Systematic Review of Thiopurine Methyltransferase Genotype and Enzymatic Testing Strategies

Optimizing thiopurine therapy in children with acute lymphoblastic leukemia: A promising “MINT” sequencing strategy and therapeutic “DNA-TG” monitoring

Thiopurines, including thioguanine (TG), 6-mercaptopurine (6-MP), and azathioprine (AZA), are extensively used in clinical practice in children with acute lymphoblastic leukemia (ALL) and inflammatory bowel diseases. However, the common adverse effects caused by myelosuppression and hepatotoxicity limit their application. Metabolizing enzymes such as thiopurine S-methyltransferase (TPMT), nudix hydrolase 15 (NUDT15), inosine triphosphate pyrophosphohydrolase (ITPA), and drug transporters like multidrug resistance-associated protein 4 (MRP4) have been reported to mediate the metabolism and transportation of thiopurine drugs. Hence, the single nucleotide polymorphisms (SNPs) in those genes could theoretically affect the pharmacokinetics and pharmacological effects of these drugs, and might also become one of the determinants of clinical efficacy and adverse effects. Moreover, long-term clinical practices have confirmed that thiopurine-related adverse reactions are associated with the systemic concentrations of their active metabolites. In this review, we mainly summarized the pharmacogenetic studies of thiopurine drugs. We also evaluated the therapeutic drug monitoring (TDM) research studies and focused on those active metabolites, hoping to continuously improve monitoring strategies for thiopurine therapy to maximize therapeutic efficacy and minimize the adverse effects or toxicity. We proposed that tailoring thiopurine dosing based on MRP4, ITPA, NUDT15, and TMPT genotypes, defined as “MINT” panel sequencing strategy, might contribute toward improving the efficacy and safety of thiopurines. Moreover, the DNA-incorporated thioguanine nucleotide (DNA-TG) metabolite level was more suitable for red cell 6-thioguanine nucleotide (6-TGNs) monitoring, which can better predict the efficacy and safety of thiopurines. Integrating the panel “MINT” sequencing strategy with therapeutic “DNA-TG” monitoring would offer a new insight into the precision thiopurine therapy for pediatric acute lymphoblastic leukemia patients.

Predictors of azathioprine toxicity in children with autoimmune hepatitis

Aim of the study

Azathioprine (AZA) is an important steroid-sparing drug in the management of autoimmune hepatitis (AIH). Avoidance of its adverse events that could be severe and carry a risk of mortality in a few cases is important, preferably with cheap and easy assessments that could be feasible in developing countries with the unavailability of molecular assays. Assessment of thiopurine methyltransferase (TPMT), the key enzyme for the inactivation of AZA, as a predictor of AZA toxicity had been a matter of conflict. This work aimed to study the role of TPMT serum level assessment and other host-, disease-, and treatment-related factors in predicting AZA toxicity.

Material and methods

Sixty-six children with AIH, divided into two groups, were recruited. Group 1 included twelve children with AZA toxicity and group 2 included fifty-four children without AZA toxicities. Both groups were compared for demographic, clinical, laboratory, histopathological, and treatment-related factors, and serum TPMT level, measured by ELISA.

Results

TPMT serum level was comparable in both groups (p = 0.363). Duration of treatment until enzyme normalization and duration of AZA therapy were significantly associated with AZA toxicity (p = 0.007 and p = 0.01, respectively). At the first follow-up treatment with AZA, total leucocyte count (TLC) and neutrophil counts were significantly lower in group 1 (p = 0.005 and p = 0.002, respectively). Moreover, the percentage reduction of TLC and neutrophil counts were significantly higher in group 1 (p < 0.001, for both).

Conclusions

Monitoring for AZA adverse events in those with the defined predictors of AZA-related adverse events is more important than TPMT assessment.

WITHDRAWN: Association between TPMT*3C and decreased thiopurine S-methyltransferase activity in patients with neuromyelitis optica spectrum disorders in China

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Implementation of Standardized Clinical Processes for TPMT Testing in a Diverse Multidisciplinary Population: Challenges and Lessons Learned

Although thiopurine S‐methyltransferase (TPMT) genotyping to guide thiopurine dosing is common in the pediatric cancer population, limited data exist on TPMT testing implementation in diverse, multidisciplinary settings. We established TPMT testing (genotype and enzyme) with clinical decision support, provider/patient education, and pharmacist consultations in a tertiary medical center and collected data over 3 years. During this time, 834 patients underwent 873 TPMT tests (147 (17%) genotype, 726 (83%) enzyme). TPMT tests were most commonly ordered for gastroenterology, rheumatology, dermatology, and hematology/oncology patients (661 of 834 patients (79.2%); 580 outpatient vs. 293 inpatient; P < 0.0001). Thirty‐nine patients had both genotype and enzyme tests (n = 2 discordant results). We observed significant differences between TPMT test use and characteristics in a diverse, multispecialty environment vs. a pediatric cancer setting, which led to unique implementation needs. As pharmacogenetic implementations expand, disseminating lessons learned in diverse, real‐world environments will be important to support routine adoption.

Association between TPMT*3C and decreased thiopurine S-methyltransferase activity in patients with neuromyelitis optica spectrum disorders in China

AIM OF THE STUDY

Thiopurines are effective drugs in treating neuromyelitis optica spectrum disorders and other diseases. Thiopurines' toxicity is mainly imputed to thiopurine S-methyltransferase activity. In Chinese population, the most common and important variation of thiopurine S-methyltransferase is TPMT*3C (rs1142345). This study aims to reveal the association between thiopurine S-methyltransferase activity and genetic polymorphisms of thiopurine S-methyltransferase in patients with neuromyelitis optica spectrum disorders in China.

MATERIAL AND METHODS

A liquid chromatography tandem mass/mass method was used to evaluate the thiopurine S-methyltransferase activity by using 6-mercapthioprine as the substrate in human erythrocyte haemolysate via 1 h incubation at 37 °C to form its methylated product 6-methylmercaptopurine. The amount of 6-methylmercaptopurine was adjusted by haematocrit and normalized to 8 × 10⁸ erythrocytes. The selected polymorphisms of thiopurine S-methyltransferase were identified using MassARRAY system (Sequenom) and multiple SNaPshot technique.

RESULTS

In 69 patients with neuromyelitis optica spectrum disorders, thiopurine S-methyltransferase activity was 80.29-154.53 (127.51 ± 16.83) pmol/h/8 × 10⁸ erythrocytes. TPMT*3C (rs1142345) was associated with lower thiopurine S-methyltransferase activity (BETA = -25.37, P = 0.011). Other selected variants were not associated with thiopurine S-methyltransferase activity.

CONCLUSIONS

TPMT*3C affects TPMT activity in Chinese patients with neuromyelitis optica spectrum disorders. Further studies are warranted to confirm the results.

ABBREVIATIONS

TPRs = thiopurines; NMOSD = neuromyelitis optica spectrum disorders; TPMT = thiopurine S-methyltransferase; LC-MS/MS = liquid chromatography tandem mass/mass; 6-MMP = 6-methylmercaptopurine; IS = internal standard; SNP = single nucleotide polymorphism; MAF = minor allele frequency; HWE = Hardy-Weinberg equilibrium; BETA = regression coefficients; UTR-3 = untranslated region 3.

Meta-analysis of the impact of thioprine S-methyltransferase polymorphisms on the tolerable 6-mercaptopurine dose considering initial dose and ethnic difference

A meta-analysis was conducted to decide whether to reduce an initial 6-mercaptopurine (6-MP) dose in TPMT heterozygote in the case of an initial 6-MP dose of <75 mg/m²/d and to compare the tolerable 6-MP dose among different ethnic groups. The study was undertaken according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The differences in mean values of the tolerable 6-MP dose were calculated by using Comprehensive Meta-Analysis version 3. The results of the meta-analysis indicated that the tolerable 6-MP dose was significantly lower in the TPMT heterozygote group (difference in mean values =11.729, 95% confidence interval =7.617-15.842, P<0.001) even when the initial 6-MP dose was <75 mg/m²/d. The TPMT*3C allele-dominant ethnic group (Asian) needed less reduction in mean 6-MP dose in comparison to the TPMT*3A allele-dominant ethnic group (Caucasian, Mediterranean, South American) (difference in mean values =8.884 vs 15.324). In conclusion, the initial 6-MP dose needs to be reduced in TPMT heterozygote when compared to the wild-type, and ethnic difference might influence the tolerable 6-MP dose in TPMT heterozygotes.

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.

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.

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.

The 3-I framework: a framework for developing public policies regarding pharmacogenomics (PGx) testing in Canada

The 3-I framework of analyzing the ideas, interests, and institutions around a topic has been used by political scientists to guide public policy development. In Canada, there is a lack of policy governing pharmacogenomics (PGx) testing compared to other developed nations. The goal of this study was to use the 3-I framework, a policy development tool, and apply it to PGx testing to identify and analyze areas where current policy is limited and challenges exist in bringing PGx testing into wide-spread clinical practice in Canada. A scoping review of the literature was conducted to determine the extent and challenges of PGx policy implementation at federal and provincial levels. Based on the 3-I analysis, contentious ideas related to PGx are (i) genetic discrimination, (ii) informed consent, (iii) the lack of knowledge about PGx in health care, (iv) the value of PGx testing, (v) the roles of health care workers in the coordination of PGx services, and (vi) confidentiality and privacy. The 3-I framework is a useful tool for policy makers, and applying it to PGx policy development is a new approach in Canadian genomics. Policy makers at every organizational level can use this analysis to help develop targeted PGx policies.

Toward a personalized medicine approach to the management of inflammatory bowel disease

The medical management of inflammatory bowel disease (IBD) is evolving toward a personalized medicine-based model. Modern therapeutic algorithms that feature use of tumor necrosis factor (TNF) antagonists in combination with immunosuppressive are highly effective when initiated in high-risk patients early in the course of disease. Defined targets that guide intensification of therapy are critical interventions. In this model, therapy is optimized through appropriate pretreatment testing, therapeutic drug monitoring, and patient-based monitoring strategies. This review discusses the current application of personalized medicine to the management of IBD.

Personalized pharmacogenomics: predicting efficacy and adverse drug reactions

Drug response varies between individuals owing to disease heterogeneity, environmental factors, and genetic factors. Genetic factors can affect both the pharmacokinetics and pharmacodynamics of a drug, leading to changes in local and systemic drug exposure and/or changes in the function of the drug target, altering drug response. Several pharmacogenetic biomarkers are already utilized in clinical practice and have been shown to improve clinical outcomes. However, a large number of other biomarkers have never made it beyond the discovery stage. Concerted effort is needed to improve the translation of pharmacogenetic biomarkers into clinical practice, and this will involve the use of standardized phenotyping and genotyping strategies, collaborative work, multidisciplinary approaches to identifying and replicating associations, and cooperation with industry to facilitate translation and commercialization. Acceptance of these approaches by clinicians, regulators, patients, and the public will be important in determining future success.

Establishment of thiopurine S-methyltransferase gene knockdown in jurkat T-lymphocytes: an in vitro model of TPMT polymorphism

BACKGROUND

Thiopurine S-methyltransferase (TPMT) is an excellent example of an enzyme whose pharmacogenetic polymorphisms affect efficacy and toxicity of a drug. The association between TPMT activity and thiopurine-related myelosuppression is well recognized. To study the significance of TPMT deficiency in thiopurine metabolism and immunosuppressive activity in vitro, we established RNA interference-based TPMT knockdown (kd) in a Jurkat cell line.

RESULTS

In Jurkat TPMT kd cells, TPMT expression was reduced to 73% at the RNA level and 83% at the protein level. TPMT kd cells were more sensitive to 6-mercaptopurine (6-MP) (10 μmol/L) and 6-thioguanine (6-TG) (8 μmol/L) than wild-type (wt) cells, (32% versus 20%) and (18% versus 9%), respectively. Both Jurkat wt and kd cells were more sensitive to 6-TG-induced apoptosis than to 6-MP. 6-TG activity was also more affected by TPMT levels than was 6-MP as reflected by IC60, concentrations that is, 6-MP [4.6 μmol/L (wt) and 4.7 μmol/L (kd)], 6-TG [2.7 μmol/L (wt) and 0.8 μmol/L (kd)]. IC60 concentrations induced significant apoptosis in both Jurkat wt and kd cells (257%, versus 314%) with 6-MP and (323% versus 306%) with 6-TG, respectively. At IC60 (6-MP) 6-thioguanine nucleotides (6-TGN) accumulation in cells was 518 versus 447 pmol/million cells in wt and kd cells, respectively. On the other hand 6-TGN accumulation at IC60 (6-TG) was 477 versus 570 pmol/million cells in wt and kd cells, respectively. 6-Methylated mercaptopurine (6-MeMP) concentrations were more affected than 6-TGN by TPMT kd (194 versus 10 pmol/million cells) in wt and kd cells, respectively.

CONCLUSION

We conclude that TPMT kd cells are an appropriate in vitro model to investigate the significance of TPMT deficiency with thiopurine therapy and could be helpful in understanding possible clinical consequences of TPMT polymorphism.

Can knowledge of germline markers of toxicity optimize dosing and efficacy of cancer therapy?

The systemic treatment of cancer with traditional cytotoxic chemotherapeutic agents and more targeted agents is often complicated by the onset of adverse drug reactions. Pharmacogenetic prediction of adverse drug reactions might have consequences for dosing and efficacy. This review discusses relevant examples where the germline variant-toxicity relationship has been validated as an initial step in developing clinically useful pharmacogenetic markers and provides examples where germline variants have influenced dosing strategies and/or survival or other outcomes of efficacy. This review will also provide insight into the reasons why more pharmacogenetic markers have not been routinely integrated into clinical practice.

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.