Curbside Consultation

How Do You Monitor Patients on Azathioprine/6-Mercaptopurine?

Miles Sparrow, MB, BS, FRACP

Why Monitor Patients on Azathioprine/6-Mercaptopurine?

The thiopurine immunomodulators azathioprine and 6-mercaptopurine (6-MP) are well established as induction, maintenance, and steroid-sparing agents in the treatment of inflammatory bowel disease (IBD). All patients taking thiopurine immunomodulators require careful ongoing blood monitoring to detect potential adverse effects such as leukopenia or hepatotoxicity, and to avoid possible associated complications.

The primary aim of immunomodulator blood monitoring is to ensure treatment safety, but it also maximizes efficacy by helping avoid dose-dependent side effects that otherwise may lead to drug cessation. More recently, thiopurine metabolite measurements have added an additional means of optimizing immunodulator dosing, but they are complementary to, and in no way replace, routine blood monitoring.

What to Monitor in Patients on Azathioprine/6-Mercaptopurine

You need to consider immunomodulator monitoring in 2 parts. The most important component of blood monitoring is the regular measurement of complete blood counts (CBC) and liver function tests (LFT), or so-called “routine bloods.” The second part of monitoring, if necessary, involves measuring the thiopurine metabolites 6-thioguanine nucleotide (6-TGN) and 6-methylmercaptopurine (6-MMP).

When and How to Monitor Patients on Azathioprine/6-Mercaptopurine

CBC and LFT

The frequency of measuring these routine bloods depends on the duration of immunomodulator therapy, as outlined in Table 9-1.

                • How do you monitor CBC and LFT when starting therapy?

                                Leukopenia and hepatotoxicity most commonly, but not exclusively, occur soon after commencing thiopurines, or upon dose escalation. This means frequent blood monitoring is needed during the first 3 months of therapy. During the first 4 weeks, I recommend weekly CBC and LFT, followed by alternate weekly for the next 4 weeks, and then again at 12 weeks. This is a cautious approach aimed at detecting any leukopenia promptly and before infectious complications can occur. Thiopurine methyltransferase (TPMT) is an enzyme crucial to thiopurine metabolism, levels of which are approximately inversely proportional to the risk of developing leukopenia. If you are able to measure TPMT activity prior to commencing immunomodulator therapy and it is normal, then it is possible to reduce the frequency of initial testing to monthly, although routine bloods are definitely still necessary.

                • How do you monitor CBC and LFT when changing immunomodulator dose?

                                At any stage during therapy, if the thiopurine dose is increased, you need to check a CBC 2 weeks later to check for leukopenia, while dose reductions do not require monitoring blood tests. It is not necessary to check LFT with each dose escalation unless patients develop symptoms suggestive of hepatotoxicity, such as nausea.

                • How do you monitor CBC and LFT when patients are on a stable dose?

                                Leukopenia can occur at any time during thiopurine treatment. Therefore, CBC and LFT are required every 3 months for the duration of therapy. This is often overlooked, but one practical way of ensuring ongoing blood monitoring is to only give immunomodulator prescriptions for 3 months at a time.

                • What CBC and LFT blood results do you aim for?

                                The aim of therapy is immunomodulation without immunosuppression, and I aim for a total white cell count of greater than 3.5 x 109/L with a lymphocyte count greater than 1.0 x 109/L, or occasionally slightly lower. Asymptomatic elevations of hepatic transaminases only require dose reduction once they reach more than twice the upper limit of normal.

Thiopurine Metabolites

Historically, weight-based dosing of immunomodulators has been employed with target doses being 2.0 to 2.5 mg/kg for azathioprine and 1.0 to 1.5 mg/kg for 6-MP. More recently, advances in pharmacogenomics and an increased understanding of thiopurine metabolism has led to the practice of measuring TPMT enzyme activity prior to commencing therapy, and then using measurements of the thiopurine metabolites 6-TGN and 6-MMP to help optimize dosage and efficacy.

Azathioprine and 6-MP are both inactive pro-drugs that are metabolized by 3 competing enzymes, including TPMT, to produce the nucleotide metabolites 6-TGN and 6-MMP (Figure 9-1). TPMT activity within the population has a trimodal distribution with 89% of people possessing normal enzyme activity (homozygous high), 11% having intermediate activity (heterozygotes), and 0.3% having essentially no functional activity of the enzyme (homozygous low), with this last group being susceptible to prompt and potentially severe leukopenia on normal thiopurine doses. Retrospective studies, supported by meta-analyzed data, have shown that 6-TGN is the active metabolite responsible for the therapeutic efficacy and myelotoxicity of thiopurines, while 6-MMP levels have no correlation with efficacy, but instead are associated with hepatotoxicity when elevated. The 6-TGN metabolites appear to exert their immunomodulatory effects through several mechanisms, including via incorporating into lymphocytic DNA and inhibiting cellular replication, and also by way of the 6-TGN triphosphate moiety inducing apoptosis of activated lymphocytes. Numerically, 6-TGN levels of greater than 235 pmol/8 x 108 RBC correlate with an increased likelihood of response, while 6-MMP levels of greater than 5,700 pmol/8 x 108 RBC are associated with potentially dose-limiting hepatotoxicity.1-3

Metabolism of azathioprine and 6-mercaptopurine. XO = xanthine oxidase; TPMT = thiopurine methyltransferase; HPRT = hypoxanthine phosphoribosyltransferase; 6-TIMP = 6-thiosine 5’-monophosphate; IMPDH = inosine monophosphate dehydrogenase; GMPS = guanosine monophosphate synthetase; 6-TGMP = 6-thioguanine mono-phosphate; 6-TGDP = 6-thioguanine di-phosphate; 6-TGTP = 6-thioguanine tri-phosphate

Figure 9-1. Metabolism of azathioprine and 6-mercaptopurine. XO = xanthine oxidase; TPMT = thiopurine methyltransferase; HPRT = hypoxanthine phosphoribosyltransferase; 6-TIMP = 6-thiosine 5’-monophosphate; IMPDH = inosine monophosphate dehydrogenase; GMPS = guanosine monophosphate synthetase; 6-TGMP = 6-thioguanine mono-phosphate; 6-TGDP = 6-thioguanine di-phosphate; 6-TGTP = 6-thioguanine tri-phosphate.

                • Do you need to measure TPMT prior to starting thiopurines?

                                Measuring TPMT prior to therapy allows you to identify the 1 in 300 patients deficient in the functional activity of the enzyme in whom these drugs should be avoided. This is very useful information, and it is recommended that if pharmacogenomic testing is available, it should be undertaken. However, if testing cannot be performed, then thiopurine therapy can still safely be commenced, provided careful blood monitoring is carried out as has been outlined.

                • When do you need to measure thiopurine metabolites?

                                Measuring the nucleotides 6-TGN and 6-MMP is only useful in patients who fail thiopurine treatment and who are unable to enter and maintain a steroid-free remission despite an adequate dose and duration of immunomodulator therapy. Conversely, if your patient is well and tolerating thiopurine therapy, then metabolite measurements do not provide any additional useful clinical information.

                • What do the results mean and how do we use them?

                                Measuring thiopurine metabolites effectively clarifies the reasons patients are not responding to immunomodulator therapy, and it identifies 4 groups of treatment failures (Table 9-2). The first group are patients with negligible or undetectable levels of both metabolites. These patients are likely to be noncompliant and can be questioned and managed accordingly. Confirming compliance before switching from thiopurine therapy is a justifiable reason for ordering metabolite testing. The second group are patients with low but detectable levels of both 6-TGN and 6-MMP. These patients are under-dosed and should tolerate dose escalation. The third group are patients with low 6-TGN levels and high 6-MMP levels. These patients have an unfavorable metabolite profile and are thiopurine “resistant.” Further dose escalation is unlikely to be successful. The last group are patients with therapeutic, or high, levels of both 6-TGN and 6-MMP, and ongoing active disease. This group is truly thiopurine “refractory” and will require a different class of therapeutic agent.4

                • How often do you need to measure thiopurine metabolites?

                                The first set of metabolite measurements, which establishes the patient’s genetically determined thiopurine metabolite profile, is the most important, and these can be performed after approximately 4 weeks of therapy. The confirmation of therapeutic metabolite levels by any subsequent dose adjustments should ideally be determined by a further set of metabolite levels 2 to 4 weeks later. Subsequent metabolite levels are thereafter only necessary if disease remission cannot be established or maintained at any time.

Summary

Immunomodulator therapy with azathioprine and 6-MP is a cornerstone of IBD treatment, and it is used with the expectation of inducing and maintaining a corticosteroid-free remission in otherwise potentially refractory patients. The benefit-to-risk profile of these agents is usually favorable, provided they are used and monitored correctly. Routine blood testing for CBC and LFT is the most important component of monitoring to ensure the safety of these agents, and this is required for the entire duration a patient is taking thiopurine therapy. More recently, metabolite testing has provided additional information clarifying the reasons for an inadequate response to immunomodulators, therefore allowing for dosage optimization. However, this is complementary to and does not replace the need for ongoing routine blood monitoring.

References

1.  Dubinsky MC, Lamothe S, Yang HY, et al. Pharmacogenomics and metabolite measurement for 6-mercaptopurine therapy in inflammatory bowel disease. Gastroenterology. 2000;118(4):705-713.

2.  Osterman MT, Kundu R, Lichtenstein GR, Lewis, JD. Association of 6-thioguanine nucleotide levels and inflammatory bowel disease activity: a meta-analysis. Gastroenterology. 2006;130(4):1047-1053.

3.  Tiede I, Fritz G, Strand S, et al. CD28-dependent Rac1 activation is the molecular target of azathioprine in primary human CD4+ T lymphocytes. J Clin Invest. 2003;111(8):1133-1145.

4.  Gearry RB, Barclay ML. Azathioprine and 6-mercaptopurine pharmacogenetics and metabolite monitoring in inflammatory bowel disease. J Gastroenterol Hepatol. 2005;20(8):1149-1157.