Thiopurine S-methyltransferase (TPMT) assessment prior to starting thiopurine drug treatment; a pharmacogenomic test whose time has come

Development of a rapid clinical TPMT genotyping assay

OBJECTIVES

Thiopurine compounds are commonly used in the treatment of childhood acute lymphoblastic leukemia, and as immunosuppressants following organ transplantation or for treatment of various autoimmune disorders. Thiopurine S-methyltransferase (TPMT) is required for detoxification, through S-methylation, of 6-thioguanine nucleotides (TGNs), a byproduct of thiopurine metabolism. Single nucleotide polymorphisms (SNPs) in the TPMT gene have been shown to affect its function, with some variants associated with serious clinical manifestations including severe to fatal myelosuppression and organ transplant rejection following treatment with standard thiopurine doses. In this study, we describe a TaqMan real time PCR allelic discrimination assay requiring minimal DNA input for TPMT genotyping.

DESIGN AND METHODS

We designed controls for the homozygous wild type and allelic variants of TPMT*2, *3B, and *3C. Genomic DNA was extracted from an additional 412 human blood samples. The samples were tested for the TPMT*2, *3B, *3C, and *3A polymorphisms by TaqMan genotyping assays using the AB 7500 FAST Real-Time PCR instrument. Allelic discrimination plots were used to identify each mutation.

RESULTS

The TaqMan assay correctly genotyped all custom control DNA samples. Of the 412 tested samples, our assay identified 375 samples as wild-type *1/*1 (91.02%), 3 as *1/*2 (0.73%), 1 as *1/*3B (0.24%), 3 as *1/*3C (0.73%), 27 presumed to be *1/*3A (6.55%), and 3 as *3B/*3A (0.73%).

CONCLUSIONS

The clinical implications of TPMT genotyping, along with the simplicity and specificity of the TaqMan genotyping assays make this test highly suitable for use in a clinical laboratory.

'Toxgnostics': an unmet need in cancer medicine

If we were to summarize the rationale that underpins medical oncology in a Latin aphorism, it might be 'veneno ergo sum'; that is, I poison, therefore I am. The burden of chemotherapy-associated toxicity is well recognized, but we have relatively few tools that increase the precision of anticancer drug prescribing. We propose a shift in emphasis from the focussed study of polymorphisms in drug metabolic pathways in small sets of patients to broader agnostic analyses to systematically correlate germline genetic variants with adverse events in large, well-defined cancer populations. Thus, we propose the new science of 'toxgnostics' (that is, the systematic, agnostic study of genetic predictors of toxicity from anticancer therapy).

TPMT genetic variants are associated with increased rejection with azathioprine use in heart transplantation

OBJECTIVES

Azathioprine (AZA) is an important immunosuppressant drug used in heart transplantation (HTX). Consensus guidelines recommend that patients with thiopurine S-methyltransferase (TPMT) genetic variants be started on lower AZA dose because of higher active metabolite levels and risk of adverse events. However, in-vitro lymphocyte proliferation assays performed in participants with inactive TPMT alleles have suggested that AZA use may result in decreased immunosuppressant efficacy as compared with wild-type (WT) individuals. The objective of this study was therefore to determine the effect of TPMT genetic variation on AZA efficacy or prevention of rejection in HTX recipients treated with AZA.

PARTICIPANTS AND METHODS

We genotyped 93 HTX recipients treated with AZA and measured erythrocyte TPMT enzyme activity. Acute rejection was monitored by routine endomyocardial biopsies.

RESULTS

There were 83 WT and 10 heterozygote (HZ) HTX recipients. TPMT activity level was lower in HZ compared with WT (13.1±2.8 vs. 21±4.5 U/ml red blood cell, P<0.001). Despite similar AZA dose, HZ developed severe rejection earlier (P<0.001), and the total rejection score was higher (P=0.02) than WT. AZA was discontinued more frequently in HZ (P=0.01) because of rejection. The incidence of leukopenia was similar between the groups (40 vs. 43%, P=1.0).

CONCLUSION

HTX recipients with TPMT genetic variant alleles who are treated with AZA develop acute rejection earlier, more frequently, and of greater severity. These patients, despite having lower TPMT enzymatic activity, should be monitored carefully for possible increased risk of acute rejection.

Enhanced specificity of TPMT*2 genotyping using unidirectional wild-type and mutant allele-specific scorpion primers in a single tube

Genotyping of thiopurine S-methyltransferase (TPMT) is recommended for predicting the adverse drug response of thiopurines. In the current study, a novel version of allele-specific PCR (AS-PCR), termed competitive real-time fluorescent AS-PCR (CRAS-PCR) was developed to analyze the TPMT*2 genotype in ethnic Chinese. This technique simultaneously uses wild-type and mutant allele-specific scorpion primers in a single reaction. To determine the optimal conditions for both traditional AS-PCR and CRAS-PCR, we used the Taguchi method, an engineering optimization process that balances the concentrations of all components using an orthogonal array rather than a factorial array. Instead of running up to 264 experiments with the conventional factorial method, the Taguchi method achieved the same optimization using only 16 experiments. The optimized CRAS-PCR system completely avoided non-specific amplification occurring in traditional AS-PCR and could be performed at much more relaxed reaction conditions at 1% sensitivity, similar to traditional AS-PCR. TPMT*2 genotyping of 240 clinical samples was consistent with published data. In conclusion, CRAS-PCR is a novel and robust genotyping method, and the Taguchi method is an effective tool for the optimization of molecular analysis techniques.

Genes and beans: pharmacogenomics of renal transplant

Advances in the management of patients after solid organ transplantation have led to dramatic decreases in rates of acute rejection, but long-term graft and patient survival have remained unchanged. Individualized therapy after transplant will ideally provide adequate immunosuppression while limiting the adverse effects of drug therapy that significantly impact graft survival. Therapeutic drug monitoring represents the best approximation of individualized drug therapy in transplant at this time; however, obtaining pharmacogenomic data in transplant patients has the potential to enhance our current practice. Polymorphisms of target genes that impact pharmacokinetics have been identified for most immunosuppressants, including tacrolimus, cyclosporine, mycophenolate, azathioprine and sirolimus. In the future, pre-emptive assessment of a patient's genetic profile may inform drug selection and provide information on specific doses that will improve efficacy and limit toxicity.

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

AIMS

In children with acute lymphoblastic leukaemia (ALL) bone marrow activity can influence red blood cell (RBC) kinetics, the surrogate tissue for thiopurine methyltransferase (TPMT) measurements. The aim of this study was to investigate TPMT phenotype-genotype concordance in ALL, and the influence of TPMT on thiopurine metabolite formation.

METHODS

We measured TPMT (activity, as units ml(-1) packed RBCs and genotype) at diagnosis (n = 1150) and TPMT and thioguanine nucleotide (TGN) and methylmercaptopurine nucleotide (MeMPN) metabolites (pmol/8 × 10(8) RBCs) during chemotherapy (n = 1131) in children randomized to thioguanine or mercaptopurine on the United Kingdom trial ALL97.

RESULTS

Median TPMT activity at diagnosis (8.5 units) was significantly lower than during chemotherapy (13.8 units, median difference 5.1 units, 95% confidence interval (CI) 4.8, 5.4, P < 0.0001). At diagnosis genotype-phenotype was discordant. During chemotherapy the overall concordance was 92%, but this fell to 55% in the intermediate activity cohort (45% had wild-type genotypes). For both thiopurines TGN concentrations differed by TPMT status. For mercaptopurine, median TGNs were higher in TPMT heterozygous genotype (754 pmol) than wild-type (360 pmol) patients (median difference 406 pmol, 95% CI 332, 478, P < 0.0001), whilst median MeMPNs, products of the TPMT reaction, were higher in wild-type (10 650 pmol) than heterozygous patients (3868 pmol) (P < 0.0001). In TPMT intermediate activity patients with a wild-type genotype, TGN (median 366 pmol) and MeMPN (median 8590 pmol) concentrations were similar to those in wild-type, high activity patients.

CONCLUSIONS

In childhood ALL, TPMT activity should not be used to predict heterozygosity particularly in blood samples obtained at disease diagnosis. Genotype is a better predictor of TGN accumulation during chemotherapy.

The cost-effectiveness of a pharmacogenetic test: a trial-based evaluation of TPMT genotyping for azathioprine

BACKGROUND

Thiopurine-methyl transferase (TPMT) testing prior to the prescription of azathioprine in autoimmune diseases is one of the few examples of a pharmacogenetic test that has made the transition from research into clinical practice. TPMT testing could lead to improved prescribing of azathioprine resulting in a reduction in adverse drug reactions as well as an improvement in effectiveness. When allocating scarce resources robust evidence on cost-effectiveness is required.

OBJECTIVE

This study aimed to evaluate the cost-effectiveness of a TPMT genotyping test to inform azathioprine prescribing in autoimmune diseases. The secondary aim of this study was to demonstrate the complexity of undertaking a trial-based evaluation of a pharmacogenetic test.

METHODS

A prospective economic evaluation was conducted alongside the TARGET (TPMT: Azathioprine Response to Genotype and Enzyme Testing) study, a pragmatic controlled trial that randomized (1:1) patients to undergo TPMT genotyping before azathioprine (n = 167) or current practice (n = 166). Assuming the UK health service perspective and a time horizon of 4 months, resource-use and health status data were collected prospectively for all recruited patients.

RESULTS

The mean incremental cost for TPMT genotyping and subsequent care pathways compared with current practice for the 4-month follow-up was -£421.06 (95% confidence interval -£925.15 to £89.75). Mean incremental quality-adjusted life-years were close to zero but negative: -0.008 (95% confidence interval -0.017 to 0.0002). Assuming a threshold of £20,000 per quality-adjusted life-year, the expected incremental net benefit of introducing the test is £256.89 (95% CI -£425.94 to £932.86).

CONCLUSIONS

TPMT genotyping potentially offers a less expensive alternative than current practice, but it may also have a small but negative effect on health status. These findings are associated with significant uncertainty, and the causal effect of TPMT genotyping on changes in health status and health care resource use remains uncertain. The results from this study therefore pose a difficult challenge to decision makers.

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.

Identification of novel predictor classifiers for inflammatory bowel disease by gene expression profiling

Background

Improvement of patient quality of life is the ultimate goal of biomedical research, particularly when dealing with complex, chronic and debilitating conditions such as inflammatory bowel disease (IBD). This is largely dependent on receiving an accurate and rapid diagnose, an effective treatment and in the prediction and prevention of side effects and complications. The low sensitivity and specificity of current markers burden their general use in the clinical practice. New biomarkers with accurate predictive ability are needed to achieve a personalized approach that take the inter-individual differences into consideration.

Methods

We performed a high throughput approach using microarray gene expression profiling of colon pinch biopsies from IBD patients to identify predictive transcriptional signatures associated with intestinal inflammation, differential diagnosis (Crohn’s disease or ulcerative colitis), response to glucocorticoids (resistance and dependence) or prognosis (need for surgery). Class prediction was performed with self-validating Prophet software package.

Results

Transcriptional profiling divided patients in two subgroups that associated with degree of inflammation. Class predictors were identified with predictive accuracy ranging from 67 to 100%. The expression accuracy was confirmed by real time-PCR quantification. Functional analysis of the predictor genes showed that they play a role in immune responses to bacteria (PTN, OLFM4 and LILRA2), autophagy and endocytocis processes (ATG16L1, DNAJC6, VPS26B, RABGEF1, ITSN1 and TMEM127) and glucocorticoid receptor degradation (STS and MMD2).

Conclusions

We conclude that using analytical algorithms for class prediction discovery can be useful to uncover gene expression profiles and identify classifier genes with potential stratification utility of IBD patients, a major step towards personalized therapy.

Advances in the development of nucleoside and nucleotide analogues for cancer and viral diseases

Nucleoside analogues have been in clinical use for almost 50 years and have become cornerstones of treatment for patients with cancer or viral infections. The approval of several additional drugs over the past decade demonstrates that this family still possesses strong potential. Here, we review new nucleoside analogues and associated compounds that are currently in preclinical or clinical development for the treatment of cancer and viral infections, and that aim to provide increased response rates and reduced side effects. We also highlight the different approaches used in the development of these drugs and the potential of personalized therapy.

Advances in the development of nucleoside and nucleotide analogues for cancer and viral diseases

Nucleoside analogues have been in clinical use for almost 50 years and have become cornerstones of treatment for patients with cancer or viral infections. The approval of several additional drugs over the past decade demonstrates that this family still possesses strong potential. Here, we review new nucleoside analogues and associated compounds that are currently in preclinical or clinical development for the treatment of cancer and viral infections, and that aim to provide increased response rates and reduced side effects. We also highlight the different approaches used in the development of these drugs and the potential of personalized therapy.

High-resolution melting analysis of the TPMT gene: a study in the Polish population

The thiopurine S-methyltransferase (TPMT) gene encoding thiopurine methyltransferase is a crucial enzyme in metabolism of thiopurine drugs: azathioprine and 6-mercoptopurine, which are used in the treatment of leukemia or inflammatory bowel diseases. Genetic polymorphism of the TPMT gene correlates with activity of this enzyme, individual reaction, and dosing of thiopurines. Thirty-one variants of the TPMT gene with low enzymatic activity have been described with three major alleles: TPMT*2 (c.238G>C), *3A (c.460 G>A, c.719A>G), and *3C (c.719A>G), accounting for 80% to 95% of inherited TPMT deficiency in different populations in the world. The aim of the study was to establish a rapid and highly sensitive method of analysis for the complete coding sequence of the TPMT gene and to determine the spectrum and prevalence of the TPMT gene sequence variations in the Polish population. Recently, high-resolution melting analysis (HRMA) has become a highly sensitive, automated, and economical technique for mutation screening or genotyping. We applied HRMA for the first time to TPMT gene scanning. In total, we analyzed 548 alleles of the Polish population. We found 11 different sequence variations, where two are novel changes: c.200T>C (p.P67S, TPMT*30) and c.595G>A (p.V199I, TPMT*31). Detection of these new rare alleles TPMT*30 and *31 in the Polish population suggests the need to analyze the whole TPMT gene and maybe also the extension of routinely used tests containing three major alleles, TPMT*2, *3A, and *3C. Identification of sequence variants using HRMA is highly sensitive and less time consuming compared to standard sequencing. We conclude that HRMA can be easy integrated into genetic testing of the TPMT gene in patients treated with thiopurines.

Personalized medicine: the impact on chemistry

An effective strategy for personalized medicine requires a major conceptual change in the development and application of therapeutics. In this article, we argue that further advances in this field should be made with reference to another conceptual shift, that of network pharmacology. We examine the intersection of personalized medicine and network pharmacology to identify strategies for the development of personalized therapies that are fully informed by network pharmacology concepts. This provides a framework for discussion of the impact personalized medicine will have on chemistry in terms of drug discovery, formulation and delivery, the adaptations and changes in ideology required and the contribution chemistry is already making. New ways of conceptualizing chemistry's relationship with medicine will lead to new approaches to drug discovery and hold promise of delivering safer and more effective therapies.

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

BACKGROUND AND AIMS

A pre-treatment determination of the thiopurine S-methyltransferase (TPMT) genotype or phenotype can identify patients at risk of developing severe adverse reactions from thiopurine treatment. The risk of misclassifying a patient might be dependent on the method used. The aim of this study was to investigate the concordance between TPMT genotyping and phenotyping.

METHODS

The data consist of 7195 unselected and consecutive TPMT genotype and phenotype determinations sent to the division of Clinical Pharmacology, Linköping, Sweden. TPMT activity was measured in red blood cells (RBC) and the genotype determined by pyrosequencing for the three most common TPMT variants (TPMT *2, *3A, *3C).

RESULTS

TPMT genotyping identified 89% as TPMT wild type (*1/*1), 10% as TPMT heterozygous and 0.5% as TMPT defective. The overall concordance between genotyping and phenotyping was 95%, while it was 96% among IBD patients (n=4024). Genotyping would have misclassified 8% of the TPMT defectives as heterozygous as compared to 11% if only TPMT activity had been measured. 11% of the heterozygous patients had a normal TPMT activity (>8.9 U/ml RBC) and 3% of the TPMT wild-type patients had an intermediate TPMT activity (2.5-8.9 U/ml RBC).

CONCLUSIONS

There is a risk for TPMT misclassification when only genotyping or phenotyping is used, but it is not reasonable to check both in all patients. Since TPMT genotyping is the more reliable test, especially in TPMT heterozygotes, we suggest that genotyping should be considered the primary choice for the pre-treatment evaluation of TPMT function before initiation of thiopurine therapy.

A functional polymorphism in UGT1A1 related to hyperbilirubinemia is associated with a decreased risk for Crohn's disease

BACKGROUND

An imbalance between the production of reactive oxygen species (ROS) and their capturing by antioxidants results in oxidative stress, this may play an important role in the pathogenesis of inflammatory bowel disease (IBD). Since bilirubin is an important endogenous antioxidant, increased levels of bilirubin may protect against IBD. UDP-glucuronosyltransferase 1A1 (UGT1A1) is the only enzyme involved in the conjugation of bilirubin and the common UGT1A1*28 allele in the UGT1A1 gene, which is strongly associated with Gilbert's syndrome in Caucasians, results in elevated plasma bilirubin levels.

AIMS

To test the hypothesis that the UGT1A1*28 allele is associated with lower disease susceptibility to, and disease behavior within, IBD. In addition, a possible altered risk for developing IBD-drug related side-effects was explored.

METHODOLOGY

Genomic DNA of 751 patients with IBD (209 patients with ulcerative colitis and 542 patients with Crohn's disease) and 930 healthy controls was genotyped for the UGT1A1*28 promoter polymorphism, and genotype distribution was compared between patients and controls. Genotype phenotype interactions were also investigated.

RESULTS

Patients with Crohn's disease significantly less often bear the UGT1A1*28 homozygous genotype compared to the control group, with an odds ratio of 0.64, 95% CI: 0.42-0.98. The ulcerative colitis group showed no significant differences compared to controls.

CONCLUSION

The homozygous state of the UGT1A1*28 polymorphism, associated with higher serum bilirubin levels, may be protective for the development of Crohn's disease, suggesting that the anti-oxidant capacity of bilirubin may play a part.

Thiopurine methyltransferase genotype and thiopurine S-methyltransferase activity in Greek children with inflammatory bowel disease

BACKGROUND

Azathioprine (AZA) and 6-mercaptopurine (6MP) are used in the treatment of pediatric inflammatory bowel disease (IBD). Genetic variations in thiopurine S-methyltranfarase (TPMT) gene have been correlated with enzyme activity and with the occurrence of adverse events to AZA and 6MP. The aim of the present study was to examine the sensitivity and specificity of TPMT genotyping for TPMT enzymatic activity, reducing harm from thiopurine by pretesting, and the association of thiopurine toxicity with TPMT status in children with IBD.

METHODS

TPMT red blood cell (RBC) activity was measured by using a radiochemical method and genotype was determined for the TPMT alleles *2, *3A, *3B and *3C in 108 thiopurinetreated pediatric IBD patients with a mean age of 11.3 years (range 3-16).

RESULTS

Significant TPMT activity differences between wild-type and heterozygous and homozygous mutated subjects were observed. We divided TPMT activity into three categories according to frequency distribution: low (16.67%), intermediate (25.92%) and high (57.41%). The whole population included a total of 77.78% of homozygous wild-type subjects, 15.74% heterozygous variants, 1.85% homozygous variants and five (4.63%) compound heterozygous variant TPMT*3B/*3C. The overall concordance rate between TPMT genotypes and phenotypes was 88.2%. Seven carriers of at least one variant allele and low or intermediate TPMT activity developed adverse effects.

CONCLUSIONS

Our findings suggest that carriers of at least one variant allele and both intermediate and absent TPMT activity have an increased risk of developing thiopurine-induced myelotoxicity compared with individuals with normal genotype and TPMT activity.

Genetic polymorphisms affecting drug metabolism: recent advances and clinical aspects

Though current knowledge of pharmacogenetic factors relevant to drug metabolism is fairly comprehensive and this should facilitate translation to the clinic, there are a number of gaps in knowledge. Recent studies using both conventional and novel approaches have added to our knowledge of pharmacogenetics of drug metabolism. Genome-wide association studies have provided new insights into the major contribution of cytochromes P450 to response to therapeutic agents such as coumarin anticoagulants and clopidogrel as well as to caffeine and nicotine. Recent advances in understanding of factors affecting gene expression, both regulation by transcription factors and by microRNA and epigenetic factors, have added to understanding of variation in expression of genes such as CYP3A4 and CYP2E1. The implementation of testing for pharmacogenetic polymorphisms in prescription of selected anticancer drugs and cardiovascular agents is considered in detail, with current controversies and barriers to implementation of pharmacogenetic testing assessed. Though genotyping for thiopurine methyltransferase is now common prior to prescription of thiopurines, genotyping for other pharmacogenetic polymorphisms prior to drug prescription remains uncommon. However, it seems likely that it will become more widespread as both increased evidence that certain pharmacogenetic tests are valuable and cost-effective and more accessible genotyping methods become available.

Pharmacogenetics in type 2 diabetes: potential implications for clinical practice

Pharmacogenetic research aims to study how genetic variation may influence drug efficacy and/or toxicity; pharmacogenomics expands this quest to the entire genome. Pharmacogenetic findings may help to uncover new drug targets, illuminate pathophysiology, clarify disease heterogeneity, aid in the fine-mapping of genetic associations, and contribute to personalized treatment. In diabetes, there is precedent for the successful application of pharmacogenetic concepts to monogenic forms of the disease, such as maturity onset diabetes of the young or neonatal diabetes. Whether similar insights will be produced for the common form of type 2 diabetes remains to be seen. With recent advances in genetic approaches, the successive application of candidate gene studies, large-scale genotyping studies and genome-wide association studies has begun to generate suggestive results that may lead to changes in clinical practice. However, many potential barriers to the translation of pharmacogenetic discoveries to the clinical management of diabetes still remain. Here, we offer a contemporary overview of the field in its current state, identify potential obstacles, and highlight future directions.

How are thiopurines used and monitored by Swedish gastroenterologists when treating patients with inflammatory bowel disease?

OBJECTIVE

To perform a survey of thiopurine treatment in inflammatory bowel disease (IBD) among Swedish gastroenterologists.

MATERIAL AND METHODS

A web-based questionnaire consisting of 25 multiple-choice questions was sent to 322 gastroenterologists in adult practice.

RESULTS

A total of 132 questionnaires were received giving a response rate of 41%. Thiopurines were used by all 122 gastroenterologists in IBD practice and azathioprine was the first-choice thiopurine among 118 (97%) of them. Almost all gastroenterologists (97%) used weight-based dosing that was gradually escalated. The vast majority (89%) considered that efficacy should be evaluated within 6 months of therapy, while opinions regarding the optimal duration of therapy varied considerably. It was seen that 74% switched thiopurine in case of intolerance to the first-line substance. Thiopurine S-methyltransferase (TPMT) determinations were performed by 74% of the gastroenterologists and 67% used metabolite measurements. TPMT analyzers were more likely to measure metabolites (74 vs. 43%, p = 0.002). A quarter of the respondents were familiar with unconventional immunomodulation (co-administration of allopurinol, 6-thioguanine, mycophenolate mofetil or tacrolimus) and these respondents were also more likely to measure metabolites (79 vs. 52%; p = 0.002).

CONCLUSIONS

Thiopurines are well established in the treatment of IBD among Swedish gastroenterologists. New and evolving knowledge about thiopurine therapy in IBD has been adapted to a large extent. Whether this change in clinical practice will have an impact on treatment outcomes has yet to be proven.

Clinical application of high throughput molecular screening techniques for pharmacogenomics

Genetic analysis is one of the fastest-growing areas of clinical diagnostics. Fortunately, as our knowledge of clinically relevant genetic variants rapidly expands, so does our ability to detect these variants in patient samples. Increasing demand for genetic information may necessitate the use of high throughput diagnostic methods as part of clinically validated testing. Here we provide a general overview of our current and near-future abilities to perform large-scale genetic testing in the clinical laboratory. First we review in detail molecular methods used for high throughput mutation detection, including techniques able to monitor thousands of genetic variants for a single patient or to genotype a single genetic variant for thousands of patients simultaneously. These methods are analyzed in the context of pharmacogenomic testing in the clinical laboratories, with a focus on tests that are currently validated as well as those that hold strong promise for widespread clinical application in the near future. We further discuss the unique economic and clinical challenges posed by pharmacogenomic markers. Our ability to detect genetic variants frequently outstrips our ability to accurately interpret them in a clinical context, carrying implications both for test development and introduction into patient management algorithms. These complexities must be taken into account prior to the introduction of any pharmacogenomic biomarker into routine clinical testing.

Thiopurine methyltransferase (TPMT) genotyping to predict myelosuppression risk

Azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine (TG) are thiopurine drugs. These agents are indicated for the treatment of various diseases including hematologic malignancies, inflammatory bowel disease (IBD), rheumatoid arthritis, and as immunosuppressants in solid organ transplants. Thiopurine drugs are metabolized, in part, by thiopurine methyltransferase (TPMT). TPMT displays genetic polymorphism resulting in null or decreased enzyme activity. At least 20 polymorphisms have been identified, of which, TPMT *2, *3A, *3B, *3C, and *4 are the most commonly studied. These polymorphisms have been associated with increased myelosuppression risk. TPMT genotyping may be useful to predict this risk.

A pragmatic randomized controlled trial of thiopurine methyltransferase genotyping prior to azathioprine treatment: the TARGET study

AIM

To conduct a pragmatic, randomized controlled trial to assess whether thiopurine methyltransferase (TPMT) genotyping prior to azathioprine reduces adverse drug reactions (ADRs).

METHODS

A total of 333 participants were randomized 1:1 to undergo TPMT genotyping prior to azathioprine or to commence treatment without genotyping.

RESULTS

There was no difference in the primary outcome of stopping azathioprine due to an adverse reaction (ADR, p = 0.59) between the two study arms. ADRs were more common in older patients (p = 0.01). There was no increase in stopping azathioprine due to ADRs in TPMT heterozygotes compared with wild-type individuals. The single individual with TPMT variant homozygosity experienced severe neutropenia.

CONCLUSION

Our work supports the strong evidence that individuals with TPMT variant homozygosity are at high risk of severe neutropenia, whereas TPMT heterozygotes are not at increased risk of ADRs at standard doses of azathioprine.

Clinical Pharmacogenetics Implementation Consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing

Thiopurine methyltransferase (TPMT) activity exhibits monogenic co-dominant inheritance, with ethnic differences in the frequency of occurrence of variant alleles. With conventional thiopurine doses, homozygous TPMT-deficient patients (~1 in 178 to 1 in 3,736 individuals with two nonfunctional TPMT alleles) experience severe myelosuppression, 30-60% of individuals who are heterozygotes (~3-14% of the population) show moderate toxicity, and homozygous wild-type individuals (~86-97% of the population) show lower active thioguanine nucleolides and less myelosuppression. We provide dosing recommendations (updates at http://www.pharmgkb.org) for azathioprine, mercaptopurine (MP), and thioguanine based on TPMT genotype.

Finding a niche with personalized generics: opportunities from systems-based therapeutic delivery in hypertension

A key principle in earlier eras of drug development was to deliver medicines with clinical benefits for populations. Individual patients frequently did not benefit, or experienced adverse drug reactions, and were at risk of exposure to a prolonged series of treatment trials before effective therapies were found, if available. A personalized medicines approach offers opportunities to select drugs likely to be effective or safer, based on knowledge, for example, of differences in disease-modulating receptors, in drug metabolism, or in drug transporters into cells and across tissue boundaries. This new genetic and phenotypic knowledge allows generics to be revisited and may also help to improve medicine adherence, by reducing predictable adverse effects from unwanted accumulation of a medicine or its metabolites. This review will consider effective population delivery of therapeutics to treat hypertension in order to illustrate the potential and current place of personalizing medicines to improve effective and safe use of therapeutics.

Expressing thiopurine S-methyltransferase activity as units per litre of whole-blood overcomes misleading high results in patients with anaemia

BACKGROUND

Thiopurine S-methyltransferase (TPMT) phenotype analysis, expressed as TPMT activity, is established as a routine pharmacogenomic test to screen patients prior to initiating thiopurine drug therapy. Conventionally measured TPMT activity is corrected for red blood cell (RBC) parameters. Here we present evidence that supports the simplification of the TPMT assay: by expressing TPMT activity in mU/L whole blood, without undertaking any haemoglobin (Hb) correction.

METHODS

Hb concentrations were compared in consecutive samples that had been received for TPMT phenotype analysis and which were stratified into samples with high (n = 111) and samples with normal (n = 50) Hb-corrected enzyme activity. TPMT activity was also measured in samples received for full blood count determination, stratified into those with low (n = 50) and normal (n = 50) Hb. A reference interval for TPMT activity in mU/L was derived from a correlation between activity expressed in conventional units and that expressed in mU/L (n = 1563), supported by comparison with associated genotype (n = 201).

RESULTS

In the high TPMT activity group, 83% of specimens had a low Hb concentration compared with 14% of specimens in the normal TPMT group. Samples with a low Hb concentration were found to have significantly higher Hb-corrected TPMT activity than samples with a normal Hb concentration: 83 versus 44 nmol 6-methyl thioguanine /g Hb/h, P < 0.0001. These results strongly suggest that misleading high Hb-corrected TPMT activity is found in anaemic patients. Based on the reference interval for enzyme activity of 70-150 mU/L, phenotype-genotype concordance compared well with the conventional approach (88% versus 89%). Furthermore, distribution of TPMT phenotypes with activity expressed in mU/L was identical: 0.5% deficient, 11% low, 86% normal and 2.5% high, to when it was expressed in conventional units.

CONCLUSION

Expressing TPMT activity in mU/L can overcome misleading high Hb-corrected TPMT results occurring in patients with anaemia, which could lead to inappropriate treatment. Removing the need to measure RBC indices further simplifies TPMT phenotyping, leading to a more robust assay, with reduced turn-around time and cost.