Patterns of azathioprine metabolites in neutrophils, lymphocytes, reticulocytes, and erythrocytes: relevance to toxicity and monitoring in recipients of renal allografts

Therapeutic drug monitoring of disease-modifying antirheumatic drugs in circulating leukocytes in immune-mediated inflammatory diseases

The treatment of immune-mediated inflammatory diseases (IMIDs) is one of the main challenges of modern medicine. Although a number of disease-modifying antirheumatic drugs (DMARDs) are available, there is wide variability in clinical response to treatment among individuals. Therapeutic drug monitoring (TDM) has been proposed to optimize treatment; however, some patients still experience unsatisfactory outcomes, although the blood concentrations of drugs in these patients remain in the therapeutic range. One possible reason for this is that the conventional samples (e.g., whole blood or plasma) used in TDM may not accurately reflect drug concentrations or concentrations of their metabolites at the target site. Hence, more refined TDM approaches to guide clinical decisions related to dose optimization are necessary. Circulating leukocytes or white blood cells have a critical role in driving the inflammatory process. They are recruited to the site of injury, infection and inflammation, and the main target of small molecule DMARDs is within immune cells. Given this, assaying drug concentrations in leukocytes has been proposed to be of possible relevance to the interpretation of outcomes. This review focuses on the clinical implications and challenges of drug monitoring of DMARDs in peripheral blood leukocytes from therapeutic or toxicological perspectives in IMIDs.

Identification of rare defective allelic variants in cases of thiopurine S-methyltransferase deficient activity

Aim: To assess rare TPMT variants in patients carrying a deficient phenotype not predicted by the four more frequent genotypes (*2, *3A, *3B and *3C). Materials & methods: Next-generation sequencing of TPMT in 39 patients with a discordant genotype. Results: None of the variants identified explained the discordances assuming that they are of uncertain significance according to the Clinical Pharmacogenetics Implementation Consortium classification. Two unknown variants were detected and predicted to result in a splicing defect. We show that TPMT*16 and TMPT*21 are defective alleles, and TPMT*8 and TPMT*24 are associated with a normal activity. Conclusion: Whole-exon sequencing for rare TPMT mutations has a low diagnostic yield. A reassessment of the functional impact of rare variants of uncertain significance is a critical issue.

Modeling the Outcome of Systematic TPMT Genotyping or Phenotyping Before Azathioprine Prescription: A Cost-Effectiveness Analysis

BACKGROUND

Thiopurine S-methyltransferase (TPMT) testing, either by genotyping or phenotyping, can reduce the incidence of adverse severe myelotoxicity episodes induced by azathioprine. The comparative cost-effectiveness of TPMT genotyping and phenotyping are not known.

OBJECTIVE

Our aim was to assess the cost-effectiveness of phenotyping-based dosing of TPMT activity, genotyping-based screening and no screening (reference) for patients treated with azathioprine.

METHODS

A decision tree was built to compare the conventional weight-based dosing strategy with phenotyping and with genotyping using a micro-simulation model of patients with inflammatory bowel disease from the perspective of the French health care system. The time horizon was set up as 1 year. Only direct medical costs were used. Data used were obtained from previous reports, except for screening test and admission costs, which were from real cases. The main outcome was the cost-effectiveness ratios, with an effectiveness criterion of one averted severe myelotoxicity episode.

RESULTS

The total expected cost of the no screening strategy was €409/patient, the total expected cost of the phenotyping strategy was €427/patient, and the total expected cost of the genotyping strategy was €476/patient. The incremental cost-effectiveness ratio was €2602/severe myelotoxicity averted in using the phenotyping strategy, and €11,244/severe myelotoxicity averted in the genotyping strategy compared to the no screening strategy. At prevalence rates of severe myelotoxicity > 1%, phenotyping dominated genotyping and conventional strategies.

CONCLUSION

The phenotype-based strategy to screen for TPMT deficiency dominates (cheaper and more effective) the genotype-based screening strategy in France. Phenotype-based screening dominates no screening in populations with a prevalence of severe myelosuppression due to azathioprine of > 1%.

The Impact of Immunosuppressive Drugs on Human Placental Explants

The use of immunosuppressive drugs guarantees the vitality of the graft and allows gestation in spite of intercurrences such as prematurity and intrauterine growth restriction. However, little is known about the direct effects of immunosuppressive drugs on placental cells. We investigated the effects of immunosuppressive drugs in the chorionic villous explants from human term placentas of healthy gestations. Human placental explants from term gestations (37-39 week gestational age, n = 12) were exposed to cyclosporine A (CSA, 0, 62.5, 125, 1250 ng/mL) or azathioprine (AZA, 0, 5, 10, 100 ng/mL) separately or, in combination for up to 48 hours. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays showed a significant decrease in the explant metabolic activity between AZA and the control group (24 hours, 100 ng/mL, 48 hours, all concentrations, P < .005). Cyclosporin A (CsA) reduced cell activity when associated with AZA (48 hours, P < .005). Fibrinoid deposits increased in AZA-treated explants alone (5 ng/mL, 48 hours; 10 ng/mL, 24-48 hours; P < .005) or when associated with CsA (10 AZA/125 CsA, P < .05), whereas in CsA treatment alone, there was an augment in syncytial knots (24-48 hours, P < .005). The sFLT1 gene (24 hours, P < .05) and protein ( P < .005) expression increased in AZA and CsA-treatments separately or in combination ( P < .05). Placental growth factor increased in AZA (24 hours, 10 ng/mL) and CsA (125 ng/mL; P < .05). In conclusion, our data indicate that AZA primarily acts on the villous metabolism, perturbing placental homeostasis. Since these drugs may alter the balance of angiogenic factors in its selection for clinical application, their impact on the behavior of placental villous should be considered.

Revisiting the Role of Thiopurines in Inflammatory Bowel Disease Through Pharmacogenomics and Use of Novel Methods for Therapeutic Drug Monitoring

Azathioprine and 6-mercaptopurine, often referred to as thiopurine compounds, are commonly used in the management of inflammatory bowel disease. However, patients receiving these drugs are prone to developing adverse drug reactions or therapeutic resistance. Achieving predefined levels of two major thiopurine metabolites, 6-thioguanine nucleotides and 6-methylmercaptopurine, is a long-standing clinical practice in ensuring therapeutic efficacy; however, their correlation with treatment response is sometimes unclear. Various genetic markers have also been used to aid the identification of patients who are thiopurine-sensitive or refractory. The recent discovery of novel Asian-specific DNA variants, namely those in the NUDT15 gene, and their link to thiopurine toxicity, have led clinicians and scientists to revisit the utility of Caucasian biomarkers for Asian individuals with inflammatory bowel disease. In this review, we explore the limitations associated with the current methods used for therapeutic monitoring of thiopurine metabolites and how the recent discovery of ethnicity-specific genetic markers can complement thiopurine metabolites measurement in formulating a strategy for more accurate prediction of thiopurine response. We also discuss the challenges in thiopurine therapy, alongside the current strategies used in patients with reduced thiopurine response. The review is concluded with suggestions for future work aiming at using a more comprehensive approach to optimize the efficacy of thiopurine compounds in inflammatory bowel disease.

Role of TPMT and ITPA variants in mercaptopurine disposition

PURPOSE

To explore the levels of thioguanine incorporated into DNA (DNA-TG), and erythrocyte levels of 6-thioguanine nucleotides (Ery-TGN) and methylated metabolites (Ery-MeMP) during 6-mercaptopurine (6MP)/Methotrexate (MTX) therapy of childhood acute lymphoblastic leukemia (ALL) and the relation to inosine triphosphatase (ITPA) and thiopurine methyltransferase (TPMT) gene variants.

METHODS

Blood samples were drawn during 6MP/MTX maintenance therapy from 132 children treated for ALL at Rigshospitalet, Copenhagen. The samples were analysed for thiopurine metabolites and compared to TPMT (rs1800460 and rs1142345) and ITPA (rs1127354) genotypes.

RESULTS

Median DNA-TG (mDNA-TG) levels were higher in TPMT and ITPA low-activity patients as compared to wildtype patients (TPMTLA 549 vs. 364 fmol/µg DNA, p = 0.007, ITPALA 465 vs. 387 fmol/µg DNA, p = 0.04). mDNA-TG levels were positively correlated to median Ery-TGN (mEry-TGN)(rs = 0.37, p = 0.001), but plateaued at higher mEry-TGN levels. DNA-TG indices (mDNA-TG/mEry-TGN) were 42% higher in TPMTWT patients as compared to TPMTLA patients but no difference in DNA-TG indices was observed between ITPAWT and ITPALA patients (median 1.7 vs. 1.6 fmol/µg DNA/ nmol/mmol Hb, p = 0.81). DNA-TG indices increased with median Ery-MeMP (mEry-MeMP) levels (rs = 0.25, p = 0.001).

CONCLUSIONS

TPMT and ITPA genotypes significantly influence the metabolism of 6MP. DNA-TG may prove to be a more relevant pharmacokinetic parameter for monitoring 6MP treatment intensity than cytosolic metabolites. Prospective trials are needed to evaluate the usefulness of DNA-TGN for individual dose adjustments in childhood ALL maintenance therapy.

Determination of Concentrations of Azathioprine Metabolites 6-Thioguanine and 6-Methylmercaptopurine in Whole Blood With the Use of Liquid Chromatography Combined With Mass Spectrometry

BACKGROUND

6-Mercaptopurine (6-MP) and its prodrug azathioprine (AZA) are used in many autoimmune diseases and after solid-organ transplantation. Their properties are mediated by active metabolites, 6-thioguanine nucleotides (6-TGN), and 6-methylmercaptopurine (6-MMP). The most common adverse effects are myelo- and hepato-toxicity. The aim of the study was quantification of 6-TG and 6-MMP, with the use of liquid chromatography combined with tandem mass spectrometry (LC/MS/MS) in solid-organ transplant recipients.

METHODS

In 33 patients, kidney transplant recipient (n = 25) and liver transplant recipient (n = 8) intra-erythrocyte concentrations of 6-TG and 6-MMP were measured with the use of LC/MS/MS.

RESULTS

The mean concentration of 6-TG was 205.35 ± 157.62 pmol/8 × 10(8) red blood cells (RBC); median concentration of 6-MMP was 1064.1 (35.78-11,552.9) pmol/8 × 10(8) RBC. There were no correlations between 6-TG levels and peripheral blood parameters (white blood cell count, WBC; hemoglobin, Hb concentration; PLT, blood platelet count) or alanine aminotransferase activity (AlAT) activity. Relationships between 6-MMP concentrations and peripheral blood parameters (WBC, Hb, PLT) or AlAT activity have not been found. Subgroups with leukopenia, anemia, thrombocytopenia, and liver dysfunction did not differ in concentrations of 6-TG or 6-MMP. We have observed a negative correlation between daily azathioprine dose and WBC count (r = -0.37, P = .04).

CONCLUSIONS

Relationships between concentrations of azathioprine metabolites and myelotoxicity or hepatotoxicity have not been confirmed. Further studies on larger groups of patients would be helpful in a more accurate understanding of the impact of azathioprine metabolites on parameters of bone marrow and liver function.

Combination treatment with 6-mercaptopurine and allopurinol in HepG2 and HEK293 cells - Effects on gene expression levels and thiopurine metabolism

Combination treatment with low-dose thiopurine and allopurinol (AP) has successfully been used in patients with inflammatory bowel disease with a so called skewed thiopurine metabolite profile. In red blood cells in vivo, it reduces the concentration of methylated metabolites and increases the concentration of the phosphorylated ones, which is associated with improved therapeutic efficacy. This study aimed to investigate the largely unknown mechanism of AP on thiopurine metabolism in cells with an active thiopurine metabolic pathway using HepG2 and HEK293 cells. Cells were treated with 6-mercaptopurine (6MP) and AP or its metabolite oxypurinol. The expression of genes known to be associated with thiopurine metabolism, and the concentration of thiopurine metabolites were analyzed. Gene expression levels were only affected by AP in the presence of 6MP. The addition of AP to 6MP affected the expression of in total 19 genes in the two cell lines. In both cell lines the expression of the transporter SLC29A2 was reduced by the combined treatment. Six regulated genes in HepG2 cells and 8 regulated genes in HEK293 cells were connected to networks with 18 and 35 genes, respectively, present at known susceptibility loci for inflammatory bowel disease, when analyzed using a protein-protein interaction database. The genes identified as regulated as well as the disease associated interacting genes represent new candidates for further investigation in the context of combination therapy with thiopurines and AP. However, no differences in absolute metabolite concentrations were observed between 6MP+AP or 6MP+oxypurinol vs. 6MP alone in either of the two cell lines. In conclusion; the effect of AP on gene expression levels requires the presence of 6MP, at least in vitro. Previously described AP-effects on metabolite concentrations observed in red blood cells in vivo could not be reproduced in our cell lines in vitro. AP’s effects in relation to thiopurine metabolism are complex. The network-identified susceptibility genes represented biological processes mainly associated with purine nucleotide biosynthetic processes, lymphocyte proliferation, NF-KB activation, JAK-STAT signaling, and apoptotic signaling at oxidative stress.

Liquid chromatography-mass spectrometry for measuring deoxythioguanosine in DNA from thiopurine-treated patients

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Thiopurines are effective immunosuppressant drugs.

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Monitoring of thiopurines is needed for research and clinical use.

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A sensitive assay of DNA-incorporated deoxythioguanosine is described.

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This method assays thiopurine nucleotides in DNA from nucleated blood cells.

Adverse reactions and non-response are common in patients treated with thiopurine drugs. Current monitoring of drug metabolite levels for guiding treatment are limited to analysis of thioguanine nucleotides (TGNs) in erythrocytes after chemical derivatisation. Erythrocytes are not the target tissue and TGN levels show poor correlations with clinical response. We have developed a sensitive assay to quantify deoxythioguanosine (dTG) without derivatisation in the DNA of nucleated blood cells. Using liquid chromatography and detection by tandem mass spectrometry, an intra- and inter-assay variability below 7.8% and 17.0% respectively were achieved. The assay had a detection limit of 0.0003125 ng (1.1 femtomoles) dTG and was quantified in DNA samples relative to endogenous deoxyadenosine (dA) in a small group of 20 patients with inflammatory bowel disease, all of whom had been established on azathioprine (AZA) therapy for more than 25 weeks. These patients had dTG levels of 20–1360 mol dTG/10⁶ mol dA; three patients who had not started therapy had no detectable dTG. This method, comparable to previous methods in sensitivity, enables the direct detection of a cytotoxic thiopurine metabolite without derivatisation in an easily obtainable, stable sample and will facilitate a better understanding of the mechanisms of action of these inexpensive yet effective drugs.

The role of thiopurine metabolites in inflammatory bowel disease and rheumatological disorders

Thiopurines have been a cornerstone of medical management of patients with inflammatory bowel disease(IBD) and many rheumatological disorders. The thiopurines are metabolized to their end products, 6-methymercaptopurine (6MMP) and the 6-thioguanine nucleotides (6TGN), with 6TGN being responsible for thiopurine efficacy by causing apoptosis and preventing activation and proliferation of T-lymphocytes. In IBD, conventional weight-based dosing with thiopurines leads to an inadequate response in many patients. Utilizing measurement of these metabolites and then employing dose optimization strategies has led to markedly improved outcomes in IBD. Switching between thiopurines as well as the addition of low-dose allopurinol can overcome adverse events and elevate 6TGN levels into the therapeutic window. There is a paucity of data on thiopurine metabolites in rheumatological diseases and further research is required.

Mercaptopurine/Methotrexate maintenance therapy of childhood acute lymphoblastic leukemia: clinical facts and fiction

The antileukemic mechanisms of 6-mercaptopurine (6MP) and methotrexate (MTX) maintenance therapy are poorly understood, but the benefits of several years of myelosuppressive maintenance therapy for acute lymphoblastic leukemia are well proven. Currently, there is no international consensus on drug dosing. Because of significant interindividual and intraindividual variations in drug disposition and pharmacodynamics, vigorous dose adjustments are needed to obtain a target degree of myelosuppression. As the normal white blood cell counts vary by patients' ages and ethnicity, and also within age groups, identical white blood cell levels for 2 patients may not reflect the same treatment intensity. Measurements of intracellular levels of cytotoxic metabolites of 6MP and MTX can identify nonadherent patients, but therapeutic target levels remains to be established. A rise in serum aminotransferase levels during maintenance therapy is common and often related to high levels of methylated 6MP metabolites. However, except for episodes of hypoglycemia, serious liver dysfunction is rare, the risk of permanent liver damage is low, and aminotransferase levels usually normalize within a few weeks after discontinuation of therapy. 6MP and MTX dose increments should lead to either leukopenia or a rise in aminotransferases, and if neither is experienced, poor treatment adherence should be considered. The many genetic polymorphisms that determine 6MP and MTX disposition, efficacy, and toxicity have precluded implementation of pharmacogenomics into treatment, the sole exception being dramatic 6MP dose reductions in patients who are homozygous deficient for thiopurine methyltransferase, the enzyme that methylates 6MP and several of its metabolites. In conclusion, maintenance therapy is as important as the more intensive and toxic earlier treatment phases, and often more challenging. Ongoing research address the applicability of drug metabolite measurements for dose adjustments, extensive host genome profiling to understand diversity in treatment efficacy and toxicity, and alternative thiopurine dosing regimens to improve therapy for the individual patient.

The role of thiopurine metabolite monitoring in inflammatory bowel disease

Thiopurines are the mainstay of medical management in inflammatory bowel disease (IBD), especially in the maintenance of disease remission. Given the limited IBD armamentarium it is important to optimize each therapy before switching to an alternative drug. Conventional weight based dosing of thiopurines in IBD leads to intolerance or inefficacy in many patients. More recently increased knowledge of their metabolism has allowed for dose optimization using thiopurine metabolite levels, namely 6-thioguanine nucleotides and 6-methylmercaptopurine, with the potential for improved outcomes in patients with IBD. This review will outline the current understanding of thiopurine metabolism and pharmacogenomics and will describe the clinical application of this knowledge in the optimization of thiopurines in individual patients.

The pharmacogenetic basis of individual variation in thiopurine metabolism

Thiopurines are an important class of immunosuppressive therapy, which have been used in clinical practice for over 50 years. Despite this extensive experience many of the pharmacodynamic and pharmacokinetic properties of these drugs remain unknown. As a consequence there is often no clear explanation for the individual variation in response to treatment, both in terms of efficacy or adverse drug reactions. This review, which emphasizes practice in gastroenterology, summarizes the current understanding of thiopurine drug metabolism and highlights the role of nongenetic and genetic factors other than TPMT, which should be a focus for future research. Correlation of polymorphic variations in these genes with clinical outcomes is expected to clarify the basis for interindividual differences in thiopurine metabolism and enable a more personalized approach to therapy.

Review article: the benefits of pharmacogenetics for improving thiopurine therapy in inflammatory bowel disease

BACKGROUND

Thiopurines represent an effective and widely prescribed therapy in inflammatory bowel disease (IBD). Concerns about toxicity, mainly resulting from a wide inter-individual variability in thiopurine metabolism, restrict their use. Optimal thiopurine dosing is challenging for preventing adverse drug reactions and improving clinical response.

AIM

To review efficacy and toxicity of thiopurines in IBD. To provide pharmacogenetic-based therapeutic recommendations.

METHODS

We conducted a query on PubMed database using 'inflammatory bowel disease', 'thiopurine', 'azathioprine', '6-mercaptopurine', 'TPMT', 'pharmacogenetics', 'TDM', and selected relevant articles, especially clinical studies.

RESULTS

Thiopurine metabolism - key enzyme: thiopurine S-methyltransferase (TPMT) - modulates clinical response, as it results in production of the pharmacologically active and toxic metabolites, the thioguanine nucleotides (6-TGN). Adjusting dosage according to TPMT status and/or metabolite blood levels is recommended for optimising thiopurine therapy (e.g. improving response rate up to 30% or decreasing haematological adverse events of 25%). Other enzymes or transporters of interest, as inosine triphosphatase (ITPase), glutathione S-transferase (GST), xanthine oxidase (XO), aldehyde oxidase (AOX), methylene tetrahydrofolate reductase (MTHFR) and ATP-binding cassette sub-family C member 4 (ABCC4) are reviewed and discussed for clinical relevance.

CONCLUSIONS

Based on the literature data, we provide a therapeutic algorithm for thiopurines therapy with starting dose recommendations depending on TPMT status and thereafter dose adjustments according to five metabolite profiles identified with therapeutic drug monitoring (TDM). This algorithm allows a dosage individualisation to optimise the management of patients under thiopurine. Furthermore, identification of new pharmacogenetic biomarkers is promising for ensuring maximal therapeutic response to thiopurines with a minimisation of the risk for adverse events.

Mass spectrometry in the quantitative analysis of therapeutic intracellular nucleotide analogs

Nucleoside analogs are widely used in anti-cancer, anti-(retro)viral, and immunosuppressive therapy. Nucleosides are prodrugs that require intracellular activation to mono-, di-, and finally triphosphates. Monitoring of these intracellular nucleotides is important to understand their pharmacology. The relatively involatile salts and ion-pairing agents traditionally used for the separation of these ionic analytes limit the applicability of mass spectrometry (MS) for detection. Both indirect and direct methods have been developed to circumvent this apparent incompatibility. Indirect methods consist of de-phosphorylation of the nucleotides into nucleosides before the actual analysis. Various direct approaches have been developed, ranging from the use of relatively volatile or very low levels of regular ion-pairing agents, hydrophilic interaction chromatography (HILIC), weak anion-exchange, or porous graphitic carbon columns to capillary electrophoresis and matrix-assisted light desorption--time of flight (MALDI-TOF) MS. In this review we present an overview of the publications describing the quantitative analysis of therapeutic intracellular nucleotide analogs using MS. The focus is on the different approaches for their direct analysis. We conclude that despite the technical hurdles, several useful MS-compatible chromatographic approaches have been developed, enabling the use of the excellent selectivity and sensitivity of MS for the quantitative analysis of intracellular nucleotides.

Thioguanine pharmacokinetics in induction therapy of children with acute myeloid leukemia

We studied the pharmacokinetics of 6-thioguanine (6TG) in 50 children treated for newly diagnosed acute myeloid leukemia, four of them with Down syndrome (DS). They received oral 6TG 100 mg/m2 body surface area twice daily for 4 days. Etoposide, 100 mg/m2/24 h, and cytarabine, 200 mg/m2/24 h, were administered concomitantly by intravenous infusion. On day 5, doxorubicin 75 mg/m2 was given as an 8-h infusion. The concentration of thioguanine nucleotides (TGN) in erythrocytes, the active metabolites of 6TG, was determined by high-performance liquid chromatography. The mean TGN concentration from 72, 95, and 106-h samples was used as a measure of drug exposure for each individual. The median TGN concentration in non-DS children above 2 years of age was 2.30 micromol/mmol Hb (range 0.57-25.3). The TGN concentrations varied widely (30-fold) also after dose normalization. We found no correlation with demographic, clinical, or biochemical parameters, and differences in bioavailability might be the most important explanation to interpatient variability. Children with high TGN concentration tended to have longer treatment interval to the next course, but we found no correlation with our predefined parameters for clinical response, that is, remission and relapse rate. Therefore, 6TG does not seem to be a candidate for therapeutic drug monitoring by TGN measurement, at least not in the setting of short multidrug treatment courses. Children with DS had significantly higher TGN concentrations, indicating that dose reduction might be considered to reach the same drug exposure as in non-DS children.

IMPDH activity in thiopurine-treated patients with inflammatory bowel disease - relation to TPMT activity and metabolite concentrations

AIMS

Azathioprine and 6-mercaptopurine are steroid-sparing drugs used in inflammatory bowel disease (IBD). The polymorphic enzyme thiopurine S-methyltransferase (TPMT) is of importance for thiopurine metabolism and occurrence of adverse events. The role of other thiopurine-metabolizing enzymes is less well known. This study investigated the role of inosine-5'-monophosphate dehydrogenase (IMPDH), which is a key enzyme in the de novo synthesis of guanine nucleotides and also strategically positioned in the metabolic pathway of thiopurines.

METHODS

IMPDH was measured in 100 healthy blood donors. IMPDH, TPMT and metabolite concentrations were studied in 50 patients with IBD on stable thiopurine therapy. IMPDH activity was measured in peripheral blood mononuclear cells. TPMT activity, 6-methylthioinosine 5'-monophosphate (meTIMP) and 6-thioguanine nucleotide (6-TGN) concentrations were measured in red blood cells, which is the current practice in clinical monitoring of thiopurines. Enzyme activities were related to metabolite concentrations and clinical characteristics.

RESULTS

A wide range of IMPDH activity was observed both in healthy blood donors (median 13.1, range 4.7-24.2 nmol mg(-1) protein h(-1)) and IBD patients (median 14.0, range 7.0-21.7). There was a negative correlation between IMPDH activity and dose-normalized meTIMP concentrations (r(s) = -0.31, P = 0.03), but no evident correlation to 6-TGN concentration or the meTIMP/6-TGN ratio. There were no significant correlations between TPMT activity and metabolite concentrations.

CONCLUSION

Even though the meTIMP concentrations correlated inversely to the IMPDH activity, the role of IMPDH in balancing the formation of methylated and phosphorylated metabolites was not evident. Taken together, the results give cause to question established opinions about thiopurine metabolism.

Pharmacokinetics of azathioprine following single-dose intravenous and oral administration and effects of azathioprine following chronic oral administration in horses

OBJECTIVE

To determine pharmacokinetics of azathioprine (AZA) and clinical, hematologic, and serologic effects of i.v. and oral administration of AZA in horses.

ANIMALS

6 horses.

PROCEDURE

In study phase 1, a single dose of AZA was administered i.v. (1.5 mg/kg) or orally (3.0 mg/kg) to 6 horses, with at least 1 week between treatments. Blood samples were collected for AZA and 6-mercaptopurine (6-MP) analysis 1 hour before and at predetermined time points up to 4 hours after AZA administration. In study phase 2, AZA was administered orally (3 mg/kg) every 24 hours for 30 days and then every 48 hours for 30 days. Throughout study phase 2, blood samples were collected for CBC determination and serum biochemical analysis.

RESULTS

Plasma concentrations of AZA and its metabolite, 6-MP decreased rapidly from plasma following i.v. administration of AZA, consistent with the short mean elimination half-life of 1.8 minutes. Oral bioavailability of AZA was low, ranging from 1% to 7%. No horses had abnormalities on CBC determination or serum biochemical analysis, other than 1 horse that was lymphopenic on day 5 and 26 of daily treatment. This horse developed facial alopecia from which 1 colony of a Trichophyton sp was cultured; alopecia resolved within 1 month after the study ended.

CONCLUSIONS AND CLINICAL RELEVANCE

Overall, no adverse effects were observed with long-term oral administration of AZA to horses, although 1 horse did have possible evidence of immunosuppression with chronic treatment. Further investigation of the clinical efficacy of AZA in the treatment of autoimmune diseases in horses is warranted.

Improved HPLC methodology for monitoring thiopurine metabolites in patients on thiopurine therapy

OBJECTIVE

Standardization of thiopurine metabolite testing is currently lacking. This paper presents in-depth methodological analysis and optimization of two currently available HPLC procedures (Lennard-Singleton [J. Chromatogr. 583 (1992) 83] and Dervieux-Boulieu [Clin. Chem. 44 (1998) 551]) to improve precision, turn-around time, ruggedness, and cost effectiveness.

DESIGN AND METHODS

Reversed-phase chromatography with UV detection was performed on a Waters HPLC system. The two protocols were improved with regards to internal standardization (IS), chromatographic conditions, as well as reagent preparation, storage, and use. 6-Thioguanine nucleotides (6-TGNs) were analyzed by our optimized techniques in samples from patients on thiopurine therapy (n = 24) and the results were compared.

RESULTS

6-Mercaptopurine (6-MP) was an ideal internal standard in either procedure. Isocratic elution with 5% acetonitrile (ACN) in 20 mmol/l phosphate buffer pH 2.5 allowed for minimal background interference in both protocols. 6-Thioguanine, 6-mercaptopurine, and 6-methylmercaptopurine (6-MMP) eluted at around 4, 5, and 6 min, respectively. Dithiothreitol (DTT) was critical only during the acid hydrolysis step. Less mercury-containing waste was generated in the Lennard-Singleton procedure. With our optimized protocols recovery of 6-TGNs was on average 1.38-fold higher in the Dervieux-Boulieu method over a range of 10-678 pmol/8 x 10(8) RBC and no interfering peaks hindered analysis. Specific extraction of thiopurines before their analysis as per Lennard-Singleton procedure may be redundant.

CONCLUSIONS

We improved the quality and cost effectiveness of two known procedures for thiopurine metabolite assay. Through common chromatographic conditions and internal standardization, future comparison studies are now facilitated a great deal. The less tedious Dervieux-Boulieu procedure for routine thiopurine metabolite testing is warranted.

Analytic aspects of monitoring therapy with thiopurine medications

The thiopurine medications 6-mercaptopurine (6-MP), 6-thioguanine (6-TG), and azathioprine are used in treatment of childhood acute lymphoblastic leukemia, autoimmune diseases, and, in the case of azathioprine, in solid organ transplantation. They are converted in vivo to the active 6-thioguanine nucleotides (6-TGN). One person in 300 in white populations has low or undetectable TPMT activity and is at risk for accumulating 6-TGN with the consequence of severe, life-threatening myelosuppression. A rational therapeutic strategy for thiopurine drug use is to first determine TPMT phenotype/genotype and then to adjust the dosage on an individual basis. Determination of erythrocyte 6-TGN levels can further help to optimize therapy. TPMT activity (phenotype) is determined in erythrocytes using radiochemical or HPLC procedures. Recent HPLC procedures show good agreement with the original radiochemical method, while offering simplified sample pretreatment and improved precision. To date, 12 mutant alleles responsible for TPMT deficiency have been published. Restriction fragment length polymorphism PCR and allele-specific PCR have been used for detection of TPMT mutations. Genotyping methods that allow a higher throughput include real-time PCR (LightCycler) and denaturing HPLC. Numerous HPLC methods have been reported for quantification of 6-TGN. The majority involve acid hydrolysis to 6-TG at high temperature. There are substantial differences in the hydrolysis step, extraction procedure, chromatographic conditions and method of detection. Erythrocyte 6-TGN concentrations can vary up to 2.6-fold depending on the HPLC method. The method that has found the greatest application in clinical studies is that of Lennard. This has served as the basis for the establishment of treatment-related therapeutic ranges for thiopurine therapy. These ranges will not necessarily be applicable when other methodology is used. There is an urgent need to harmonize the analytic procedures for 6-TGN.

Benefit-Risk Assessment of Ciclosporin Withdrawal in Renal Transplant Recipients

Ciclosporin is associated with significant toxicity, including nephrotoxicity, and with an increased risk of cardiovascular events. Many attempts have been made to wean patients from ciclosporin. Before the availability of new immunosuppressive drugs, the acute rejection rate observed after ciclosporin withdrawal did not permit the widespread use of withdrawal regimens even though meta-analysis did not show that they adversely affected patient or graft survival. Nevertheless, maintenance therapy with azathioprine and corticosteroids has not become routine practice. The introduction of mycophenolate mofetil and subsequently sirolimus has increased the number of clinical studies of the effects of ciclosporin withdrawal. In stable patients, this withdrawal is associated with a small but significant increase in the incidence of acute rejection episodes. Declining renal function and other forms of ciclosporin-related toxicity have improved. However, this improvement was also observed when ciclosporin was only reduced (and not withdrawn), which did not increase the risk of acute rejection. More precise definition of the patients who could benefit from ciclosporin-withdrawal may help to optimise the immunosuppressive regimen in this setting. In patients with chronic allograft deterioration, ciclosporin withdrawal together with mycophenolate mofetil introduction has been shown to improve renal function significantly in many small studies, and a large prospective randomised study. For the time being, ciclosporin withdrawal is a good therapeutic option for patients with declining renal function and signs of chronic ciclosporin nephrotoxicity on renal biopsy. Finally, recent preliminary studies have reported the results of complete avoidance of calcineurin inhibitors after renal transplantation. These results are promising as regards the incidence of acute rejection, renal function and safety, but need confirmation in larger trials with a longer follow-up. Nevertheless, it has become clear that the concept of an immunosuppressive regimen with little or no nephrotoxicity after renal transplantation is more and more important and plays a crucial part in tailoring immunosuppression to the needs of specific patient populations.

Differences in nucleotide hydrolysis contribute to the differences between erythrocyte 6-thioguanine nucleotide concentrations determined by two widely used methods

BACKGROUND

Measurement of 6-thioguanine nucleotide (6-TGN) concentrations in erythrocytes is widely accepted for use in optimization of thiopurine therapy. Various chromatographic methods have been developed for this purpose. In preliminary experiments we observed a considerable difference between 6-TGN concentrations determined with two widely used methods published by Lennard (Lennard L. J Chromatogr 1987;423:169-78) and by Dervieux and Boulieu (Dervieux T, Boulieu R. Clin Chem 1998;44:551-5). We therefore investigated methodologic differences between the two procedures with respect to hydrolysis of 6-TGNs to 6-thioguanine (6-TG) in more detail.

METHODS

We analyzed 6-TGNs in erythrocyte preparations (n = 50) from patients on azathioprine therapy by both methods, using the original protocols. In one set of experiments, we replaced the 0.5 mol/L sulfuric acid in the Lennard method with the 1 mol/L perchloric acid used by Dervieux and Boulieu. In a second set of experiments, we investigated the effect of various dithiothreitol (DTT) concentrations on 6-TG recovery with both methods. In a third set of experiments, we determined the effect of hydrolysis time on both protocols.

RESULTS

Direct comparison of both methods showed that 6-TGN concentrations were, on average, 2.6-fold higher in the Dervieux-Boulieu method over the concentration range tested, although the correlation (r = 0.99; P <0.001) was good. Replacement of sulfuric acid by perchloric acid reduced this difference to approximately 1.4-fold (r = 0.99; P <0.001). Increasing the DTT concentration enhanced 6-TG recovery. The hydrolysis time used in the Lennard method (1 h) was not sufficient to achieve complete hydrolysis.

CONCLUSIONS

The difference between 6-TGN concentrations measured by the two methods is attributable, at least in part, to differences in the extent of nucleotide hydrolysis. For optimization of thiopurine therapy, method-dependent therapeutic ranges are necessary, which precludes comparison of results from clinical studies derived with these methods. Efforts must therefore be made to standardize the analytical procedures for the determination of 6-TGN.

Improved methods for determining the concentration of 6-thioguanine nucleotides and 6-methylmercaptopurine nucleotides in blood

The conversion of the cytotoxic and immunosuppressive 6-mercaptopurine (6MP) to the active 6-thioguanine nucleotides (6TGN) is necessary for clinical efficacy of 6MP and its prodrug azathioprine. Another metabolite, 6-methylmercaptopurine nucleotide (6MMPN), is formed via a competing pathway by thiopurine methyl transferase. The concentrations of 6TGN and 6MMPN are measured in washed erythrocytes as a surrogate to the intracellular levels of these metabolites in the target tissues. Analysis of 6TGN and 6MMPN in multi-center clinical studies is more complicated because of the requirement to wash erythrocytes. In this investigation, we found no differences in the concentrations of 6TGN and 6MMPN in blood versus washed erythrocytes in samples obtained from patients taking therapeutic doses of oral 6MP or azathioprine for inflammatory bowel disease. We concluded that whole blood could be used for the analysis of these analytes, thus saving sample preparation time. We also found that the erythrocyte 6TGN concentration in blood at ambient temperature declined 2-4% per day, a loss that can be avoided by shipping blood samples frozen. The loss of 6TGN in blood stored at approximately -80 degrees C was 1% after 1 week and 12% after 24 weeks, indicating the analyte was moderately stable. 6MMPN in blood did not significantly change after 24 weeks of storage at approximately -80 degrees C. In addition, the sensitivity of the 6TGN assay was improved by modifying the HPLC conditions, which made the method more suitable for quantifying low levels of 6TGN in human intestinal biopsy samples and blood.

Recommendations for the outpatient surveillance of renal transplant recipients. American Society of Transplantation

Many complications after renal transplantation can be prevented if they are detected early. Guidelines have been developed for the prevention of diseases in the general population, but there are no comprehensive guidelines for the prevention of diseases and complications after renal transplantation. Therefore, the Clinical Practice Guidelines Committee of the American Society of Transplantation developed these guidelines to help physicians and other health care workers provide optimal care for renal transplant recipients. The guidelines are also intended to indirectly help patients receive the access to care that they need to ensure long-term allograft survival, by attempting to systematically define what that care encompasses. The guidelines are applicable to all adult and pediatric renal transplant recipients, and they cover the outpatient screening for and prevention of diseases and complications that commonly occur after renal transplantation. They do not cover the diagnosis and treatment of diseases and complications after they become manifest, and they do not cover the pretransplant evaluation of renal transplant candidates. The guidelines are comprehensive, but they do not pretend to cover every aspect of care. As much as possible, the guidelines are evidence-based, and each recommendation has been given a subjective grade to indicate the strength of evidence that supports the recommendation. It is hoped that these guidelines will provide a framework for additional discussion and research that will improve the care of renal transplant recipients.

Therapeutic drug monitoring of antimetabolic cytotoxic drugs

Therapeutic drug monitoring is not routinely used for cytotoxic agents. There are several reasons, but one major drawback is the lack of established therapeutic concentration ranges. Combination chemotherapy makes the establishment of therapeutic ranges for individual drugs difficult, the concentration-effect relationship for a single drug may not be the same as that when the drug is used in a drug combination. Pharmacokinetic optimization protocols for many classes of cytotoxic compounds exist in specialized centres, and some of these protocols are now part of large multicentre trials. Nonetheless, methotrexate is the only agent which is routinely monitored in most treatment centres. An additional factor, especially in antimetabolite therapy, is the existence of pharmacogenetic enzymes which play a major role in drug metabolism. Monitoring of therapy could include assay of phenotypic enzyme activities or genotype in addition to, or instead of, the more traditional measurement of parent drug or drug metabolites. The cytotoxic activities of mercaptopurine and fluorouracil are regulated by thiopurine methyltransferase (TPMT) and dihydropyrimidine dehydrogenase (DPD), respectively. Lack of TPMT functional activity produces life-threatening mercaptopurine myelotoxicity. Very low DPD activity reduces fluorouracil breakdown producing severe cytotoxicity. These pharmacogenetic enzymes can influence the bioavailability, pharmacokinetics, toxicity and efficacy of their substrate drugs.

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.