Genetic polymorphism of thiopurine methyltransferase and its clinical relevance for childhood acute lymphoblastic leukemia

Impact of the loss of slc43a3 on 6-mercaptopurine absorption and tissue distribution in mice

6-Mercaptopurine (6-MP) is a nucleobase analog used in the therapy of acute lymphoblastic leukemia and inflammatory bowel disease. It is associated with numerous side effects including myelotoxicity, hepatotoxicity, and gastrointestinal complications, which can lead to patient adherence issues or discontinuation of treatment. This is further complicated by the wide variability in plasma levels of 6-MP and the therapeutic response to a standard dose. Although a number of enzyme polymorphisms have been linked to therapeutic response, it is unclear what factors underlie the variability in plasma levels. We have established that SLC43A3-encoded equilibrative nucleobase transporter 1 mediates the transport of 6-MP into cells in both mice and humans. To determine whether this transporter is critical for 6-MP absorption and biodistribution, we examined the effect of the genetic deletion of slc43a3 in mice on the absorption and tissue distribution of orally administered 6-MP. A high-performance liquid chromatography method was developed to measure tissue levels of 6-MP and its key metabolites, 6-methylmercaptoprine, 6-thiourate, and 6-thioguanine nucleotides. The results of this study show that loss of slc43a3 dramatically reduces the absorption of 6-MP from the gastrointestinal tract and attenuates the levels achieved in peripheral tissues. Furthermore, the loss of slc43a3 decreases the tissue:blood concentration ratios of 6-MP and its metabolites, particularly in those tissues that show high levels of expression of slc43a3, such as the heart and lungs. Therefore, it is possible that differences in SLC43A3 expression in humans may contribute to the variability seen in 6-MP plasma levels and therapeutic response. SIGNIFICANCE STATEMENT: The loss of slc43a3 in mice dramatically reduces the absorption and the biodistribution of the chemotherapeutic drug 6-mercaptopurine. These data suggest that variations in SLC43A3 expression in humans may contribute to the variability in plasma levels that have been reported when using this drug therapeutically.

Thiopurine S-methyltransferase - An important intersection of drug-drug interactions in thiopurine treatment

Understanding the molecular mechanisms of medicines is crucial for developing novel drugs, for repurposing existing medicines, and for predicting toxicities. Thiopurine S-methyltransferase (TPMT) serves as an exemplary case in personalized medicine, as its activity is influenced by genetic variants, co-factors, substrates, and inhibitors, which lead to diverse outcomes in thiopurine therapy. This comprehensive review explores the role of TPMT in drug-drug interactions by investigating its interactions with co-factors, substrates, and inhibitors. We focus on the principal interactions of TPMT with clinically relevant inhibitors, and add to this information with molecular docking analyses for the substrate and co-factor binding sites of TPMT. Notably, methotrexate and sulfasalazine emerged as the top-ranked compounds with favorable docking scores for the co-factor binding site, while furosemide is presented as the highest ranked inhibitor for the substrate binding site. Furthermore, we highlight the chemical and structural properties governing ligand binding to TPMT. We support the molecular characteristics by using a summary of clinical implications. Examining the molecular interactions between substrates or inhibitors and TPMT not only addresses therapeutic consequences, but also reveals potential novel indications of interacting compounds. These insights are also invaluable for identifying endogenous ligands and enhancing our understanding of TPMT's biological function.

Importance of Sex-Dependent Differences for Dosing Selection and Optimization of Chemotherapeutic Drugs

BACKGROUND

Despite major advances in cancer treatment in the past years, there is a need to optimize chemotherapeutic drug dosing strategies to reduce toxicities, suboptimal responses, and the risk of relapse. Most cancer drugs have a narrow therapeutic index with substantial pharmacokinetics variability. Yet, current dosing approaches do not fully account for the complex pathophysiological characteristics of the patients. In this regard, the effect of sex on anticancer chemotherapeutic drugs' disposition is still underexplored. In this article, we review sex differences in chemotherapeutic drug pharmacokinetics; we suggest a novel approach that integrates sex into the traditional a priori body surface area (BSA) dosing selection model, and finally, we provide an overview of the potential benefits of a broader use of therapeutic drug monitoring (TDM) in oncology.

SUMMARY

To date, anticancer chemotherapeutic drug dosing is most often determined by BSA, a method widely used for its ease of practice, despite criticism for not accounting for individual factors, notably sex. Anatomical, physiological, and biological differences between males and females can affect pharmacokinetics, including drug metabolism and clearance. At equivalent doses, females tend to display higher circulating exposure and more organ toxicities, which has been formally demonstrated at present for about 20% of chemotherapeutic drugs. An alternative could be the sex-adjusted BSA (SABSA), incorporating a 10% increase in dosing for males and a 10% decrease for females, though this approach still lacks formal clinical validation. Another strategy to reduce treatment-related toxicity and potentially enhance clinical outcomes could be a more widespread use of TDM, for which a benefit has been demonstrated for 5-fluorouracil, busulfan, methotrexate, or thiopurines.

KEY MESSAGES

The inclusion of sex besides BSA in an easy-to-implement formula such as SABSA could improve a priori chemotherapy dosing selection, even though it still requires clinical validation. The a posteriori use of TDM could further enhance treatment efficacy and safety in oncology.

Differential molecular mechanisms of substrate recognition by selenium methyltransferases, INMT and TPMT, in selenium detoxification and excretion

It is known that the recommended dietary allowance of selenium (Se) is dangerously close to its tolerable upper intake level. Se is detoxified and excreted in urine as trimethylselenonium ion (TMSe) when the amount ingested exceeds the nutritional level. Recently, we demonstrated that the production of TMSe requires two methyltransferases: thiopurine S-methyltransferase (TPMT) and indolethylamine N-methyltransferase (INMT). In this study, we investigated the substrate recognition mechanisms of INMT and TPMT in the Se-methylation reaction. Examination of the Se-methyltransferase activities of two paralogs of INMT, namely, nicotinamide N-methyltransferase and phenylethanolamine N-methyltransferase, revealed that only INMT exhibited Se-methyltransferase activity. Consistently, molecular dynamics simulations demonstrated that dimethylselenide was preferentially associated with the active center of INMT. Using the fragment molecular orbital method, we identified hydrophobic residues involved in the binding of dimethylselenide to the active center of INMT. The INMT-L164R mutation resulted in a deficiency in Se- and N-methyltransferase activities. Similarly, TPMT-R152, which occupies the same position as INMT-L164, played a crucial role in the Se-methyltransferase activity of TPMT. Our findings suggest that TPMT recognizes negatively charged substrates, whereas INMT recognizes electrically neutral substrates in the hydrophobic active center embedded within the protein. These observations explain the sequential requirement of the two methyltransferases in producing TMSe.

Population-Specific Distribution of TPMT Deficiency Variants and Ancestry Proportions in Ecuadorian Ethnic Groups: Towards Personalized Medicine

Purpose

Thiopurine S-methyltransferase (TPMT) is an enzyme that metabolizes purine analogs, agents used in the treatment of acute lymphoblastic leukemia. Improper drug metabolism leads to toxicity in chemotherapy patients and reduces treatment effectiveness. TPMT variants associated with reduced enzymatic activity vary across populations. Therefore, studying these variants in heterogeneous populations, such as Ecuadorians, can help identify molecular causes of deficiency for this enzyme.

Methods

We sequenced the entire TPMT coding region in 550 Ecuadorian individuals from Afro-Ecuadorian, Indigenous, Mestizo, and Montubio ethnicities. Moreover, we conducted an ancestry analysis using 46 informative ancestry markers.

Results

We identified 8 single nucleotide variants in the coding region of TPMT. The most prevalent alleles were TPMT*3A, TPMT*3B, and TPMT*3C, with frequencies of 0.055, 0.012, and 0.015, respectively. Additionally, we found rare alleles TPMT*4 and TPMT*8 with frequencies of 0.005 and 0.003. Correlating the ancestry proportions with TPMT-deficient genotypes, we observed that the Native American ancestry proportion influenced the distribution of the TPMT*1/TPMT*3A genotype (OR = 5.977, p = 0.002), while the contribution of African ancestral populations was associated with the TPMT*1/TPMT*3C genotype (OR = 9.769, p = 0.003). The rates of TPMT-deficient genotypes observed in Mestizo (f = 0.121) and Indigenous (f = 0.273) groups provide evidence for the influence of Native American ancestry and the prevalence of the TPMT*3A allele. In contrast, although Afro-Ecuadorian groups demonstrate similar deficiency rates (f = 0.160), the genetic factors involved are associated with contributions from African ancestral populations, specifically the prevalent TPMT*3C allele.

Conclusion

The distribution of TPMT-deficient variants offers valuable insights into the populations under study, underscoring the necessity for genetic screening strategies to prevent thiopurine toxicity events among Latin American minority groups.

Clinical characteristics, diagnosis and management of Sweet syndrome induced by azathioprine

Sweet syndrome is a rare complication of azathioprine treatment with unelucidated clinical features. The purpose of this study was to investigate the clinical characteristics of azathioprine-induced Sweet syndrome (AISS) and provide a reference for diagnosis, treatment and prognosis. We collected relevant case reports of AISS by searching Chinese and English databases from 1960 to December 31, 2022, extracted the data and carried out a retrospective analysis. The median age of the 44 patients was 50 (range 9-89) years, and they included 32 males (72.7%). Fever (86.4%) and arthralgia (31.8%) were the most common clinical symptoms. The skin lesions were mainly pustules (54.5%), papules (40.9%), plaques (40.9%) and nodules (31.8%), which were mainly distributed on the extremities (54.5%), face (38.6%) and hands (36.4%). Laboratory examination revealed neutropenia (65.9%) as well as elevated C-reactive protein (63.6%) and erythrocyte sedimentation (40.9%) rates. Histopathology of the lesioned skin showed neutrophil infiltration (93.2%) and dermal edema (38.6%). Symptom relief was achieved at a median time of 7 days (range 2-28 days) after azathioprine discontinuation in all patients. Nine patients (20.5%) had skin lesions that recurred within 24 h after taking azathioprine again. Clinicians and pharmacists should grasp the regularity and characteristics of AISS and should not recommend the readministration of azathioprine, to avoid the recurrence of Sweet syndrome.

Technical Validation and Clinical Utility of an NGS Targeted Panel to Improve Molecular Characterization of Pediatric Acute Leukemia

Development of next-generation sequencing (NGS) has provided useful genetic information to redefine diagnostic, prognostic, and therapeutic strategies for the management of acute leukemia (AL). However, the application in the clinical setting is still challenging. Our aim was to validate the AmpliSeq™ for Illumina® Childhood Cancer Panel, a pediatric pan-cancer targeted NGS panel that includes the most common genes associated with childhood cancer, and assess its utility in the daily routine of AL diagnostics. In terms of sequencing metrics, the assay reached all the expected values. We obtained a mean read depth greater than 1000×. The panel demonstrated a high sensitivity for DNA (98.5% for variants with 5% variant allele frequency (VAF)) and RNA (94.4%), 100% of specificity and reproducibility for DNA and 89% of reproducibility for RNA. Regarding clinical utility, 49% of mutations and 97% of the fusions identified were demonstrated to have clinical impact. Forty-one percent of mutations refined diagnosis, while 49% of them were considered targetable. Regarding RNA, fusion genes were more clinically impactful in terms of refining diagnostic (97%). Overall, the panel found clinically relevant results in the 43% of patients tested in this cohort. To sum up, we validated a reliable and reproducible method to refine pediatric AL diagnosis, prognosis, and treatment, and demonstrated the feasibility of incorporating a targeted NGS panel into pediatric hematology practice.

Pharmacogenomics in Children

Historically genetics has not been considered when prescribing drugs for children. However, it is clear that genetics are not only an important determinant of disease in children but also of drug response for many important drugs that are core agents used in the therapy of common problems in children. Advances in therapy and in the ethical construct of children's research have made pharmacogenomic assessment for children much easier to pursue. It is likely that pharmacogenomics will become part of the therapeutic decision-making process for children, notably in areas such as childhood cancer where weighing benefits and risks of therapy is crucial.

NUDT15 is a key genetic factor for prediction of hematotoxicity in pediatric patients who received a standard low dosage regimen of 6-mercaptopurine

6-Mercaptopurine (6-MP) is commonly used for treatment of acute lymphoblastic leukemia (ALL). The incidence of hematotoxicity caused by this drug is quite high in Asians even using a standard low dosage regimen. The present study was aimed to elucidate the impact of thiopurine S-methyltransferase (TPMT), a nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15), inosine triphosphatase (ITPA) and ATP Binding Cassette Subfamily C Member 4 (ABCC4) polymorphisms on hematotoxicity in pediatric patients who received a standard low starting dose of 6-MP. One hundred and sixty-nine pediatric patients were enrolled and their genotypes were determined. Patients who carried NUDT15^∗3 and NUDT15^∗2 genotypes were at a 10-15 fold higher risk of severe neutropenia than those of the wild-type during the early months of the maintenance phase. Risk of neutropenia was not significantly increased in patients with other NUDT15 variants as well as in patients with TPMT, ITPA or ABCC4 variants. These results suggest that NUDT15 polymorphisms particularly, NUDT15^∗3 and NUDT15^∗2, play major roles in 6-MP-induced severe hematotoxicity even when using a standard low dosage of 6-MP and genotyping of these variants is necessary in order to obtain precise tolerance doses and avoid severe hematotoxicity in pediatric patients.

Genetic Polymorphisms in Pharmaceuticals and Chemotherapy

The study of genetic polymorphisms has significantly advanced the field of personalized medicine. Polymorphism of genes influence the efficacy of drugs used for treating medical conditions such as depression, cardiac diseases, thromboembolic disorders, oncological diseases, etc. The study of genetic polymorphism is beneficial for drug safety as well as for assessing therapeutic outcomes. Understanding and detecting genetic polymorphisms early on in patients can be useful in selecting the correct chemotherapeutic agent and appropriate dosage for a patient. Knowing the genetic profile of a patient and the interindividual response to various drugs significantly influences the proper selection of medication - a key step towards personalized medicine. Polymorphisms also make patients susceptible to certain cancers and identification of these polymorphisms early can be useful for a personalized treatment plan. The Genome-Wide Association Studies (GWAS) project where millions of genetic variants in the genomes of many individuals are studied to identify connections between what is present on the gene and the phenotype of the patient has enhanced the prospect of personalized medicine. GWAS has been used to identify hundreds of diseases associated to genetic polymorphisms. Individual pharmacokinetic profiles of patients to drugs enable the development of early surveillance protocols to prophylactically prevent patients from having adverse reactions. Furthermore, patient-derived cellular organoids are another advancement that allows researchers to screen for polymorphisms of the patient for adverse reactions from chemotherapy and will allow for the development of new medications that are specific to the profile of the patient’s tumor. These advances have led to significant progress towards personalized medicine. The functional consequences of genetic polymorphism on cancer drugs and treatment are studied here.

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.

NUDT15 polymorphism and NT5C2 and PRPS1 mutations influence thiopurine sensitivity in acute lymphoblastic leukaemia cells

In chemotherapy for childhood acute lymphoblastic leukaemia (ALL), maintenance therapy consisting of oral daily mercaptopurine and weekly methotrexate is important. NUDT15 variant genotype is reportedly highly associated with severe myelosuppression during maintenance therapy, particularly in Asian and Hispanic populations. It has also been demonstrated that acquired somatic mutations of the NT5C2 and PRPS1 genes, which are involved in thiopurine metabolism, are detectable in a portion of relapsed childhood ALL. To directly confirm the significance of the NUDT15 variant genotype and NT5C2 and PRPS1 mutations in thiopurine sensitivity of leukaemia cells in the intrinsic genes, we investigated 84 B‐cell precursor‐ALL (BCP‐ALL) cell lines. Three and 14 cell lines had homozygous and heterozygous variant diplotypes of the NUDT15 gene, respectively, while 4 and 2 cell lines that were exclusively established from the samples at relapse had the NT5C2 and PRPS1 mutations, respectively. Both NUDT15 variant genotype and NT5C2 and PRPS1 mutations were significantly associated with DNA‐incorporated thioguanine levels after exposure to thioguanine at therapeutic concentration. Considering the continuous exposure during the maintenance therapy, we evaluated in vitro mercaptopurine sensitivity after 7‐day exposure. Mercaptopurine concentrations lethal to 50% of the leukaemia cells were comparable to therapeutic serum concentration of mercaptopurine. Both NUDT15 variant genotype and NT5C2 and PRPS1 mutations were significantly associated with mercaptopurine sensitivity in 83 BCP‐ALL and 23 T‐ALL cell lines. The present study provides direct evidence to support the general principle showing that both inherited genotype and somatically acquired mutation are crucially implicated in the drug sensitivity of leukaemia cells.

Hepatic sinusoidal obstruction syndrome and short-term application of 6-thioguanine in pediatric acute lymphoblastic leukemia

Long-term treatment with 6-thioguanine (6-TG) for pediatric acute lymphoblastic leukemia (ALL) is associated with high rates of hepatic sinusoidal obstruction syndrome (SOS). Nevertheless, current treatment continues to use short-term applications of 6-TG with only sparse information on toxicity. 6-TG is metabolized by thiopurine methyltransferase (TPMT) which underlies clinically relevant genetic polymorphism. We analyzed the association between hepatic SOS reported as a serious adverse event (SAE) and short-term 6-TG application in 3983 pediatric ALL patients treated on trial AIEOP-BFM ALL 2000 (derivation cohort) and defined the role of TPMT genotype in this relationship. We identified 17 patients (0.43%) with hepatic SOS, 13 of which with short-term exposure to 6-TG (P < 0.0001). Eight of the 13 patients were heterozygous for low-activity TPMT variants, resulting in a 22.4-fold (95% confidence interval 7.1-70.7; P ≤ 0.0001) increased risk of hepatic SOS for heterozygotes in comparison to TPMT wild-type patients. Results were supported by independent replication analysis. All patients with hepatic SOS after short-term 6-TG recovered and did not demonstrate residual symptoms. Thus, hepatic SOS is associated with short-term exposure to 6-TG during treatment of pediatric ALL and SOS risk is increased for patients with low-activity TPMT genotypes.

Cost-effectiveness analysis of genotype screening and therapeutic drug monitoring in patients with inflammatory bowel disease treated with azathioprine therapy: a Chinese healthcare perspective using real-world data

Background

This study aimed to analyze the cost-effectiveness of combining screening for thiopurine methyl transferase (TPMT) and nucleotide triphosphate diphosphatase (NUDT15) defective alleles with therapeutic drug monitoring (TDM) in Chinese patients with inflammatory bowel disease (IBD) treated with azathioprine (AZA).

Methods

We evaluated the cost-effectiveness of combining screening for NUDT15 and TPMT deficiency with TDM in patients receiving AZA treatment over a 1-year horizon by developing a decision tree model. Real-world data and published literature were used to derive model inputs. The model’s primary outcomes included quality-adjusted life-years (QALYs) and incremental cost-effectiveness ratios (ICERs). One-way and probabilistic sensitivity analyses were used to address uncertainty.

Results

Compared to NUDT15 genotyping, the combined TPMT/NUDT15 genotyping strategy cost an additional $13.83, yielding an ICER of $3,929.54/QALY, which was under the willingness-to-pay level of $30,425 per QALY in China. Compared to strategies with singular TPMT genotyping or no genotyping, the combined TPMT/NUDT15 genotyping strategy gained 0.00406 and 0.00782 QALYs and reduced the cost by $25.15 and $99.06, respectively. Additionally, incorporating TDM of AZA was more effective and less expensive than strategies without TDM. One-way sensitivity analysis revealed the expense attached to severe myelotoxicity to be the factor with the greatest influence in the present research. The application of the combined genotype screening strategy with TDM of AZA treatment was found to have a 91.7% chance of being cost-effective.

Conclusions

For Chinese patients with IBD who receive an AZA regimen, a strategy involving combined NUDT15/TPMT genotype screening prior to treatment initiation and incorporating TDM for treatment management is cost-effective compared to strategies involving genotyping of NUDT15 or TPMT alone or genotyping without TDM.

Insights into S-adenosyl-l-methionine (SAM)-dependent methyltransferase related diseases and genetic polymorphisms

Enzymatic methylation catalyzed by methyltransferases has a significant impact on many human biochemical reactions. As the second most ubiquitous cofactor in humans, S-adenosyl-l-methionine (SAM or AdoMet) serves as a methyl donor for SAM-dependent methyltransferases (MTases), which transfer a methyl group to a nucleophilic acceptor such as O, As, N, S, or C as the byproduct. SAM-dependent methyltransferases can be grouped into different types based on the substrates. Here we systematically reviewed eight types of methyltransferases associated with human diseases. Catechol O-methyltransferase (COMT), As(III) S-adenosylmethionine methyltransferase (AS3MT), indolethylamine N-methyltransferase (INMT), phenylethanolamine N-methyltransferase (PNMT), histamine N-methyltransferase (HNMT), nicotinamide N-methyltransferase (NNMT), thiopurine S-methyltransferase (TPMT) and DNA methyltansferase (DNMT) are classic SAM-dependent MTases. Correlations between genotypes and disease susceptibility can be partially explained by genetic polymorphisms. The physiological function, substrate specificity, genetic variants and disease susceptibility associated with these eight SAM-dependent methyltransferases are discussed in this review.

Individual prediction of nonadherence to oral mercaptopurine in children with acute lymphoblastic leukemia: Results from COG AALL03N1

BACKGROUND

Poor mercaptopurine (6MP) adherence (mean adherence rate < 90%) increases the relapse risk among children with acute lymphoblastic leukemia (ALL). 6MP adherence remains difficult to measure in real time. Easily measured patient-level factors could identify patients at risk for poor adherence.

METHODS

The authors measured 6MP adherence via electronic monitoring for 6 months per patient. Using data from month 3, they created a risk prediction model for 6MP nonadherence in 407 children with ALL (mean age, 7.7 ± 4.4 years); they used receiver operating characteristic analyses in the training set (n = 250) and replicated this in the test set (n = 157).

RESULTS

Age, race/ethnicity, 6MP dose intensity, absolute neutrophil count, 6MP ingestion patterns, and household structure were retained in the prediction model. The model yielded areas under the receiver operating characteristic curve (AUCs) of 0.79 (95% confidence interval [CI], 0.71-0.85) and 0.74 (95% CI, 0.63-0.85) in the training and test sets, respectively. The model performed better for those who were ≥12 years old (AUC, 0.79; 95% CI, 0.59-0.99) than those <12 years old (AUC, 0.70; 95% CI, 0.58-0.81). Using the predicted probability of nonadherence based on receiver operating characteristic analysis, the authors developed a binary risk classifier to classify patients with a high or low probability of nonadherence. The sensitivity and specificity of the binary risk classifier were 71% and 76%, respectively. Adjusted for clinical prognosticators, the risk of relapse was 2.2-fold higher (95% CI, 0.94-5.1; P = .07) among patients with a high probability of nonadherence in comparison with those with a low probability, as identified by the risk prediction model.

CONCLUSIONS

The risk prediction model identified patients with a high probability of nonadherence and could be used in real time to personalize recommendations and interventions in the clinic.

LAY SUMMARY

The vast majority of children with acute lymphoblastic leukemia, the most common childhood cancer, are cured. The treatment of acute lymphoblastic leukemia includes taking an oral chemotherapy medicine (mercaptopurine) for approximately 2 years. Children who miss doses of this medicine (specifically children who take the medicine less than 90% of the time that it is prescribed) are more likely to suffer leukemia relapse. The authors of this article have measured mercaptopurine adherence with electronic bottle caps to determine characteristics of patients that predict nonadherence, and they have created a prediction tool that could allow physicians to identify and intervene with patients at high risk of nonadherence.

TPMT polymorphisms and minimal residual disease after 6-mercaptopurine post-remission consolidation therapy of childhood acute lymphoblastic leukaemia

Bone marrow minimal residual disease (MRD) is the strongest predictor of relapse in children with acute lymphoblastic leukemia (ALL). 6-mercaptopurine (6MP) in ALL therapy has wide inter-individual variation in disposition and is strongly influenced by polymorphisms in the thiopurine methyltransferase (TPMT) gene. In 952 patients treated according to the NOPHO ALL2008 protocol, we explored the association between thiopurine disposition, TPMT genotypes and MRD levels after consolidation therapy with 6MP, high-dose methotrexate (HD-MTX), asparaginase, and vincristine. The levels of the cytotoxic DNA-incorporated thioguanine were significantly higher on day 70-79 in G460A/A719G TPMT heterozygous (TPMT_HZ) compared to TPMT wild type (TPMT_WT) patients (mean: 230.7 vs. 149.7 fmol/µg DNA, p = 0.002). In contrast, TPMT genotype did not associate with the end of consolidation MRD levels irrespective of randomization of the patients to fixed dose (25 mg/m²/day) or 6MP escalation (up to 50 or 75 mg/m²/day) during consolidation therapy.

Genetic Polymorphisms of Drug-Metabolizing Enzymes Involved in 6-Mercaptopurine-Induced Myelosuppression in Thai Pediatric Acute Lymphoblastic Leukemia Patients

Genetic polymorphisms of thiopurine S-methyltransferase (TPMT) and nucleoside diphosphate-linked moiety X-type motif 15 ( NUDT15 ) genes have been proposed as key determinants of 6-mercaptopurine (6-MP)-induced myelosuppression in pediatric acute lymphoblastic leukemia (ALL). In the present study, genotypes of TPMT and NUDT15 were investigated in 178 Thai pediatric patients with ALL by the TaqMan SNP genotyping assay and DNA sequencing. The frequency of TPMT*3C was 0.034. Among NUDT15 variants, NUDT15*3 is the most common variant with the allele frequency of 0.073, whereas those of NUDT15*2 , NUDT15*5 , and NUDT15*6 variants were 0.022, 0.011, and 0.039. These data suggest that a high proportion of Thai pediatric ALL patients may be at risk of thiopurine-induced myelosuppression.

Pharmacogenomics with red cells: a model to study protein variants of drug transporter genes

The PharmacoScan pharmacogenomics platform screens for variation in genes that affect drug absorption, distribution, metabolism, elimination, immune adverse reactions and targets. Among the 1,191 genes tested on the platform, 12 genes are expressed in the red cell membrane: ABCC1, ABCC4, ABCC5, ABCG2, CFTR, SLC16A1, SLC19A1, SLC29A1, ATP7A, CYP4F3, EPHX1 and FLOT1. These genes represent 5 ATP-binding cassette proteins, 3 solute carrier proteins, 1 ATP transport protein and 3 genes associated with drug metabolism and adverse drug reactions. Only ABCG2 and SLC29A1 encode blood group systems, JR and AUG, respectively. We propose red cells as an ex vivo model system to study the effect of heritable variants in genes encoding the transport proteins on the pharmacokinetics of drugs. Altered pharmacodynamics in red cells could also cause adverse reactions, such as haemolysis, hitherto unexplained by other mechanisms.

Pharmacogenetics of inflammatory bowel disease

Patients with inflammatory bowel disease (IBD) show large variability in disease course, and also treatment response. The variability in treatment response has led to many initiatives in search of genetic markers to optimize treatment and avoid severe side effects. This has been very successful for thiopurines, one of the drugs used to induce and maintain remission in IBD. However, for the newer treatment options for IBD, like biologicals, the search for genetic predictors has not yielded any candidate biomarkers with clinical utility. In this review, a summary of recent advances in pharmacogenetics focusing on thiopurines and anti-TNF agents is given.

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.

Pharmacogenetic studies of thiopurine methyltransferase genotype-phenotype concordance and effect of methotrexate on thiopurine metabolism

The discovery and implementation of thiopurine methyltransferase (TPMT) pharmacogenetics has been a success story and has reduced the suffering from serious adverse reactions during thiopurine treatment of childhood leukaemia and inflammatory bowel disease. This MiniReview summarizes four studies included in Dr Zimdahl Kahlin's doctoral thesis as well as the current knowledge on this field of research. The genotype‐phenotype concordance of TPMT in a cohort of 12 663 individuals with clinically analysed TPMT status is described. Notwithstanding the high concordance, the benefits of combined genotyping and phenotyping for TPMT status determination are discussed. The results from the large cohort also demonstrate that the factors of gender and age affect TPMT enzyme activity. In addition, characterization of four previously undescribed TPMT alleles (TPMT*41, TPMT*42, TPMT*43 and TPMT*44) shows that a defective TPMT enzyme could be caused by several different mechanisms. Moreover, the folate analogue methotrexate (MTX), used in combination with thiopurines during maintenance therapy of childhood leukaemia, affects the metabolism of thiopurines and interacts with TPMT, not only by binding and inhibiting the enzyme activity but also by regulation of its gene expression.

Pharmacokinetics and population pharmacokinetics in pediatric oncology

Pharmacokinetic research has become increasingly important in pediatric oncology as it can have direct clinical implications and is a crucial component in individualized medicine. Population pharmacokinetics has become a popular method especially in children, due to the potential for sparse sampling, flexible sampling times, computing of heterogeneous data, and identification of variability sources. However, population pharmacokinetic reports can be complex and difficult to interpret. The aim of this article is to provide a basic explanation of population pharmacokinetics, using clinical examples from the field of pediatric oncology, to facilitate the translation of pharmacokinetic research into the daily clinic.

Screening of Novel Pharmacogenetic Candidates for Mercaptopurine-Induced Toxicity in Patients With Acute Lymphoblastic Leukemia

A small proportion of patients with acute lymphoblastic leukemia (ALL) may experience severe leukopenia after treating with 6-mercaptopurine (6MP), which can be largely explained by germline variants in TPMT and NUDT15. However, a minority of patients who suffered such adverse drug reaction have NUDT15 ^wt/wt TPMT ^wt/wt genotype, indicating that other genetic factors may take part in. In this study, we genotyped 539 exon-located nonsilent pharmacogenetic variants in genes involved in phase I/II of drug metabolism in 173 pediatric patients with ALL and conducted association screening for 6MP-induced leukopenia. Besides NUDT15 (rs116855232, P = 6.4 × 10^-11) and TPMT (rs1142345, P = 0.003), a novel variant was identified in CYP2A7 gene (i.e., rs73032311, P = 0.0007), which is independent of NUDT15/TPMT variant. In addition, a variant (i.e., rs4680) in COMT is significantly associated with 6MP-induced hepatotoxicity (P = 0.007). In conclusion, variants in CYP2A7 and COMT may be considered as novel potential pharmacogenetic markers for 6MP-induced toxicities, but additional independent validations with large sample size and investigations on related mechanisms are further needed.

The Role of TPMT, ITPA, and NUDT15 Variants during Mercaptopurine Treatment of Swedish Pediatric Patients with Acute Lymphoblastic Leukemia

OBJECTIVE

To evaluate the roles of thiopurine methyltransferase (TPMT), inosine triphosphatase (ITPA), and Nudix hydrolase 15 (NUDT15) in 6-mercaptopurine (6-MP) sensitivity during treatment of pediatric patients with acute lymphoblastic leukemia (ALL).

STUDY DESIGN

The study included 102 pediatric patients with ALL subject to the Nordic society Of Paediatric Haematology and Oncology (NOPHO) ALL-2000 and ALL-2008 protocols. Episodes of neutropenia and febrile neutropenia, TPMT sequence variants, as well as 6-MP end doses, were collected retrospectively from medical records. TPMT, ITPA, and NUDT15 sequence variants were analyzed using pyrosequencing.

RESULTS

TPMT variants were associated with a reduced risk of neutropenia and febrile neutropenia during the maintenance II period (P = .019 and P < .0001, respectively). In addition, a NUDT15 variant was associated with a lower end dose of 6-MP (P = .0097), but not with neutropenia and febrile neutropenia. ITPA variants were not associated with an increased risk of neutropenia, febrile neutropenia, nor lower end dose of 6-MP. However, when analyzing the entire treatment period, ITPA variants were associated with a decreased risk of febrile neutropenia.

CONCLUSIONS

White blood cell count-based dose adjustments are regularly performed for known TPMT- deficient patients and results in a reduced risk of neutropenia and febrile neutropenia. Also in NUDT15-deficient patients dose adjustments are performed as indicated by low end dose of 6-MP. ITPA-deficient patients had a decreased risk of febrile neutropenia when analyzing the entire treatment period. Our data suggest that NUDT15 plays an important role in 6-MP treatment and the results should be confirmed in larger cohorts. Future studies should also follow up whether white blood cell count-based dose adjustments affect the risk of relapse.

Drug-induced interstitial lung disease: role of pharmacogenetics in predicting cytotoxic mechanisms and risks of side effects

PURPOSE OF REVIEW

The diagnosis of drug-induced interstitial lung disease (DI-ILD) is challenging and mainly made by exclusion of other possible causes. Toxicity can occur as a cause of drug(s) or drug-drug interactions. In this review, we summarize the possible role of pharmacogenetics of metabolizing enzymes in DI-ILD.

RECENT FINDINGS

Knowledge of the genetic predispositions of enzymes involved in drug metabolization and their relation with proposed cytotoxic mechanisms of DI-ILD, in particular direct cell toxicity and free oxygen radical production is increasing. The cytochrome P450 enzyme family and other enzymes play an important role in the metabolism of all sorts of ingested, injected, or inhaled xenobiotic substances. The liver is the major site for metabolism. Metabolic cytotoxic mechanisms have however also been detected in lung tissue. Polymorphisms in genes coding for enzymes that influence metabolic activity may lead to localized (toxic) reactions and tissue damage. This knowledge may be helpful in preventing the risk of DI-ILD.

SUMMARY

Drug toxicity can be the consequence of absence or very poor enzyme activity, especially if no other metabolic route is available. In the case of reduced enzyme activity, it is recommended to reduce the dose or to prescribe an alternative drug, which is metabolized by a different, unaffected enzyme system to prevent toxic side effects. However, enhanced enzyme activity may lead to excessive formation of toxic and sometimes reactive metabolites. Therefore, knowing a patient's drug-metabolizing profile before drug prescription is a promising way to prevent or explain DI-ILD.

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%.

Pharmacogenetics of treatments for inflammatory bowel disease

Inflammatory bowel disease is a chronic inflammation of the gut whose pathogenesis is still unclear. Although no curative therapy is currently available, a number of drugs are used in induction and maintenance therapy; however, for most of these drugs, a high inter-individual variability in response is observed. Among the factors of this variability, genetics plays an important role. Areas covered: This review summarizes the results of pharmacogenetic studies, considering the most important drugs used and in particular aminosalycilates, glucocorticoids, thiopurines, monoclonal antibodies and thalidomide. Most studies used a candidate gene approach, even if significant breakthroughs have been obtained recently from applying genome-wide studies. When available, also investigations considering epigenetics and pharmacogenetic dosing guidelines have been included. Expert opinion: Only for thiopurines, genetic markers identified as predictors of efficacy or adverse events have allowed the development of dosing guidelines. For the other drugs, encouraging results are available and great expectations rely on the study of epigenetics and integration with pharmacokinetic information, especially useful for biologics. However, to improve therapy of IBD patients with these drugs, for implementation in the clinics of pharmacogenetics, informatic clinical decision support systems and training about pharmacogenetics of health providers are needed.

Thiopurine Methyltransferase Genetic Polymorphisms and Activity and Metabolic Products of Azathioprine in Patients with Inflammatory Bowel Disease

BACKGROUND

Thiopurine S-methyltransferase (TPMT) is a cytoplasmic enzyme that catalyzes thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. There is a correlation between thiopurine drug metabolism, response, and toxicity and genetic polymorphism of TPMT. The aim of this study is to assess TPMT genetic polymorphisms activity and metabolic products of AZA in patients with IBD.

METHODS

Blood samples were obtained from 50 IBD unrelated patients from a private laboratory. We used polymerase chain reaction-restriction length polymorphism (PCR-RFLP) and allele-specific PCRbased assays to determine the TPMT gene for the different variants. A high-performance liquid chromatography system (HPLC) was carried out to determine the whole blood 6-TGN concentration. Determination of serum TMPT activity was done by ELISA kit.

RESULTS

In IBD patients, 46/50 (92%) subjects were homozygous for the wild-type allele (TPMT*1/*1). Mutant TPMT*1/*2 and TPMT*1/*3C alleles were found in 4/46 (8%) and 3/47 (6%) of IBD patients, respectively. TPMT*1/*3B variant was not detected in any of the IBD patients. TPMT enzyme activity was higher in wild-type than that mutant variants TPMT*1/*2 and TPMT*1/*3C, suggesting that there are statistically significant differences between 6-TG levels and polymorphisms of TMPT enzyme. 6-TG levels significantly increased in IBD patients mutant variants TPMT*1/*2 and TPMT*1/*3C.

CONCLUSIONS

Our results showed that TPMT polymorphisms are associated with 6-TGN levels in patients using AZA. This study suggests that AZA dosage may be determined according to the high or low prevalence of a TPMT genotype. Moreover, the results present the determination of metabolite for assessing possible safe effective dosage of the drug.

Genetic polymorphisms of pharmacogenomic VIP variants in the Lisu population of southwestern China: A cohort study

Pharmacogenomic studies of different ethnic or racial groups have been used to develop personalized therapies specific to subjects. This study aimed to identify the distribution differences of very important pharmacogenetic (VIP) variants between the Lisu population from southwestern China and other ethnic groups.Eighty VIP variants in 37 genes were selected from the pharmacogenomic knowledge base (PharmGKB), and compared with genotype data of the Lisu population then compared with other 11 populations from the HapMap dataset and previously published data including Miao, Li, Deng, Sherpa, Lhoba, Tibetan, Kirghiz, Tajik, Mongol, Shaanxi Han ethnic, and Uygur populations.VDR rs1540339, MTHFR rs1801131, P2RY1 rs701265, and PTGS2 rs689466 were significantly different between Lisu and 11 HapMap populations. ANKK1 rs1800497 was the least statistical significant locus among selected single nucleotide polymorphisms. In addition, genetic background of Lisu was strongly closest to Shaanxi Han ethnic cohort, and followed by Chinese in metropolitan Denver population based on population structure and F-statistics analyses.Our results showed significant interethnic differences between Lisu and other populations, which will give useful information for prospective studies and better individualized treatments.

Nucleobase and Nucleoside Analogues: Resistance and Re-Sensitisation at the Level of Pharmacokinetics, Pharmacodynamics and Metabolism

Antimetabolites, in particular nucleobase and nucleoside analogues, are cytotoxic drugs that, starting from the small field of paediatric oncology, in combination with other chemotherapeutics, have revolutionised clinical oncology and transformed cancer into a curable disease. However, even though combination chemotherapy, together with radiation, surgery and immunotherapy, can nowadays cure almost all types of cancer, we still fail to achieve this for a substantial proportion of patients. The understanding of differences in metabolism, pharmacokinetics, pharmacodynamics, and tumour biology between patients that can be cured and patients that cannot, builds the scientific basis for rational therapy improvements. Here, we summarise current knowledge of how tumour-specific and patient-specific factors can dictate resistance to nucleobase/nucleoside analogues, and which strategies of re-sensitisation exist. We revisit well-established hurdles to treatment efficacy, like the blood-brain barrier and reduced deoxycytidine kinase activity, but will also discuss the role of novel resistance factors, such as SAMHD1. A comprehensive appreciation of the complex mechanisms that underpin the failure of chemotherapy will hopefully inform future strategies of personalised medicine.

Implementation of genomic medicine for gastrointestinal tumors

Genomic medicine is an approach to take advantage of genomic data in medical practice and health care. The advancement of sequencing technologies has enabled the determination of individual genomes as well as the genome in neoplasms. In the field of human cancer, understanding genomic alterations in tumors and variations associated with drug responses has paved the way towards the development of new drugs and personalized medicine. International collaborations of cancer genome analyses have accumulated a huge body of information about somatic mutations, and identified new driver mutations and pathways in a wide range of cancers. In particular, a growing body of evidence has shown that information about mutations in neoplasms helps to assess the efficacy and resistance of anti-cancer drugs. Information about germline mutations associated with hereditary cancer has been shown to benefit patients by enabling early detection of their tumors and disease-specific treatment, as well as reducing the risk for those at risk. To promote personalized medicine in a more cost-effective and personalized way, further inter-institutional, nationwide, and international collaboration is needed. This article summarizes the background and current situation of genomic medicine in the field of gastrointestinal tumors to help physicians and medical coworkers by assisting their better understanding of genomic medicine and strengthening their confidence of its clinical use.

Drug metabolizing enzymes and their inhibitors' role in cancer resistance

Despite continuous research on chemotherapeutic agents, different mechanisms of resistance have become a major pitfall in cancer chemotherapy. Although, exhaustive efforts are being made by several researchers to target resistance against chemotherapeutic agents, there is another class of resistance mechanism which is almost carrying on unattended. This class of resistance includes pharmacokinetics resistance such as efflux by ABC transporters and drug metabolizing enzymes. ABC transporters are the membrane bound proteins which are responsible for the movement of substrates through the cell membrane. Drug metabolizing enzymes are an integral part of phase-II metabolism that helps in the detoxification of exogenous, endogenous and xenobiotics substrates. These include uridine diphospho-glucuronosyltransferases (UGTs), glutathione-S-transferases (GSTs), dihydropyrimidine dehydrogenases (DPDs) and thiopurine methyltransferases (TPMTs). These enzymes may affect the role of drugs in both positive as well negative manner, depending upon the type of tissue and cells present and when present in tumors, can result in drug resistance. However, the underlying mechanism of resistance by drug metabolizing enzymes is still not clear. Here, we have tried to cover various aspects of these enzymes in relation to anticancer drugs.

Pharmacogenetics: Applications to Pediatric Patients

Individual genomic differences may affect drug disposition and effects of many drugs, and identification of biomarkers are crucial to personalize dosage and optimize response. In children, developmental changes associated with growth and maturation translate into different relationships between genotype and phenotype and different responses to treatment compared to adults. This review aims to summarize some developmental aspects of pharmacogenetics, based on practical examples.

Pharmacogenetics and application in pediatrics

Identification of markers involved in drug disposition is crucial for drugs with a narrow therapeutic index. Individual genomic differences can affect the pharmacology of some drugs and participate to inter-individual variability in drug response. Pharmacogenetics is a useful tool in clinical practice for dosage adjustment and to limit drug toxicities. In pediatrics, physiological changes can also influence the disposition of drugs in infants, children and adolescents. The importance of ontogeny translates into different responses to the same drug in children and adults. Thus, interactions between the maturation of metabolism enzymes or transporters and genetics have a major impact on drug exposure leading to age-specific dosage requirements. This review aims to describe implementation of pharmacogenetics in personalized medicine and specifies pediatric characteristics with ethical considerations.

[Usefulness of thiopurine methyltransferase polymorphism study and metabolites measurement for patients treated by azathioprine]

Azathioprine is widely used in internal medicine and frequently implicated in occurrence of adverse events. Among these adverse events the bone marrow suppression, a dose-related one, is the most serious because of is potential morbidity and mortality. Severe myelosuppression, associated with abnormal AZA metabolism, is linked to the thiopurine methyltransferase (TPMT) genetic polymorphism that results in a high variability of its activity with 89% of patients with a normal activity, 11% with an intermediate activity, and 0.3% with very low activity leading to a very high risk of bonne marrow suppression. TPMT status can be assessed prior to AZA treatment by measuring enzyme activity or genotyping techniques to identify patients for which the standard dose is not advisable. Furthermore, azathioprine metabolites monitoring is helpful for the follow up of patients, especially in therapeutic failure, to distinguish non-compliant patients from under-dosed, "shunters" or resistant patients.

DNA variants in DHFR gene and response to treatment in children with childhood B ALL: revisited in AIEOP-BFM protocol

AIM

We have previously reported an association of dihydrofolate reductase promoter polymorphisms with reduced event-free survival in childhood acute lymphoblastic leukemia (ALL) patients treated with Dana Farber Cancer Institute protocol. Here, we assessed whether these associations are applicable to other protocol, based on different methotrexate doses.

METHODS

Genotypes for six tag polymorphisms and resulting haplotypes were analyzed for an association with ALL outcome.

RESULTS

The association was found with the polymorphisms A-680C, A-317G and C-35T in high-risk group patients. Carriers of haplotype *1 had a remarkably higher risk of events compared with noncarriers and a lower probability of event-free survival (21.4 vs 81.3%).

CONCLUSION

The role of DHFR variants in predicting the outcome of childhood ALL extends beyond single-treatment protocol and can be useful biomarker in personalizing treatment.

Analysis of Thiopurine S-Methyltransferase Deficient Alleles in Acute Lymphoblastic Leukemia Patients in Mexican Patients

BACKGROUND AND AIMS

It has been demonstrated that heterozygote and homozygote thiopurine S-methyltransferase (TPMT) mutant allele carriers are at high risk to develop severe and potentially fatal hematopoietic toxicity after treatment with standard doses of 6-mercaptopurine (6-MP) and methotrexate (MX). Those drugs are the backbone of acute lymphoblastic leukemia (ALL) and several autoimmune disease treatments. We undertook this study to determine the frequency of the TPMT deficient alleles in children with ALL and non-ALL subjects from Mexico City and Yucatan, Mexico.

METHODS

We included 849 unrelated subjects, of which 368 ALL children and 342 non-ALL subjects were from Mexico City, and 60 ALL cases and 79 non-ALL individuals were from Yucatan. Genotyping of the rs1800462, rs1800460 and rs1142345 SNPs was performed by 5'exonuclease technique using TaqMan probes (Life Technologies Foster City, CA).

RESULTS

The mutant TPMT alleles were present in 4.8% (81/1698 chromosomes) and only 0.2% were homozygote TPMT*3A/TPMT*3A. We did not find statistically significant differences in the distribution of the mutant alleles between patients from Mexico City and Yucatan in either ALL cases or non-ALL. Nonetheless, the TPMT*3C frequency in ALL patients was higher than non-ALL subjects (p = 0.03). To note, the null homozygous TPMT*3A/TPMT*3A genotype was found in 2.5% of the non-ALL subjects.

CONCLUSIONS

TPMT mutant alleles did not exhibit differential distribution between both evaluated populations; however, TPMT*3C is overrepresented in ALL cases in comparison with non-ALL group. Assessing the TPMT mutant alleles could benefit the ALL children and those undergoing 6-MP and MX treatment.

Differential effects of thiopurine methyltransferase (TPMT) and multidrug resistance-associated protein gene 4 (MRP4) on mercaptopurine toxicity

PURPOSE

Mercaptopurine plays a pivotal role in treatment of acute lymphoblastic leukemia (ALL) and autoimmune diseases, and inter-individual variability in mercaptopurine tolerance can influence treatment outcome. Thiopurine methyltransferase (TPMT) and multi-drug resistant Protein 4 (MRP4) have both been associated with mercaptopurine toxicity in clinical studies, but their relative contributions remain unclear.

METHODS

We studied the metabolism of and tolerance to mercaptopurine in murine knockout models of Tpmt, Mrp4, and both genes simultaneously.

RESULTS

Upon mercaptopurine treatment, Tpmt ^-/- Mrp4 ^-/- mice had the highest concentration of bone marrow thioguanine nucleotides (8.5 pmol/5 × 10⁶ cells, P = 7.8 × 10^-4 compared with 2.7 pmol/5 × 10⁶ cells in wild-types), followed by those with Mrp4 or Tpmt deficiency alone (6.1 and 4.3 pmol/5 × 10⁶ cells, respectively). Mrp4-deficient mice accumulated higher concentrations of methylmercaptopurine metabolites compared with wild-type (76.5 vs. 23.2 pmol/5 × 10⁶ cells, P = 0.027). Mice exposed to a clinically relevant mercaptopurine dosing regimen displayed differences in toxicity and survival among the genotypes. The double knock-out of both genes experienced greater toxicity and shorter survival compared to the single knockout of either Tpmt (P = 1.7 × 10^-6) or Mrp4 (P = 7.4 × 10^-10).

CONCLUSIONS

We showed that both Tpmt and Mrp4 influence mercaptopurine disposition and toxicity.

Precision Medicine in Pediatric Oncology: Translating Genomic Discoveries into Optimized Therapies

Survival of children with cancers has dramatically improved over the past several decades. This success has been achieved through improvement of combined modalities in treatment approaches, intensification of cytotoxic chemotherapy for those with high-risk disease, and refinement of risk stratification incorporating novel biologic markers in addition to traditional clinical and histologic features. Advances in cancer genomics have shed important mechanistic insights on disease biology and have identified "driver" genomic alterations, aberrant activation of signaling pathways, and epigenetic modifiers that can be targeted by novel agents. Thus, the recently described genomic and epigenetic landscapes of many childhood cancers have expanded the paradigm of precision medicine in the hopes of improving outcomes while minimizing toxicities. In this review, we will discuss the biologic rationale for molecularly targeted therapies in genomically defined subsets of pediatric leukemias, solid tumors, and brain tumors. Clin Cancer Res; 23(18); 5329-38. ©2017 AACR.

Genotypes Affecting the Pharmacokinetics of Anticancer Drugs

Cancer treatment is becoming more and more individually based as a result of the large inter-individual differences that exist in treatment outcome and toxicity when patients are treated using population-based drug doses. Polymorphisms in genes encoding drug-metabolizing enzymes and transporters can significantly influence uptake, metabolism, and elimination of anticancer drugs. As a result, the altered pharmacokinetics can greatly influence drug efficacy and toxicity. Pharmacogenetic screening and/or drug-specific phenotyping of cancer patients eligible for treatment with chemotherapeutic drugs, prior to the start of anticancer treatment, can identify patients with tumors that are likely to be responsive or resistant to the proposed drugs. Similarly, the identification of patients with an increased risk of developing toxicity would allow either dose adaptation or the application of other targeted therapies. This review focuses on the role of genetic polymorphisms significantly altering the pharmacokinetics of anticancer drugs. Polymorphisms in DPYD, TPMT, and UGT1A1 have been described that have a major impact on the pharmacokinetics of 5-fluorouracil, mercaptopurine, and irinotecan, respectively. For other drugs, however, the association of polymorphisms with pharmacokinetics is less clear. To date, the influence of genetic variations on the pharmacokinetics of the increasingly used monoclonal antibodies has hardly been investigated. Some studies indicate that genes encoding the Fcγ-receptor family are of interest, but more research is needed to establish if screening before the start of therapy is beneficial. Considering the profound impact of polymorphisms in drug transporters and drug-metabolizing enzymes on the pharmacokinetics of chemotherapeutic drugs and hence, their toxicity and efficacy, pharmacogenetic and pharmacokinetic profiling should become the standard of care.

Survival Outcome of Filipino Children With Acute Lymphoblastic Leukemia Treated With Modified Berlin-Frankfurt-Muenster/Hong Kong Acute Lymphoblastic Leukemia (BFM95/HKALL97) Protocol in a Tertiary General Hospital From January 2005 to December 2009: A Retrospective Cohort Study

This retrospective cohort study is primarily aimed to evaluate the outcome of children ages 0 to 18 years old, with acute lymphoblastic leukemia and treated with a modified Berlin-Frankfurt-Muenster/Hong Kong Acute Lymphoblastic Leukemia (BFM95-HKALL97) protocol at University of Santo Tomas Hospital from January 2005 to December 2009. Seventy-eight patients were included. Majority were between 1 and 10 years old (87.2%), male (61.5%), with normal nutritional status, and classified as upper socioeconomic class (65.3%), mainly from National Capital Region (24.3%). Eighty percent had mild anemia and a white cell count <50,000/mm. No patient had an initial platelet count <20,000/mm. More than 90% were standard risk, with FAB L1 morphology and pre-B immunophenotype. Five-year overall survival (OS) and event-free survival (EFS) rates were 86.94% and 86.2%, respectively. Among the 69 patients in the efficacy subset analysis, the 5-year OS and EFS rates were 98.36% and 86.80%, respectively. Relapse rate was 14.5%. Only FAB morphology and risk classification were correlated with relapse. Most common complications were febrile neutropenia, sepsis, and oral mucositis during induction phase. No deaths occurred due to treatment complications. In conclusion, using higher doses of methotrexate during consolidation phase improved the 5-year OS and EFS rates of our patients, without an increase in complications or deaths. Other contributing factors include improved adherence to treatment and risk-based treatment classification.

Cancer Drug Delivery

Advancements in cancer drug delivery have led to the development of personalized oncology care through molecularly-driven targeted therapies. Understanding molecular and cellular mechanisms which drive tumor progression and resistance is critical in managing new treatment strategies which have shifted from empiric to biomarker-directed therapy selection. Biomarker-directed therapies have improved clinical outcomes in multiple malignancies as monotherapy and in combination with other treatment modalities, however the changing scope of treatment options presents new opportunities and challenges for research. Furthermore, pharmacogenetics may provide a rationale method of personalizing anticancer drug dosing and supportive care management for oncology patients. This chapter reviews biomarker classifications and pharmacogenetics in anticancer therapy and supportive care. Examples of biomarker-directed therapies and clinical assays, in addition to future directions of molecular profiling in oncology therapy management are discussed.

Genomewide Approach Validates Thiopurine Methyltransferase Activity Is a Monogenic Pharmacogenomic Trait

We performed a genomewide association study (GWAS) of primary erythrocyte thiopurine S-methyltransferase (TPMT) activity in children with leukemia (n = 1,026). Adjusting for age and ancestry, TPMT was the only gene that reached genomewide significance (top hit rs1142345 or 719A>G; P = 8.6 × 10^-61 ). Additional genetic variants (in addition to the three single-nucleotide polymorphisms [SNPs], rs1800462, rs1800460, and rs1142345, defining TPMT clinical genotype) did not significantly improve classification accuracy for TPMT phenotype. Clinical mercaptopurine tolerability in 839 patients was related to TPMT clinical genotype (P = 2.4 × 10^-11 ). Using 177 lymphoblastoid cell lines (LCLs), there were 251 SNPs ranked higher than the top TPMT SNP (rs1142345; P = 6.8 × 10^-5 ), revealing a limitation of LCLs for pharmacogenomic discovery. In a GWAS, TPMT activity in patients behaves as a monogenic trait, further bolstering the utility of TPMT genetic testing in the clinic.