Understanding the Use of Immunosuppressive Agents

Current Drug Targets, 2011, 12, 00-00

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Understanding the Use of Immunosuppressive Agents in the Clinical Management of IBD O.R. Waters1,2 and I.C. Lawrance*,1, 3 1

Centre for Inflammatory Bowel Diseases, Department of Gastroenterology, Fremantle Hospital, WA, Australia

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Department of Gastroenterology, Royal Devon and Exeter Hospital Foundation Trust, Exeter, Devon, United Kingdom

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University Department of Medicine and Pharmacology, University of Western Australia, Fremantle Hospital, Fremantle, WA, Australia Abstract: The advent of the biological era has seen many improvements in the management of inflammatory bowel disease (IBD). These agents, however, are not a ubiquitous panacea as they are neither universally available nor are they universally efficacious in the short or long-term. There is, therefore, still a need for other therapies and it is important to remember about the medications that have been effective in the past. The use of azathioprine and 6-mercoptopurine has been the mainstay of long-term therapy for many IBD patients for many years. Their role as steroid sparing agents and in the maintenance of remission is well recognized, and with the advent of metabolite testing their use has been refined. Methotrexate is a second line immunomodulator with less impressive data but still with observed benefits in Crohn’s disease (CD) and two newer immunosuppressive agents, mycophenylate mofetil and tacrolimus have sparked some interest as they appear to be efficacious in some patients. As IBD is a chronic incurable condition that primarily presents in young patients, the treating clinician’s goal is to induce and maintain long-term remission. So when one agent is ineffective, or unavailable, other agents need to be considered. This review aims to provide clinicians with practical and up to date knowledge about the use of the immunomodulators in the management of IBD, which is vital in order to offer the best management for their patients.

Keywords: Azathioprine, immunosuppression, inflammatory bowel disease, methotrexate, mycophenoylate mofetil, tacrolimus, 6-mercaptopurine. INTRODUCTION

AZATHIOPRINE AND 6-MERCAPTOPURINE

The natural history, of both forms of the inflammatory bowel diseases (IBDs), Crohn’s disease (CD) and ulcerative colitis (UC), is characterized by a lifelong course of remissions and relapses. A proportion of these patients are steroid dependent, or refractory, necessitating the use of other medications. Up to 50% of CD and 20% of UC patients will require hospitalization for an acute severe episode at some stage of their disease, and almost half will require rescue medical therapy or surgery [1, 2]. The anti-tumour necrosis factor alpha (TNF) medications can be extremely effective in both CD and UC, but not all patients will respond. Medical remission can also be achieved in acute severe UC with cyclosporine, but over 65% of these patients will relapse by 12 months [3]. Thus, despite the advent of new biological agents and salvage therapy, efficacy is not universal and the need for other medical options remains. This paper presents a users guide to the immunomodulators azathioprine (AZA), 6-mercaptopurine (6MP), methotrexate (MTX), mycophenoylate mofetil (MFM) and tacrolimus in CD and UC so that these options may be considered in the hard to manage CD and UC patient.

Both AZA and 6MP are derivatives of thioguanine and act as anti-metabolites by competition with purines in the synthesis of nucleic acids. The resulting reduction of intracellular purine metabolism leads to a decrease in the number of circulating B and T lymphocytes [4], decreased levels of immunoglobulin synthesis [5] as well as interleukin (IL)-2 levels [6]. AZA also induces T lymphocyte apoptosis, however their full mode of action has yet to be fully elucidated.

*Address correspondence to this author at the The School of Medicine and Pharmacology, University of Western Australia, T Block, Fremantle Hospital, Alma Street, Fremantle, 6059, WA, Australia; Tel: 618 9431 6347; Fax: 618 9431 3160; E-mail: [email protected] 1389-4501/11 $58.00+.00

Metabolism AZA is a prodrug with 40% excreted unchanged in the urine. The rest is converted by red blood cell (RBC) glutathione into 6MP [7]. 6MP undergoes further anabolic metabolism into its active metabolite 6-thioguanine (6TGN). Thiopurine methyltransferase (TPMT) is the primary determinant of AZA/6MP metabolism (Fig. 1) and catalyses the production of the hepatotoxic metabolite 6-methylmercaptopurine (6-MMP). TMPT activity is genetically determined with 89% of people homozygous for the wildtype TPMT allele [8] with high enzymic TMPT activity. Approximately 11% of the population has one mutated TPMT allele and lower enzymic activity, while 0.3% of the population are homozygous with two mutated TPMT genes resulting in negligible enzymic activity [8]. Genotyping can identify the mutant TPMT alleles with low activity, which results in more AZA/6MP being © 2011 Bentham Science Publishers Ltd.

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metabolized into 6-TGN and higher serum levels. These patients are, therefore, prone to suffer leucopenia [9] and in these patients, commencing AZA/6MP at lower doses with more frequent monitoring is suggested. TPMP genotyping, however, does not always predict AZA/6MP-induced myelosuppression or intolerability as the majority of patients suffering bone marrow suppression do not carrying a mutant TMPT allele [10] and 25% of patients regardless of their TMPT genotype are intolerant to AZA/6MP [11]. This lack of predictability has led to TMPT phenotyping that measures enzymic activity within the RBCs. This identifies patients with low TMPT activity (12 U/ml) are very likely to require higher than normal doses. It must be noted, however, that TPMT activity is dynamic and AZA/6MP treatment can induce TPMT enzymic activity and while TPMT genotyping and phenotyping may help reduce the risk of myelosuppression, it does not negate the need for frequent FBC monitoring in all patients for at least six to eight weeks following initiation or dose escalation. Rapidly achieving therapeutic 6-TGN levels does not reduce the time to efficacy and so slow dose escalations are both safer and preferable [13]. Certain drugs can significantly alter AZA/6-MP metabolism. 5-aminosalicylates result in mild reversible inhibition of TPMT activity, whilst allopurinol can inhibit xanthine oxidase activity and greatly increase 6-TGN metabolite levels (Fig. 1) [14]. Great care should thus be taken when allopurinol is used in combination with AZA/6MP. This combination, however, can be extremely effective when AZA/6MP metabolism is preferentially directed away from 6-TGN towards the 6MMP metabolite [15] which is associated with high TPMT activity. Allopurinol in these circumstances can reduce 6-MMP, while increasing 6-TGN, levels. A reduction in the AZA/6MP dose by 75% is suggested when allopurinol is started [16].

Waters and Lawrance

Induction of Remission There are numerous trials examining AZA/6MP for the induction of CD remission. Many used concomitant steroid therapy, but included highly heterogeneous patient populations with a range of follow-up. The doses of AZA/6MP also varied. Correlation of eight [17-24] of these heterogeneous studies has been undertaken in two meta-analyses [25, 26] from which the following conclusions were made: •

The overall response rate for AZA/6MP was 55% (95% CI 47-61%) compared to 33% (95% CI 2740%) for placebo - odds ratio 2.43 (95% CI 1.62 to 3.64).

•

The difference in response only becomes significant after 17 weeks of therapy, demonstrating the slow onset of AZA/6MP activity.

•

To achieve one remission, the number needed to treat (NNT) was 5 for AZA and 3 for 6MP.

Mucosal Healing No longer is the goal of treatment merely to induce a clinical response or remission. With advancements in therapies the new goal is now to achieve complete mucosal healing and the evidence would suggest that AZA/6MP are effective for this. Various studies have reported the effect of AZA/6MP in CD [27] with complete mucosal healing with monotherapy reported to be as high as 73% [28, 29] and complete or near complete healing as high as 83% [28, 29]. The use of AZA/6MP in combination with infliximab is now also regarded as more effective for the induction and maintenance of steroid-free remission and mucosal healing than with the use of either drug alone for up to 1 year in patients who are naïve to both agents [30, 31]. In UC the evidence is not as strong, but a role of AZA/6MP in mucosal healing, and protection against the development of colorectal cancer, has been presented [32]. Closure of Fistulae Data is limited for the role of AZA/6MP in the management of CD fistulae with no specifically designed studies. In a meta-analysis of 5 randomized trials in which closure of various fistulae was a secondary end point, a beneficial

Fig. (1). Metabolism of azathioprine and 6-mercaptopurine. Thiopurine methyltransferase (TPMT) is the primary determinant of AZA/ 6MP metabolism with high TPMT activity shunting metabolism towards 6-methylmercaptopurine (6-MMP) production. Allopurinol inhibits xanthine oxidase activity and can greatly increases 6-thioguanine (6-TGN) metabolite levels. 6-TIMP - 6-thioinosine monophosphate, 6TXMP - 6-thioxanthine monophosphate, 6-TGMP - 6-thioguanine mono phosphate, HPRT - hypoxanthine phosphoribosyltransferase, IMPDH - inosine monophosphate dehydrogenase, GMPS - guanosine monophosphate synthetase.

Immunosuppressants in IBD

Current Drug Targets, 2011, Vol. 12, No. 5

effect of AZA/6MP over placebo was demonstrated after 6 months of therapy (54% vs. 21%) giving a pooled odds ratio of 4.45 (95% CI 1.5-13.2). Numbers, however, were low and a clear role for AZA/6MP in fistulae management is lacking [25]. Steroid Sparing AZA/6MP reduces patient steroid requirements and maintains clinical remission following steroid cessation [1720, 24] and although the various study designs differ, each assessed patient steroid requirements. Reduced steroid requirements occurred in 65% (95% CI 56-74%) of patients on AZA/6MP compared with 36% (95% CI 27-45%) receiving placebo [25] equating to a NNT of 3 to reduce steroid requirements. Maintenance Therapy Both CD and UC are life long conditions so the management goal is maintenance of remission. Five controlled studies [33-37] and one meta-analysis [38] have examined AZA/MP in maintaining CD remission. Together these studies examined 208 CD patients receiving AZA, 47 on 6MP and 295 placebo. Compared to placebo, AZA (OR 2.32) and 6-MP (3.32) maintained remission better. Overall the NNT to prevent one flare was 6 for AZA and 4 for 6-MP. The number needed to harm was 20 [38]. A recent metaanalysis in UC also suggested that AZA/6MP effectively prevents relapse, with a NNT of 5 and an absolute risk reduction of 23% [39] Drug Level Monitoring As the availability of metabolite testing improves, there is a move towards measuring both 6-TGN and 6-MMP levels and correlating these with clinical response. 6-TGN levels between 230-400pmol/8x108 RBC are associated with a clinical response. 6-TGN levels >400pmol/8x108 RBC are associated with myelosuppression, while 6-MMP levels >5700pmol/8x108 RBC are associated with hepatotoxicity and other AZA/6MP-induced side effects [40-43]. There is, however, considerable intra-patient variability of metabolite levels over time with up to 5-fold differences observed despite the drug doses remaining unchanged [43]. The most useful strategy in assessing AZA/6MP management can be considered in 4 ways (Table 1). Low 6-TGN and 6-MMP levels indicate patient non-compliance or underdosing of the drug. High 6-MMP and low 6-TGN may indicate high TMPT activity and shunting towards 6-MMP and away from 6-TGN. The addition of allopurinol (100mg/ Table 1.

Interpretation of 6-Methylmercaptopurine (6-MMP) and 6-Thioguanine (6-TGN) Metabolite Levels Low 6-TGN

High 6-TGN

Low 6MMP

Patient is non-compliant or under dosing of AZA/6MP

Dose is appropriate

High 6MMP

Shunting away from 6-TG consider allopurinol 100mg/day and a reduced AZA/6MP dose

AZA/6MP dose is too high

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day) can inhibit xanthine oxidase activity and increase 6TGN levels [14, 44]. This should be done in conjunction with a marked reduction in the AZA/6MP dose. Patients with raised 6-MMP and 6-TGN are taking too much AZA/6MP, while patients with a low 6-MMP and 6-TGN in the therapeutic range can be considered to be taking the appropriate dose. Toxicity Up to 25% of patients with IBD are unable to tolerate AZA/6MP [11]. The most common adverse affects are gastrointestinal and experienced by 23% of patients shortly after commencing therapy [45]. Commencing AZA/6MP at a low dose with slow dose escalation can minimize these. Liver aminotransferase elevation occurs in 5-10% of patients and is usually associated with elevated 6-MMP levels. These changes are reversible following drug withdrawal or dose reduction. Severe cholestatic hepatitis, however, may persist with potential progression to cirrhosis [46, 47]. Any unexplained hyperbilirubinaemia should, therefore, lead to cessation of these medications and further investigations to exclude other causes. Monitoring liver function is recommended and required as frequently as the FBC with drug initiation, following dose escalation and with long-term use. Idiosyncratic hypersensitivity reactions occur in 2% of patients [25, 48] usually within the first few weeks of commencing therapy but can occur at any time without correlation to metabolite levels. The monitoring of metabolite levels is thus not a substitute for the regular patient review. Patients suffer gastrointestinal symptoms, but may also complain of fever, rashes, myalgia and fatigue. There may be associated liver function abnormalities and rarely hypotension. Pancreatitis occurs in 3% of patients [48]. Again the onset is usually within the first 4 weeks of therapy, with middle-aged women having the highest risk [49]. It should be noted, however, that corticosteroids and 5-ASAs are well documented to induce pancreatitis. An episode of pancreatitis should, therefore, not automatically be attributed to AZA/6-MP therapy. Patients suffering a hypersensitivity reaction should cease the AZA/6-MP. Significant bone marrow suppression occurs in up to 5% of patients. Infective complications with AZA therapy can occur and bacterial infections are associated with leucopenia and concurrent steroid use. Viral infections occur in 6% of patients, with herpes zoster and viral hepatitis exacerbations most common, suggesting that immunization against these may be beneficial prior to initiating of therapy [10, 50]. There is also a 4-fold increase in the relative risk of lymphoma in IBD patients taking AZA/6MP [51]. Lymphoma, however, occurs in IBD patients not taking AZA/6MP and it is unclear if the increased risk is due to AZA/6MP use, IBD or both [52]. Overall the risk is very small and decision analysis modeling demonstrates that IBD patients using of AZA have an increase in quality adjusted life expectancy despite the potential increase in lymphoma [51]. Fertility and Pregnancy Much of the data for AZA/6MP safety in pregnancy comes from transplant patients. The fetus is protected from 6MP/AZA as the fetal liver lacks the ability to convert AZA


into its active metabolites. The majority of experience, however, is with AZA and although it is assumed that the risk is similar for 6MP, more data is required. Prospective and retrospective analysis of AZA/6MP use in transplant, SLE and IBD patients, however, are associated with good fetal and maternal outcomes [53-55], with no increase in fetal abnormalities. It is now recommended that AZA/6MP should not be ceased during pregnancy, but nor should they be commenced, as they take months to become efficacious and hypersensitivity and dosage associated side effects are best avoided. The AZA/6MP and their metabolites are detected in the breast milk. The maximum concentration an infant would receive from a mother taking AZA 75-200mg/day was 50% reduction in the number of draining fistulae for > 4 weeks and was reached by 43% (10/23) receiving tacrolimus compared to 8% on placebo (p=0.01). Remission rates, however, were low (8%) with no difference between tacrolimus and placebo. The second study included 65 hospitalized patients with moderate/severe steroid refractory UC [99], not on AZA/6MP. Patients received tacrolimus at high target serum concentrations (1015 ng/ml), low target serum concentration (5-10 ng/ml) or placebo. After two weeks, the rate of clinical improvement with tacrolimus was statistically significant and dose dependent, however there were no statistical differences in the rates of remission. The effect of tacrolimus on mucosal healing was also examined in this study with healing achieved in 78.9% (15/19) of patients with trough levels of 10-15 ng/ml compared to 44.4% (8/18) of patients with a trough level of 5-10 ng/ml. This was significantly better than that observed in placebo-treated patients who had a healing rate of 12.5% (2/16) [99]. Data for luminal CD comes from retrospective case series focusing on patients failing, or intolerant, to AZA/ 6MP. These studies do suggest efficacy in inducing and maintaining remission [91, 93, 95]. Most patients were commenced on 0.1mg/kg tacrolimus twice a day aiming for a trough level in the therapeutic range of 5-20ng/ml. It would appear, however, that the higher the trough level the better


Waters and Lawrance

the response and remission rates as patients levels >10ng/ml can be associated with a remission rate in CD of 64% [95] and response rates of 68% to 83% [100]. It must be noted, however, that patient relapse after initial response does occur [95].

REFERENCES

Overall, a Cochran review [101] concluded that tacrolimus was effective in inducing a clinical improvement in a dose-dependent manner in treatment-resistant UC with the NNT being 3.

[3]

Topical Tacrolimus

[5]

There have been a promising small feasibility study [102], case series [103] and one placebo controlled trial [99] looking at topical tacrolimus for refractory distal colitis and perianal ulcerating and fistulizing CD. The doses ranged from 1mg to 4mg/day in a variety of formulations including ointments, 100ml enema and suppositories. 75% of patients showed a global improvement at 12 weeks with external ulcerating perianal CD compared to none on placebo. However, no difference between placebo and topical tacrolimus for perianal fistulizing disease was noted [99]. In proctitis, topical tacrolimus induced remission in 75% of patients at eight weeks with significantly reduced steroid requirements [103]. Similarly 63% of left-sided colitis and 83% of proctitis improved with therapy after four weeks [102]. Tacrolimus trough levels were generally very low and the agents were well tolerated. Further work on the use of topical tacrolimus, however, is still required.

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Adverse Effects Oral tacrolimus is generally well tolerated. The most common adverse effects are headache, tremor, parasaethesia, insomnia, gastrointestinal upset and arthralgia. Tacrolimus is nephrotoxic with dose-dependent rises in creatinine. Adverse events in clinical trial were significantly more common in with tacrolimus than placebo [99, 100]. Most adverse events, however, are reversible and can be managed by dose reduction. Tacrolimus has been shown to cross the placenta with episodes transient neonatal hyperkalaemia and renal dysfunction being reported. The use of tacrolimus, however, in pregnancy is not contraindicated nor has the risk been fully quantified. CONCLUSION The current paradigm in the management of IBD is to use immunomodulators or the biological agents early in the disease in an attempt to alter disease course. The biological agents are, however, not always effective or available. As an alternative the immunomodulators AZA/6MP can reduce disease recurrences and steroid use in UC and CD, but there is a lack of long-term data with MTX, MMF and tacrolimus. Even in the biological era, AZA/6MP remain the beststudied and simplest drug to manage IBD. The other immunomodulators offer alternatives to patients who are intolerant to AZA/6MP and the anti-TNF therapies. Ultimately, due to the complex nature of IBD, therapy must be individualized to each patient and this frequently requires different medications at different times in the disease course.

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Revised: ???????????, 2010

Accepted: ??????????, 2010