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

Thiopurine S-methyltransferase and Pemphigus Vulgaris: A Phenotype-Genotype Study

Background & Objective:

Thiopurine drugs are considered as a treatment modality in various autoimmune disorders including pemphigus vulgaris (PV). These drugs are metabolized by an enzyme “Thiopurine S-methyl transferase” (TPMT). Various variants of this enzyme may have decreased activity leading to serious drug side effects. To investigate the phenotype and genotype of TPMT in PV patients receiving thiopurine drugs.

Methods:

A total of 50 patients (29 women and 21 men) with pemphigus vulgaris treating with standard dose of Thiopurine drugs were selected. Sex, age, result of liver function test and complete blood count were recorded. Genotyping of two common non-functional allele (TPMT*2 and TPMT*3C) by Allele-specific and RFLP-PCR was performed. TPMT enzymatic level was determined by an ELISA based method.

Results:

Of patients, 36 (72%) were found to have normal TPMT level; and 12, (24%) had higher level of enzyme and 2, 4% had low TPMT enzyme, but none of the patients showed mutant TPMT*2 and TPMT*3C alleles. None of the patients showed hepatotoxicity and bone marrow suppression.

Conclusion:

The phenotypic assay based on ELISA method may have false positive and misleading results but genotyping using PCR-RFLP and allele specific PCR is accurate, simple and cost-effective and can be used in patients decided to undergo thiopurine treatment.

Importance of 6-Thioguanine Nucleotide Metabolite Monitoring in Inflammatory Bowel Disease Patients Treated with Azathioprine

The active metabolite of azathioprine, 6-thioguanine nucleotide (6-TGN) is the main component responsible for the immunosuppressive effect in treatment of inflammatory bowel disease (IBD). The aim of this study was to assess the correlation between the concentration of 6-thioguanine nucleotide and disease activity, azathioprine-related adverse effects and time duration of treatment in patients with inflammatory bowel disease. Thirty-four patients were included in this study. Type of disease, gender, time duration of therapy and adverse effects were recorded. Metabolite concentration was determined by high performance liquid chromatography. Twenty-one percent of patients have experienced an adverse effect, with leucocytopenia most commonly occurring (42.9%). More adverse effects were registered when patients were treated with azathioprine in a period of less than 3 months in comparison to the group of patients that have been under therapy between 3-12 months and more than 12 months (p˂0.05). Most of the patients that presented any adverse effect had high 6-TGN concentration (>450 pmol/8x108 Er). The mean value of 6-TGN metabolite concentration in IBD patients treated with azathioprine was 437.46 pmol/8x108 Er ± 198.82 pmol/8x108. The time duration of azathioprine treatment did not have any significant impact on the achieved 6-TGN concentration (p>0.05).Twenty patients (58.9%) had achieved remission after therapy initiation with azathioprine. More alertness is recommended to clinicians towards patients in the first 3 months of the therapy. Our study demonstrated that higher 6-TGN concentration is associated with azathioprine toxicity.

LC-MS/MS Analysis of Erythrocyte Thiopurine Nucleotides and Their Association With Genetic Variants in Patients With Neuromyelitis Optica Spectrum Disorders Taking Azathioprine

BACKGROUND

Azathioprine is a first-line drug in treating neuromyelitis optica spectrum disorders (NMOSD). To exhibit its bioactivity, azathioprine needs to be converted to thiopurine nucleotides (TPNs) including 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine nucleotides (6-MMPNs) that are affected by genetic polymorphisms. This study aims to develop an LC-MS/MS method for the analysis of erythrocyte concentrations of TPNs and to evaluate their associations with variants of various genes (MTHFR, TPMT, HLA, SLC29A1, SLC28A2, SLC28A3, ABCB1, and ABCC4) in patients with NMOSD.

METHODS

Erythrocyte 6-TGNs and 6-MMPNs were converted to their free bases 6-thioguanine and 6-methylmercaptopurine derivative by 1-hour acid hydrolysis at 95°C. An LC-MS/MS method was developed, validated, and used to study 32 patients with NMOSD to determine these free bases. Genetic variants were identified by MassARRAY (Sequenom) and multiple SNaPshot techniques. The associations between genetic variants and the concentrations of TPNs or the 6-MMPNs:6-TGNs ratio were evaluated by PLINK software using linear regression.

RESULTS

Methanol and water were used for separation with a total run time of 6.5 minutes. The lowest limit of quantification was 0.1 μmol/L with an injection volume of 10 μL. rs10868138 (SLC28A3) was associated with a higher erythrocyte concentration of 6-TGNs (P = 0.031), whereas rs12378361 (SLC28A3) was associated with a lower erythrocyte concentration of 6-TGNs (P = 0.0067). rs507964 (SLC29A1) was significantly associated with a lower erythrocyte concentration of 6-MMPNs (P = 0.024) and a lower 6-MMPNs:6-TGNs ratio (P = 0.029).

CONCLUSIONS

An LC-MS/MS method for the analysis of erythrocyte TPNs was developed, validated, and used to study 32 patients with NMOSD. SLC29A1 and SLC28A3 were associated with the erythrocyte concentrations of TPNs and 6-MMPNs:6-TGNs ratio. Further studies are needed to confirm these results.

Ethnicity and lupus nephritis: an Australian single centre study

BACKGROUND

The clinical impression of Australian physicians is that systemic lupus erythematosus (SLE) is more prevalent and more severe in Asian patients than in their Caucasian counterparts. The presence and severity of lupus nephritis is a major determinant of prognosis in SLE, and largely determines disease impact.

AIM

To analyse the relationships between ethnicity and the prevalence and severity of lupus nephritis (LN) in patients attending a tertiary referral centre (The Royal Melbourne Hospital (RMH)).

METHODS

The ethnicity of all known patients with biopsy-proven LN was determined according to three definitions of ethnicity - ancestry, country of origin and primary language spoken. The prevalence of Asian ethnicity in the LN cohort was analysed across severity class, and was compared with the prevalences of Asian ethnicity in the general population within the hospital's geographic area, and with that in the relevant RMH cohorts of inpatients and outpatients, over the same time period.

RESULTS

Within this single tertiary centre, Asian patients were disproportionately represented in both the systemic lupus erythematosus (SLE) and the LN patient groups, although the distribution of histological severity of LN was not significantly different from Caucasian patients.

CONCLUSION

This study supports the common clinical impression that SLE is more common and more severe in the Asian-Australian population. Asian patients with SLE were more commonly diagnosed with LN. However, the spectrum of histological severity of LN was similar in Asian and Caucasian patients.

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.

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.