Thiopurine methyltransferase genotype-phenotype discordance and thiopurine active metabolite formation in childhood acute lymphoblastic leukaemia

The evaluation of the impact of NUDT15 variants on thiopurine metabolism in Japanese children with acute lymphoblastic leukemia

PURPOSE

This study aimed to evaluate the impact of Nudix hydrolase 15 (NUDT15) variants on thiopurine metabolites, DNA-incorporated thioguanine nucleotides (DNA-TG), erythrocyte thioguanine nucleotides (Ery-TGNs) and methyl mercaptopurine nucleotide (Ery-MMPN) levels, and the association among the levels of these 6-MP metabolites in Japanese children with acute lymphoblastic leukemia (ALL).

METHODS

DNA-TG, Ery-TGNs, and Ery-MMPN levels were measured in 20 Japanese patients with childhood ALL (171 sampling points) on consecutive clinical visits, using liquid chromatography with tandem mass spectrometry. NUDT15 was genotyped using Sanger sequencing.

RESULTS

Three NUDT15 intermediate metabolizers (IM, *1/*2 or *1/*3) and two poor metabolizers (PM, *3/*3) were identified. DNA-TG/dose was significantly higher in PM than in normal metabolizers (NM). Intra-patient coefficients of variation (CV) of DNA-TG levels were similar in NUDT15 genotypes, and inter-patient CV was higher in IM and PM than in NM. The DNA-TG/Ery-TGNs ratio was higher in IM and PM than in NM (p < 0.01). The ranges of DNA-TG/dose and DNA-TG/Ery-TGNs ratio were not different within NUDT15 phenotypes. In NUDT15 NM, patients with high Ery-TGNs/dose showed high DNA-TG/dose. Absolute lymphocyte count was significantly correlated with DNA-TG, Ery-TGNs, and Ery-MMPN levels (p < 0.001). White blood cell counts were significantly correlated with Ery-TGNs levels (p < 0.02), and the levels of aspartate and alanine aminotransferases were significantly correlated with Ery-MMPN levels (p < 0.001).

CONCLUSION

NUDT15 phenotype is a strong factor for thiopurine trialability in Japanese children with ALL. Ery-TGNs levels may associate with difference of individual response.

British Society of Gastroenterology guidelines for diagnosis and management of autoimmune hepatitis

Autoimmune hepatitis (AIH) is a chronic inflammatory liver disease which, if untreated, often leads to cirrhosis, liver failure and death. The last British Society of Gastroenterology (BSG) guideline for the management of AIH was published in 2011. Since then, our understanding of AIH has advanced in many areas. This update to the previous guideline was commissioned by the BSG and developed by a multidisciplinary group. The aim of this guideline is to review and summarise the current evidence, in order to inform and guide diagnosis and management of patients with AIH and its variant syndromes. The main focus is on AIH in adults, but the guidelines should also be relevant to older children and adolescents.

TPMT and NUDT15 genotyping, TPMT enzyme activity and metabolite determination for thiopurines therapy: a reference laboratory experience

AIM

To share the experience of a US national reference laboratory, offering genotyping for TPMT and NUDT15, TPMT enzyme phenotyping and detection of thiopurine metabolites.

METHODS

Retrospective review of archived datasets related to thiopurines drug therapy including patients' data that underwent TPMT and NUDT15 genotyping, and smaller data sets where genotyping was performed with TPMT enzyme levels (phenotyping) +/- therapeutic drug monitoring (TDM).

RESULTS

Thirteen percent of patients had variants in one or both genes tested. Testing for NUDT15 revealed 3.9% additional patients requiring thiopurines dosing recommendations. A correlation between TPMT enzyme activity and TPMT polymorphisms (odds ratio OD = 71.41, p-value <0.001) and between older age and higher enzyme levels (OD = 0.98, p-value = 0.002) was identified. No correlation between sex and TPMT enzyme levels, nor between TPMT genotyping and the level of thiopurine metabolites was found.

CONCLUSION

Adding NUDT15 to TPMT genotyping, identified additional 3.9% patients to benefit from thiopurine dose modifications. A significant correlation between genetic variants in TPMT and TPMT enzyme levels and between age and enzyme levels was established, while no correlation was identified between sex and enzyme levels nor between TPMT variation and thiopurine metabolites. Providers rely more significantly on genotyping only approach, rather than genotyping and phenotyping.

Competing Benefits and Competing Hazards: The Benefit to Harm Balance in Individual Patients in Rational Therapeutics

For any therapeutic intervention in an individual, there is a balance between the potential benefits and the possible harms. The extent to which the benefits are desirable in a given condition depends on the efficacy of the intervention, the chance of obtaining it and the seriousness and intensity of the condition. The extent to which the harms are undesirable depends on the nature of the hazard that can lead to harm, the chance that the harm will occur and its seriousness and intensity. Rational therapeutic decisions require clinicians to consider competing courses of action, with potential benefits of different desirability and potential harms of different undesirability. They also have a duty to explain to the patient, for the contemplated interventions, both the possible benefits and the potential harms that the patient may consider significant. In an individual patient, it is necessary to consider (a) the probabilities of benefit from both intervention and non-intervention and (b) the probabilities of harm from both intervention and non-intervention. However, there are several potential problems. Here, we consider how failure to distinguish maximum benefits from probable benefits, or hazards (potential harms) from probable harms, and failure to consider all the competing probabilities may lead to imperfect therapeutic decisions. We also briefly discuss methods to assess the benefit to harm balance.

Transcriptome analysis reveals involvement of thiopurine S-methyltransferase in oxidation-reduction processes

Thiopurine S-methyltransferase (TPMT) is an important enzyme involved in the deactivation of thiopurines and represents a major determinant of thiopurine-related toxicities. Despite its well-known importance in thiopurine metabolism, the understanding of its endogenous role is lacking. In the present study, we aimed to gain insight into the molecular processes involving TPMT by applying a data fusion approach to analyze whole-genome expression data. The RNA profiling was done on whole blood samples from 1017 adult male and female donors to the Estonian biobank using Illumina HTv3 arrays. Our results suggest that TPMT is closely related to genes involved in oxidoreductive processes. The in vitro experiments on different cell models confirmed that TPMT influences redox capacity of the cell by altering S-adenosylmethionine (SAM) consumption and consequently glutathione (GSH) synthesis. Furthermore, by comparing gene networks of subgroups of individuals, we identified genes, which could have a role in regulating TPMT activity. The biological relevance of identified genes and pathways will have to be further evaluated in molecular studies.

Adherence to Oral Chemotherapy in Acute Lymphoblastic Leukemia during Maintenance Therapy in Children, Adolescents, and Young Adults: A Systematic Review

Acute lymphoblastic leukemia (ALL) is the most common malignancy in children and young adults. Treatment is long and involves 2-3 years of a prolonged maintenance phase composed of oral chemotherapies. Adherence to these medications is critical to achieving good outcomes. However, adherence is difficult to determine, as there is currently no consensus on measures of adherence or criteria to determine nonadherence. Furthermore, there have been few studies in pediatric B-ALL describing factors associated with nonadherence. Thus, we performed a systematic review of literature on oral chemotherapy adherence during maintenance therapy in ALL following PRISMA guidelines. Published studies demonstrated various objective and subjective methods of assessing adherence without generalizable definitions of nonadherence. However, the results of these studies suggested that nonadherence to oral maintenance chemotherapy was associated with increased risk of relapse. Future studies of B-ALL therapy should utilize a uniform assessment of adherence and definitions of nonadherence to better determine the impact of nonadherence on B-ALL outcomes and identify predictors of nonadherence that could yield targets for adherence improving interventions.

A bioinformatics approach to the identification of novel deleterious mutations of human TPMT through validated screening and molecular dynamics

Polymorphisms of Thiopurine S-methyltransferase (TPMT) are known to be associated with leukemia, inflammatory bowel diseases, and more. The objective of the present study was to identify novel deleterious missense SNPs of TPMT through a comprehensive in silico protocol. The initial SNP screening protocol used to identify deleterious SNPs from the pool of all TPMT SNPs in the dbSNP database yielded an accuracy of 83.33% in identifying extremely dangerous variants. Five novel deleterious missense SNPs (W33G, W78R, V89E, W150G, and L182P) of TPMT were identified through the aforementioned screening protocol. These 5 SNPs were then subjected to conservation analysis, interaction analysis, oncogenic and phenotypic analysis, structural analysis, PTM analysis, and molecular dynamics simulations (MDS) analysis to further assess and analyze their deleterious nature. Oncogenic analysis revealed that all five SNPs are oncogenic. MDS analysis revealed that all SNPs are deleterious due to the alterations they cause in the binding energy of the wild-type protein. Plasticity-induced instability caused by most of the mutations as indicated by the MDS results has been hypothesized to be the reason for this alteration. While in vivo or in vitro protocols are more conclusive, they are often more challenging and expensive. Hence, future research endeavors targeted at TPMT polymorphisms and/or their consequences in relevant disease progressions or treatments, through in vitro or in vivo means can give a higher priority to these SNPs rather than considering the massive pool of all SNPs of TPMT.

Pharmacogenomic testing in paediatrics: clinical implementation strategies

Pharmacogenomics (PGx) relates to the study of genetic factors determining variability in drug response. Implementing PGx testing in paediatric patients can enhance drug safety, helping to improve drug efficacy or reduce the risk of toxicity. Despite its clinical relevance, the implementation of PGx testing in paediatric practice to date has been variable and limited.

As with most paediatric pharmacological studies, there are well‐recognised barriers to obtaining high‐quality PGx evidence, particularly when patient numbers may be small, and off‐label or unlicensed prescribing remains widespread. Furthermore, trials enrolling small numbers of children can rarely, in isolation, provide sufficient PGx evidence to change clinical practice, so extrapolation from larger PGx studies in adult patients, where scientifically sound, is essential.

This review paper discusses the relevance of PGx to paediatrics and considers implementation strategies from a child health perspective. Examples are provided from Canada, the Netherlands and the UK, with consideration of the different healthcare systems and their distinct approaches to implementation, followed by future recommendations based on these cumulative experiences.

Improving the evidence base demonstrating the clinical utility and cost‐effectiveness of paediatric PGx testing will be critical to drive implementation forwards. International, interdisciplinary collaborations will enhance paediatric data collation, interpretation and evidence curation, while also supporting dedicated paediatric PGx educational initiatives. PGx consortia and paediatric clinical research networks will continue to play a central role in the streamlined development of effective PGx implementation strategies to help optimise paediatric pharmacotherapy.

Biomarkers for gastrointestinal adverse events related to thiopurine therapy

Thiopurines are immunomodulators used in the treatment of acute lymphoblastic leukemia and inflammatory bowel diseases. Adverse reactions to these agents are one of the main causes of treatment discontinuation or interruption. Myelosuppression is the most frequent adverse effect; however, approximately 5%-20% of patients develop gastrointestinal toxicity. The identification of biomarkers able to prevent and/or monitor these adverse reactions would be useful for clinicians for the proactive management of long-term thiopurine therapy. In this editorial, we discuss evidence supporting the use of PACSIN2, RAC1, and ITPA genes, in addition to TPMT and NUDT15, as possible biomarkers for thiopurine-related gastrointestinal toxicity.

Thiopurine pharmacogenomics and pregnancy in inflammatory bowel disease

The thiopurine drugs azathioprine and 6-mercaptopurine are widely used for the maintenance of clinical remission in steroid-dependent inflammatory bowel disease (IBD). Thiopurines are recommended to be continued throughout pregnancy in IBD patients, but conclusive safety data in pregnant patients remain still insufficient. On the other hand, a strong association between a genetic variant of nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15 p.Arg139Cys) and thiopurine-induced myelotoxicity has been identified. Pharmacokinetic studies have revealed that thiopurine metabolism is altered in pregnant IBD patients and suggested that the fetus may be exposed to the active-thiopurine metabolite, 6-thioguaninetriphosphate, in the uterus. A recent study using knock-in mice harboring the p.Arg138Cys mutation which corresponds to human p.Arg139Cys showed that oral administration of 6-MP at clinical dose induces a severe toxic effect on the fetus harboring the homozygous or heterozygous risk allele. This suggests that NUDT15 genotyping may be required in both women with IBD who are planning pregnancy (or pregnant) and their partner to avoid adverse outcomes for their infant. The risk to the fetus due to maternal thiopurine use is minimal but there are some concerns that are yet to be clarified. In particular, a pharmacogenomic approach to the fetus is considered necessary.

Relationship Between Thiopurine S-Methyltransferase Genotype/Phenotype and 6-Thioguanine Nucleotide Levels in 316 Patients With Inflammatory Bowel Disease on 6-Thioguanine

BACKGROUND

In inflammatory bowel disease (IBD), conventional thiopurine users cease treatment in 60% of cases within 5 years, mostly because of adverse events or nonresponse. In this study, the authors aimed to investigate the role of 6-thioguanine nucleotide (TGN) measurements, geno/phenotyping of thiopurine S-methyltransferase (TPMT), and their mutual relationship with TG therapy in IBD.

METHODS

An international retrospective, multicenter cohort study was performed at 4 centers in the Netherlands (Máxima Medical Centre) and the United Kingdom (Guy's and St. Thomas' Hospital, Queen Elizabeth Hospital, and East Surrey Hospital).

RESULTS

Overall, 526 6-TGN measurements were performed in 316 patients with IBD. The median daily dosage of TG was 20 mg/d (range 10-40 mg/d), and the median duration of TG use was 21.1 months (SD, 28.0). In total, 129 patients (40.8%) had a known TPMT status. In the variant-type and wild-type TPMT genotype metabolism groups, median 6-TGN values were 1126 [interquartile range (IQR) 948-1562] and 467.5 pmol/8 × 10E8 red blood cells (RBCs) (IQR 334-593). A significant difference was observed between the 2 groups (P = 0.0001, t test). For TPMT phenotypes, in the slow, fast, and normal metabolism groups, the median 6-TGN values were 772.0 (IQR 459-1724), 296.0 (IQR 200-705), and 774.5 pmol/8 × 10E8 RBCs (IQR 500.5-981.5), with a significant difference observed between groups (P < 0.001, analysis of variance).

CONCLUSIONS

Our findings indicated that TPMT measurements at TG initiation can be useful but are not necessary for daily practice. TPMT genotypes and phenotypes are both associated with significant differences in 6-TGN levels between metabolic groups. However, the advantage of TG remains that RBC 6-TGN measurements are not crucial to monitor treatments in patients with IBD because these measurements did not correlate with laboratory result abnormalities. This presents as a major advantage in countries where patients cannot access these diagnostic tests.

TPMT Genetic Variability and Its Association with Hematotoxicity in Indonesian Children with Acute Lymphoblastic Leukemia in Maintenance Therapy

PURPOSE

Hematotoxicity monitoring in children with acute lymphoblastic leukemia (ALL) is critical to preventing life-threatening infections and drug discontinuation. The primary drug that causes hematotoxicity in ALL children is 6-mercaptopurine (6-MP). Genetic variability of the drug-metabolizing enzymes of 6-MP, thiopurine S-methyltransferase (TPMT), is one factor that might increase the susceptibility of children to hematotoxicity. The present study aimed to determine the variability in TPMT genotypes and phenotypes and its association with the occurrence of hematotoxicity in ALL children in maintenance therapy.

PATIENTS AND METHODS

A cross-sectional study was conducted at Cipto Mangunkusumo and Dharmais National Cancer Hospital, Jakarta, Indonesia, from June 2017 to October 2018. We included ALL patients, 1-18 years, who were receiving at least one month of 6-MP during maintenance therapy according to the Indonesian protocol for ALL 2013. Direct sequencing was used to determine TPMT*3A, *3B, and *3C genotypes, and LC-MS/MS analysis was performed to measure the plasma concentrations of 6-MP and its metabolites. Association analysis between the TPMT genotype and hematotoxicity was evaluated using the unpaired t-test or Mann-Whitney's test.

RESULTS

The prevalence of neutropenia, anemia, and thrombocytopenia in ALL children during maintenance therapy was 51.9%, 44.3%, and 6.6%, respectively. We found a low frequency of TPMT*3C, which is 0.95%. No association was found between hematotoxicity and TPMT genotypes or age, nutritional status, serum albumin levels, risk stratification, the daily dose of 6-MP, and cotrimoxazole co-administration. However, hematotoxicity was associated with 6-methylmercaptopurine (6-MeMP) plasma concentrations and the ratio 6-MeMP/6-thioguanine (6-TGN). We also found no association between TPMT genotypes and TPMT phenotypes.

CONCLUSION

The 6-MeMP/6-TGN ratio is associated with hematotoxicity in ALL children during maintenance therapy but is not strong enough to predict hematotoxicity.

Identification of rare defective allelic variants in cases of thiopurine S-methyltransferase deficient activity

Aim: To assess rare TPMT variants in patients carrying a deficient phenotype not predicted by the four more frequent genotypes (*2, *3A, *3B and *3C). Materials & methods: Next-generation sequencing of TPMT in 39 patients with a discordant genotype. Results: None of the variants identified explained the discordances assuming that they are of uncertain significance according to the Clinical Pharmacogenetics Implementation Consortium classification. Two unknown variants were detected and predicted to result in a splicing defect. We show that TPMT*16 and TMPT*21 are defective alleles, and TPMT*8 and TPMT*24 are associated with a normal activity. Conclusion: Whole-exon sequencing for rare TPMT mutations has a low diagnostic yield. A reassessment of the functional impact of rare variants of uncertain significance is a critical issue.

Modeling the Outcome of Systematic TPMT Genotyping or Phenotyping Before Azathioprine Prescription: A Cost-Effectiveness Analysis

BACKGROUND

Thiopurine S-methyltransferase (TPMT) testing, either by genotyping or phenotyping, can reduce the incidence of adverse severe myelotoxicity episodes induced by azathioprine. The comparative cost-effectiveness of TPMT genotyping and phenotyping are not known.

OBJECTIVE

Our aim was to assess the cost-effectiveness of phenotyping-based dosing of TPMT activity, genotyping-based screening and no screening (reference) for patients treated with azathioprine.

METHODS

A decision tree was built to compare the conventional weight-based dosing strategy with phenotyping and with genotyping using a micro-simulation model of patients with inflammatory bowel disease from the perspective of the French health care system. The time horizon was set up as 1 year. Only direct medical costs were used. Data used were obtained from previous reports, except for screening test and admission costs, which were from real cases. The main outcome was the cost-effectiveness ratios, with an effectiveness criterion of one averted severe myelotoxicity episode.

RESULTS

The total expected cost of the no screening strategy was €409/patient, the total expected cost of the phenotyping strategy was €427/patient, and the total expected cost of the genotyping strategy was €476/patient. The incremental cost-effectiveness ratio was €2602/severe myelotoxicity averted in using the phenotyping strategy, and €11,244/severe myelotoxicity averted in the genotyping strategy compared to the no screening strategy. At prevalence rates of severe myelotoxicity > 1%, phenotyping dominated genotyping and conventional strategies.

CONCLUSION

The phenotype-based strategy to screen for TPMT deficiency dominates (cheaper and more effective) the genotype-based screening strategy in France. Phenotype-based screening dominates no screening in populations with a prevalence of severe myelosuppression due to azathioprine of > 1%.

Thiopurine methyltransferase activity in children with acute myeloid leukemia

Activity of the enzyme thiopurine methyltransferase (TPMT) determines the anti-leukemic effect of thiopurines used in the chemotherapy of acute lymphoblastic leukemia (ALL) and acute myeloblastic leukemia (AML). TPMT status and its effects on treatment outcome have been studied extensively in ALL and autoimmune disorders, but few data is available on TPMT in AML. The present study assessed the genetic polymorphisms and activity of TPMT in children with AML at different treatment stages, and compared the results with those obtained for children with ALL. The study included 33 children with AML (0.7-19.7 years) treated with 6-thioguanine (6-TG) according to the AML-BFM 2004 Protocol. Blood samples were collected at diagnosis, during and following maintenance chemotherapy from 8, 10 and 17 patients with AML (the assay was performed at two time points in 2 patients), respectively. Blood samples from 105 children with ALL were obtained at diagnosis, during the maintenance chemotherapy and following the cessation of the chemotherapy from 16, 55 and 34 children, respectively. The activity of TPMT in red blood cells lysates was measured using an enzymatic reaction based on the conversion of 6-mercaptopurine into 6-methylmercaptopurine, involving S-adenozyl-L-methionine as the methyl group donor. TPMT mutations were determined using a polymerase chain reaction/restriction fragment length polymorphism method. Median TPMT activity at diagnosis, during maintenance chemotherapy and following chemotherapy was 43.1, 47,3 and 41.7 nmol 6-mMP g^-1 Hb h^-1, respectively. All patients with AML exhibited the homozygous TPMT*1/*1 genotype, with the exception of 1, who was a heterozygote with the TPMT*1/*3C genotype and demonstrated a TPMT activity level at diagnosis of 42.5 nmol 6-mMP g^-1 Hb h^-1. At each chemotherapy stage, the median TPMT activities in children with AML were significantly increased compared with the median TPMT activities in children with ALL. The preliminary results suggest that the TPMT activity in AML may be increased compared with that in ALL. Comprehensive studies on the association between thiopurine metabolism and treatment outcome in AML are required, with regard to the cytogenetic and molecular factors currently used for AML risk stratification.

Genetic Polymorphism of Thiopurine S-methyltransferase in Children with Acute Lymphoblastic Leukemia in Jordan

Background and Aims:

It has been demonstrated that homozygote and heterozygote mutant allele carriers for thiopurine S-methyltransferase (TPMT) are at high risk of developing myelosuppression after receiving standard doses of 6-mercaptopurine (6-MP). The aim of this study was to determine the frequency of TPMT deficient alleles in children with acute lymphoblastic leukemia (ALL) in Jordan and to compare it with other ethnic groups.

Methods:

We included 52 ALL childhood cases from King Hussein Cancer Research Center in Jordan. Genotyping of the rs1800460, rs1800462, and rs1142345 SNPs was performed by polymerase chain reaction (PCR) followed by sequencing. Comparisons were made with historical data for controls and for both volunteers and cases from other middle-eastern countries.

Results:

Mutant TPMT alleles were present in 3.8% (2/52) of patients. Allelic frequencies were 1.0% for both TPMT*B and TPMT*C. None of the patients were heterozygous or homozygous for TPMT*3A or TPMT *2. We did not find statistically significant differences in the distribution of mutant alleles between Jordan and other middle-eastern countries for both healthy volunteers or ALL patients.

Conclusions:

The overall frequency of TPMT mutant alleles was low and did not exhibit differences compared to other middle-eastern countries, including Jordanian studies assessing TPMT mutant alleles in healthy volunteers. The current results question the value of TPMT genotyping in the Jordanian population.

Polymorphic variation in TPMT is the principal determinant of TPMT phenotype: A meta-analysis of three genome-wide association studies

Thiopurine-related hematotoxicity in pediatric acute lymphoblastic leukemia (ALL) and inflammatory bowel diseases has been linked to genetically defined variability in thiopurine S-methyltransferase (TPMT) activity. While gene testing of TPMT is being clinically implemented, it is unclear if additional genetic variation influences TPMT activity with consequences for thiopurine-related toxicity. To examine this possibility, we performed a genome-wide association study (GWAS) of red blood cell TPMT activity in 844 Estonian individuals and 245 pediatric ALL cases. Additionally, we correlated genome-wide genotypes to human hepatic TPMT activity in 123 samples. Only genetic variants mapping to chromosome 6, including the TPMT gene region, were significantly associated with TPMT activity (P < 5.0 × 10^-8 ) in each of the three GWAS and a joint meta-analysis of 1,212 cases (top hit P = 1.2 × 10^-72 ). This finding is consistent with TPMT genotype being the primary determinant of TPMT activity, reinforcing the rationale for genetic testing of TPMT alleles in routine clinical practice to individualize mercaptopurine dosage.

Pharmacology and Optimization of Thiopurines and Methotrexate in Inflammatory Bowel Disease

Improving the efficacy and reducing the toxicity of thiopurines and methotrexate (MTX) have been areas of intense basic and clinical research. An increased knowledge on pharmacodynamics and pharmacokinetics of these immunomodulators has optimized treatment strategies in inflammatory bowel disease (IBD). This review focuses on the metabolism and mode of action of thiopurines and MTX, and provides an updated overview of individualized treatment strategies in which efficacy in IBD can be increased without compromising safety. The patient-based monitoring instruments adapted into clinical practice include pretreatment thiopurine S-methyltransferase testing, thiopurine metabolite monitoring, and blood count measurements that may help guiding the dosage to improve clinical outcome. Other approaches for optimizing thiopurine therapy in IBD include combination therapy with allopurinol, 5-aminosalicylates, and/or biologics. Similar strategies are yet to be proven effective in improving the outcome of MTX therapy. Important challenges for the management of IBD in the future relate to individualized dosing of immunomodulators for maximal efficacy with minimal risk of side effects. As low-cost conventional immunomodulators still remain a mainstay in pharmacotherapy of IBD, more research remains warranted, especially to substantiate these tailored management strategies in controlled clinical trials.

Pharmacogenomics in Pediatric Oncology: Review of Gene-Drug Associations for Clinical Use

During the 3rd congress of the European Society of Pharmacogenomics and Personalised Therapy (ESPT) in Budapest in 2015, a preliminary meeting was held aimed at establishing a pediatric individualized treatment in oncology and hematology committees. The main purpose was to facilitate the transfer and harmonization of pharmacogenetic testing from research into clinics, to bring together basic and translational research and to educate health professionals throughout Europe. The objective of this review was to provide the attendees of the meeting as well as the larger scientific community an insight into the compiled evidence regarding current pharmacogenomics knowledge in pediatric oncology. This preliminary evaluation will help steer the committee’s work and should give the reader an idea at which stage researchers and clinicians are, in terms of personalizing medicine for children with cancer. From the evidence presented here, future recommendations to achieve this goal will also be suggested.

Thiopurine S-methyltransferase testing for averting drug toxicity: a meta-analysis of diagnostic test accuracy

Thiopurine S-methyltransferase (TPMT) deficiency increases the risk of serious adverse events in persons receiving thiopurines. The objective was to synthesize reported sensitivity and specificity of TPMT phenotyping and genotyping using a latent class hierarchical summary receiver operating characteristic meta-analysis. In 27 studies, pooled sensitivity and specificity of phenotyping for deficient individuals was 75.9% (95% credible interval (CrI), 58.3-87.0%) and 98.9% (96.3-100%), respectively. For genotype tests evaluating TPMT*2 and TPMT*3, sensitivity and specificity was 90.4% (79.1-99.4%) and 100.0% (99.9-100%), respectively. For individuals with deficient or intermediate activity, phenotype sensitivity and specificity was 91.3% (86.4-95.5%) and 92.6% (86.5-96.6%), respectively. For genotype tests evaluating TPMT*2 and TPMT*3, sensitivity and specificity was 88.9% (81.6-97.5%) and 99.2% (98.4-99.9%), respectively. Genotyping has higher sensitivity as long as TPMT*2 and TPMT*3 are tested. Both approaches display high specificity. Latent class meta-analysis is a useful method for synthesizing diagnostic test performance data for clinical practice guidelines.The Pharmacogenomics Journal advance online publication, 24 May 2016; doi:10.1038/tpj.2016.37.

PACSIN2 polymorphism is associated with thiopurine-induced hematological toxicity in children with acute lymphoblastic leukaemia undergoing maintenance therapy

Adequate maintenance therapy for childhood acute lymphoblastic leukemia (ALL), with 6-mercaptopurine as an essential component, is necessary for retaining durable remission. Interruptions or discontinuations of the therapy due to drug-related toxicities, which can be life threatening, may result in an increased risk of relapse. In this retrospective study including 305 paediatric ALL patients undergoing maintenance therapy, we systematically investigated the individual and combined effects of genetic variants of folate pathway enzymes, as well as of polymorphisms in PACSIN2 and ITPA, on drug-induced toxicities by applying a multi-analytical approach including logistic regression (LR), classification and regression tree (CART) and generalized multifactor dimensionality reduction (GMDR). In addition to the TPMT genotype, confirmed to be a major determinant of drug related toxicities, we identified the PACSIN2 rs2413739TT genotype as being a significant risk factor for 6-MP-induced toxicity in wild-type TPMT patients. A gene-gene interaction between MTRR (rs1801394) and MTHFR (rs1801133) was detected by GMDR and proved to have an independent effect on the risk of stomatitis, as shown by LR analysis. To our knowledge, this is the first study showing PACSIN2 genotype association with hematological toxicity in ALL patients undergoing maintenance therapy.

Thiopurine S-methyltransferase testing for averting drug toxicity in patients receiving thiopurines: a systematic review

AIM

Thiopurine S-methyltransferase (TPMT) testing is used in patients receiving thiopurines to identify enzyme deficiencies and risk for adverse drug reactions. It is uncertain whether genotyping is superior to phenotyping. The objectives were to conduct a systematic review of TPMT-test performance studies.

MATERIALS & METHODS

Electronic and grey literature sources were searched for studies reporting test performance compared with a reference standard. Sixty-six eligible studies were appraised for quality.

RESULTS

Thirty phenotype-genotype and six phenotype-phenotype comparisons were of high quality. The calculated sensitivity and specificity for genotyping to identify a homozygous mutation ranged from 0.0-100.0% and from 97.8-100.0%, respectively.

CONCLUSION

Clinical decision-makers require high-quality evidence of clinical validity and clinical utility of TPMT genotyping to ensure appropriate use in patients.

How can we improve on the already impressive results in pediatric ALL?

The past 70 years have seen childhood acute lymphoblastic leukemia move from a fatal disease with a survival of barely 4 months to a curable disease in >85% of patients. It has become clear that as treatment has intensified, more children are cured but at the expense of increased toxicity which for some can cause significant long-term morbidity and even mortality. The drive in more recent years has been to identify sensitive markers of disease and response to treatment to allow a reduction in therapy in those who do not require it and more intensive treatment in those who do. Clinical characteristics have been used to stratify patients into different risk groups and this, coupled with following response at a molecular level, has done much to tailor treatment to the patient. Considerable research has been focused on the molecular characteristics of the leukemia itself to elucidate the biologic mechanisms underlying both the disease and the comparative or absolute resistance of some types of leukemia. These molecular markers can also act as targets for novel therapies, which require newer trial methodologies to prove their utility. There has been less focus on the biology of the patient but it is clear that some patients are more susceptible to adverse events and toxicities than others. Through the use of pharmacogenomics, modification to therapy may be appropriate in certain patients based on their genetic profile. As novel therapies become available, suitable controlled trials in children are essential for their safe use in this population and will ensure that children are not denied timely access to advances in treatment.

Association between Thiopurine S-Methyltransferase Polymorphisms and Azathioprine-Induced Adverse Drug Reactions in Patients with Autoimmune Diseases: A Meta-Analysis

Purpose

Azathioprine (AZA) is widely used as an immunosuppressive drug in autoimmune diseases, but its use is limited by significant adverse drug reactions (ADRs). Thiopurine S-methyltransferase (TPMT) is an important enzyme involved in AZA metabolism. Several clinical guidelines recommend determining TPMT genotype or phenotype before initiating AZA therapy. Although several studies have investigated the association between TPMT polymorphisms and AZA-induced ADRs, the results are inconsistent. The purpose of this study is to evaluate whether there is an association between TPMT polymorphisms and AZA-induced ADRs using meta-analysis.

Methods

We explored PubMed, Web of Science and Embase for articles on TPMT polymorphisms and AZA-induced ADRs. Studies that compared TPMT polymorphisms with-ADRs and without-ADRs in patients with autoimmune diseases were included. Relevant outcome data from all the included articles were extracted and the pooled odds ratios (ORs) with corresponding 95% confidence intervals (CIs) were calculated using Revman 5.3 software.

Results

Eleven published studies, with a total of 651 patients with autoimmune diseases, investigated associations between TPMT polymorphisms and AZA-induced ADRs, were included in this meta-analysis. Our meta-analysis demonstrated that TPMT polymorphisms were significantly associated with AZA-induced overall ADRs, bone marrow toxicity and gastric intolerance; pooled ORs were 3.12 (1.48–6.56), 3.76 (1.97–7.17) and 6.43 (2.04–20.25), respectively. TPMT polymorphisms were not associated with the development of hepatotoxicity; the corresponding pooled OR was 2.86 (95%CI: 0.32–25.86). However, the association in GI subset could be driven by one single study. After this study was excluded, the OR was 2.11 (95%CI: 0.36–12.42); namely, the association became negative.

Conclusions

Our meta-analysis demonstrated an association of TPMT polymorphisms with overall AZA-induced ADRs, bone marrow toxicity and gastric intolerance, but not with hepatotoxicity. The presence of the normal TPMT genotypes cannot preclude the development of ADRs during AZA treatment, TPMT genotyping prior to commencing AZA therapy cannot replace, may augment, the current practice of regular monitoring of the white blood cell. Because of small sample sizes, large and extensive exploration was required to validate our findings.

From pharmacogenetics to pharmacometabolomics: SAM modulates TPMT activity

AIM

In the present study, the influence of SAM on TPMT activity in vivo on human subjects was investigated.

SUBJECTS & METHODS

A total of 1017 donors from the Estonian Genome Center of the University of Tartu (Estonia) were genotyped for common TPMT variants, evaluated for TPMT activity, SAM levels, a set of 19 biochemical and ten hematological parameters and demographic data.

RESULTS

After adjustment in multiple regression models and correction for multiple testing, from the 43 factors that were tested, only TPMT genotype (p = 1 × 10(-13)) and SAM levels (p = 1 × 10(-13)) were found to significantly influence TPMT activity. The influence of SAM on TPMT activity was more pronounced in TPMT-heterozygous than wild-type individuals.

CONCLUSION

SAM represents a potential pharmacometabolomic marker and therapeutic agent in TPMT-heterozygous subjects.

Complete sequence-based screening of TPMT variants in the Korean population

Thiopurine S-methyltransferase (TPMT) is a cytoplasmic enzyme involved in the metabolism of thiopurine drugs and its activity is largely influenced by polymorphisms of the TPMT gene. To date, more than 35 TPMT variants are known to be associated with reduced enzyme activity, but most studies on the TPMT genotype have included only common nonfunctional variants, such as TPMT*2 and TPMT*3. In this study, we carried out a complete sequencing analysis to screen all TPMT variants in Korean patients. A total of 900 Korean patients were genotyped for TPMT and 30 patients (3.3%) had the known TPMT variant alleles. TPMT*3C was found in 25 patients (2.8%): 24 patients with TPMT*1/*3 and one with TPMT*3/*3. Rare TPMT variants including TPMT*6, TPMT*16, and TPMT*32 were detected in five patients (0.6%) and a novel variant, TPMT*38 (c.514T>C, p.S172P), was identified in two patients. This is the first complete sequence-based screening study evaluating all TPMT variants in Asian populations.

Getting the best out of thiopurine therapy: thiopurine S-methyltransferase and beyond

Thiopurines are the cornerstone of treatment for a wide variety of medical disorders, ranging from pediatric leukemia to inflammatory bowel disease. Because of their complex metabolism and potential toxicities, the use of biomarkers to predict risk and response is paramount. Thiopurine S-methyltransferase and thiopurine metabolite levels have emerged as companion diagnostics with crucial roles in facilitating safe and effective treatment. This review serves to update the reader on how these tools are being developed and implemented in clinical practice. A useful paradigm in thiopurine therapeutic strategy is presented, along with fresh insights into the mechanisms underlying these approaches. We elaborate on potential future developments in the optimization of thiopurine therapy.

Physiologically based pharmacokinetic model for 6-mercpatopurine: exploring the role of genetic polymorphism in TPMT enzyme activity

AIMS

To extend the physiologically based pharmacokinetic (PBPK) model developed for 6-mercaptopurine to account for intracellular metabolism and to explore the role of genetic polymorphism in the TPMT enzyme on the pharmacokinetics of 6-mercaptopurine.

METHODS

The developed PBPK model was extended for 6-mercaptopurine to account for intracellular metabolism and genetic polymorphism in TPMT activity. System and drug specific parameters were obtained from the literature or estimated using plasma or intracellular red blood cell concentrations of 6-mercaptopurine and its metabolites. Age-dependent changes in parameters were implemented for scaling, and variability was also introduced for simulation. The model was validated using published data.

RESULTS

The model was extended successfully. Parameter estimation and model predictions were satisfactory. Prediction of intracellular red blood cell concentrations of 6-thioguanine nucleotide for different TPMT phenotypes (in a clinical study that compared conventional and individualized dosing) showed results that were consistent with observed values and reported incidence of haematopoietic toxicity. Following conventional dosing, the predicted mean concentrations for homozygous and heterozygous variants, respectively, were about 10 times and two times the levels for wild-type. However, following individualized dosing, the mean concentration was around the same level for the three phenotypes despite different doses.

CONCLUSIONS

The developed PBPK model has been extended for 6-mercaptopurine and can be used to predict plasma 6-mercaptopurine and tissue concentration of 6-mercaptopurine, 6-thioguanine nucleotide and 6-methylmercaptopurine ribonucleotide in adults and children. Predictions of reported data from clinical studies showed satisfactory results. The model may help to improve 6-mercaptopurine dosing, achieve better clinical outcome and reduce toxicity.

Thiopurine methyltransferase and treatment outcome in the UK acute lymphoblastic leukaemia trial ALL2003

The influence of thiopurine methyltransferase (TPMT) genotype on treatment outcome was investigated in the United Kingdom childhood acute lymphoblastic leukaemia trial ALL2003, a trial in which treatment intensity was adjusted based on minimal residual disease (MRD). TPMT genotype was measured in 2387 patients (76% of trial entrants): 2190 were homozygous wild-type, 189 were heterozygous for low activity TPMT alleles (166 TPMT*1/*3A, 19 TPMT*1/*3C, 3 TPMT*1/*2 and 1 TPMT*1/*9) and 8 were TPMT deficient. In contrast to the preceding trial ALL97, there was no difference in event-free survival (EFS) between the TPMT genotypes. The 5-year EFS for heterozygous TPMT*1/*3A patients was the same in both trials (88%), but for the homozygous wild-type TPMT*1/*1 patients, EFS improved from 80% in ALL97% to 88% in ALL2003. Importantly, the unexplained worse outcome for heterozygous TPMT*1/*3C patients observed in ALL97 (5-year EFS 53%) was not seen in ALL2003 (5-year EFS 94%). In a multivariate Cox regression analysis the only significant factor affecting EFS was MRD status (hazard ratio for high-risk MRD patients 4·22, 95% confidence interval 2·97–5·99, P < 0·0001). In conclusion, refinements in risk stratification and treatment have reduced the influence of TPMT genotype on treatment outcome in a contemporary protocol.

Relationship between azathioprine dosage, 6-thioguanine nucleotide levels, and therapeutic response in pediatric patients with IBD treated with azathioprine

BACKGROUND

Azathioprine (AZA) is commonly used to treat IBD either alone or in combination with mesalazine. However, there are relatively few studies concerning the relationship between AZA dose, thiopurine metabolite levels, and therapeutic response in pediatric patients treated with both AZA and mesalazine.

METHODS

We retrospectively investigated the relationship between AZA dose, thiopurine metabolite levels, and therapeutic response in 137 pediatric patients with IBD treated with AZA using multilevel analysis. Additional factors affecting metabolite levels and therapeutic response were also analyzed.

RESULTS

A positive correlation was observed between AZA dosage and 6-thioguanine nucleotide (6-TGN) level (P < 0.0001). Variant TPMT genotype (P < 0.001) and concomitant use of mesalazine (P < 0.001) were predictors of higher 6-TGN levels. Leukopenia (P = 0.025) and lymphopenia (P = 0.045) were associated with higher levels of 6-TGN. Poor AZA compliance affected median 6-TGN levels (P < 0.001). The frequency of patients with median 6-TGN levels >235 pmol per 8 × 10(8) red blood cells was the highest in the sustained therapeutic response group (P = 0.015). Age, sex, IBD type, and duration of AZA therapy did not influence 6-TGN levels or therapeutic effect.

CONCLUSIONS

AZA dosage is positively correlated with 6-TGN level. Higher 6-TGN levels are related to leukopenia, lymphopenia, and concurrent use of mesalazine. These results provide the rationale for monitoring metabolites to optimize drug dosing and minimize drug-related toxicity. In addition, maintenance of 6-TGN levels within a beneficial therapeutic range by direct monitoring should be helpful in attaining therapeutic efficacy, although this possibility should be verified in prospective studies.

Association between thiopurine S-methyltransferase polymorphisms and thiopurine-induced adverse drug reactions in patients with inflammatory bowel disease: a meta-analysis

Purpose

Thiopurine drugs are well established treatments in the management of inflammatory bowel disease (IBD), but their use is limited by significant adverse drug reactions (ADRs). Thiopurine S-methyltransferase (TPMT) is an important enzyme involved in thiopurine metabolism. Several clinical guidelines recommend determining TPMT genotype or phenotype before initiating thiopurine therapy. Although several studies have investigated the association between TPMT polymorphisms and thiopurine-induced ADRs, the results are inconsistent. The purpose of this study is to evaluate whether there is an association between TPMT polymorphisms and thiopurine-induced ADRs using meta-analysis.

Methods

We explored PubMed, Web of Science and Embase for articles on TPMT polymorphisms and thiopurine-induced ADRs. Studies that compared TPMT polymorphisms with-ADRs and without-ADRs in IBD patients were included. Relevant outcome data from all the included articles were extracted and the pooled odds ratio (OR) with corresponding 95% confidence intervals were calculated using Revman 5.3 software.

Results

Fourteen published studies, with a total of 2,206 IBD patients, which investigated associations between TPMT polymorphisms and thiopurine-induced ADRs were included this meta-analysis. Our meta-analysis demonstrated that TPMT polymorphisms were significantly associated with thiopurine-induced overall ADRs and bone marrow toxicity; pooled ORs were 3.36 (95%CI: 1.82–6.19) and 6.67 (95%CI: 3.88–11.47), respectively. TPMT polymorphisms were not associated with the development of other ADRs including hepatotoxicity, pancreatitis, gastric intolerance, flu-like symptoms and skin reactions; the corresponding pooled ORs were 1.27 (95%CI: 0.60–2.71), 0.97 (95%CI: 0.38–2.48), 1.82 (95%CI: 0.93–3.53), 1.28 (95%CI: 0.47–3.46) and 2.32 (95%CI: 0.86–6.25), respectively.

Conclusions

Our meta-analysis demonstrated an association of TPMT polymorphisms with overall thiopurine-induced ADRs and bone marrow toxicity, but not with hepatotoxicity, pancreatitis, flu-like symptoms, gastric intolerance and skin reactions. These findings suggest that pretesting the TPMT genotype could be helpful in clinical practice before initiating thiopurine therapy. However, white blood cell count analysis should be the mainstay for follow-up.

Interindividual variability in TPMT enzyme activity: 10 years of experience with thiopurine pharmacogenetics and therapeutic drug monitoring

BACKGROUND & AIMS

TPMT activity and metabolite determination (6-thioguanine nucleotides [6-TGN] and 6-methylmercaptopurine nucleotides [6-MMPN]) remain controversial during thiopurine management. This study assessed associations between patient characteristics and TPMT activity, and their impact on metabolite levels.

PATIENTS & METHODS

A retrospective review of the laboratory database from a French university hospital identified 7360 patients referred for TPMT phenotype/genotype determination, and/or for 6-TGN/6-MMPN monitoring.

RESULTS

Four TPMT phenotypes were identified according to TPMT activity distribution: low, intermediate, normal/high and very high. Based on 6775 assays, 6-TGN concentrations were 1.6-fold higher in TPMT-deficient patients compared with TPMT-normal patients. Azathioprine dose and TPMT genotype were significant predictors of metabolite levels. Furthermore, 6-MMPN and 6-MMPN: 6-TGN ratios were, respectively, 1.6- and 2.2-fold higher in females than in males, despite similar TPMT, 6-TGN and azathioprine doses. An unfavorable ratio (≥20) was associated with a slightly higher TPMT activity.

CONCLUSION

These results illustrate the usefulness of pharmacogenomics and metabolite measurement to improve the identification of noncompliance and patients at high risk for toxicity or therapeutic resistance. Original submitted 13 November 2013; Revision submitted 30 January 2014.

Implementation of TPMT testing

The activity of the enzyme thiopurine methyltransferase (TPMT) is regulated by a common genetic polymorphism. One in 300 individuals lack enzyme activity and 11% are heterozygous for a variant low activity allele and have an intermediate activity. The thiopurine drugs azathioprine, mercaptopurine and thioguanine are substrates for TPMT; these drugs exhibit well documented myelosuppressive effects on haematopoietic cells and have a track record of idiosyncratic drug reactions. The development of severe bone marrow toxicity, in patients taking standard doses of thiopurine drugs, is associated with TPMT deficiency whilst the TPMT heterozygote is at an increased risk of developing myelosuppression. Factors influencing TPMT enzyme activity, as measured in the surrogate red blood cell, are discussed in this review to enable an appreciation of why concordance between TPMT genotype and phenotype is not 100%. This is particularly important for lower/intermediate TPMT activities to avoid misclassification of TPMT status. TPMT testing is now widely available in routine service laboratories. The British National Formulary suggests TPMT testing before starting thiopurine drugs. Dermatologists were quick to adopt routine TPMT testing whilst gastroenterologists do not specifically recommend TPMT screening. TPMT testing is mandatory prior to the use of mercaptopurine in childhood leukaemia. Thiopurine drug dose and other treatment related influences on cell counts explain some of the differing recommendations between clinical specialities. TPMT testing is cost-effective and the major role is in the identification of the TPMT deficient individual prior to the start of thiopurine drugs.

Thiopurine dose intensity and treatment outcome in childhood lymphoblastic leukaemia: the influence of thiopurine methyltransferase pharmacogenetics

The impact of thiopurine methyltransferase (TPMT) genotype on thiopurine dose intensity, myelosuppression and treatment outcome was investigated in the United Kingdom childhood acute lymphoblastic leukaemia (ALL) trial ALL97. TPMT heterozygotes had significantly more frequent cytopenias and therefore required dose adjustments below target levels significantly more often than TPMT wild‐type patients although the average dose range was similar for both genotypes. Event‐free survival (EFS) for patients heterozygous for the more common TPMT*1/*3A variant allele (n = 99, 5‐year EFS 88%) was better than for both wild‐type TPMT*1/*1 (n = 1206, EFS 80%, P = 0·05) and TPMT*1/*3C patients (n = 17, EFS 53%, P = 0·002); outcomes supported by a multivariate Cox regression analysis. Poor compliance without subsequent clinician intervention was associated with a worse EFS (P = 0·02) and such non‐compliance may have contributed to the poorer outcome for TPMT*1/*3C patients. Patients prescribed escalated doses had a worse EFS (P = 0·04), but there was no difference in EFS by dose intensity or duration of cytopenias. In contrast to reports from some USA and Nordic trials, TPMT heterozygosity was not associated with a higher rate of second cancers. In conclusion, TPMT*1/*3A heterozygotes had a better EFS than TPMT wild‐type patients. Thiopurine induced cytopenias were not detrimental to treatment outcome.

Single nucleotide polymorphism and its dynamics for pharmacogenomics

Pharmacogenomics is the study of how the genetic makeup determines the response to a therapeutic intervention. It has the capability to revolutionize the practice of medicine by personalized approach for treatment through the use of novel diagnostic tools. Pharmacogenomic based approaches reduce the trial-and-error approach and restrict the exposure of patients to those drugs which are not effective or are toxic for them. Single Nucleotide Polymorphisms (SNPs) hold the key in defining the risk of an individual's susceptibility to various illnesses and response to drugs. There is an ongoing process of identifying the common, biologically relevant SNPs, in particular those that are associated with the risk of disease and adverse drug reaction. The identification and characterization of these SNPs are necessary before their use as genetic tools. Most of the ongoing SNP related studies are biased deliberately towards coding regions and the data generated from them are therefore unlikely to reflect genome wide distribution of SNPs. To avoid this biasing towards the coding regions SNP, SNP consortium protocol was designed. Though, projects like the HapMap increase credibility and use of SNPs, still there are some concern like the required sample (patient) sizes, the number of SNPs required for mapping, number of association studies, the cost of SNP genotyping, and the interpretation and explanation of results are some of the challenges that surround this field.

Revitalizing personalized medicine: respecting biomolecular complexities beyond gene expression

Despite recent advancements in "omic" technologies, personalized medicine has not realized its fullest potential due to isolated and incomplete application of gene expression tools. In many instances, pharmacogenomics is being interchangeably used for personalized medicine, when actually it is one of the many facets of personalized medicine. Herein, we highlight key issues that are hampering the advancement of personalized medicine and highlight emerging predictive tools that can serve as a decision support mechanism for physicians to personalize treatments.

Nomenclature for alleles of the thiopurine methyltransferase gene

The drug-metabolizing enzyme thiopurine methyltransferase (TPMT) has become one of the best examples of pharmacogenomics to be translated into routine clinical practice. TPMT metabolizes the thiopurines 6-mercaptopurine, 6-thioguanine, and azathioprine, drugs that are widely used for treatment of acute leukemias, inflammatory bowel diseases, and other disorders of immune regulation. Since the discovery of genetic polymorphisms in the TPMT gene, many sequence variants that cause a decreased enzyme activity have been identified and characterized. Increasingly, to optimize dose, pretreatment determination of TPMT status before commencing thiopurine therapy is now routine in many countries. Novel TPMT sequence variants are currently numbered sequentially using PubMed as a source of information; however, this has caused some problems as exemplified by two instances in which authors' articles appeared on PubMed at the same time, resulting in the same allele numbers given to different polymorphisms. Hence, there is an urgent need to establish an order and consensus to the numbering of known and novel TPMT sequence variants. To address this problem, a TPMT nomenclature committee was formed in 2010, to define the nomenclature and numbering of novel variants for the TPMT gene. A website (http://www.imh.liu.se/tpmtalleles) serves as a platform for this work. Researchers are encouraged to submit novel TPMT alleles to the committee for designation and reservation of unique allele numbers. The committee has decided to renumber two alleles: nucleotide position 106 (G>A) from TPMT*24 to TPMT*30 and position 611 (T>C, rs79901429) from TPMT*28 to TPMT*31. Nomenclature for all other known alleles remains unchanged.

Methotrexate binds to recombinant thiopurine S-methyltransferase and inhibits enzyme activity after high-dose infusions in childhood leukaemia

PURPOSE

Important drugs in the treatment of childhood acute lymphoblastic leukaemia (ALL) are 6-mercaptopurine (6-MP) and methotrexate (MTX). Thiopurine methyltransferase (TPMT) is a polymorphic enzyme causing variability in 6-MP response and toxicity. The aim of this study was to investigate the fluctuation in TPMT enzyme activity over time and the effect of high-dose MTX infusions on TPMT enzyme activity and 6-MP metabolites in paediatric ALL patients.

METHODS

Fifty-three children with ALL treated according to the NOPHO-ALL 2000 protocol were included in the study. TPMT enzyme activity was measured at six different times starting from diagnosis until after the end of maintenance treatment. TPMT and 6-MP metabolites were measured before the initiation of high-dose MTX (HD-MTX) infusions and at 66 h post-infusion. The interaction between MTX and TPMT was investigated in vitro using recombinant TPMT protein and a leukaemic cell line.

RESULTS

Forty percent of TPMT wild-type individuals had deceptively low TPMT enzyme activity according to genotype at the time of diagnosis. TPMT activity had decreased significantly 66 h after the start of HD-MTX infusions (-9.2 %; p = 0.013). MTX bound to recombinant TPMT protein severely inhibiting TPMT enzyme activity (remaining activity 16 %).

CONCLUSIONS

Our results show that TPMT genotyping should be performed in children with ALL, since 40 % of the children in our study who carried the wild-type TPMT gene were at risk of initial underdosing of 6-MP in cases where only TPMT enzyme activity was determined. MTX inhibits the TPMT enzyme activity after HD-MTX infusions due to protein binding.