Thiopurine methyl transferase activity: new extraction conditions for high-performance liquid chromatographic assay

Development and validation of a UPLC-UV method for the quantification of thiopurine methyltransferase enzyme activity in human erythrocytes

Thiopurines are drugs widely used for the treatment of autoimmune conditions, inflammatory bowel disease or acute lymphoblastic leukemia. Determination of thiopurine methyltransferase activity (TPMT), a major determinant of thiopurines toxicity, has been suggested before implementing thiopurine treatment. An ultraperformance liquid chromatography (UPLC) method was developed and validated for the quantification of TPMT enzyme activity based on the conversion of 6-mercaptopurine (6-MP) to 6-methylmercaptopurine (6-MMP) using S-adenosyl-L-methionine (SAM) as methyl donor in red blood cell lysates (RBC). This method was improved from a previous laborious high performance liquid chromatography (HPLC) method, using a lower volume of injection and with a shorter runtime. After incubation and protein precipitation 6-MMP was separated on a HSS-T3 (2.1 × 50 mm, 1.8 μm) column and monitored by UV detection (290 nm). A change on the organic solvent used to dissolve 6-MP resulted in a reduction of interference by endogenous or non-enzymatic methylated 6-MMP. A full validation of the 6-MMP assay was performed according to the FDA and EMA guidelines. The method was linear from 0.125 to 2 nmol/mL, with acceptable values of accuracy and precision. The method was applied in 106 patients treated with thiopurines whose TPMT activity was previously quantified by HPLC. Evaluation through Bland-Altman plot showed that TPMT activities were in agreement between both methods.

Correlation of thiopurine methyltransferase activity and 6-thioguanine nucleotide concentration in Han Chinese patients treated with azathioprine 25 to 100 mg: A 1-year, single-center, prospective study

BACKGROUND

Of the enzymes involved in the metabolism of azathioprine, thiopurine methyltransferase (TPMT) is the one characterized by genetic polymorphisms and ethnic variations. There have been several studies of the ethnic variations in phenotype and genotype of TPMT, although few have assessed the possible correlation between TPMT activity and 6-thioguanine nucleotide (6-TGN) concentrations.

OBJECTIVE

The aim of this study was to examine the relationship between TPMT activity and the steady-state concentration (Css) of 6-TGN, the primary active metabolite of azathioprine, in red blood cells (RBCs) in Han Chinese patients treated with azathioprine.

METHODS

Han Chinese patients aged 18 to 60 years with immunosuppression and normal hepatic and renal function who had been receiving a stable dose (25-100 mg/d) of oral azathioprine as a part of their regular anti-immunosuppression regimen for at least 10 days were recruited for this 1-year, single-center, prospective study. Azathioprine was administered PO QD in the morning, in combination with a stable regimen of other immunosuppressive drugs, for 1 year. At 1 year, blood samples were drawn just before the ingestion of azathioprine. TPMT activity and 6-TGN Css in RBCs were determined in our laboratory using high-performance liquid chromatography. Adverse drug events were monitored by a patient questionnaire and laboratory testing. Out of the initial cohort, several patients were concurrently enrolled in a subanalysis in which the effect of TPMT polymorphism on the pharmacokinetic properties of 6-mercaptopurine, the intermediate metabolite of azathioprine, was examined.

RESULTS

Nineteen patients (14 women, 5 men; mean [SD] age, 41 [9.6] years [range, 22-59 years]; mean [SD] weight, 62 [12] kg) were included in the study; 7 were included in the subanalysis. A significant negative correlation was found between TPMT activity and 6-TGN Css in RBCs (r = -0.712; P = 0.001); when the outlier data were removed, no significant correlation was found. Mean (SD) TPMT activity was 12.95 (3.07) nmol/h · mL(-1) RBCs and the interindividual CV was 23.68%. Mean (SD) 6-TGN CSS was 42.95 (41.98) ng/8 × 108 RBCs and the interindividual CV was 97.74% (N = 19), while the intraindividual CV of 6-TGNs within 8 hours after azathioprine ingestion was between 4.23% and 7.37% (n = 7). No significant correlation was found between 6-TGN Css in RBCs and the dose of azathioprine used. One patient's treatment was discontinued because her white blood cell count decreased to < 4 × 109 cells/L, indicating myelotoxicity; the t/12 of 6-TGNs in this patient was 5.85 days. Treatment was well tolerated by all other patients.

CONCLUSION

In this small study, a significant negative correlation was found between TPMT activity and 6-TGN concentration in the RBCs of these Han Chinese patients. However, the correlation was not significant when data from 1 patient with low TPMT activity were excluded.

Analysis of thiopurine S-methyltransferase phenotype-genotype in a Tunisian population with Crohn's disease

This study was conducted to investigate the thiopurine S-methyltransferase TPMT activity distribution and gene mutations in Tunisian population with positive diagnostic for Crohn's disease. TPMT activity was measured in Tunisian population (n = 88) by a high performance liquid chromatography assay. Polymerase chain reaction-based methods were used to determine the frequency of TPMT mutant alleles TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C. TPMT activity was normally distributed, ranging from 4.58 to 35.27 nmol/(h ml) RBC with a mean of 18.67 ± 7.10 nmol/(h ml) RBC. Seven TPMT*3A heterozygotes and one TPMT*3C homozygote were found in 88 patients, with allele frequencies of 0.039 and 1.13, respectively. TPMT*3A and the TPMT*3C, which cause the largest decrease in enzyme activity, were both variant alleles detected in the Tunisian population.

Pre-analytic and analytic sources of variations in thiopurine methyltransferase activity measurement in patients prescribed thiopurine-based drugs: A systematic review

OBJECTIVES

Low thiopurine S-methyltransferase (TPMT) enzyme activity is associated with increased thiopurine drug toxicity, particularly myelotoxicity. Pre-analytic and analytic variables for TPMT genotype and phenotype (enzyme activity) testing were reviewed.

DESIGN AND METHODS

A systematic literature review was performed, and diagnostic laboratories were surveyed.

RESULTS

Thirty-five studies reported relevant data for pre-analytic variables (patient age, gender, race, hematocrit, co-morbidity, co-administered drugs and specimen stability) and thirty-three for analytic variables (accuracy, reproducibility). TPMT is stable in blood when stored for up to 7 days at room temperature, and 3 months at -30°C. Pre-analytic patient variables do not affect TPMT activity. Fifteen drugs studied to date exerted no clinically significant effects in vivo. Enzymatic assay is the preferred technique. Radiochemical and HPLC techniques had intra- and inter-assay coefficients of variation (CVs) below 10%.

CONCLUSION

TPMT is a stable enzyme, and its assay is not affected by age, gender, race or co-morbidity.

Thiopurine S-methyltransferase pharmacogenetics in a large-scale healthy Italian-Caucasian population: differences in enzyme activity

AIMS

To investigate the influence of genotype, age and gender on the thiopurine S-methyltransferase (TPMT) phenotype in healthy Italian-Caucasian subjects.

MATERIALS & METHODS

The study investigated the TPMT genotype and the TPMT phenotype of 943 healthy Italian-Caucasian subjects of different age and gender (age range: 0.08-68 years; 623 males 320 females). TPMT red blood cell activity was measured in all samples and genotype was determined for the TPMT alleles *2, *3A, *3B and *3C.

RESULTS

TPMT activity levels in our whole population ranged from 1.6 up to 75.2 U/gHb. Significant TPMT activity differences between wild-type and heterozygous subjects were observed. We divided our TPMT activity into four categories according to our frequency distribution: low (0.1%), intermediate (32.9%), normal (60%) and high (7%), with arbitrary cut-off values of 8.0, 19.4 and 37.0 U/gHb, respectively. The whole population had a total of 94.5% of homozygous wild-type subjects, 5.4% heterozygous variants and one (0.1%) compound heterozygous variant TPMT*3B/*3C. The overall concordance rate between TPMT genotypes and phenotypes was 71.6%. The TPMT activity was significantly higher in wild-type children (0.08-17 years) than in wild-type adults (aged 18-68 years). Moreover, it was noted that wild-type infants from 0.08 to 5 years had a 9% higher average TPMT activity than the other wild-type groups, and only in children from 0.08 to 2 years was the TPMT activity higher in males than in females.

CONCLUSION

The data obtained in this study show that genetic factors seem to be the major aspect in TPMT phenotype variability in adults, whilst, in children, other physiological factors should be taken into consideration when assessing the TPMT phenotype, such as age and gender.

Pharmacogenetic differences and drug-drug interactions in immunosuppressive therapy

With the advent of new immunosuppressants and formulations, the elucidation of molecular targets and the evolution of therapeutic drug monitoring, the field of organ transplantation has witnessed significant reductions in acute rejection rates, prolonged graft survival and improved patient outcome. Nonetheless, challenges persist in the use of immunosuppressive medications. Marked interindividual variability remains in drug concentrations and drug response. As medications with narrow therapeutic indices, variations in immunosuppressant concentrations can result in acute toxicity or transplant rejection. Recent studies have begun to identify factors that contribute to this variability with the promise of tailoring immunosuppressive regimens to the individual patient. These advances have uncovered differences in genetic composition in drug-metabolising enzymes, drug transporters and drug targets. This review focuses on commonly used maintenance immunosuppressants (including cyclosporin, mycophenolate mofetil, tacrolimus, sirolimus, everolimus, azathioprine and corticosteroids), examines current studies on pharmacogenetic differences in drug-metabolising enzymes, drug transporters and drug targets and addresses common drug-drug interactions with immunosuppressant therapies. The potential role of drug-metabolising enzymes in contributing to these drug-drug interactions is briefly considered.

Efficient screening method of the thiopurine methyltransferase polymorphisms for patients considering taking thiopurine drugs in a Chinese Han population in Henan Province (central China)

BACKGROUND

Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzed the S-methylation of thiopurine drugs. TPMT activity exhibits an interindividual variability, mainly as a result of genetic polymorphism. Patients with intermediate or deficient TPMT activity are at risk for toxicity after receiving standard doses of thiopurine drugs. We determined a cut-off concentration of the TPMT activity assay less than which genotyping of the TPMT gene should be performed. In addition, the influence of hemodialysis on TPMT activity in uremic patients was examined.

METHODS

In 248 healthy subjects and 30 uremic patients, PCR-based methods were used to analyze the most common functional mutations TPMT2, 3A, 3B and 3C. A HPLC assay was used to measure erythrocyte TPMT activity in the whole population.

RESULTS

Seven TPMT3C heterozygotes were identified, while TPMT2, 3A and 3B alleles were not detected in 248 healthy subjects. The frequency of TPMT3C allele was 1.4% (7/496). The TPMT activity in healthy subjects was normally distributed, ranged from 6.09 to 28.65 nmol/h/ml pRBC with a mean of 16.03 +/- 4.16 nmol/h/ml pRBC. The cut-off for high TPMT activity and intermediate TPMT activity was 10.07 nmol/h/ml pRBC. There were 19 intermediate activity healthy subjects (7.7%) and 229 high activity healthy subjects (92.3%), and no TPMT deficiency subject was found. All of the 229 healthy subjects with high activity had no mutant alleles, while 7 of the 19 subjects with intermediate activity had a mutant allele. Phenotypes were in good agreement with genotypes for 95% of subjects. The uremic patients were all homozygous for the wild-type allele whose TPMT activity was activated significantly before hemodialysis compared with TPMT activity after hemodialysis.

CONCLUSIONS

We defined the cut-off values for the TPMT phenotyping assay at 10.07 nmol/h/ml pRBC, less than which additional genotyping elucidates the individual risk for drug therapy. In uremic patients, TPMT activity is increased by some uremic factors, and dialysis shifted their TPMT activity close to that of a healthy control group.

Determination of thiopurine S-methyltransferase (TPMT) activity by comparing various normalization factors: Reference values for Estonian population using HPLC-UV assay

Thiopurine S-methyltransferase (TPMT; EC 2.1.1.67) is the key enzyme in the metabolism of thiopurine drugs. Determination of TPMT activity has been used for the individualization of thiopurine dose. We developed HPLC-UV assay for the determination of TPMT activity in human erythrocytes using 6-mercaptopurine as a substrate. Various extraction and chromatographic conditions were compared. In-house developed extraction with acetonitrile provided the lowest limit of quantification. TPMT activity was determined in 99 previously genotyped healthy Estonians. TPMT activity was expressed as the formation of 6-methylmercaptopurine ng/ml/h and normalized either to haemoglobin, haematocrit, erythrocyte count or protein content. The receiver-operating characteristic curve analysis revealed similar accuracy values for TPMT activity in predicting heterozygous and wild type individuals for each method of calculation. In healthy Estonians, TPMT activity varied from 21.5 to 129.6 ng/ml/h. For heterozygous individuals (n = 18), TPMT activity was 48.1 +/- 11.7 ng/ml/h. Wild type individuals (n = 81) revealed significantly higher TPMT activity 79.3 +/- 20.7 ng/ml/h (P < 0.001). This sensitive HPLC assay for quantitative determination of TPMT activity could easily be used in clinical settings. Under constant experimental conditions for haemolysate preparation no normalization is required.

Interference free and simplyfied liquid chromatography-based determination of thiopurine S-methyltransferase activity in erythrocytes

The determination of the thiopurine S-methyltransferase activity (TPMT; EC 2.1.1.67) has become an important issue during thiopurine therapy due to its known genetic polymorphism resulting in a wide range of TPMT activity. Therefore, the standard thiopurine drug regimen is associated with increased hematopoetic toxicity in patients with low or absent TPMT activity, whereas patients with high activity may be insufficiently treated. However, presently available methods are labour intensive and time consuming and tend towards too high or too low enzyme activity due to their methodological approach. The use of instable substrate solutions (6-MP or 6-TG), organic solvents like dimethyl sulfoxide and too high substrate and co-substrate saturation concentrations contribute to this phenomenon. We therefore, established an optimized and fast isocratic HPLC linked TPMT assay based on the enzymatic methylation of mercaptopurine or thioguanine in RBC lysates with S-adenosyl-l-methionine as methyl donor. Unspecific non-enzymatic methylation was not detectable. The recovery of 6-methyl-mercaptopurine was 97-102%, the intra- and interday variation between 1.0 and 5.0%, respectively. The assay dispenses with a time consuming extraction procedure with organic solvents, a heating step, and a gradient elution and is therefore, favourable for clinical routine application. The TPMT activity was measured in 62 untreated children with acute lymphoblastic leucemia at the time of diagnosis (activity = 34.0+/-10.6 nmol/g Hb/h, range: 11.5-55.4 nmol/g Hb/h) and in 12 adult healthy volunteers (62.8+/-7.7 nmol/g Hb/h, range: 48-82 nmol/g Hb/h) reflecting the wide measurable TPMT activity found in erythrocytes.

Thiopurine Methyltransferase Genotype and Phenotype Status in Japanese Patients with Systemic Lupus Erythematosus

We investigated the genotypic status of thiopurine methyltransferase (TPMT) polymorphism to evaluate the possible risk of the toxicity of azathioprine (AZA) in 68 patients with systemic lupus erythematosus (SLE). The allele frequency of TPMT mutation in the SLE group (2.9%) was higher than that in 174 Japanese healthy volunteers (1.1%), although it did not reach statistically significant difference (p=0.23). The mean value of TPMT activities in 51 subjects with TPMT*1/*1 was 40% higher than that of 4 subjects with TPMT*1/*3C in SLE group (18.1+/-6.1 nmol/h/ml packed red blood cells (pRBC) versus 13.2+/-3.2 nmol/h/ml pRBC; p=0.11). Two out of 4 SLE patients with TPMT*1/*3C had been treated with AZA, and one patient showed a leucopenia. The TPMT genotyping before AZA treatment is recommended for Japanese SLE patient group to avoid the AZA-induced adverse events, although detection of the patient with low TPMT activity by genotyping is still imperfect.

Phenotype and genotype for thiopurine methyltransferase activity in the French Caucasian population: impact of age

OBJECTIVE

Thiopurine drugs are commonly used in pediatric patients for the treatment of acute leukemia, organ transplantation and inflammatory diseases. They are catabolized by the cytosolic thiopurine methyltransferase (TPMT), which is subject to a genetic polymorphism. In children, enzyme activities are immature at birth and developmental patterns vary widely from one enzyme to another. The present study was undertaken to evaluate erythrocyte TPMT activity and the correlation between genotype and phenotype in different age groups from birth to adolescence and adulthood.

METHODS

The study included 304 healthy adult blood donors, 147 children and 18 neonates (cord bloods). TPMT activity was measured by liquid chromatography, and genotype was determined using a polymerase chain reaction reverse dot-blot analysis identifying the predominant TPMT mutant alleles (TPMT*3A, TPMT*3B, TPMT*3C, TPMT*2).

RESULTS

There was no significant difference in TPMT activity between cord bloods ( n=18) and children ( n=147) (17.48+/-4.04 versus 18.62+/-4.14 respectively, P=0.424). However, TPMT was significantly lower in children than in adults (19.34+/-4.09) ( P=0.033). In the whole population, there were 91.9% homozygous wild type, 7.9% heterozygous mutants and 0.2% homozygous mutants. The frequency of mutant alleles was 3.0% for TPMT*3A, 0.7% for TPMT*2 and 0.4% for TPMT*3C.

CONCLUSION

No impact of child development on TPMT activity could be evidenced, suggesting that TPMT activity is already mature at birth. The difference between children and adults was low with reduced clinical impact expected. When individual TPMT activity was compared with genotype, there was an overlapping region where subjects (4.5%, 12 adults, 9 children) were either homozygous wild type or heterozygous, with a TPMT activity below the antimode value. This result highlighted the importance of measuring TPMT activity to detect all patients at risk of thiopurine toxicity.

Phenotypic and genotypic analysis of thiopurine s-methyltransferase polymorphism in the bulgarian population

Genetic polymorphism of TPMT activity is an important factor responsible for large individual differences in thiopurine toxicity and therapeutic efficacy. The aim of this study was to determine the distribution of TPMT activity as well as the types and frequencies of mutant alleles in a Bulgarian population sample. TPMT activity was measured in 313 Bulgarians, using an established HPLC procedure. All individuals with TPMT activity less than 12.0 nmol/(mL Ery.h) (n = 76) were additionally genotyped using a color multiplex hybridization assay. The samples were tested for TPMT*2, *3A, *3B, *3C, *3D, *4, and *6 mutant alleles. TPMT activities varied from 1.1 to 24.0 nmol/(mL Ery.h) [mean 14.2 +/- 3.2 nmol/(mL Ery.h)]: 92.3% of the individuals investigated had high TPMT activity [>10 nmol/(mL Ery. h)], whereas 7.4% were intermediate [2.8-10 nmol/(mL Ery.h)], and 0.3% were low metabolizers [< 2.8 nmol/(mL Ery.h)]. A significant gender-related difference in TPMT activity (P = 0.02) was observed with 6.2% higher values in men than in women. There was no significant correlation between age and enzyme activity (r = 0.06, P = 0.27). Genotype analysis revealed three mutant TPMT alleles: 2, 3A, and 3C. The frequency of these alleles among the TPMT-deficient individuals was 2.17%, 30.4%, and 2.17%, respectively. These data show a similar distribution of TPMT activity among the Bulgarian population investigated as in most other white populations with the frequency of intermediate metabolizers being somewhat lower (7.4% versus approximately 11%) in the Bulgarians. The most common variant allele was TPMT-3A, as in other white populations.

Determination of Thiopurine Methyltransferase Phenotype in Isolated Human Erythrocytes Using a New Simple Nonradioactive HPLC Method

Genetic polymorphism of the S-methylation pathway catalyzed by thiopurine methyltransferase (TPMT) is responsible for variation in the metabolism, toxicity, and therapeutic efficacy of thiopurine drugs. This paper describe a new simple, nonradioactive HPLC method for determination of TPMT activity in isolated erythrocytes (Ery), based on the conversion of 6-mercaptopurine (pH 7.5, 37 degrees C) to 6-methylmercaptopurine (6-MMP) using S-adenosyl-l-methionine as methyl donor. The incubation step was stopped by a mixture of trichloroacetic acid/acetonitrile containing the internal standard 4-aminoacetophenone. 6-MMP was quantified by absorbance at 290 nm after chromatographic separation on a Zorbax SB-Phenyl column (5 microm, 4.6 x 250 mm) using mobile phases (flow rate 1.1 mL/min) consisting of acetonitrile, phosphate buffer pH 3.0, triethylamine, and dithiothreitol. The assay was linear up to 50 nmol/(mL Ery. h), and the detection limit was 0.3 nmol/(mL Ery. h). The extraction efficiency of 6-MMP was 95-103% (n = 3), and its analytic recovery ranged between 98.3% and 101.8% (n = 12). The within-day imprecision using pooled human erythrocytes (n = 12) was 4.4% at a TPMT activity of 14.3 nmol/(mL Ery.h) and 4.9% at 6.5 nmol/(mL Ery.h). The between-day imprecision (n = 12) was 6.8% and 7.5% nmol/(mL Ery.h), respectively. A very good agreement was found between TPMT activity determined with this method (y) and a widely used radiochemical procedure (x) (r = 0.94; n = 130; y = 0.502 + 0.946x; P < 0.05). Genotype analysis of all individuals with TPMT activity under 12.5 nmol/(mL Ery.h) revealed a genotype/phenotype concordance of 86%. The new HPLC method for determination of TPMT activity in Ery is a simple, rapid, and reliable nonradioactive procedure that can be successfully used for both research and routine clinical analysis.

Thiopurine methyltransferase activity: new conditions for reversed-phase high-performance liquid chromatographic assay without extraction and genotypic-phenotypic correlation

Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the metabolism of thiopurine drugs. A genetic polymorphism is responsible for large inter-individual differences observed in TPMT activity. We report a new HPLC technique, which avoids an extraction step and the use of radioactive reagents, based on the conversion of 6-mercaptopurine (6-MP) to 6-methylmercaptopurine (6-MMP) using S-adenosyl-L-methionine (SAM) as methyl donor in red blood cell lysates (RBC). Intra- and inter-assay variation, within-day, within-run, between-day, and between-run variations showed high precision. The formation of 6-MMP was linear with respect to the lysate concentration and time. In a blinded assay of 61 samples, the results of HPLC method correlated with those of the radiochemical method (r2=0.82, P<0.0001). Using a cut-off point of 8.5 nmol/h/ml packed RBC, positive predictive value of HPLC was 100% for heterozygous patients. Because of the absence of extraction step, this new HPLC technique of TPMT activity determination reduces analysis variation and is time-saving. This rapid, sensitive, and reproducible method is suitable for routine monitoring of TPMT activity and for fundamental studies.

Determination of thiopurine methyltransferase activity in human erythrocytes by high-performance liquid chromatography: comparison with the radiochemical method

The current article describes a new assay to measure thiopurine methyltransferase (TPMT) activity from red blood cells. This method is based on the measurement of the reaction product 6-methylmercaptopurine (6-MMP) by high-performance liquid chromatography (HPLC). 6-MMP is extracted by ethyl acetate with recoveries of 85%, 80%, 80%, and 92% for 50, 250, 500, and 1,000 ng/100 microL packed red blood cells, respectively. 6-MMP was identified and measured by a Zorbax CN column installed in an HPLC system. The chromatograms were resolved using a mobile phase consisting of 40 mmol/L sodium phosphate buffer (pH 3) and methanol in a gradient from 1% to 20% of methanol. Under these conditions 6-MMP is well resolved from substrates (6-mercaptopurine and S-adenosyl-L-methionine) and endogenous peaks. When the TPMT activity from 20 patients was measured by the HPLC-linked assay and the classic radiochemical method, a linear correlation was obtained between both procedures ( y = 0.99x + 0.33; x-axis, radiochemical assay; y-axis, HPLC-linked assay; r = 0.98). In conclusion, the current report describes a new, reliable, safe, and nonradioactive method to measure TPMT activity that is shorter and simpler than the previously described ones.