TPMT genotype and the use of thiopurines in paediatric inflammatory bowel disease

Azathioprine dose tailoring based on pharmacogenetic information: Insights of clinical implementation

Azathioprine is commonly used as an immunosuppressive antimetabolite in the treatment of acute lymphoblastic leukemia, autoimmune disorders (such as Crohn's disease and rheumatoid arthritis), and in patients receiving organ transplants. Thiopurine-S-methyltransferase (TPMT) is a cytoplasmic trans-methylase catalyzing the S-methylation of thiopurines. The active metabolites obtained from thiopurines are hydrolyzed into inactive forms by the Nudix hydrolase 15 (NUDT15). The TPMT*2 (defined by rs1800462), *3A (defined by rs1800460 and rs1142345), *3B (defined by rs1800460), *3C (defined by rs1142345), *6 (defined by rs75543815), and NUDT15 rs116855232 genetic variant have been associated, with the highest level of evidence, with the response to azathioprine, and, the approved drug label for azathioprine and main pharmacogenetic dosing guidelines recommend starting with reduced initial doses in TPMT intermediate metabolizer (IM) patients and considering an alternative treatment in TPMT poor metabolizer (PM) patients. This study aims to assess the clinical impact of azathioprine dose tailoring based on TPMT genotyping studying the azathioprine toxicity and efficacy, treatment starts, and dose adjustments during follow-up, comparing TPMT IM/PM and normal metabolizer (NM) patients. It also studied the association of NUDT15 rs116855232 with response to azathioprine in patients receiving a tailored treatment based on TPMT and characterized the TMPT and NUDT15 studied variants in our population. Results show that azathioprine dose reduction in TPMT IM patients (TPMT*1/*2, *1/*3A, or *1/*3C genotypes) is related to lower toxicity events compared to TPMT NM (TPMT *1/*1 genotype), and lower azathioprine dose adjustments during follow-up without showing differences in the efficacy. The results support the hypothesis of existing other genetic variants affecting azathioprine toxicity.

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.

Usefulness and difficulties with the thiopurine pharmacogenomic NUDT15 genotyping test: Analysis of real-world data in Japan

The usefulness of NUDT15 genotyping as a pharmacogenomic test for thiopurine has been established. The first such test developed to date, NUDT15 genotyping was approved for reimbursement in Japan in February 2019 for all indicated patients. We retrospectively examined claims data in Japan and confirmed that the proportion of patients who undergo genotyping before initiating a new thiopurine regimen has increased; furthermore, genotyping has improved the rate of treatment continuation and reduced on-treatment hospitalization. However, the genotyping rate before thiopurine induction was >50% for patients with inflammatory bowel disease and <20% for those with other immune-related diseases, indicating significant variation by disease field. Additionally, over 10% of tests were found to have been performed inappropriately, such as multiple genotyping of the same patient or testing more than 2 weeks after starting treatment. Although NUDT15 genotyping for patients requiring thiopurine treatment has been shown to improve thiopurine treatment continuation rate, measures are required to address the systematic issues identified in our analysis.

The Role of Pharmacogenetics in the Therapeutic Response to Thiopurines in the Treatment of Inflammatory Bowel Disease: A Systematic Review

This study focuses on the use of thiopurines for treating inflammatory bowel diseases (IBD). These drugs undergo enzymatic changes within the body, resulting in active and inactive metabolites that influence their therapeutic effects. The research examines the role of genetic polymorphisms in the enzyme thiopurine S-methyltransferase (TPMT) in predicting the therapeutic response and adverse effects of thiopurine treatment. The TPMT genotype variations impact the individual responses to thiopurines. Patients with reduced TPMT activity are more susceptible to adverse reactions (AEs), such leukopenia, hepatotoxicity, pancreatitis, and nausea, which are common adverse effects of thiopurine therapy. The therapeutic monitoring of the metabolites 6-thioguanine nucleotides (6-TGN) and 6-methyl mercaptopurine (6-MMP) is proposed to optimize treatment and minimize AEs. Patients with higher 6-TGN levels tend to have better clinical responses, while elevated 6-MMP levels are linked to hepatotoxicity. Genotyping for TPMT before or during treatment initiation is suggested to tailor dosing strategies and enhance treatment efficacy while reducing the risk of myelosuppression. In conclusion, this study highlights the importance of considering genetic variations and metabolite levels in optimizing thiopurine therapy for IBD patients, focusing on balance therapeutic efficacy with the prevention of adverse effects and contributing to personalized treatment and better patient outcomes.

Combined evaluation of genotype and phenotype of thiopurine S-methyltransferase (TPMT) in the clinical management of patients in chronic therapy with azathioprine

Background The thiopurine S-methyltransferase (TPMT)/azathioprine (AZA) gene-drug pair is one of the most well-known pharmacogenetic markers. Despite this, few studies investigated the implementation of TPMT testing and the combined evaluation of genotype and phenotype in multidisciplinary clinical settings where patients are undergoing chronic therapy with AZA. Methods A total of 356 AZA-treated patients for chronic autoimmune diseases were enrolled. DNA was isolated from whole blood and the samples were analyzed for the c.460G>A and c.719A>G variants by the restriction fragment length polymorphism (RFLP) technique and sequenced for the c.238G>C variant. The TPMT enzyme activity was determined in erythrocytes by a high-performance liquid chromatography (HPLC) assay. Results All the patients enrolled were genotyped while the TPMT enzyme activity was assessed in 41 patients. Clinical information was available on 181 patients. We found no significant difference in the odds of having adverse drug reactions (ADRs) in wild-type patients and variant allele carriers, but the latter had an extra risk of experiencing hematologically adverse events. The enzyme activity was significantly associated to genotype. Conclusions TPMT variant allele carriers have an extra risk of experiencing hematologically adverse events compared to wild-type patients. Interestingly, only two out of 30 (6.6%) patients had discordant results between genotype, phenotype and onset of ADRs.

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.

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

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

Thiopurine monitoring in children with inflammatory bowel disease: a systematic review

AIMS

The aim was to systematically review the evidence on the clinical usefulness of thiopurine metabolite and white blood count (WBC) monitoring in the assessment of clinical outcomes in children with inflammatory bowel disease (IBD).

METHODS

Medline, Embase, Cochrane Central Register of controlled trials and http://www.clinicaltrials.gov were screened in adherence to the PRISMA statement by two independent reviewers for identification of eligible studies. Eligible studies were randomized controlled trials (RCTs), cohort studies and large case series of children with inflammatory bowel disease (IBD) (<18 years) who underwent monitoring of thiopurine metabolites and/or WBC.

RESULTS

Fifteen papers were identified (n = 1026). None of the eligible studies were RCTs. High 6-thioguanine nucleotide (6TGN) concentrations were not consistently associated with leucopenia. Leucopenia was not associated with achievement of clinical remission. A positive but not consistent correlation between 6TGN and clinical remission was reported. Haematological toxicity could not be reliably assessed with 6TGN measurements only. A number of studies supported the use of high 6-methylmercaptopurine ribonucleotides (6MMPR) as an indicator of hepatotoxicity. Low thiopurine metabolite concentration may be indicative of non-compliance.

CONCLUSION

Thiopurine metabolite testing does not safely predict clinical outcome, but may facilitate toxicity surveillance and treatment optimization in poor responders. Current evidence favours the combination of thiopurine metabolite/WBC monitoring and clinic follow-up for prompt identification of haematologic/hepatic toxicity safe dose adjustment, and treatment modification in cases of suboptimal clinical outcome or non-compliance. Well designed RCTs for the identification of robust surrogate markers of thiopurine efficacy and toxicity are required.

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.

Phenome-wide association studies on a quantitative trait: application to TPMT enzyme activity and thiopurine therapy in pharmacogenomics

Phenome-Wide Association Studies (PheWAS) investigate whether genetic polymorphisms associated with a phenotype are also associated with other diagnoses. In this study, we have developed new methods to perform a PheWAS based on ICD-10 codes and biological test results, and to use a quantitative trait as the selection criterion. We tested our approach on thiopurine S-methyltransferase (TPMT) activity in patients treated by thiopurine drugs. We developed 2 aggregation methods for the ICD-10 codes: an ICD-10 hierarchy and a mapping to existing ICD-9-CM based PheWAS codes. Eleven biological test results were also analyzed using discretization algorithms. We applied these methods in patients having a TPMT activity assessment from the clinical data warehouse of a French academic hospital between January 2000 and July 2013. Data after initiation of thiopurine treatment were analyzed and patient groups were compared according to their TPMT activity level. A total of 442 patient records were analyzed representing 10,252 ICD-10 codes and 72,711 biological test results. The results from the ICD-9-CM based PheWAS codes and ICD-10 hierarchy codes were concordant. Cross-validation with the biological test results allowed us to validate the ICD phenotypes. Iron-deficiency anemia and diabetes mellitus were associated with a very high TPMT activity (p = 0.0004 and p = 0.0015, respectively). We describe here an original method to perform PheWAS on a quantitative trait using both ICD-10 diagnosis codes and biological test results to identify associated phenotypes. In the field of pharmacogenomics, PheWAS allow for the identification of new subgroups of patients who require personalized clinical and therapeutic management.

The use of underlying molecular mechanisms and other factors to describe and classify diseases is a major challenge for future treatment strategies. New methods are needed to achieve this goal. The phenome wide association study (PheWAS) methodology was initially developed to unveil unknown associations between a specific genetic status and phenotypic features (e.g. diagnoses from electronic health records). We initially propose to extend this method to assessment of the relationships between the levels of a quantitative trait and diagnosis codes. We also assess the relationships between this quantitative trait and the biological test results. We tested this method using the levels of enzymatic activity of thiopurine S-methyltransferase (TPMT) that is involved in the metabolism of thiopurine drugs used in inflammatory bowel diseases for example. We discovered an association between a very high TPMT activity and nutritional anemia and diabetes. These results could be used to describe a new subgroup of patients in order to optimize drug treatments.

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.

APEX microarray panel for genotyping polymorphisms in cancer chemotherapy and estimation frequencies in a Slovak population

Aim: Many studies focus on monitoring response to chemotherapy, adverse effects and prediction of therapeutic effects, which depend on individual gene variability. The amount of various polymorphisms in genes involved in the folate cycle, and other metabolic pathways involved in the metabolism of chemotherapeutic drugs, are an essential topic of such studies. This work focuses on the design and establishment of a pharmacogenetically relevant panel, which could be applied to the rapid genotyping of patients treated with thiopurines, 5-fluorouracil, methotrexate, irinotecan and glucocorticoids. Materials & methods: A total of 97 variations in 36 genes associated with side effects of chemotherapeutic treatment were selected. Of these, 94 SNPs were genotyped by the arrayed primer extension (APEX; Asper Biotech Ltd) microarray method or direct sequencing. Variations of tandem repeats or gene deletions were genotyped by capillary electrophoresis and PCR detection. A total of 300 DNA samples from healthy volunteers were tested to estimate genotype frequencies for a Slovak population. All data were checked for Hardy–Weinberg equilibrium and genetic linkage between variations. Results: We designed an APEX microarray for genotyping pharmacologically relevant polymorphisms in patients undergoing chemotherapy. We estimated genotype frequencies for all 97 polymorphisms testing 300 individuals from the Slovak population, which may also serve as an estimate of central European frequencies. These data also allowed for the testing of genetic linkage between loci. Many of the determined genotype frequencies in this study were in similar ranges found in other European populations but four SNPs, rs11760837 (p = 0.018), rs1801265 (p = 0.0375), rs1801394 (p = 0.0066) and rs182455 (p = 0.0083), demonstrated stronger deviation. Conclusion: Genetic variability in genes involved in metabolic pathways of chemotherapeutic drugs, such as methotrexate, 5-fluorouracil, thiopurines or irinotecan, is responsible for individual therapy response and development of side effects. A comprehensive approach in genotyping of numerous variants is aimed to improve individual access to patients and the selection of appropriate drugs for treatment. The APEX microarray method is a valuable tool for fast, reliable and cost-effective genotyping of variants which can be used for the typing of known variants in patients prior to treatment as well as in studies searching for new genotype–phenotype associations. The opportunity of adding additional variants during the study makes the APEX microarray technology flexible and suitable for such trials.

Original submitted 4 October 2010; Revision submitted 23 November 2010.

Thiopurine S-methyltransferase genotype and the use of thiopurines in paediatric inflammatory bowel disease Greek patients

BACKGROUND AND OBJECTIVE

Azathioprine (AZA) and 6-mercaptopurine (6MP) are used in the treatment of paediatric 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 investigate the frequency of the functional TPMT polymorphisms and their association with the occurrence of adverse events during azathioprine therapy in a paediatric IBD cohort.

METHODS

Ninety-seven thiopurine-treated paediatric IBD patients (41.24% boys and 58.76% girls) with a mean age 11.25 years (range 3-16), were assessed for TPMT polymorphisms and adverse events.

RESULTS

Of the 97 patients enrolled in the study, 18 (18.56%) were heterozygous mutated; two (2.06%) were homozygous for a mutated TPMT gene. Ten patients (10.31%) developed adverse effects, and four of them (40%) had one of the variant alleles.

CONCLUSIONS

In this small cohort of subjects, no association was found between TPMT polymorphisms and the occurrence of thiopurines-related adverse events.

Are patients with intermediate TPMT activity at increased risk of myelosuppression when taking thiopurine medications?

Thiopurine S-methyltransferase (TPMT) metabolizes thiopurine medications, including azathioprine and 6-mercaptopurine. Absent TPMT activity (i.e., in individuals homozygous for a variant TPMT allele) is associated with an increased risk of myelosuppression in patients taking thiopurine drugs. However, it is not clear if there is also an increased risk for patients with intermediate TPMT activity (i.e., in individuals heterozygous for a variant TPMT allele).

AIMS

To quantify the increased risk of myelosuppression for patients with intermediate TPMT activity.

MATERIALS & METHODS

A systematic review identified published studies, up to 29 September 2008, that explored the relationship between TMPT and hematological adverse drug reactions to thiopurines. Following a critical appraisal of the quality of published studies, a meta-analysis calculated the odds ratio of myelosuppression for patients with intermediate TPMT activity compared with wild-type.

RESULTS

A total of 67 studies were identified, the majority retrospective cohort in design. Patients with two TPMT variant alleles who are TPMT deficient have a substantial increase in their risk of myelotoxicity (86% of deficient patients developed myelosuppression). The increase in odds ratio of developing leukopenia for patients with intermediate TPMT activity or one TPMT variant allele compared with wild-type was 4.19 (95% CI: 3.20-5.48).

CONCLUSION

This meta-analysis suggests that individuals with both intermediate and absent TPMT activity have an increased risk of developing thiopurine-induced myelosuppression, compared with individuals with normal activity. However, there is significant variability in the quality of the reported studies and large prospective studies to clarify the size of the effect of TPMT variant alleles on the risk of myelosuppression should be conducted. Accurate risk assessments will provide important data to inform clinical guidelines.

Genetic polymorphism of inosine-triphosphate-pyrophosphatase influences mercaptopurine metabolism and toxicity during treatment of acute lymphoblastic leukemia individualized for thiopurine-S-methyl-transferase status

IMPORTANCE OF THE FIELD

Although genetic polymorphisms in the gene encoding human thiopurine methyltransferase (TPMT) are known to have a marked effect on mercaptopurine metabolism and toxicity, there are many patients with wild-type TPMT who develop toxicity. Furthermore, when mercaptopurine dosages are adjusted in patients who are heterozygous at the TPMT locus, there are still some patients who develop toxicity for reasons that are not fully understood. Therefore, we recently studied the effects of a common polymorphism in another gene encoding an enzyme involved in mercaptopurine metabolism (SNP rs1127354 in inosine-triphospate-pyrophosphatase, ITPA), showing that genetic polymorphism of ITPA is a significant determinant of mercaptopurine metabolism and of febrile neutropenia following combination chemotherapy of acute lymphoblastic leukemia (ALL) in which mercaptopurine doses are individualized based on TPMT genotype.

AREA COVERED IN THIS REVIEW

In this review, we summarize the knowledge available about the effect and clinical relevance of TPMT and ITPA on mercaptopurine pharmacogenomics, with a particular focus on the use of this medication in pediatric patients with ALL.

WHAT THE READER WILL GAIN

Reader will gain insights into: i) the effects of pharmacogenomic traits on mercaptopurine toxicity and efficacy for the treatment of ALL and ii) individualization strategies that can be used to mitigate toxicity without compromising efficacy in pediatric patients with ALL.

TAKE HOME MESSAGE

Mercaptopurine dose can be adjusted on the basis of TPMT genotype to mitigate toxicity in pediatric patients with ALL. As treatment is individualized in this way for the most relevant genetic determinant of drug response (i.e., for mercaptopurine, TPMT), the importance of other genetic polymorphisms emerges (e.g., ITPA).

Thiopurine-induced myelotoxicity in patients with inflammatory bowel disease: a review

AIM

Probably, the most important and potentially lethal adverse event of azathioprine (AZA) and mercaptopurine (MP) is myelosuppression. Our aim was to conduct a review of AZA/MP-induced myelotoxicity in inflammatory bowel disease (IBD) patients.

METHODS

Bibliographical searches were performed in MEDLINE/EMBASE. The studies evaluating thiopurine-induced myelotoxicity in patients with IBD were reviewed. The cumulative incidence and the incidence rate of AZA/MP-induced myelotoxicity were calculated by a meta-analysis.

RESULTS

In total, 66 studies (8,302 patients) were included. The cumulative incidence of AZA/MP-induced myelotoxicity was 7% (95% confidence interval [CI] 6-8%). The incidence rate (per patient and year of treatment) of the drug-induced myelotoxicity was 3% (95% CI 3-4%). The risk was roughly similar with AZA and with MP (7%vs 9%). The duration of AZA/MP treatment in patients with myelotoxicity ranged from 12 days to 27 yr. The cumulative incidence of infections among AZA/MP-induced myelotoxicity patients was 6.5%. The cumulative incidence of severe myelotoxicity was 1.1% (incidence rate 0.9%). Three deaths were reported due to myelotoxicity (cumulative incidence 0.06%, 95% CI 0.02-0.17%). The risk of death among patients who developed myelotoxicity was 0.94% (95% CI 0.32-2.70%).

CONCLUSION

The incidence rate of myelotoxicity in IBD patients receiving AZA/MP is approximately 3% per patient and year of treatment. Although bone marrow toxicity may develop at any time after starting the therapy, this happens more frequently during the first months. The incidence rate of severe myelotoxicity is less than 1% per patient and year of treatment, and the mortality risk is less than 0.1% (which means that the risk of death among IBD patients who develop myelotoxicity is approximately 1%).

Glutathione-S-transferase genotypes and the adverse effects of azathioprine in young patients with inflammatory bowel disease

BACKGROUND

Adverse drug reactions to azathioprine, the prodrug of 6-mercaptopurine, occur in 15%-38% of patients and the majority are not explained by thiopurine-S-methyltransferase (TPMT) deficiency. Azathioprine is known to induce glutathione depletion and consumption of glutathione is greater in cells with high glutathione-S-transferase (GST) activity compared with those with low activity; moreover, some reports indicate that GST might play a direct role in the reaction of glutathione with azathioprine. The association between polymorphisms of GST-M1, GST-P1, GST-T1, and TPMT genes and the adverse effects of azathioprine was therefore investigated.

METHODS

Seventy patients with inflammatory bowel disease (IBD), treated with azathioprine, were enrolled and clinical data were retrospectively determined. TPMT and GST genotyping were performed by polymerase chain reaction (PCR) assays on DNA extracted from blood samples.

RESULTS

Fifteen patients developed adverse effects (21.4%); there was a significant underrepresentation of the GST-M1 null genotype among patients developing adverse drug reactions to azathioprine (odds ratio [OR] = 0.18, 95% confidence interval [CI] = 0.037-0.72, P = 0.0072) compared with patients who did not develop adverse effects. Patients heterozygous for TPMT mutations presented a marginally significant increased probability of developing adverse effects (OR = 6.38, 95% CI = 0.66-84.1, P = 0.062). Moreover, among the 55 patients who did not develop adverse effects, there was a significant underrepresentation of the GST-M1 null genotype among patients who displayed lymphopenia as compared with those that did not display this effect of azathioprine (OR = 0.15, 95% CI = 0.013-1.08, P = 0.032).

CONCLUSION

Patients with IBD with a wildtype GST-M1 genotype present increased probability of developing adverse effects and increased incidence of lymphopenia during azathioprine treatment.