Intracellular thiopurine nucleotides and azathioprine myelotoxicity in organ transplant patients

Thiopurines' Metabolites and Drug Toxicity: A Meta-Analysis

Many questions remain unanswered regarding therapeutic drug monitoring (TDM) utility with thiopurines. This study aims to establish a relationship between thiopurines' metabolites and drug toxicity. We performed a systematic review with inclusion of studies evaluating the relationship between thiopurines' metabolites and drug toxicity. Meta-analysis of mean difference (MD), correlations and odds ratio (OR) was performed. We identified 21,240 records, 72 of which were eligible for meta-analysis. Levels of 6-thioguanine nucleotides (6-TGN) were higher in patients with leukopenia (MD 127.06 pmol/8 × 10⁸ RBC) and gastrointestinal intolerance (MD 201.46 pmol/8 × 10⁸ RBC), and lower in patients with hepatotoxicity (MD -40.6 pmol × 10⁸ RBC). We established a significant correlation between 6-TGN and leukocytes (r = -0.21), neutrophils (r = -0.24) and alanine aminotransferase levels (r = -0.24). OR for leukopenia in patients with elevated 6-TGN was 4.63 (95%CI 2.24; 9.57). An optimal cut-off of 135 pmol/8 × 10⁸ RBC for leukopenia was calculated (sensitivity 75.4%; specificity 46.4%). 6-methylmercaptopurine ribonucleotides (6-MMPR) were significantly associated with hepatotoxicity (MD 3241.2 pmol/8 × 10⁸ RBC; OR 4.28; 95%CI 3.20; 5.71). Levels of 6-MMPR measured in the first 8 weeks of treatment were associated with leukopenia. We conclude that TDM could be used to prevent thiopurines' toxicity. As optimal metabolites level may vary according to indication, physicians may adapt posology to decrease toxicity without compromising efficacy.

Pharmacogenetic association with adverse drug reactions to azathioprine immunosuppressive therapy following liver transplantation

Azathioprine (AZA) is a thiopurine prodrug commonly used in triple-immunosuppressive therapy following liver transplantation. Approximately 1 in 10 patients suffers side effects in response to the drug, the most problematic being bone marrow toxicity. There is evidence that polymorphisms in the genes encoding thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPase) predict adverse drug reactions to AZA therapy. Furthermore, common genetic polymorphisms in the gene encoding methylenetetrahydrofolate reductase (MTHFR) may have an indirect impact on thiopurine drug methylation by influencing levels of the methyl donor S-adenosylmethionine (SAM). The aim of this study was to determine whether polymorphisms in these candidate pharmacogenetic loci predict adverse drug reactions to AZA immunosuppressive therapy in liver transplant patients. A series of 65 liver transplant recipients were recruited to the study from the Liver Transplant Out-Patient clinic at The Royal Infirmary of Edinburgh. Clinical response to AZA was retrospectively correlated against TPMT activity, TPMT*2, *3A, and *3A genotypes, inosine triphosphatase (ITPA) 94C>A and IVS2+21A>C genotypes, and MTHFR 677C>T and 1298A>C genotypes. Variant TPMT, ITPA, and MTHFR genotypes were not significantly associated with adverse drug reactions to AZA, including bone marrow suppression. However, the 2 patients who suffered nodular regenerative hyperplasia (NRH) were both heterozygous for the TPMT*3A mutation. In conclusion, our findings suggest that TPMT, ITPA, and MTHFR genotypes do not predict adverse drug reactions, including bone marrow suppression, in liver transplant patients. However, the possible association between NRH and a heterozygous TPMT genotype should be investigated further.

Thiopurine methyltransferase (TPMT) genotype distribution in azathioprine-tolerant and -intolerant patients with various disorders. The impact of TPMT genotyping in predicting toxicity

OBJECTIVE

To study the distribution of the thiopurine methyltransferase (TPMT) genotype among azathioprine (Aza)-tolerant and -intolerant patients with various disorders, and to investigate a possible relationship with the Aza metabolite levels.

METHODS

Forty-six Aza-tolerant and six Aza-intolerant patients had the TPMT genotype distribution determined using a polymerase chain reaction (PCR) assay and the forty-six Aza-tolerant patients had the Aza metabolite levels determined using a high-pressure liquid chromatography (HPLC) analysis.

RESULTS

One non-functional TPMT mutant allele was demonstrated in 2 of the 46 Aza-tolerant patients (4.4%) and one or two non-functional mutant alleles in 2 of the 6 Aza-intolerant patients (33.3%). Of the 4 patients, with one or two non-functional mutant alleles 2 (50%) were intolerant to Aza compared with 4 of the 48 patients (8.3%) with no mutations detected. The time to hepatotoxicity did not differ significantly between the 2 patients with one or two non-functional mutant alleles and the remaining 3 patients ( P=0.5). The TPMT genotype distribution differed slightly in the three different categories of disorders ( P=0.05). The median E-6-TGN level among the 2 TPMT heterozygous patients was 275 pmol/8x10(8) RBC (range 240-310), whereas the remaining 44 patients had a median E-6-TGN level of 110 pmol/8x10(8) RBC (range 0-440) ( P=0.07).

CONCLUSION

Although TPMT genotyping cannot be recommended on behalf of the present study, it is to be expected that half of the patients with one or two non-functional TPMT mutant alleles will develop Aza intolerance leading to withdrawal of therapy. Thus, clinicians may anticipate about 5% of the patients to develop intolerance to Aza therapy solely for that reason.

Differences between children and adults in thiopurine methyltransferase activity and metabolite formation during thiopurine therapy: possible role of concomitant methotrexate

This study examined the role of thiopurine methyltransferase (TPMT) polymorphism in the metabolism and clinical effects of azathioprine and 6-mercaptopurine in the treatment of inflammatory bowel disease and childhood leukemia. The current hypothesis is that the cytotoxic effects of thiopurines are caused by the incorporation of thioguanine nucleotides into DNA. In this context, S-methylation catalyzed by TPMT can be regarded as a competing metabolic pathway. The authors assayed the TPMT activity in red blood cells from 122 patients treated with azathioprine or 6-mercaptopurine (83 adults with inflammatory bowel disease and 39 children with acute lymphoblastic leukemia) and in 290 untreated controls (219 adult blood donors and 71 children). The concentrations of thioguanine nucleotides and methylthioinosine monophosphate were also assayed in red blood cells from the patients. The TPMT activity and the concentrations of methylthioinosine monophosphate and thioguanine nucleotides were higher in children than in adults. All children but no adult patient received concomitant methotrexate. Interaction between methotrexate and 6-mercaptopurine has been described, and may explain the results. Low TPMT activity in adult patients with inflammatory bowel disease correlated to an increased incidence of adverse drug reactions. However, there was no correlation between TPMT activity and the red blood cell concentrations of methylthioinosine monophosphate or thioguanine nucleotides, or between the concentrations of these metabolites and the occurrence of adverse effects. The results show that the role of thiopurine metabolism for drug effects is complex.

Identification of 6-methylmercaptopurine derivative formed during acid hydrolysis of thiopurine nucleotides in erythrocytes, using liquid chromatography–mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance assay

6-Thioguanine and 6-methylmercaptopurine (Me6-MP) nucleotides are the two major thiopurine metabolites of azathioprine found in erythrocytes. During the acid hydrolysis required for the conversion of thiopurine nucleotides into their free bases, Me6-MP was converted into a compound that could be analyzed on a Purospher RP18-e column with dihydrogen phosphate-methanol buffer as eluent. The pH of the acid extract strongly influenced the conversion of Me6-MP into its derivative. The Me6-MP derivative was identified using liquid chromatography–mass spectrometry and infrared and nuclear magnetic resonance spectrometric methods. During the acid hydrolysis of thiopurine nucleotides in erythrocytes, Me6-MP undergoes degradation, leading to 4-amino-5-(methylthio)carbonyl imidazole.

Monitored high-dose azathioprine treatment reduces acute rejection episodes after renal transplantation

BACKGROUND

Azathioprine (AZA) is widely used in organ transplantation. Common practice is to adjust dose according to body weight only, despite documented pharmacokinetic variability. The purpose of this study was to investigate whether high-dose AZA treatment monitored by 6-thioguanine nucleotides (6-TGN) levels reduces the incidence of rejection episodes in renal transplantation without a corresponding increase in myelotoxicity.

METHODS

Patients receiving cyclosporine, steroids, and AZA were randomized into either the low-dose AZA group (3 mg/kg on day 0, then 2 mg/kg/day the first week and 1 mg/kg/day thereafter) or the high-dose AZA group. In the latter, AZA was started at 5 mg/kg/day and then adjusted to keep 6-TGN concentrations (measured twice weekly) between 100 and 200 pmol/8 x 10(8) RBCs.

RESULTS

A total of 360 transplant recipients were included in the final analysis. The cumulative incidence of first rejection episodes was reduced by 21%, from 62.8% in the low-dose group to 49.4% in the high-dose group (difference: 13.3%; 95% confidence interval: 3.2-23.5). Similar results were found in subgroups according to HLA-DR match. The 6-TGN concentration was significantly higher in the high-dose AZA group during the first month, and the reduction in rejection episodes was achieved in the same period. A larger proportion of patients in the high-dose group had nadir white blood cell count below 2.0 x 10(9) leukocytes/L (13.3% vs. 4.4%; difference: 8.9%; confidence interval: 3.1-14.7).

CONCLUSIONS

High-dose AZA therapy in a triple-drug regimen, monitored by 6-TGN, will keep myelotoxicity within acceptable limits with the benefit of a reduction in acute rejection episodes.

Simultaneous determination of 6-thioguanine and methyl 6-mercaptopurine nucleotides of azathioprine in red blood cells by HPLC

6-thioguanine (6-TGN) and methyl 6-mercaptopurine nucleotides (Me6-MPNs) are the two major metabolites found in erythrocytes after administration of azathioprine. In an attempt to understand the role of these metabolites in the pharmacologic and toxic activity of thiopurines, we have developed a HPLC method for the simultaneous determination of 6-TGNs and Me6-MPNs in erythrocytes. A simple and rapid treatment procedure based on deproteinization by perchloric acid with dithiothreitol is described. The nucleotides were hydrolyzed to their own bases by heating the sample for 45 min at 100 °C. During acid hydrolysis Me6-MP was converted into a compound analyzed on a Purospher RP18-e column with 0.02 mol/L dihydrogenophosphate buffer–methanol as eluents. With this procedure, mean recoveries of 73.1% and 84.0% for 6-TGN and Me6-MPN derivatives, respectively, were found