Azathioprine in inflammatory bowel disease: improved molecular insights and resulting clinical implications

Azathioprine and 6-mercaptopurine represent important first-line immunosuppressive drugs in the treatment of inflammatory bowel disease. Owing to 45 years of clinical experience with thiopurines in inflammatory bowel disease, there currently exist strong data from numerous clinical trials and meta-analyses, which clearly document the therapeutic efficacy of azathioprine and 6-mercaptopurine in the treatment of inflammatory bowel disease. However, the exact molecular mechanism of action of these drugs was insufficiently understood for a long time. During the last few years, important new insights into the intracellular effects of azathioprine have been gained and thiopurines have been identified as strong inducers of T-cell apoptosis. This article aims to summarize traditional and current concepts of azathioprine-mediated effects and endeavors to discuss the resulting clinical implications.     

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.     

Efficacy of 6-mercaptopurine treatment after azathioprine hypersensitivity in inflammatory bowel disease

AIM：To investigate the efficacy of 6-mercaptopurine （6-MP） in cases of azathioprine （AZA） hypersensitivity in patients with inflammatory bowel disease. METHODS： Twenty nine previously confirmed Crohn’s disease （CD） （n = 14） and ulcerative colitis （UC） （n = 15） patients with a known previous （AZA） hypersensitivity reaction were studied prospectively. The 6-MP doses were gradually increased from 0.5 up to 1.0-1.5 mg/kg per day. Clinical activity indicies （CDAI/CAI）, laboratory variables and daily doses of oral 5-ASA, corticosteroids, and 6-MP were assessed before and in the first, sixth and twelfth months of treatment. RESULTS： In 9 patients, 6-MP was withdrawn in the first 2 wk due to an early hypersensitivity reaction. Medication was ineffective within 6 mo in 6 CD patients, and myelotoxic reaction was observed in two. Data were evaluated at the end of the sixth month in 12 （8 UC, 4 CD） patients, and after the first year in 9 （6 UC, 3 CD） patients. CDAI decreased transiently at the end of the sixth month, but no significant changes were observed in the CDAI or the CAI values at the end of the year. Leukocyte counts （P = 0.01）, CRP （P = 0.02）, and serum iron （P = 0.05） values indicated decreased inflammatory reactions, especially in the UC patients at the end of the year, making the possibility to taper oral steroid doses. CONCLUSION： About one-third of the previously AZA- intolerant patients showed adverse effects on taking 6MP. In our series, 20 patients tolerated 6MP, but it was ineffective in 8 CD cases, and valuable mainly in ulcerative colitis patients.     

Measurement of thiopurine methyltransferase activity and azathioprine metabolites in patients with inflammatory bowel disease

BACKGROUND: Measurement of 6-thioguanine nucleotide concentrations may be useful for optimising treatment with azathioprine and 6-mercaptopurine. METHODS: We conducted a study of 170 patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine to determine the relationship between 6-thioguanine nucleotide concentrations and both disease activity, as measured by the inflammatory bowel disease questionnaire (active disease < 170, remission > or = 170) and leucopenia. Blood was submitted for whole blood 6-thioguanine nucleotide concentration and leucocyte count. RESULTS: Mean (SD) inflammatory bowel disease questionnaire score was 176 (32). There was no correlation between inflammatory bowel disease questionnaire scores and 6-thioguanine nucleotide concentrations (r(s) = -0.09, p = 0.24). Median 6-thioguanine nucleotide concentrations in 56 patients with active disease and 114 patients in remission were similar (139 v 131 pmol/8 x 10(8) red blood cells; p = 0.26). There was no correlation between 6-thioguanine nucleotide concentrations and leucocyte counts. CONCLUSIONS: In patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine, 6-thioguanine nucleotide concentrations did not correlate with disease activity, as measured by the inflammatory bowel disease questionnaire, or leucocyte count. These findings are discrepant with most previous studies, possibly due to selection of responding patients who tolerated the medications. A prospective, randomised, dose optimisation trial using 6-thioguanine nucleotide concentrations is warranted.     

6-mercaptopurine and spermatogenesis in the young rat

In recent years, 6-MP treatment has been beneficial in the treatment of inflammatory bowel disease (IBD). Since 6-MP and its metabolites interfere with various steps in nucleic acid biosynthesis, chronic use of 6-MP could theoretically alter normal cell turnover, including spermatogenesis. Therefore, we have investigated the effect of daily 6-MP administration on spermatogenesis in the young rat. 6-Mercaptopurine was administered in clinically relevant doses 24 and 40 mg/m². Testicular weights of rats treated with 24 mg/m² for 75 days or 40 mg/m² for 25 days were not significantly different among 6-MP, pair-fed, orad libitum chow-fed groups. Quantitation of the stages of seminiferous tubules or the number of homogenization-resistant, mature spermatids per testis were not affected by 6-MP treatment. In addition, 6-MP had no effect on serum testosterone or on HCG-stimulated testosterone release by the testes. These results suggest that chronic low-dose 6-MP therapy, as used in the treatment of IBD, may not carry as great a risk for suppression of spermatogenesis as theorized. Our study in animals indicates that evaluation of 6-MP and spermatogenesis in man is warranted.This project was funded by a grant from Reach Out for Youth with IBD.     

Long term efficacy and safety of allopurinol and azathioprine or 6-mercaptopurine in patients with inflammatory bowel disease

Background and aimsWe previously reported that IBD patients who are non-responders to thiopurines with preferential shunting of metabolites to hepatotoxic 6-methylmercaptopurine ribonucleotides compared to 6-thioguanine nucleotides can reverse the ratio of 6-MMP/6-TGN and respond to thiopurines with the addition of allopurinol. The objective of this study is to report long term efficacy and safety, along with results for an additional 11 patients.MethodsRetrospective chart review of patients at the University of Chicago IBD Center treated with allopurinol in addition to thiopurines.ResultsTwenty five patients with Crohn's disease or ulcerative colitis were enrolled. Within the first month of therapy 6-TGN metabolite levels increased from a mean of 186.5 ± 17.4 (SE) to 352.8 ± 37.8 pmol/8 × 10⁸ (p = 0.0001). Over the same period 6-MMP levels decreased from a mean of 11,966 ± 1697 to 2004 ± 536 pmol/8 × 10⁸ (p < 0.0001). The mean daily dosage of prednisone decreased from 19.8 ± 3.8 mg to 5.3 ± 2.7 mg (p = 0.03). Thirteen patients have a minimum of one year follow-up. Nine of these thirteen patients have continued on therapy for at least 2 years. All thirteen of these patients continue to be in clinical remission at the last follow-up visit. No patients have had evidence of sustained thrombocytopenia or abnormal liver enzymes.ConclusionsIn AZA/6-MP non-responders with increased 6-MMP/6-TGN ratios, addition of allopurinol continues to demonstrate safety and efficacy for long-term maintenance and steroid-sparing in IBD.     

Thiopurine S-methyltransferase and inosine triphosphate pyrophosphohydrolase genes in Japanese patients with inflammatory bowel disease in whom adverse drug reactions were induced by azathioprine/6-mercaptopurine treatment

Background  The main cause of azathioprine (AZA)/6-mercaptopurine (6MP)-induced adverse reactions is a reduction in the activities of the metabolizing enzymes thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphohydrolase (ITPA). Adverse reactions develop at a high frequency in Japanese patients at half the dose required for European and American patients; however, the association with TPMT and ITPA gene polymorphisms in Japanese has not been fully investigated. Methods  Gene mutations of TPMT and ITPA, the major AZA/6-MP -metabolizing enzymes, were investigated retrospectively in 16 Japanese patients with inflammatory bowel disease (IBD) in whom AZA/6MP treatment induced adverse reactions. Results  The TPMT gene was found to have a wild-type sequence in all patients, but in the ITPA gene a mutation, 94C>A, was detected at a rate of 50% (8/16), with 83.3% (5/6) occurring in patients with acute bone marrow suppression and 75% (3/4) in those with agranulocytosis. The 94C>A allele frequency was 10 of 32 (0.313; 95% CI, 0.180–0.486). Adverse reactions developed earlier in patients with the 94C>A mutation. However, in half the patients, no gene polymorphism was noted. Conclusions  It is suggested that the ITPA gene mutation is closely related to the adverse reactions of AZA/6-MP in Japanese patients, and screening for the mutant allele is useful for predicting the most serious adverse reactions, agranulocytosis and acute bone marrow suppression.     

Monitoring 6-thioguanine nucleotide concentrations in Japanese patients with inflammatory bowel disease

Background and Aim: There have been no reports on 6‐thioguanine nucleotide (6‐TGN) concentrations in Japanese patients with inflammatory bowel disease (IBD) undergoing azathioprine (AZA) or 6‐mercaptopurine (6‐MP) therapy. The aim of this study was to assess 6‐TGN concentrations in Japanese IBD patients. Methods: Eighty‐three patients with Crohn's disease (n = 42) and ulcerative colitis (n = 41) were enrolled. In 69 patients, AZA was prescribed at 50 mg/day, and seven patients were given 75 (n = 5) or 100 mg/day (n = 2). 6‐MP was administered at 30 mg/day (n = 7). The 6‐TGN concentrations were then assayed by high‐performance liquid chromatography. Results: The mean 6‐TGN concentrations of the entire study population (n = 83) were 277.9 ± 179.8 pmol/8 × 10⁸ red blood cells (RBC). The mean 6‐TGN concentrations in those patients with active disease (n = 38) and those in remission (n = 45) were 232.9 ± 159.7(mean ± SD) and 342.8 ± 184.6 pmol/8 × 10⁸ RBC, respectively (P < 0.05). The odds ratio of being in remission and having a 6‐TGN value >235 pmol/8 × 10⁸ RBC was 2.6 (95% CI 1.05–6.2). A significant inverse correlation was found between the white blood cell (WBC) counts and 6‐TGN concentrations (r = ?0.301, P < 0.05, n = 83); the mean WBC counts of the active patients (6780 ± 2412) were significantly higher than the patients in clinical remission (5468 ± 1920, P < 0.05). Three patients with severe leukopenia and 10 patients with high 6‐TGN concentrations had no thiopurine S‐methyl transferase mutations. Conclusion: The 6‐TGN concentrations in Japanese patients with IBD on low‐dose AZA and 6‐MP therapy were comparable to those reported from Western countries. The monitoring of 6‐TGN concentrations may be helpful for developing a therapeutic strategy for Japanese IBD patients.     

TPMT genotype and the use of thiopurines in paediatric inflammatory bowel disease

BACKGROUND: Thiopurines are used in the treatment of inflammatory bowel disease. They are metabolised via methylation by thiopurine-S-methyltransferase (TPMT), which displays a genetically determined polymorphic activity. Subjects with reduced TPMT activity have a higher concentration of active thiopurine metabolites and may be at increased risk of bone-marrow suppression. AIMS: To evaluate the relevance of TPMT genotyping in the management of thiopurines therapy in inflammatory bowel disease patients. PATIENTS AND METHODS: Adverse effects and clinical response were determined retrospectively and correlated with TPMT genotype in 70 paediatric inflammatory bowel disease patients. RESULTS: Nineteen patients (27.1%) developed adverse effects; of the 51 who did not, 34 (66.7%) responded to treatment. Five patients (7.1%) were heterozygous for a variant TPMT allele; two of these (40%) were intolerant to thiopurines, compared to 17 of the 65 patients (26.2%) with a wild type gene (O.R. 1.88, 95% CI 0.29-12.2, p=0.61); among the 34 responders, the median dosage of the drug required to obtain remission was lower for mutated than for wild type patients (1.6mgkg(-1)day(-1) versus 2.0mgkg(-1)day(-1), p=0.043). CONCLUSIONS: There was no significant association between adverse effects of thiopurines and TPMT heterozygous genotype, but TPMT genotyping could be useful in establishing the most appropriate dose of thiopurines to start treatment.     

Allopurinol might improve response to azathioprine and 6-mercaptopurine by correcting an unfavorable metabolite ratio

Background and Aim: Allopurinol potentiates azathioprine and 6‐mercaptopurine (6‐MP) by increasing 6‐thioguanine nucleotide (6‐TGN) metabolite concentrations. The outcome might also be improved by adding allopurinol in individuals who preferentially produce 6‐methylmercaptopurine nucleotides (6‐MMPN), rather than 6‐TGN. The aim of the present study was to investigate the effect of allopurinol on concentrations of 6‐MMPN and 6‐TGN in individuals with a high ratio of these metabolites (>20), which is indicative of a poor thiopurine response. Methods: Sixteen individuals were identified who were taking azathioprine or 6‐MP, and were commenced on allopurinol to improve a high 6‐MMPN : TGN ratio. Metabolite concentrations were compared before and after commencing allopurinol, and markers of disease control were compared. Results: The addition of 100–300 mg allopurinol daily and thiopurine dose reduction (17–50% of the original dose) resulted in a reduction of the median (and range) 6‐MMPN concentration, from 11 643 (3 365–27 832) to 221 (55–844) pmol/8 × 10⁸ red blood cells (RBC; P = 0.0005), increased 6‐TGN from 162 (125–300) to 332 (135–923) pmol/8 × 10⁸ RBC (P = 0.0005), and reduced the 6‐MMPN : 6‐TGN ratio from 63 (12–199) to 1 (0.1–4.5) (P = 0.0005). There was a significant reduction in steroid dose requirements at 12 months (P = 0.04) and trends for improvement in other markers of disease control. One patient developed red cell aplasia that resolved upon stopping azathioprine and allopurinol. Conclusions: In those with a high 6‐MMPN : 6‐TGN ratio (>20), response to thiopurine treatment might be improved by the addition of allopurinol, together with a reduced thiopurine dose and close hematological monitoring.     

Sequential Group Trial to Determine Gastrointestinal Site of Absorption and Systemic Exposure of Azathioprine

Azathioprine (AZA) is used in the treatment of patients with refractory inflammatory bowel disease; however, its use is limited because of systemic toxicity associated with long-term use. Ileocecal delivery of AZA might be advantageous if local intestinal therapeutic effects could be provided with decreased systemic side effects. Decreased cecal systemic absorption would allow higher dosages of AZA to be administered. A two-phase study was performed to compare the systemic exposure of AZA and 6-mercaptopurine (6-MP) following administration of AZA into the stomach, jejunum, and cecum and to compare the systemic exposure to AZA and 6-MP following administration of three different dosages of AZA into the cecum. In phase I, six healthy male volunteers received three 50 mg sequential doses of AZA via an oral tube directly placed into the stomach, jejunum, and cecum, respectively. In phase II, six healthy male volunteers received three different dosages (50, 300, 600 mg of AZA) into the cecum. Plasma concentrations of AZA and 6-MP at various times were quantified and area under the plasma concentration-time curve (AUC) and mean residence time (MRT) were determined. No significant differences in the AUC of AZA were seen at the different sites. The AUC of 6-MP following administration of AZA into the jejunum (67.0 ± 30.1 ng×hr/ml) was higher compared to the stomach (39.9 ± 38.1 ng/hr/ml) and cecum (29.2 ± 10.9 ng×hr/ml). Jejunal absorption was 68% higher than absorption from the stomach and 129% higher than that of the cecum. Gastric absorption was 27% higher than that of the cecum. Increased dosages given into the cecum resulted in increased AUCs of AZA and 6-MP. The AUCs of AZA following 50, 300, and 600 mg dosages were 16.9 ± 7.4, 52.3 ± 67.2, and 132 ± 151 ng×hr/ml, respectively, and the AUCs of 6-MP were 22.2 ± 14.9, 63.4 ± 50.6, and 104 ± 115 ng×hr/ml, respectively. Systemic exposure to 6-MP is reduced following administration of AZA into the cecum, most likely secondary to reduced absorption of 6-MP from the colon. Higher dosages of AZA presented to the cecum do result in increased systemic absorption, but may still allow more drug to be administered with less toxicity than the same dose received orally.     

Thiopurines in inflammatory bowel disease: pharmacogenetics, therapeutic drug monitoring and clinical recommendations

There is a growing interest in the use of thiopurines (azathioprine, 6-mercaptopurine and 6-thioguanine) for the management of inflammatory bowel disease. The genetically controlled thiopurine (S)-methyltransferase enzyme is involved in the metabolism of these agents and is hypothesised to determine the clinical response to thiopurines. Diminished activity of this enzyme decreases the methylation of thiopurines, theoretically resulting in potential overdosing, while high thiopurine (S)-methyltransferase status leads to overproduction of toxic metabolites and ineffectiveness of azathioprine and 6-mercaptopurine. In practice, controversies exist regarding the utility of standard thiopurine (S)-methyltransferase pheno- and genotyping. Current pharmacogenetic insights suggest that another enzyme system may participate in the efficacy and toxicity of thiopurines; inosine triphosphate pyrophosphatase. Other topics discussed in this review are the utilisation of therapeutic drug monitoring and the experimental use of 6-thioguanine in the treatment of inflammatory bowel disease. 6-Thioguanine has a less genetically controlled metabolism and skips genetically determined metabolic steps. On theoretical basis, 6-thioguanine might therefore have a more predictable profile than azathioprine and 6-mercaptopurine. However, the use of 6-thioguanine has been associated with an increased risk of nodular regenerative hyperplasia of the liver and veno-occlusive disease. Further research is warranted before 6-thioguanine can be considered as a treatment option for inflammatory bowel disease.     

Epstein-Barr virus-positive lymphoma in patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine.

BACKGROUND & AIMS: The use of azathioprine and 6-mercaptopurine for inflammatory bowel disease increased in the early 1990s. We sought to determine the effect of this change in therapy on the risk of lymphoma in patients with inflammatory bowel disease. METHODS: All patients with inflammatory bowel disease at a single tertiary care medical center who developed lymphoma between 1985-2000 were identified and the pathologic features of the lymphoma including presence of Epstein- Barr virus were determined. The patients were divided into two 8-year periods (1985-1992, 1993-2000) corresponding with the introduction of azathioprine and 6-mercaptopurine in 1993. RESULTS: Eighteen patients with lymphoma were identified, 6 between 1985-1992 and 12 between 1993-2000. Six of 18 lymphomas occurred in patients treated with azathioprine or 6-mercaptopurine, all between 1993-2000. Seven patients developed Epstein-Barr virus-positive lymphoma (1 from 1985-1992, 6 from 1993-2000). Five of 7 Epstein-Barr virus-positive lymphomas occurred in patients treated with azathioprine or 6-mercaptopurine compared with 1 of 11 Epstein-Barr virus-negative lymphomas (P = 0.01). Approximately 1200 patients with inflammatory bowel disease were treated with these agents between 1993-2000. CONCLUSIONS: Treatment of inflammatory bowel disease with azathioprine or 6-mercaptopurine appears to be associated with a small increased risk of Epstein-Barr virus-positive lymphoma.     

Association of 6-thioguanine nucleotide levels and inflammatory bowel disease activity: a meta-analysis

BACKGROUND & AIMS: 6-Thioguanine nucleotide (6-TGN) levels have been proposed to correlate with inflammatory bowel disease (IBD) activity among patients treated with azathioprine or 6-mercaptopurine (6-MP). Previous studies, most with small sample sizes, yielded conflicting conclusions. Our aim was to pool the available data to provide a more precise estimate of the association between 6-TGN levels and IBD activity. METHODS: We searched Medline and PubMed (from 1966 to November 2004) and reviewed the reference lists of selected articles. Fixed and random-effects models were used to test whether mean/median 6-TGN levels differed among patients with active disease vs remission and whether 6-TGN levels above a threshold of 230-260 pmol/8 x 10(8) red blood cells were associated with clinical remission. When studies reported multiple 6-TGN threshold values, we used the data for the lower value. RESULTS: We identified 55 articles, 12 of which contained data sufficient for inclusion. The mean/median 6-TGN levels were higher among patients in remission than in those with active IBD (pooled difference, 66 pmol/8 x 10(8) red blood cells; 95% confidence interval, 18-113; P = .006), but with significant heterogeneity. Excluding the 1 outlier study eliminated this heterogeneity. Patients with 6-TGN levels above the threshold value were more likely to be in remission (62%) than those below the threshold value (36%) (pooled odds ratio, 3.3; 95% confidence interval, 1.7-6.3; P < .001), but with significant heterogeneity. Again, excluding the 1 outlier study eliminated this heterogeneity. CONCLUSIONS: Although prior studies yielded inconsistent conclusions, this analysis strongly supports that higher 6-TGN levels are associated with clinical remission.     

Use of 6-Mercaptopurine in Patients with Inflammatory Bowel Disease Previously Intolerant of Azathioprine

Both azathioprine and its active metabolite, 6-mercaptopurine, are of benefit in the treatment of inflammatory bowel disease, either in resistant cases, or for their steroid-sparing effect. Azathioprine treatment is limited in some patients by hypersensitivity reactions or other side effects. We report our experience in 11 patients previously unable to tolerate azathioprine for a variety of reasons, who were switched to 6-mercaptopurine. Of seven patients with ulcerative colitis and four patients with Crohn's disease who were treated with 6-mercaptopurine following failed azathioprine therapy, six were able to successfully tolerate the substitute medication, with good response. Where patients have previously been intolerant of azathioprine yet ongoing indications for immunosuppressive therapy remain, a trial of 6-mercatopurine may be warranted. Given the similar efficacies of the two drugs in inflammatory bowel disease, these findings also favor the use of 6-mercaptopurine rather than the parent compound as initial therapy.     

How are thiopurines used and monitored by Swedish gastroenterologists when treating patients with inflammatory bowel disease?

Abstract

Objective. To perform a survey of thiopurine treatment in inflammatory bowel disease (IBD) among Swedish gastroenterologists. Material and methods. A web-based questionnaire consisting of 25 multiple-choice questions was sent to 322 gastroenterologists in adult practice. Results. A total of 132 questionnaires were received giving a response rate of 41%. Thiopurines were used by all 122 gastroenterologists in IBD practice and azathioprine was the first-choice thiopurine among 118 (97%) of them. Almost all gastroenterologists (97%) used weight-based dosing that was gradually escalated. The vast majority (89%) considered that efficacy should be evaluated within 6 months of therapy, while opinions regarding the optimal duration of therapy varied considerably. It was seen that 74% switched thiopurine in case of intolerance to the first-line substance. Thiopurine S-methyltransferase (TPMT) determinations were performed by 74% of the gastroenterologists and 67% used metabolite measurements. TPMT analyzers were more likely to measure metabolites (74 vs. 43%, p = 0.002). A quarter of the respondents were familiar with unconventional immunomodulation (co-administration of allopurinol, 6-thioguanine, mycophenolate mofetil or tacrolimus) and these respondents were also more likely to measure metabolites (79 vs. 52%; p = 0.002). Conclusions. Thiopurines are well established in the treatment of IBD among Swedish gastroenterologists. New and evolving knowledge about thiopurine therapy in IBD has been adapted to a large extent. Whether this change in clinical practice will have an impact on treatment outcomes has yet to be proven.     

Optimizing 6-mercaptopurine and azathioprine therapy in the management of inflammatory bowel disease

The thiopurine drugs,6-mercaptopurine(6-MP) and azathioprine,are efficacious in the arsenal of inflammatory bowel disease(IBD) therapy.Previous reports indicate that 6-thioguanine nucleotide(6-TGN) levels correlate with therapeutic efficacy,whereas high 6-methylmercaptopurine(6-MMP) levels are associated with hepatotoxicity and myelotoxicity.Due to their complex metabolism,there is wide individual variation in patient response therein,both in achieving therapeutic drug levels as well as in developing advers...     

6-MP metabolite profiles provide a biochemical explanation for 6-MP resistance in patients with inflammatory bowel disease

BACKGROUND & AIMS: Approximately 40% of inflammatory bowel disease (IBD) patients fail to benefit from 6-mercaptopurine (6-MP)/azathioprine (AZA). Recent reports suggest 6-thioguanine nucleotide (6-TGN) levels (>235) independently correlate with remission. An inverse correlation between 6-TGN and thiopurine methyltransferase (TPMT) has been described. The objectives of this study were to determine whether dose escalation optimizes both 6-TGN levels and efficacy in patients failing therapy because of subtherapeutic 6-TGN levels and its effect on TPMT. METHODS: Therapeutic efficacy and adverse events were recorded at baseline and upon reevaluation after dose escalation in 51 IBD patients. 6-MP metabolite levels and TPMT activity were recorded blinded to clinical information. RESULTS: Fourteen of 51 failing 6-MP/AZA at baseline achieved remission upon dose escalation, which coincided with significant rises in 6-TGN levels. Despite increased 6-MP/AZA doses, 37 continued to fail therapy at follow-up. Dose escalation resulted in minor changes in 6-TGN, yet a significant increase in 6-methylmercaptopurine ribonucleotides (6-MMPR) (P < or = 0.01) and 6-MMPR:6-TGN ratio (P < 0.001). 6-MMPR rises were associated with dose-dependent hepatotoxicity in 12 patients (24%). TPMT was not influenced by dose escalation. CONCLUSIONS: Serial metabolite monitoring identifies a novel phenotype of IBD patients resistant to 6-MP/AZA therapy biochemically characterized by suboptimal 6-TGN and preferential 6-MMPR production upon dose escalation.