Case Report: Azathioprine: An Old and Wronged Immunosuppressant

Mycophenolate rapidly substituted azathioprine (AZA) in transplant immunosuppression regimens since the 1990s, when early clinical trials indicated better outcomes, although opposite results were also observed. However, none of these trials used the well-established optimization methods for AZA dosing, namely, thiopurine methyltransferase pharmacogenetics combined with monitoring of the thiopurine metabolites 6-thioguanine nucleotides (6-TGN) and 6-methylmercaptopurine (6-MMP). Resistance to optimize AZA therapy remains today in transplant therapy, despite the fact that thiopurine metabolite testing is being used by other medical disciplines with evident improvement in clinical results. In a previous analysis, we found that active 6-TGN metabolites were not detectable in about 30% of kidney transplant patients under continuous use of apparently adequate azathioprine dosage, which demonstrates the need to monitor these metabolites for therapeutic optimization. Two of four case studies presented here exemplifies this fact. On the other hand, some patients have toxic 6-TGN levels with a theoretically appropriate dose, as seen in the other two case studies in this presentation, constituting one more important reason to monitor the AZA dose administered by its metabolites. This analysis is not intended to prove the superiority of one immunosuppressant over another, but to draw attention to a fact: there are thousands of patients around the world receiving an inadequate dose of azathioprine and, therefore, with inappropriate immunosuppression. This report is also intended to draw attention, to clinicians using thiopurines, that allopurinol co-therapy with AZA is a useful therapeutic pathway for those patients who do not adequately form active thioguanine metabolites.

Current status and future perspectives on the use of therapeutic drug monitoring of thiopurine metabolites in patients with inflammatory bowel disease

INTRODUCTION

Despite new treatment options for inflammatory bowel disease (IBD), conventional thiopurines remain a common treatment option for maintaining remission, particularly in non-Westernized countries. Therapeutic drug monitoring (TDM) is advised in standard care for optimizing therapy strategies to improve effectiveness, reveal nonadherence and reduce toxicity. Still, the rationale of TDM is debated.

AREAS COVERED

Key insights on TDM of thiopurine metabolites are discussed. The pharmacology of thiopurines is described, emphasizing the interindividual differences in pharmacogenetics, pharmacokinetics and pharmacodynamics. Pharmacological differences between conventional thiopurines and tioguanine are outlined. Finally, several optimization strategies for thiopurine therapy in IBD are discussed.

EXPERT OPINION

TDM has been a useful, but limited, tool to individualize thiopurine therapy. Pharmacokinetic data on the active thiopurine metabolites, derived from measurements in erythrocytes, associated with clinical response only partially predict effectiveness and toxicity. An additional pharmacodynamic marker, such as Rac1/pSTAT3 expression in leukocytes, may improve applicability of TDM in the future.

Thiopurines with low-dose allopurinol (ThiLDA)-a prospective clinical one-way crossover trial

PURPOSE

Many patients with Crohn's disease (CD) and ulcerative colitis (UC) who have a high 6-methylmercaptopurine/6-thioguanine (6-MMP/6-TGN) ratio receive allopurinol 100 mg in addition to thiopurines to optimize metabolite concentrations. However, some patients do not tolerate allopurinol at this dosage. The aim of this study was to determine the intra-patient effect of reducing the allopurinol dosage from 100 to 50 mg, in terms of metabolite concentrations, enzyme activities, efficacy, and tolerability.

METHODS

A prospective non-inferiority one-way crossover study was performed. CD and UC patients with stable disease using a thiopurine and allopurinol 100 mg were switched to 50 mg for 1 month. Primary outcomes were thiopurine metabolite concentrations. Secondary outcomes were enzyme activities of xanthine oxidase, thiopurine methyltransferase and hypoxanthine-guanine phosphoribosyltransferase, disease activity, and tolerability.

RESULTS

Twenty-two patients were included. Treatment with allopurinol 50 mg compared with 100 mg resulted in a significant decrease in mean 6-TGN levels (761 to 625 pmol/8 × 10⁸ RBC; p = 0.005) and a significant increase in mean 6-MMP levels (451 to 665 pmol/8 × 10⁸ RBC; p = 0.01). However, the mean metabolite concentrations were still therapeutic. Enzyme activities, disease activity scores, and patient experiences did not alter significantly. Generally, UC patients were more positive about their improved treatment than CD patients.

CONCLUSION

Combination therapy with 50 mg allopurinol led to a decrease of 6-TGN levels compared with 100 mg allopurinol. Disease activity, side effects, and patient experience, however, were similar between allopurinol 100 and 50 mg. UC patients seem to benefit and prefer lower doses whereas the contrary is seen in CD patients.

TRIAL REGISTRATION

EudraCT trial registry - number 2016-001638-84.

Low 6-thioguanine nucleotide level: Effective in maintaining remission in Chinese patients with Crohn's disease

BACKGROUND AND AIM

Measuring 6-thioguanine nucleotide (6-TGN) level is useful in optimizing dose of azathioprine (AZA) and monitoring for toxicity. Lower dose of AZA was suggested for maintenance of clinical remission in Asian patients than Caucasian patients with Crohn's disease (CD). However, the optimal 6-TGN threshold required in Asian patients is undetermined. Therefore, the aim of the current study is to explore the optimal 6-TGN threshold required in Asian patients with CD for maintenance of clinical remission.

METHODS

A retrospective cohort study in a tertiary referral center recruited 252 CD patients. The primary endpoint was disease relapse. The levels of 6-TGN and AZA dose were compared in remission group and relapse group. Remission rate was compared across the increased 6-TGN level and dose range.

RESULTS

Patients with 6-TGN range of 0-180.94 pmol/8 × 10⁸ red blood cells (RBC) had lower remission rate compared with those with 180.94-255.50 pmol/8 × 10⁸ RBC (P = 0.020). Quartile analysis showed that increasing 6-TGN level beyond 180 pmol/8 × 10⁸ RBC produced negligible gain in rate of remission. Frequency of adverse events significantly increased in patients with 6-TGN level > 355 pmol/8 × 10⁸ RBC (8.0% with 6-TGN > 355 pmol/8 × 10⁸ RBC vs 2.7% with 6-TGN < 355 pmol/8 × 10⁸ RBC, P = 0.035).

CONCLUSION

Our study suggested that optimal 6-TGN threshold required to maintain clinical remission in Chinese patients was 180-355 pmol/8 × 10⁸ RBC.

Measurement of red blood cell 6-thioguanine nucleotide is beneficial in azathioprine maintenance therapy of Chinese Crohn's disease patients

OBJECTIVE

It remains controversial whether 6-thioguanine nucleotide (6-TGN)-based dose adjusting can be beneficial in azathioprine (AZA) therapy. This study is designed to assess the role of 6-TGN concentrations in maintaining clinical remission in Chinese patients with Crohn's disease (CD).

MATERIAL AND METHOD

We performed a prospective observational study and collected data of CD patients in the First Affiliated Hospital of Anhui Medical University from June 2013 to April 2014. Demographic material, CD activity index, 6-TGN concentration, and laboratory tests were recorded at baseline and at each visit. In addition, 6-TGN was measured when drug adverse effects occurred. All patients achieved maintenance stage were administered a stable AZA dose at least 3 months before enrollment and were followed up at least 12 months. Thiopurine S-methyltransferase (TPMT) genotype was measured before AZA treatment.

RESULTS

Sixty-nine patients receiving maintenance therapy were analyzed. A positive correlation was found between 6-TGN levels and AZA dose (r = 0.258, p = 0.032). The mean 6-TGN concentration was 302.06 ± 115.84 in the remission group vs. 264.94 ± 164.53 pmol/8 × 10(8) RBC in those with active disease (t = 0.847, p = 0.40), and 197.74 ± 66.54 pmol/8 × 10(8) RBC in patients who relapsed vs. 310.26 ± 122.38 pmol/8 × 10(8) RBC for those in sustained remission (t= -2.541, p = 0.013). In the leukopenia group, the 6-TGN concentration was 469.11 ± 115.53 pmol/8 × 10(8) RBC vs. 257.31 ± 83.74 pmol/8 × 10(8) RBC in the non-leukopenia group (t = 7.622, p < 0.001). There was a significant negative correlation between leukocyte count and 6-TGN concentration (r= -0.326, p = 0.006).

CONCLUSIONS

6-TGN measurement is a helpful method of preventing disease relapse and avoiding leukopenia in individual azathioprine maintenance therapy.

Update on thiopurine pharmacogenetics in inflammatory bowel disease

Azathioprine and 6-mercaptopurine remain pivotal therapies for the maintenance of disease remission in patients with Crohn's disease and ulcerative colitis. While thiopurine S-methyltransferase deficiency was the first pharmacogenetic phenomenon to be recognized to influence thiopurine toxicity and reliably predict leukopenia, it does not predict other adverse effects, nor does it explain most cases of thiopurine resistance. In recent years, a number of other genetic polymorphisms have received increasing attention in the literature. In particular, SNPs in NUDT15 and in the class II HLA locus have been shown to predict thiopurine-related leukopenia and pancreatitis. The aim of this review is to provide a concise update of genetic variability which may influence patient response to azathioprine and 6-mercaptopurine.

Interindividual variability in TPMT enzyme activity: 10 years of experience with thiopurine pharmacogenetics and therapeutic drug monitoring

BACKGROUND & AIMS

TPMT activity and metabolite determination (6-thioguanine nucleotides [6-TGN] and 6-methylmercaptopurine nucleotides [6-MMPN]) remain controversial during thiopurine management. This study assessed associations between patient characteristics and TPMT activity, and their impact on metabolite levels.

PATIENTS & METHODS

A retrospective review of the laboratory database from a French university hospital identified 7360 patients referred for TPMT phenotype/genotype determination, and/or for 6-TGN/6-MMPN monitoring.

RESULTS

Four TPMT phenotypes were identified according to TPMT activity distribution: low, intermediate, normal/high and very high. Based on 6775 assays, 6-TGN concentrations were 1.6-fold higher in TPMT-deficient patients compared with TPMT-normal patients. Azathioprine dose and TPMT genotype were significant predictors of metabolite levels. Furthermore, 6-MMPN and 6-MMPN: 6-TGN ratios were, respectively, 1.6- and 2.2-fold higher in females than in males, despite similar TPMT, 6-TGN and azathioprine doses. An unfavorable ratio (≥20) was associated with a slightly higher TPMT activity.

CONCLUSION

These results illustrate the usefulness of pharmacogenomics and metabolite measurement to improve the identification of noncompliance and patients at high risk for toxicity or therapeutic resistance. Original submitted 13 November 2013; Revision submitted 30 January 2014.

The pharmacokinetic effect of adalimumab on thiopurine metabolism in Crohn's disease patients

BACKGROUND AND AIMS

A drug interaction between infliximab and azathioprine has previously been reported in Crohn's disease patients: the concentration of the main active thiopurine metabolites, the 6-thioguanine nucleotides (6-TGN), increased 1-3 weeks after the first infliximab infusion by 50% compared to baseline. The aim of this prospective study was to determine the effect of adalimumab on thiopurine metabolism in Crohn's disease patients, evaluated by 6-TGN and 6-methylmercaptopurine ribonucleotides (6-MMPR) concentration measurement.

METHODS

Crohn's disease patients on azathioprine or mercaptopurine maintenance therapy starting with concomitant adalimumab treatment were included. 6-TGN and 6-MMPR concentrations were determined before initiation of adalimumab and after 2, 4, 6 and 12 weeks of combination therapy. The activity of three essential enzymes involving thiopurine metabolism, thiopurine S-methyltransferase (TPMT), hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and inosine-triphosphate pyrophosphatase (ITPase), was evaluated at baseline and week 4. Clinical outcome was evaluated by the Crohn's disease activity index and C-reactive protein concentrations at baseline, week 4 and week 12.

RESULTS

Twelve Crohn's disease patients were analyzed. During the follow-up period of 12 weeks the median 6-TGN and 6-MMPR concentrations did not significantly change compared to baseline. TPMT, ITPase and HGPRT enzyme activity did not change either after 4 weeks. In two patients (17%) myelotoxicity was observed within 2-4 weeks, in whom both low therapeutic 6-TGN and 6-MMPR concentrations were found.

CONCLUSIONS

In this study in Crohn's disease patients no pharmacokinetic interaction was shown between adalimumab and the conventional thiopurines, azathioprine and mercaptopurine.

Efficacy and safety of treatment of hepatitis C in patients with inflammatory bowel disease

BACKGROUND & AIMS

There is uncertainty about the efficacy and safety of treatment for hepatitis C virus (HCV) infection in patients with inflammatory bowel disease (IBD). IBD can become exacerbated during treatment with interferon (IFN), and serious adverse events, such as pancytopenia or hepatotoxicity, can be compounded by drug interactions. We investigated the risk of exacerbation of IBD during HCV therapy and the rate of adverse effects of concomitant therapy for HCV and IBD. We also evaluated the efficacy of HCV treatment in the IBD population.

METHODS

We conducted a retrospective review of all patients who underwent IFN-based treatment for HCV at the Mayo Clinic in Rochester, Minnesota from 2001 to 2012. Exacerbation of IBD was evaluated by clinical, endoscopic, and histologic parameters during antiviral therapy and the ensuing 12 months. Hematologic toxicity was assessed by levels of all 3 cell lineages at baseline and during therapy. Efficacy of antiviral treatment was assessed by serum levels of HCV RNA until 24 weeks after completion of therapy. We also conducted a detailed MEDLINE database search and reviewed the literature on this topic.

RESULTS

We identified 15 subjects with concomitant IBD (8 with ulcerative colitis and 7 with Crohn's disease). Only 1 patient experienced exacerbation of the disease during therapy; symptoms were controlled with mesalamine enemas. Another patient developed a flare shortly after completing antiviral therapy; symptoms returned spontaneously to baseline 2 weeks later. All subjects experienced an anticipated degree of pancytopenia while on IFN-based therapy. The rate of sustained virologic response was 67%. A concise review of available literature regarding the safety and efficacy of HCV treatment in IBD patients is also presented; although limited, the published data appear to support the safety of treatment with IFN in patients whose IBD is under control.

CONCLUSIONS

In conjunction with data from the literature, our findings indicate that the efficacy and safety of HCV therapy with IFN and ribavirin for patients with IBD are comparable to those of subjects without IBD.

Beyond TPMT: genetic influences on thiopurine drug responses in inflammatory bowel disease

Azathioprine and 6-mercaptopurine are widely used in the management of inflammatory bowel disease (IBD). However, approximately 25% of IBD patients experience toxicity, and up to 10% show resistance to these thiopurine drugs. The importance of genetic variability in determining thiopurine toxicity was first recognized over 25 years ago with the discovery of the thiopurine S-methyltransferase (TPMT) polymorphism and the occurrence of azathioprine-induced myelosuppression in TPMT-deficient patients. In the intervening period, TPMT has become the foremost example of pharmacogenetics, and TPMT deficiency represents one of the few pharmacogenetic phenomena that have successfully made the transition from the research laboratory to diagnostics. While TPMT activity predicts some cases of myelosuppression, deficiency in this enzyme is neither predictive of other adverse drug reactions, nor resistance to thiopurine therapy. As myelosuppression only accounts for approximately 2.5% of adverse reactions in IBD patients, researchers are increasingly turning their attention to other enzymes involved in thiopurine metabolism to find molecular explanations for intolerance and resistance to azathioprine and 6-mercaptopurine. In this review, we summarize the current state of knowledge with regards to TPMT, and also explore genetic variability, beyond TPMT, that may contribute to thiopurine response in IBD patients.