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Clinical Chemistry 53: 1306-1314, 2007. First published May 10, 2007; 10.1373/clinchem.2007.086215
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Right arrow Drug Monitoring and Toxicology
(Clinical Chemistry. 2007;53:1306-1314.)
© 2007 American Association for Clinical Chemistry, Inc.

Drug Monitoring and Toxicology

6-Thioguanine Nucleotide–Adapted Azathioprine Therapy Does Not Lead to Higher Remission Rates Than Standard Therapy in Chronic Active Crohn Disease: Results from a Randomized, Controlled, Open Trial

Max Reinshagen1,a, Ekkehard Schütz2,2, Victor W. Armstrong2, Christoph Behrens3, Christian von Tirpitz4, Andreas Stallmach5, Hans Herfarth6, Jürgen Stein7, Peter Bias3, Guido Adler4, Maria Shipkova2,3, Wolfgang Kruis8, Michael Oellerich2 and Nicolas von Ahsen2

1 Department of Medicine I, Klinikum Braunschweig, Germany.
2 Department of Clinical Chemistry, University of Göttingen, Germany.
3 Clinical Research and Development, Merckle GmbH Ulm, Germany.
4 Department of Medicine I, University of Ulm, Germany.
5 Department of Internal Medicine II, Saarland University, Homburg, Germany.
6 Department of Medicine I, University of Regensburg, Germany.
7 Department of Medicine I - ZAFES, University of Frankfurt, Germany.
8 Evangelisches Krankenhaus Kalk, University of Cologne, Germany.

aAddress correspondence to this author at: Medizinische Klinik I, Klinikum Braunschweig, Salzdahlumer Straße 90 8, 38126 Braunschweig, Germany. Fax 49-531-595-2653; e-mail m.reinshagen{at}klinikum-braunschweig.de.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Background: A prospective randomized trial in patients with Crohn disease studied whether 6-thioguanine nucleotide (6-TGN) concentration–adapted azathioprine (AZA) therapy is clinically superior to a standard dose of 2.5 mg/kg/day AZA.

Methods: After 2 weeks of standard therapy, patients (n = 71) were randomized into standard (n = 32) or adapted-dose (n = 25) groups; 14 patients dropped out before randomization. In the adapted group, the AZA dose was adjusted to maintain 6-TGN concentrations between 250 and 400 pmol/8 x 108 erythrocytes (Ery). Response criteria were the number of patients in remission after 16 weeks without steroids (primary) and remission after 24 weeks, frequency of side effects, and quality of life (secondary).

Results: After 16 weeks, 14 of 32 (43.8%) patients in the standard group vs 11 of 25 (44%) in the adapted group were in remission without steroids (intent-to-treat analysis). After 24 weeks, 43.8% vs 40% were in remission. No significant differences were found concerning quality of life, disease activity, 6-TGN concentrations, AZA dose, or dropouts due to side effects. Sixty-six patients had a wild-type thiopurine S-methyltransferase (TPMT) genotype, with TPMT activities of 8 to 20 nmol/(mL Ery x h). Five patients (dropouts after randomization) were heterozygous, with TPMT activities <8 nmol/(mL Ery x h). 6-Methyl mercaptopurine (6-MMP) concentrations >5700 pmol/8 x 108 Ery were not associated with hepatotoxicity.

Conclusion: Standard and adapted dosing with the provided dosing scheme led to identical 6-TGN concentrations and remission rates. Adapted dosing had no apparent clinical benefit for patients with TPMT activity between 8 and 20 nmol/(mL Ery x h). Additionally, 6-MMP monitoring had no predictive value for hepatotoxicity.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Discussion
 References
 
Treatment of chronic active Crohn disease with azathioprine (AZA) 1 or 6-mercaptopurine (6-MP) is effective in both inducing and maintaining remission in steroid-dependent and steroid-resistant patients (1)(2)(3). AZA is rapidly converted in vivo to 6-MP and is further metabolized to 6-thioguanine nucleotide (6-TGN). The immunosuppressive properties of AZA and its thiopurine metabolites are complex (4)(5)(6)(7). In addition to the anabolic pathway, 6-MP is catabolized to 6-thiouric acid by xanthine oxidase and to 6-methyl mercaptopurine (6-MMP) by thiopurine S-methyltransferase (TPMT) (8). TPMT deficiency is associated with an increased risk for AZA-mediated bone marrow toxicity (9). Treatment with AZA or 6-MP is effective in 60%–70% of all patients (1). Several studies reported an association between erythrocyte (Ery) 6-TGN concentrations and clinical response in patients with chronic active Crohn disease (10)(11)(12)(13)(14)(15). Patients with 6-TGN concentrations >230–260 pmol/8 x 108 Ery were more likely to be in remission than patients with lower concentrations. Cuffari et al. (12) reported that optimization of the AZA dosage based on 6-TGN concentrations led to disease remission in 18 of 21 patients with incompletely responsive Crohn disease. This was supported by a metaanalysis of retrospective and cross-sectional studies published before 2004, concluding that there is a strong association between 6-TGN concentrations and induction of remission among patients with inflammatory bowel disease (16). In contrast, other investigations found no association between 6-TGN concentrations and clinical response (17)(18)(19).

Increased concentrations of 6-MMP have been implicated in the pathogenesis of AZA-induced hepatotoxicity (10) and myelotoxicity (20). It was suggested that 6-MMP concentrations <5700 pmol/8 x 108 Ery were less often associated with increased transaminase activities (10). However, these results from pediatric studies have not been confirmed in adults (19), and caution is warranted regarding premature dose adjustments (21).

The role of 6-TGN for assessing compliance, and in combination with 6-MMP to assess drug-failure attributable to high methylator status, seems well accepted (21)(22) but not yet substantiated by rigorous study data.

We therefore conducted a prospective, randomized, controlled open study to investigate whether adjusting the AZA dose to achieve 6-TGN concentrations of 250–400 pmol/8 x 108 Ery would lead to an improved remission rate.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Discussion
 References
 
selection of patients
Patients were between 18 and 75 years of age and had active Crohn disease with a Crohn disease activity index (CDAI) of 150–450 (23). They had received either a cumulative prednisone dose >300 mg in the previous 4 weeks, had suffered a relapse within 6 months after steroid therapy for an acute flare of the disease, or had ≥3 acute flares within the last 3 years. All patients had disease that involved the ileum, ileocolon, or colon, as verified previously by standard clinical, radiological, and histological criteria. Exclusion criteria were active Crohn disease isolated to the duodenum, jejunum, or perianal region; current ileostomy or colostomy, septic complications, abscess, perforation with acute abdomen, or fistulas involving the skin, bladder, or vagina; and stricture of the ileum or colon resulting in symptomatic obstruction (confirmed by endoscopic or radiological studies) within 6 months or requiring immediate surgery. No patient had received biological therapies within 6 months or immune-modifier drugs within 3 months of the start of this study. Patients with a history of cancer of any type, definite dysplasia of the colon within the last 5 years, or clinically significant renal or hepatic disease were ineligible, as were pregnant or breast-feeding women, patients who were allergic to AZA or 6-MP, and patients receiving allopurinol.

Patients who developed the following AZA-related side effects during the prerandomization phase were excluded from the dosage-optimization phase: (a) leukocytopenia <2.5 x 10 1 /L, (b) thrombocytopenia <100 x 10 1 /L, (c) pancreatitis: upper abdominal pain and a 2-fold increase of amylase or lipase activities above the upper reference interval limit, and (d) hepatotoxicity: 2.5-fold increase of alanine aminotransferase and aspartate aminotransferase activities above the upper reference interval limit and/or increased bilirubin concentration >51.3 µmol/L (3 mg/dL).

The study was approved by the institutional review boards at each study center. All participants provided written informed consent.

design of the study
The prospective, randomized, controlled open study was performed at 11 centers in Germany from January 2001 until December 2002. Study duration was 24 weeks, including a 2-week prerandomization phase in which all patients received 2.5 mg/kg/day AZA.

For 80% power to detect a significant difference (P <0.05, single-sided Fisher exact test) between treatment groups, assuming a CDAI response rate of ≥80% for the adapted group and ≤57.5% in the standard group, 59 patients were needed in each study arm. With a 10% surplus to compensate for loss, the study was designed to include 132 patients (66 in each group) within 18 months. Only 71 patients were recruited, however, and the study was pursued with this number. The sponsor provided a computer-generated fixed block randomization scheme. For randomization, the study physician opened a sealed envelope (numbered in ascending order per center) that contained the random treatment group allocation. Patients were randomized to either receive further standard AZA treatment (2.5 mg/kg/day) or an AZA dose-adapted therapy according to 6-TGN concentrations (target 6-TGN 250–400 pmol/8 x 108 Ery). Dose adaptation was possible at weeks 1, 4, 8, 12, and 16, depending on the 6-TGN concentrations (measured a minimum 8 days before adaptation). If the previous 6-TGN concentration was <150 pmol/8 x 108 Ery, the AZA dose was increased by 50 mg/day; if the 6-TGN concentration was between 150 and 250 pmol/8 x 108 Ery, AZA was increased by 25 mg/day; if the 6-TGN concentration was >400 pmol/8 x 108 Ery, AZA was decreased by 50 mg/day.

At entry, each patient’s demographic characteristics, medical history, and current medications were recorded. Disease activity (CDAI) was assessed before treatment (–2 weeks) and at weeks 0, 1, 4, 8, 12, 16, and 24. At each visit, a physical examination, laboratory tests, and full evaluation were conducted. Patients were asked to identify adverse events since the previous visit. All adverse events were recorded whether or not they were related to the study medication. No medications for Crohn disease other than prednisone, the study drug, mesalamine (stable dose), and metronidazole or ciprofloxacin (stable dose) were allowed. Prednisone doses were tapered according to a standard protocol with the aim of achieving steroid-free therapy at week 12.

Quality of life was assessed by use of the Inflammatory Bowel Disease Questionnaire (IBDQ) at –2 weeks, 0 weeks, and the last available visit. The IBDQ scores of patients in remission usually range from 170 to 190, with higher scores indicating better quality of life (24).

During the 7 days before each visit, patients kept records of study medication and prednisone intake, frequency of loose stools, extent of abdominal pain, and general well being. We collected blood samples for hematological and biochemical assessments including blood count, aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, {gamma}-glutamyltransferase, bilirubin, lipase, creatinine, electrolytes, C-reactive protein, and urinalysis. We drew blood for measurement of Ery TPMT enzyme activity at the first visit (–2 week) and at week 4, and for measurement of Ery 6-TGN and 6-MMP concentrations and at weeks 0, 1, 4, 8, 12, 16, and 24.

Results regarding the effects of variant inosine triphosphatase on premature study termination have been published (25).

aza metabolite measurements
Concentrations of 6-TGN were measured by use of a HPLC method (26), essentially according to the original procedure of Lennard and Singleton (27). Between-day imprecision in our laboratory was 4.9% (CV) at 240 pmol/8 x 108 Ery. Concentrations of 6-MMP in Ery were determined using the method of Dervieux and Boulieu (28). Between-day imprecision was 5.6% (CV) at 7000 pmol/8 x 108 Ery.

tpmt enzyme activity and tpmt genotype
Ery TPMT enzyme activity was measured using a modification (29) of the method of Weinshilboum et al. (30). Between-day imprecision was 12.4% (CV) at 12.9 nmol/(mL Ery x h) TPMT activity and 7.0% (CV) at 4.9 nmol/(mL Ery x h). Intermediate and low activity cutoffs were 10 and <2.8 nmol/(mL Ery x h) (31). TPMT genotyping for TPMT2, 3A, 3B, and 3C mutations was performed by use of a published LightCycler (Roche) assay (31).

primary outcome
The primary outcome criterion was the number of patients in each group in remission [defined as a CDAI score <150 points (23)] at week 16, with total steroid withdrawal. Analysis was by intention-to-treat (ITT) of randomized patients.

secondary outcome
Secondary outcome criteria were: (a) patients in remission (defined as a CDAI score <150 points and total steroid withdrawal) in each group at week 24, (b) quality of life measured by the IBDQ at baseline and last value compared to baseline between both groups, and (c) number of patients in each group with AZA-related side effects.

statistical analysis
Inferential statistics included analyses of odds ratios of the response rates in the treatment groups as well as a Fisher-Pitman test between the treatment groups. All other statistical tests for differences between the treatment groups were exploratory. Two-sided 95% CIs were calculated for the effects within each treatment group as well as for the differences between the treatment groups. Tests were 1-sided at the {alpha} = 0.025 level of significance, if not stated otherwise. Missing values of the main variable CDAI at visit 7 and visit 8 were replaced using the last observation carried forward technique. Missing values of other data were not replaced. Statistical analyses were performed using SAS version 8.2 and SPSS version 11.5 on a Windows platform.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Discussion
 References
 
A detailed presentation of patient enrollment, randomization, and allocation to treatment arms is presented in Fig. 1 . Patients received either standard AZA treatment of 2.5 mg/kg/day (n = 33) or AZA dose adaptation (n = 25) according to 6-TGN. One patient was excluded from the ITT population (Fig. 1 ). The baseline characteristics of the 2 groups of patients were similar (Table 1 ). No significant differences were observed between the groups regarding the stratification of patients according to the Vienna classification of Crohn disease.


Figure 1
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  		Figure 1. Flowchart according to Consort criteria for study participants.

Note that during follow-up, week 0 refers to the time point of randomization, which was after 2 weeks of 2.5 mg/kg/day AZA.


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  		Table 1. Baseline characteristics of the patients who were randomized to the standard or adapted groups.1

primary outcome
Rates of remission without steroids at week 16 were similar in both groups. Fourteen of 32 patients (44%) in the standard group were in remission (CDAI <150) without steroids compared to 11 of 25 (44%) in the adapted group (P = 0.99). The stratified odds ratio for treatment success rates at 16 weeks for the adapted therapy vs the standard therapy was 1.02, 95% CI 0.34–3.09, in the ITT population.

secondary outcome
At week 24, 14 of 32 patients (44%) in the standard group were in remission without steroids (CDAI <150) compared to 10 of 25 (40%) in the adapted group (P = 0.76). The stratified odds ratio for the treatment success rates at 24 weeks for the adapted therapy vs the standard therapy was 0.85, 95% CI 0.29–2.56, in the ITT population.

The quality of life measured by the IBDQ increased in the standard group from mean (SD) 132 (30) at –2 weeks to 187 (20) at week 16 and 172 (32) at week 24 [not significant (NS]. In the dose-adapted group, the IBDQ increased from 140 (33) at –2 weeks to 194 (21) at week 16 and 196 (20) at week 24 (NS).

Ten patients from the standard group and 9 patients from the AZA-adapted group dropped out of the study for various reasons (Fig. 1Up ). There was no significant difference in the proportion of dropouts between the groups. Four patients from each group left the study due to AZA-related side effects (NS).

The mean (SD) CDAI at –2 weeks in the standard group [240.3 (75.2)] did not significantly differ from that in the dose-adapted group [234.8 (56.7)]. At week 16, the mean (SD) CDAIs were significantly (P <0.001) lower than at –2 weeks, but no significant difference was observed between the standard [102.2 (65.8)] and adapted [129.2 (87.6)] groups. At week 24, the mean (SD) CDAI was 105.0 (78.7) in the standard group and 117.0 (85.7) in the dose-adapted group. When only the patients remaining in the study at each visit were analyzed, again no significant differences were observed between the groups with regard to the number of patients in remission at each visit (Fig. 2 ).


Figure 2
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  		Figure 2. Relative proportion of patients in remission (CDAI <150) at each study visit.

Note that week 0 refers to the time point of randomization, which was after 2 weeks of 2.5 mg/kg/day AZA.

tpmt activity
TPMT activities before treatment with AZA were similar in the adapted [11.8 (2.7) nmol/(mL Ery x h)] and standard [12.3 (3.5) nmol/(mL Ery x h)] groups. TPMT activity was induced after 4 weeks of AZA treatment [15.2 (2.7) and 14.8 (4.0) nmol/(mL Ery x h) in the adapted and standard groups, respectively]. This increase in TPMT activity compared with baseline was significant within groups (P = 0.010 and P <0.001, respectively). No significant differences were observed between the groups at either visit.

Patients with TPMT activity <10 nmol/(mL Ery x h) (n = 21) were genotyped for TPMT. Of these 21 patients, 5 were heterozygous carriers of a mutant TPMT allele (1 TPMT*1/*2, 4 TPMT*1/*3A). The TPMT activities of the patients with a heterozygous genotype were the lowest of all patients [median 6.6; range 5.3–7.5 nmol/(mL Ery x h); P <0.001] at –2 weeks.

tpmt phenotype/genotype association with aza-related side effects
Eight patients with TPMT activities <10 nmol/(mL Ery x h) dropped out during the prerandomization period (all wild-type genotype). This early dropout was significantly (P <0.05) linked to TPMT <10 nmol/(mL Ery x h). Of the remaining 13 patients with TPMT activity <10 nmol/(mL Ery x h) who were randomized to the study, 7 concluded the study and 6 dropped out.

Of the 5 patients with TPMT mutant alleles, 3 patients dropped out after 1 week and 2 patients after 4 weeks. Reasons included nausea (n = 1), subileus due to Crohn disease (n = 1), pancreatitis (n = 1), and personal reasons (n = 2).

leukocytopenia, thrombocytopenia, mean corpuscular volume
Leukocytopenia <2.5 x 10 1 /L or thrombocytopenia <100 x 10 1 /L were not observed throughout the study. As noted above, all 5 patients with a TPMT mutant allele dropped out of the study. Their 6-TGN concentrations after the 2-week run-in phase were significantly higher [616 (285) vs 242 (136) pmol/8 x 108 Ery, P = 0.001] than those of the other patients. Five patients in the standard group and 5 patients in the adapted group had 6-TGN concentrations >450 pmol/8 x 108 Ery (maximum 1034 pmol/8 x 108 Ery in a patient with heterozygous TPMT genotype). The Ery mean corpuscular volume (MCV) was higher and leukocyte counts were lower in patients in remission at week 16 compared with patients not in remission, but this difference was not statistically significant (Table 2 ). Neither MCV nor leukocyte counts were associated with 6-TGN concentrations (data not shown).


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  		Table 2. Ery MCV, blood leukocytes, and Ery 6-TGN concentrations stratified according to CDAI at 16 weeks.1

6-tgn
6-TGN concentrations did not differ between the 2 study arms during the study period (Fig. 3A ). There was a similar wide variation in 6-TGN concentrations in the dose-adjusted group compared to the standard dose group, even at the later sampling time points when the variation in the former group might have been expected to be lower.


Figure 3
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  		Figure 3. Note that week 0 refers to the time point of randomization, which was after 2 weeks of 2.5 mg/kg/day AZA.

(A), Tukey box-plots for 6-TGN concentrations in the standard and adapted groups at each study visit. (B), Tukey box-plots for 6-MMP concentrations in the standard and adapted groups at each study visit.

6-mmp and hepatotoxicity
6-MMP concentrations were not significantly different between the groups at any visit (Fig. 3BUp ). There was a wide range of 6-MMP concentrations in both groups. Sixteen of 32 patients (50%) in the standard group and 14 of 25 (56%) in the dose-adapted group had 6-MMP concentrations >5700 pmol/8 x 108 Ery. Only 1 patient in the standard group and none in the dose-adapted group with 6-MMP concentrations >5700 pmol/8 x 108 Ery developed hepatotoxicity, defined as a >2.5-fold increase of transaminases. Nine of 32 patients (28.1%) in the standard group and 7 of 25 (28%) in the adapted group showed 6-MMP concentrations >10 000 pmol/8 x 108 Ery, but none of these patients developed hepatotoxicity.

aza dose and thioguanine metabolite formation
The AZA dose was increased in 18 of 25 patients in the adapted group during the study. A single dose increase was required in 9 patients, and multiple increases were conducted in a further 9 patients in an attempt to achieve 6-TGN concentrations of 250–400 pmol/8 x 108 Ery. In 8 of 25 patients, multiple dose increases did not lead to the desired 6-TGN target concentrations. A dose decrease was necessary in 11 of 25 patients. Multiple dose decreases were required in 2 of 25 patients in the dose-adapted group. AZA doses, summarized in Table 3 , did not differ significantly between the groups.


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  		Table 3. AZA doses observed throughout the study.1

adverse events
The numbers of patients with adverse events were similar in both groups (Fig. 1Up ). Moderate adverse events and severe adverse events were also similar in both groups (data not shown). There were no clinically significant changes in any of the hematological or biochemical factors monitored throughout the study.


   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
References
 
This study failed to demonstrate that a 6-TGN concentration–controlled adaptation of the AZA dose increases the proportion of Crohn disease patients in remission without steroids at 16 or 24 weeks of treatment. The quality of life as measured by the IBDQ was not significantly different between the 2 treatment groups, and dose adaptation did not change the rate of AZA-related side effects. 6-TGN concentrations in the standard group without dose adaptation averaged between 216 and 266 pmol/8 x 108 Ery throughout the investigation, which is close to the lower end of the suggested therapeutic range (16). The dose adaptation did not lead to significant differences in 6-TGN concentrations between groups throughout this study. With this in mind, we do not believe that a larger study would be worthwhile in this context. As evident from Fig. 3AUp , there was even a tendency toward higher 6-TGN concentrations in the standard group from week 12 onwards. This suggests that the AZA dose in the standard arm leaves more room for dose reduction than for escalation and is, if tolerated, close to a maximal effective dose. In the case of subtherapeutic 6-TGN concentrations in patients on a dose of 2.5 mg AZA/kg/day, further dose escalation would appear to be futile.

Despite dose adjustment, 6-TGN concentrations were still highly variable in the adapted group. Thus, dose adjustment in inflammatory bowel disease patients, even based on serial 6-TGN measurements, may be of limited value [cf (15)]. It should be noted that in our study the initial AZA dose was high compared with previous studies that reported an association between 6-TGN concentrations and efficacy (12)(18)(19)(32). This led to target 6-TGN concentrations in 17 of 25 patients, but in the remaining 8 patients even multiple dose increases did not result in 6-TGN concentrations >250 pmol/8 x 108 Ery. This could be due to preferential 6-MMP production upon dose escalation, as reported for a group of patients who failed to respond clinically to an increase in 6-MP/AZA dose (33). The preferential increase of 6-MMP but not 6-TGN concentrations was observed in 5 of 8 patients with multiple dose escalations without the desired rise of 6-TGN >250 pmol/8 x 108 Ery. Because AZA dosage increases based on 6-TGN concentrations did not show the desired increase in the latter, irrespective of TPMT, other independent variables must influence metabolite steady-state concentrations; variable bioavailability in active inflammatory bowel disease (34)(35) is one candidate. Our previous observations in active Crohn disease compared with other chronic inflammatory systemic diseases are also in agreement with these findings (36). Similar to others (19) but in contrast to Dubinsky et al. (10), we did not observe 6-MMP–related hepatotoxicity, even in patients with 6-MMP concentrations >10 000 pmol/8 x 108 Ery.

TPMT activity increased significantly by 24% after 4 weeks of AZA treatment. Five patients with mutant TPMT alleles had significantly higher 6-TGN concentrations than the other patients. All heterozygotes dropped out of the study at a relatively early stage due to AZA-related (n = 2) or -unrelated (n = 1) side effects as well as personal reasons (n = 2). Because patients carrying TPMT mutations are at risk of developing myelotoxicity within 7 months of starting therapy (37), it would appear prudent to control 6-TGN concentrations in such patients.

Measurement of TPMT activity before treatment with AZA or 6-MP is therefore advisable and cost-effective (38). In the case of TPMT activity between 8 and 20 nmol/(mL Ery x h), the standard AZA dose of 2.5 mg/kg/day should be used. Dose adaptation based on single 6-TGN measurements seems useful only to avoid toxicity by dose reduction. Finally, our data do not support a role of 6-MMP as a predictor for hepatotoxicity.


   Acknowledgments
 
Grant/funding support: This study was supported by Merckle GmbH, Ulm, Germany.

Financial disclosures: While this study was conducted, C.B. and P.B. were employees of Merckle GmbH, Ulm, Germany. Merckle (now Merckle Recordati GmbH) sells an azathioprine generic drug. Some authors have received lecture fees from Merckle.

Acknowledgments: We thank Helmut Malchow, Leverkusen, for his valuable contribution to the study.


   Footnotes
 
2 Current affiliation: Chronix biomedical, Göttingen, Germany.

3 Current affiliation: Institute for Clinical Chemistry and Laboratory Medicine, Katharinenhospital, Stuttgart, Germany.

1 Nonstandard abbreviations: AZA, azathioprine; 6-MP, 6-mercaptopurine; 6-TGN, 6-thioguanine nucleotide; 6-MMP, 6-methyl mercaptopurine; TPMT, thiopurine S-methyltransferase; Ery, erythrocyte; CDAI, Crohn disease activity index; IBDQ, Inflammatory Bowel Disease Questionnaire; ITT, intention-to-treat; MCV, mean corpuscular volume.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 

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P. de Graaf, N. K.H. de Boer, B. Jharap, C. J.J. Mulder, A. A. van Bodegraven, and A. I. Veldkamp
Stability of Thiopurine Metabolites: A Potential Analytical Bias
Clin. Chem., January 1, 2008; 54(1): 216 - 218.
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