terms preferentially) in multiple combinations: 'azathioprine', '6-mercaptopurine', '6-MP', '6-thioguanine', '6-TG', 'thiopurine(s)', 'metabolites', 'level(s)', 'TDM', 'TMPT', 'ITPA', 'genotype(s)', 'phenotype(s)', 'inflammatory bowel disease', 'Crohn('s) disease', 'ulcerative colitis'.

RESULTS

Strategies for optimization of pharmacotherapy include therapeutic drug monitoring of thiopurine metabolites, geno- or phenotyping crucial enzymes in thiopurine metabolism like thiopurine S-methyltransferase and inosine triphosphate pyrophosphatase, and the use of thioguanine as such.

CONCLUSIONS

Thiopurine S-methyltransferase genotyping and therapeutic drug monitoring are useful instruments for individualizing thiopurine pharmacotherapy of inflammatory bowel disease. Inosine triphosphate pyrophosphatase genotyping may be helpful in case of unexplainable myelotoxicity. In case of azathioprine- or mercaptopurine-intolerance, thioguanine seems a promising alternative. However, more knowledge needs to be gathered about its potential hepatotoxicity.

Pharmacogenetics in inflammatory bowel disease

Pharmacogenetics is the study of the association between variability in drug response and (or) drug toxicity and polymorphisms in genes. The goal of this field of science is to adapt drugs to a patient's specific genetic background and therefore make them more efficacious and safe. In this article we describe the variants in genes that influence either the efficacy or toxicity of common drugs used in the treatment of inflammatory bowel diseases (IBD), ulcerative colitis (UC), and Crohn's disease (CD) including sulfasalazine and mesalazine, azathioprine (AZA) and 6-mercaptopurine (6-MP), methotrexate (MTX), glucocorticosteroids (CSs) and infliximab. Furthermore, difficulties with pharmacogenetic studies in general and more specifically in IBD are described. Although pharmacogenetics is a promising field that already contributed to a better understanding of some of the underlying mechanisms of action of drugs used in IBD, the only discovery translated until now into daily practice is the relation between thiopurine S-methyltransferase (TPMT) gene polymorphisms and hematological toxicity of thiopurine treatment. In the future it is necessary to organize studies in well characterized patient cohorts who have been uniformly treated and systematically evaluated in order to quantitate drug response more objectively. An effort should be made to collect genomic DNA from all patients enrolled in clinical drug trials after appropriate informed consent for pharmacogenetic studies.

Pharmacoeconomic analyses of azathioprine, methotrexate and prospective pharmacogenetic testing for the management of inflammatory bowel disease

OBJECTIVES

To compare the cost effectiveness of azathioprine (AZA), methotrexate (MTX) and no immunosuppression for maintaining remission of moderate to severe inflammatory bowel disease (IBD) in New Zealand Caucasians, and to determine whether prospective testing for poor metabolisers of AZA by genotype or phenotype is cost effective.

METHODS

Pharmacoeconomic models were developed to compare treatment costs and effects (QALYs) in theoretical populations of 1,000 IBD patients over a 1-year period. Efficacy and tolerability profiles for AZA and MTX were taken from the literature. The costs (year 2004 values) of the drugs and treatment of adverse effects were estimated from New Zealand drug and service costs. Representations of the patients' health-related quality of life (HR-QOL) were obtained from clinicians via the EQ-5D health state classification system and valued using the New Zealand EQ-5D social tariff. The effects of genotyping or phenotyping a population for thiopurine methyltransferase (TPMT) status were compared using the prevalence of TPMT deficiency in Caucasians, the relative risks of neutropenia and the associated costs.

RESULTS

Net cost savings (vs no immunosuppressant treatment) of approximately 2.5 million and 1 million New Zealand dollars were realised for AZA and MTX, respectively, for the theoretical 1,000 patients, and AZA generated 877 QALYs compared with 633 for MTX. Phenotype and genotype testing generated net cost savings (vs no testing) of 120,000 and 11,000 New Zealand dollars, respectively. Savings related to phenotype tests were greater because of the lower assay costs of phenotype testing and a greater likelihood of pre-empting neutropenia.

CONCLUSION

Our model suggests that both MTX and AZA may generate significant net cost savings and benefits for patients with IBD in New Zealand, with AZA likely to be more cost effective than MTX. Prospective testing for poor metabolisers of AZA may also be cost effective, with phenotype testing likely to be more cost effective than genotype testing.

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.

Comprehensive analysis of thiopurine S-methyltransferase phenotype-genotype correlation in a large population of German-Caucasians and identification of novel TPMT variants

The thiopurine S-methyltransferase (TPMT) genetic polymorphism has a significant clinical impact on the toxicity of thiopurine drugs. It has been proposed that the identification of patients who are at high risk for developing toxicity on the basis of genotyping could be used to individualize drug treatment. In the present study, phenotype-genotype correlation of 1214 healthy blood donors was investigated to determine the accuracy of genotyping for correct prediction of different TPMT phenotypes. In addition, the influence of gender, age, nicotine and caffeine intake was examined. TPMT red blood cell activity was measured in all samples and genotype was determined for the TPMT alleles *2 and *3. Discordant cases between phenotype and genotype were systematically sequenced. A clearly defined trimodal frequency distribution of TPMT activity was found with 0.6% deficient, 9.9% intermediate and 89.5% normal to high methylators. The frequencies of the mutant alleles were 4.4% (*3A), 0.4% (*3C) and 0.2% (*2). All seven TPMT deficient subjects were homozygous or compound heterozygous carriers for these alleles. In 17 individuals with intermediate TPMT activity discordant to TPMT genotype, four novel variants were identified leading to amino acid changes (K119T, Q42E, R163H, G71R). Taking these new variants into consideration, the overall concordance rate between TPMT genetics and phenotypes was 98.4%. Specificity, sensitivity and the positive and negative predictive power of the genotyping test were estimated to be higher than 90%. Thus, the results of this study provide a solid basis to predict TPMT phenotype in a Northern European Caucasian population by molecular diagnostics.

Thiopurine methyltransferase in acute lymphoblastic leukaemia: biochemical and molecular biological aspects

Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme, catalysing S-methylation of aromatic and heterocyclic sulphhydryl compounds. TPMT activities and genotypes have been determined in patients with acute lymphoblastic leukaemia (ALL) and in control children. Median red blood cell (RBC) TPMT activity in ALL patients at diagnosis was significantly lower than in controls (median 11.5 pmol/10(7) RBC*hr; range 1.7-30.7; n = 191 vs. 14.6 pmol/10(7) RBC*hr; range 1.6-50.7; n = 140). This reduction of TPMT activity in ALL patients was not due to differences in the frequency of mutations in the TPMT gene. In concordance with other authors, we found a higher TPMT activity during maintenance treatment with 6-mercaptopurine (6MP) than at diagnosis and in controls. However, we observed that TPMT activity was already significantly increased after the induction therapy, before the patients received 6MP (median 17.5; range 3.9-40.3 pmol/10(7) RBC*hr; n = 139). In vitro experiments indicate that the early increase of TPMT activity during treatment may be explained by the use of antifolates, e.g., methotrexate and trimethoprim.

Thioguanine-nucleotides do not predict efficacy of tioguanine in Crohn's disease

BACKGROUND

6-Thioguanine-nucleotides seem to be the active metabolites of thiopurine therapy, and their monitoring has been considered a useful tool for optimizing response in inflammatory bowel diseases. Tioguanine (thioguanine) therapy results in much higher levels of 6-thioguanine-nucleotide levels when compared with azathioprine or mercaptopurine.

AIM

To elucidate the influence of 6-thioguanine-nucleotide and methylated 6-thioguanine-nucleotide levels under tioguanine on efficacy and toxicity in Crohn's disease.

METHODS

6-Thioguanine-nucleotide and methylated 6-tioguanine-nucleotide levels were measured regularly in 26 Crohn's disease patients treated with tioguanine. Nucleotide levels were related to efficacy and toxicity.

RESULTS

6-Thioguanine-nucleotide levels rose very high [median 1241 pmol/8 x 10(8) red blood cells (range 313-1853)]. Methylated 6-thioguanine-nucleotide levels were detected in all patients [491 pmol/8 x 10(8) red blood cells (154-1775)]. 6-Thioguanine-nucleotide and methylated 6-thioguanine-nucleotide concentrations correlated significantly (r = 0.7, P < 0.0001). Nucleotide levels from patients achieving remission (n = 14) did not differ significantly from non-remitters (n = 12) [6-thioguanine-nucleotide: 1077 (599-2160) vs. 1210 (534-4665); methylated 6-thioguanine-nucleotide: 510 (214-1222) vs. 421 (145-1284)]. One patient with intermediate thiopurine S-methyltransferase activity experienced bone marrow toxicity upon dose escalation parallel with excessively high thioguanine-nucleotide levels.

CONCLUSIONS

6-Thioguanine-nucleotide as well as methylated 6-thioguanine-nucleotide levels under tioguanine therapy were not related to efficacy. This suggests that monitoring of 6-thioguanine-nucleotide levels is not a useful tool to predict response to thiopurines.

RT-PCR permits simultaneous genotyping of thiopurine S-methyltransferase allelic variants by multiplex induced heteroduplex analysis

Thiopurine-based drugs are a widely prescribed group of medications. Their tolerance and effectiveness is dependent on an individual's ability to metabolize these compounds. An essential enzyme for the metabolism of these drugs is thiopurine S-methyltransferase (TPMT), whose activity is subject to genetic variation. Genotyping of the most frequent allelic variants in TPMT affords an extremely accurate prediction of the three clinical phenotypes: high, intermediate, and low enzyme activity. One constraint of most genotyping methods is the inability to demonstrate physical linkage between two sequence variants that occur in different exons, e.g., c.460G>A and c.719A>G, which give rise to TPMT*3, the most common defective allele in Caucasian populations. Using mRNA/cDNA as a template enables analysis of both sequence variants in a single assay. This approach could be applicable to other genes where allelic variation (in-cis and in-trans) is due to alterations in different exons. Induced heteroduplex generator analysis has previously been shown to discriminate in-cis and has also been suitable for multiplexing. In this method we have exploited both these features and for the first time have applied them to a RT-PCR analysis. The primary reagent developed allows unequivocal resolution of TPMT*3A and the alleles carrying the c.719A>G allelic variant, TPMT*3C, as well as the silent alteration c.474T>C. The TPMT*3B variant has not been observed. A secondary reagent, which can be multiplexed, identifies the TPMT*2 allele.

Determination of Thiopurine Methyltransferase Phenotype in Isolated Human Erythrocytes Using a New Simple Nonradioactive HPLC Method

Genetic polymorphism of the S-methylation pathway catalyzed by thiopurine methyltransferase (TPMT) is responsible for variation in the metabolism, toxicity, and therapeutic efficacy of thiopurine drugs. This paper describe a new simple, nonradioactive HPLC method for determination of TPMT activity in isolated erythrocytes (Ery), based on the conversion of 6-mercaptopurine (pH 7.5, 37 degrees C) to 6-methylmercaptopurine (6-MMP) using S-adenosyl-l-methionine as methyl donor. The incubation step was stopped by a mixture of trichloroacetic acid/acetonitrile containing the internal standard 4-aminoacetophenone. 6-MMP was quantified by absorbance at 290 nm after chromatographic separation on a Zorbax SB-Phenyl column (5 microm, 4.6 x 250 mm) using mobile phases (flow rate 1.1 mL/min) consisting of acetonitrile, phosphate buffer pH 3.0, triethylamine, and dithiothreitol. The assay was linear up to 50 nmol/(mL Ery. h), and the detection limit was 0.3 nmol/(mL Ery. h). The extraction efficiency of 6-MMP was 95-103% (n = 3), and its analytic recovery ranged between 98.3% and 101.8% (n = 12). The within-day imprecision using pooled human erythrocytes (n = 12) was 4.4% at a TPMT activity of 14.3 nmol/(mL Ery.h) and 4.9% at 6.5 nmol/(mL Ery.h). The between-day imprecision (n = 12) was 6.8% and 7.5% nmol/(mL Ery.h), respectively. A very good agreement was found between TPMT activity determined with this method (y) and a widely used radiochemical procedure (x) (r = 0.94; n = 130; y = 0.502 + 0.946x; P < 0.05). Genotype analysis of all individuals with TPMT activity under 12.5 nmol/(mL Ery.h) revealed a genotype/phenotype concordance of 86%. The new HPLC method for determination of TPMT activity in Ery is a simple, rapid, and reliable nonradioactive procedure that can be successfully used for both research and routine clinical analysis.

Thiopurine S-methyltransferase (TPMT) genotype does not predict adverse drug reactions to thiopurine drugs in patients with inflammatory bowel disease

BACKGROUND

Azathioprine and mercaptopurine (MP) are well established treatments for inflammatory bowel disease but they have severe adverse effects that prevent their use in some patients. The likelihood and type of adverse effect may relate to thiopurine methyltransferase (TPMT) enzyme activity and genotype.

AIM

To compare the TPMT genotype frequencies in patients with inflammatory bowel disease who have had severe adverse effects to those who tolerate azathioprine or MP (controls).

METHODS

Patients with inflammatory bowel disease who had been treated with azathioprine or MP in Christchurch between 1996 and 2002 were identified. Patients with adverse effects, and controls, were invited to provide a peripheral blood sample for analysis of TPMT genotype. The genotype frequencies were then compared between the two groups.

RESULTS

Fifty-six patients were identified with adverse effects requiring cessation of therapy, of which 50 were genotyped. Reactions included allergic-type (25%), hepatitis (33%), nausea/vomiting (14%), bone marrow suppression (10%), pancreatitis (6%) and other (12%). Five of 50 patients with reactions had TPMT genotype *1/*3, one had *3/*3, and the rest had the wildtype genotype *1/*1. The patient with genotype *3/*3 had severe pancytopenia requiring hospitalization. Three of 50 controls had the *1/*3 genotype and the rest were *1/*1.

CONCLUSIONS

The TPMT allele frequency in our population with inflammatory bowel disease is similar to that reported elsewhere. There was a slight trend for more frequent TPMT mutations in the patients with adverse reactions, but this was not statistically significant. Most patients with reactions did not have gene mutations.

Remission maintenance by tioguanine in chronic active Crohn's disease

BACKGROUND

Tioguanine may offer an alternative for immunosuppression in chronic active Crohn's disease. Recently, we have shown that tioguanine is effective in inducing rapid remission.

AIM

To evaluate the role of tioguanine in the maintenance of remission in chronic active Crohn's disease.

METHODS

A follow-up study was performed to investigate the long-term efficacy and safety of and tolerance to tioguanine in chronic active Crohn's disease. Sixteen patients who had successfully received 6-tioguanine for remission induction were enrolled. The reasons for immunosuppressive therapy were steroid dependence (n = 10), steroid refractoriness (n = 6) and intolerance (n = 6) or refractoriness (n = 1) to azathioprine. After remission induction therapy for 6 months, patients were treated for another 6 months with a daily dose of 20-40 mg tioguanine. Primary outcomes were remission (Crohn's disease activity index < 150) and complete steroid reduction in steroid-dependent patients at 12 months. Laboratory controls of white blood count and liver enzymes, as well as erythrocyte tioguanine nucleotide levels, were performed regularly.

RESULTS

After 12 months of treatment, 14 of 16 (88%) patients were in remission, and 12 of these were completely free of systemic steroids. Adverse events during maintenance therapy included photosensitivity (one patient), minor viral infections (one), headache (four) and mild alopecia (one). One patient developed elevated liver enzymes, splenomegaly and thrombocytopenia, indicative of nodular regenerative hyperplasia of the liver.

CONCLUSIONS

In responders to tioguanine, the drug appears to be very effective in maintaining remission of chronic active Crohn's disease. Unfortunately, long-term hepatotoxicity seems to be an unpredictable and potentially severe adverse drug reaction. Therefore, to date, tioguanine cannot be recommended for general use outside clinical trials.

6-thioguanine--efficacy and safety in chronic active Crohn's disease

BACKGROUND

: Azathioprine and mercaptopurine are commonly used in chronic active Crohn's disease. They share the disadvantage of a delayed onset of action and potentially serious side-effects, and are metabolized to thioguanine nucleotides which are thought to be the active metabolites. The direct use of 6-thioguanine may offer a more rapid and safer alternative. We conducted an open prospective study to investigate the efficacy and safety of 6-thioguanine in chronic active Crohn's disease.

METHODS

: Thirty-seven patients with chronic active Crohn's disease and a Crohn's disease activity index of > 150 were enrolled in this study. Inclusion criteria were steroid dependence (n = 19), steroid refractoriness (n = 9) and/or intolerance (n = 16) or refractoriness (n = 6) to azathioprine. Patients were treated with 40 mg/day of 6-thioguanine for 24 weeks; a dose escalation to 80 mg was allowed at week 12. Remission was defined as a Crohn's disease activity index of < 150 associated with a decrease of > 70 points; response was defined as a decrease of > 70 points in the Crohn's disease activity index.

RESULTS

: In the intention-to-treat analysis, 13 of 37 patients achieved remission (35%). Twelve of these 13 patients achieved remission after 4 weeks. Fifty-seven per cent of patients (21/37) achieved a response. The mean Crohn's disease activity index decreased from 284 +/- 74 to 153 +/- 101. 6-Thioguanine was more effective in azathioprine-intolerant than in azathioprine-refractory patients. Twelve of 16 patients intolerant to azathioprine tolerated 6-thioguanine. Adverse events included phototoxicity, pancreatitis, headache, nausea, alopecia, arthralgia, minor infections and reversible elevation of transaminases. Six patients required discontinuation of medication, two because of leucopenia.

CONCLUSIONS

: In this patient group with chronic active Crohn's disease, 6-thioguanine appeared to be effective with acceptable short-term toxicity, but long-term controlled trials are clearly needed to further define its role.

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.