Pretreatment determination of TPMT – state of the art in clinical practice

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.

Mucosal improvement in patients with moderate to severe postoperative endoscopic recurrence of Crohn's disease and azathioprine metabolite levels

BACKGROUND

The value of azathioprine metabolites (6-thioguanine nucleotides [6-TGN]) in monitoring clinical treatment response is still controversially discussed. Data regarding thiopurine metabolite levels and endoscopic improvement are lacking.

METHODS

Data were analyzed post hoc from a 1-year, multicenter, double-blind, double-dummy, randomized trial comparing azathioprine 2.0 to 2.5 mg/kg per day versus mesalamine 4 g/d in a subset of 23 postoperative patients with Crohn's disease (CD) treated with azathioprine and having moderate-to-severe endoscopic recurrence according to a modified 6-grade score. Red blood cell (RBC) concentrations of 6-TGN, 6-methyl-mercaptopurine ribonucleotides (6-MMPR), and 6-methyl-thioguanine nucleotides (6-MTGN) were indicated as follows: area under the concentration-time curve, average concentration (C av), and concentration at the final study visit.

RESULTS

Overall, 74% of patients showed an improvement in the modified endoscopic score (P = 0.022). Median endoscopic score reduced from 4 at the baseline to 2 at the final visit. Patients with a high C av for 6-TGN (≥ 193 pmol/8 × 10(8) RBC; P = 0.017) or 6-MTGN (≥ 79.2 pmol/8 × 10(8) RBC; P = 0.035) significantly improved in endoscopic score, and the improvement in endoscopic score correlated with C av for 6-TGN (r = -0.51; P = 0.013). For concentration at the final visit, higher values for 6-TGN (≥ 142 pmol/8 × 10(8) RBC; P = 0.017) were associated with a better postoperative score. Sensitivity analysis revealed a significant correlation between 6-TGN (area under the concentration-time curve) and postoperative endoscopic improvement.

CONCLUSIONS

Our post hoc analysis from a double-blind, randomized trial suggests that higher RBC 6-TGN levels are associated with endoscopic improvement in patients with severe postoperative endoscopic recurrence of CD. Thus, our study provides first evidence on the utility of monitoring of thiopurine metabolites to achieve mucosal response in CD.

Validation of new allele-specific real-time PCR system for thiopurine methyltransferase genotyping in Korean population

Introduction. Thiopurine drugs are metabolized via S-methylation and catalyzed by thiopurine S-methyltransferase (TPMT). Patients with very low TPMT activity are at high risk of fatal bone marrow toxicity when standard doses of thiopurine drugs are administered. TPMT genotyping can predict TPMT activity and is not affected by transfusion or red blood cell defects. Here, we report a new allele-specific real-time polymerase chain reaction (PCR) system for thiopurine methyltransferase genotyping that is validated in Korean population. Materials and Methods. Three major TPMT single-nucleotide polymorphisms (TPMT∗2, ∗3B, and ∗3C) were genotyped using real-time PCR with the allele-specific primers and probes. Internal positive controls were included in each well, and an automatic interpretative algorithm was applied. This system was validated using 244 clinical samples and 2 commercial DNA samples that had been previously genotyped using PCR-direct sequencing. Results. All of the obtained results are concordant with those of the reference method. All of the internal positive control reactions were successful. The allele frequency of TPMT∗3C was 2.05% (10 of 488 alleles). All of the patients with variant alleles were heterozygotes, and no homozygotes were detected. No TPMT∗2, ∗3A, or ∗3B alleles were observed in this Korean population. Conclusion. This rapid, accurate, and user-friendly genotyping system can be readily used to improve the efficacy and safety of thiopurine treatments in clinical practice.

Allele frequency of inosine triphosphate pyrophosphatase (ITPA) and thiopurine-S-methyl transferase (TPMT) genes in the Tunisian population

AIM

The aim of this study was to determine the frequencies of TPMT and ITPA polymorphisms in Crohn's disease patients of Tunisian origin and to compare them with allele frequencies previously reported in other populations of various ethnic origins.

METHODS

ITPA (c.94C>A and IVS2+21A>C) and TPMT (c.238G>C, c.460G>A and c.719A>G) mutations and genotypes were assessed in 208 Tunisian subjects (78 males/130 females) by polymerase chain reaction-restriction fragment length polymorphism and allele-specific-PCR methods.

RESULTS

Genotyping of ITPA revealed frequencies of 6% and 7.9% for c.94C>A and IVS2+21A>C, respectively. Accordingly, deficient or diminished ITPA phenotype can be predicted to concern 2.4% of Tunisians. The observed frequencies of the c. 238G>C, c.460G>A and c.719A>G TPMT polymorphisms were 0, 0.24 and 1.44%, respectively.

CONCLUSION

This study provides the first analysis of TPMT and ITPA mutant allele frequency in individuals of Tunisian origin. Unlike in Caucasians, TPMT*3C which harbours the c.719A>G polymorphism appears to be the most common mutant allele in Tunisians. In contrast, ITPA mutant allele frequency distribution appears to be similar to that observed in Caucasians.

Practical recommendations for pharmacogenomics-based prescription: 2010 ESF-UB Conference on Pharmacogenetics and Pharmacogenomics

The present article summarizes the discussions of the 3rd European Science Foundation-University of Barcelona (ESF-UB) Conference in Biomedicine on Pharmacogenetics and Pharmacogenomics, which was held in June 2010 in Spain. It was focused on practical applications in routine medical practice. We provide practical recommendations for ten different clinical situations, that have either been approved or not approved by regulatory agencies. We propose some comments that might accompany the results of these tests, indicating the best drug and doses to be prescribed. The discussed examples include KRAS, cetuximab, panitumumab, EGFR-gefitinib, CYP2D6-tamoxifen, TPMT-azathioprine-6-mercaptopurine, VKORC1/CYP2C9-warfarin, CYP2C19-clopidogrel, HLA-B*5701-abacavir, HLA-B*5701-flucloxacillin, SLCO1B1-statins and CYP3A5-tacrolimus. We hope that these practical recommendations will help physicians, biologists, scientists and other healthcare professionals to prescribe, perform and interpret these genetic tests.