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Revista Española de Enfermedades Digestivas

Print version ISSN 1130-0108

Rev. esp. enferm. dig. vol.109 n.1 Madrid Jan. 2017 



Mercaptopurine and inflammatory bowel disease: the other thiopurine



Fernando Bermejo1, Alicia Algaba1, Sergio López-Durán2, Iván Guerra1, Marta Aicart-Ramos2, María Hernández-Tejero1, Elena Garrido2, María de-Lucas1, Daniel Bonillo1 and Antonio López-Sanromán2

1Department of Gastroenterology. Hospital Universitario de Fuenlabrada. Fuenlabrada, Madrid. Spain.
2Department of Gastroenterology. Hospital Universitario Ramón y Cajal. Madrid, Spain





Background: Data about use and effectiveness of mercaptopurine in inflammatory bowel disease are relatively limited.
Aims: To assess the possible therapeutic indications, efficacy and safety of mercaptopurine as an alternative to azathioprine in inflammatory bowel disease.
Methods: Retrospective observational study in patients treated with mercaptopurine in a total cohort of 1,574 patients with inflammatory bowel disease.
Results: One hundred and fifty-two patients received mercaptopurine, 15.7% of these patients as an initial thiopurine, 5.3% after azathioprine failure, and 79% after azathioprine intolerance. In 52.6% of patients (n = 80), adverse effects of mercaptopurine occurred, resulting in withdrawal in 49 of them. Mercaptopurine was effective in 39% of cases (95% CI 31-48%). In the remaining patients, failure was due mainly to withdrawal due to side effects (55.1%) and therapeutic step-up (33.7%). The average total time of mercaptopurine exposure was 36 months (IQR: 2-60). Myelotoxicity with mercaptopurine was more common in patients with intermediate TPMT activity than in those with normal activity (p = 0.046).
Conclusions: In our setting, mercaptopurine is primarily used as a rescue therapy in patients with azathioprine adverse effects. This could explain its modest efficacy and the high rate of adverse effects. However, this drug is still an alternative in this group of patients, before a therapeutic step-up to biologics is considered.

Key words: Mercaptopurine. Immunosuppressant drugs. Crohn's disease. Ulcerative colitis.



Thiopurines are immunomodulatory drugs that are widely used in the treatment of inflammatory bowel disease (IBD). Both azathioprine (AZA) and mercaptopurine (MP) are effective in maintaining remission (1,2). Azathioprine is a prodrug that is converted by the liver into MP (3). Subsequently, processing continues and the metabolic activity of the enzyme thiopurine methyltransferase (TPMT) largely determines the extent to which MP is metabolized.

The main factor limiting the use of thiopurine is the possible occurrence of adverse effects in a subset of patients (4). MP is not the first choice thiopurine in our setting, and is used less frequently than AZA (5). However, some of the side effects that can potentially occur during treatment with these drugs, such as hepatotoxicity or digestive intolerance, can be resolved using MP instead of AZA (6,7), which offers the possibility of optimizing thiopurine therapy in AZA-intolerant or AZA-unresponsive patients in which thiopurine therapy is still a good option.

Current data on the therapeutic indications and effectiveness of MP are relatively limited. Therefore, our objectives included: a) identifying key situations in which MP is used as an immunomodulatory therapy in IBD, either as first choice thiopurine or following AZA discontinuation; b) assessing its effectiveness and tolerance by analyzing related adverse events; and c) determining whether AZA-related adverse events recur after MP exposure.




This was a retrospective observational study that included all patients treated with MP over a 15 year period (2000-2014) in a cohort of 1,574 patients with IBD, included in the databases of two referral hospitals. This cohort included 851 patients with Crohn's disease (CD), 676 with ulcerative colitis (UC), and 47 patients with IBD unclassified, according to classic criteria (8).

The following epidemiological and clinical data were collected for each patient: age, sex, active tobacco exposure, type of IBD, extent of IBD according to the Montreal classification (9), reason for thiopurine therapy, TPMT phenotype (U/mL erythrocytes), time lapse from the IBD diagnosis to the first thiopurine exposure, reason for MP monotherapy use (first choice vs AZA adverse effects), total time of MP exposure, MP dose, type of AZA adverse effects (if any) that lead to switching to MP, recurrence of these adverse effects, efficacy of MP, reason for MP discontinuation, MP adverse effects, and concomitant medication.

The study was reviewed and approved by the Research Ethics Committee at the Hospital Universitario de Fuenlabrada. All study participants, or their legal guardian, provided informed written consent prior to study enrolment.

Treatment efficacy and side effects

The dose of mercaptopurine was adjusted by weight, according to routine clinical practice, and therapy was started with the full dose. Monotherapy with MP was considered as effective in patients not requiring a pharmacological therapy step-up, or a surgical procedure to achieve disease control. Therapy was monitored according to the commonly used recommendations. Regarding adverse effects, myelotoxicity was described as any degree of significant anemia, leukopenia or thrombocytopenia, alone or in combination, which was clinically attributed to this cause. Leucopenia was defined as a leucocyte count below 3 x 109/L and neutropenia, as a neutrophil count < 1.5 x 109/L. Regarding hepatotoxicity, and following the recommendations issued by the Council for International Organizations of Medical Sciences (CIOMS) (10), liver damage or hepatotoxicity was defined as an increase in transaminases (ALT, AST), alkaline phosphatase, gamma-glutamyl transferase (GGT) or total bilirubin more than twice the upper normal limit, with the exclusion of other causes by appropriate serological tests and abdominal ultrasound. Pancreatitis was diagnosed when observing an episode with typical abdominal pain, associated with elevated amylase or lipase > 3 times the upper limit of the normal value. For the diagnosis of digestive intolerance, the onset of nausea, vomiting and/or abdominal pain coinciding with drug exposure, and ceasing upon discontinuation, was considered. Other adverse effects included general malaise, fever, myalgia, arthralgia, rash, headache, severe infections, and tumors.

Thiopurine methyltransferase

Thiopurine methyltransferase (TPMT) activity in erythrocytes was measured in 110 patients by a previously described radiochemical method (11). This procedure is based on the conversion of 6-MP to 6-methylmercaptopurine using 3H-S-adenosyl-L-methionine (3H-SAM) as a methyl donor. Ten mL of whole blood were obtained in lithium heparin tubes, chilled and sent for further analysis. The enzyme activity was expressed in international units (nmol of 6-methylmercaptopurine formed per hour) per milliliter of packed erythrocytes (U/mL erythrocytes). TMPT values were considered to be low at < 5 U/mL, intermediate between 5 and 13.7 U/mL and normal > 13.7 U/mL (12).

Statistical analysis

Frequencies and percentages with their corresponding 95% confidence intervals were calculated for categorical variables. With regard to quantitative variables, these were expressed as mean ± standard deviation or median and interquartile range when the variable did not follow a normal distribution. The Kolmogorov-Smirnov test was used to assess the normality of continuous variables. The comparison between categorical variables was performed using the Chi-squared test. The comparison between quantitative variables and categorical variables of two groups was performed using the Student's t test or the Mann-Whitney U test when the variable did not follow a normal distribution. Comparisons between quantitative and qualitative variables with three or more categories were made via the analysis of variance test (ANOVA) or by the nonparametric Kruskal-Wallis. A value of p < 0.05 was considered as statistically significant.



A total of 61% (n = 961) of the cohort of 1,574 patients had been treated with thiopurines at some point. Of these, 152 (51% women, 25.9% smokers, median age 48 years [IQR: 38-59]) received treatment with MP (98 CD, 52 UC, 2 IBD unclassified), which represents 9.6% (95% CI 8.2-11.1%) of the total cohort, and 15.8% (13.5-18.2%) of all patients exposed to thiopurines. Table I describes the baseline characteristics of the patients treated with MP, 50% of them with an inflammatory phenotype. The mean dose of MP was 1.4 ± 0.3 mg/kg/day.



In patients treated with MP, this drug was chosen as the initial thiopurine in 15.7% of cases (n = 24), whereas in 5.3% (n = 8) MP was prescribed after AZA failure (steroid withdrawal was not possible in steroid-dependent patients) and in 79% (n = 120) after the appearance of AZA-related adverse effects, including digestive intolerance (n = 48), hepatotoxicity (n = 46), myelotoxicity (n = 3), pancreatitis (n = 2) and other adverse effects (n = 31).

The efficacy of treatment with MP was similar in patients receiving MP as an initial immunomodulatory therapy compared to patients in which MP was prescribed as a second option (52.3% vs 37.6% efficiency, p = 0.2). Regarding safety, the development of AE related to MP was more frequent in the group of patients who began MP after AZA than in patients receiving MP as a first immunomodulator therapy (56.2% vs 31.87%, p = 0.03).

In 80 of the 152 patients treated with MP (52.6%; 47 CD, 32 UC, 1 CI) adverse effects appeared with this drug (Table II), notably myelotoxicity (n = 28), hepatotoxicity (n = 23) and digestive intolerance (n = 20). Forty-nine of these cases required withdrawal from treatment (32.2% of those treated with MP, 13 patients due to digestive intolerance, 12 due to myelotoxicity, 11 after hepatotoxicity, 3 due to pancreatitis and 10 because of other different adverse effects). The mean dose of MP in patients who developed adverse events with MP was 78 ± 29 mg vs 84.7 ± 28.3 mg in non-affected patients (p = 0.16). Table III shows an analysis of different demographic factors and stratified phenotypic factors with regard to tolerance or non-tolerance in the MP patients. In 35 of the patients with adverse effects to MP, the adverse effect was identical to that previously suffered with AZA: 14 suffered hepatotoxicity, 12 had digestive intolerance, 8 suffered various idiosyncratic reactions (including two with pancreatitis) and one, myelotoxicity. No patient had serious consequences and the adverse effect was completely reversible in all cases. In 32 cases the adverse effect with MP was different from the one patient that had previously presented with AZA, and 13 patients suffered adverse effects with MP, without previous AZA therapy.




Regarding the efficacy of MP, this reached 39% of cases (95% CI 31-48; n = 58), with 41% efficacy (30.3-51.4%) in CD and 36.5% efficacy (22.5-50.6%) in UC (p = 0.76). In patients with MP failure, this was due to the discontinuation of treatment as a result of adverse effects (55.1%, n = 49), a need to step up drug treatment (33.7%, n = 30), need for surgery (10.1%, n = 9) or drug supply problems (1.1%, n = 1) (Fig. 1). The median overall treatment time with MP was 26 months (IQR: 2.0-60.0); during treatment, 31 patients (20%) had at least one hospitalization due to the disease. The median average MP treatment duration was five months (IQR: 1.0-42.0) in the group of patients with adverse effects and 36.5 months (IQR: 18.5-36.2) in the group without adverse effects. In patients with idiosyncratic adverse events with MP, treatment was permanently withdrawn in all cases (including pancreatitis, myalgia-arthralgia, febrile syndrome, rash, and vasculitis).



TPMT activity was available in 110 cases treated with MP, and the average value was 18.6 ± 5.8 U/mL. Values were intermediate (5-13.8 U/mL) in 19% (n = 22), and none of the MP patients had low TMPT levels (< 5 U/mL). No differences in TPMT phenotype were found when comparing patients developing MP-related adverse effects and those who did not (18 ± 6 vs 19 ± 5.6; p = 0.32). No significant differences were found when comparing the overall proportions with the range of TPMT activity. Fourteen of the 22 patients with intermediate TMPT did not tolerate MP, compared to 50 of 88 (56.8%) who had normal TMPT (p = 0.63). Bone marrow toxicity was the most frequent cause for discontinuing MP (14 of 47 patients withdrew from treatment for this reason) and this adverse effect was more common in patients with intermediate TPMT activity than in those with normal activity: of 23 patients who suffered myelotoxicity, 8 (36%) had TPMT with intermediate values and only 15 (17%) had normal values (p = 0.046). On the other hand, there was a non-significant trend for hepatotoxicity to occur in patients with intermediate TPMT activity compared with patients with normal activity (4.5% vs 20.4%; p = 0.07, respectively). Finally, no significant differences were found when comparing the proportions in patients with gastric intolerance (17.0% of cases with intermediate TPMT vs 13.6% with normal TPMT, p = 0.7).



Thiopurines are used in various clinical settings to maintain remission in IBD patients (13,14), notably corticosteroid dependence. They are also useful in other settings (15,16), such as maintaining remission achieved with drugs like cyclosporine or infliximab in patients with steroid-refractory disease, or in the prophylaxis of CD recurrence (1,16). In our experience, MP is used mainly as a rescue therapy in IBD patients who developed adverse effects with AZA, which explains the modest efficacy (39%) obtained by the drug in our patients. However, nearly four in ten patients who use it avoid step-up therapy and achieve control of IBD, which is an acceptable result in this scenario.

Thiopurines have been used for decades, so there is extensive clinical experience regarding potential adverse events and follow-up performed in patients treated with these drugs (17). Adverse events occur in one out of 4-5 patients, with an annual risk of 7% per patient-year of treatment (17,18). The most common side effect is digestive intolerance with nausea/vomiting, followed by hepatotoxicity, myelotoxicity and acute pancreatitis, many of which are serious enough to discontinue treatment (13). Some of these adverse events have an allergic or idiosyncratic basis, while others are dose-dependent. These idiosyncratic adverse events include fever, myalgia, arthralgia, skin rash and pancreatitis (19). The main dose dependent adverse effects are myelosuppression and digestive intolerance. Hepatotoxicity may follow a dose dependent or idiosyncratic pattern. Administration of MP instead of AZA makes less sense in patients who develop idiosyncratic reactions since replacement almost inevitably leads to the recurrence of adverse effects. Most authors agree on the appearance of a cross-reaction after re-exposure to the drug related to acute pancreatitis (19,20). In our series, we observed a tendency for patients who have had IBD for less time and patients with inflammatory CD to have a greater probability of adverse effects to MP. On the other hand, patients with more aggressive phenotypes (stenosing, fistulising) whose frequency increases with a longer course of the disease (21) seem to tolerate better the treatment with MP, although a larger sample size would be needed to confirm this. Some authors have described an increased risk of adverse effects with thiopurines in female patients (18,22), which was not observed in our series.

Although there is no head-to-head study on the toxicity of the two thiopurines, there are data in the literature implying that the MP can be well tolerated in patients with AZA toxicity, especially if there is hepatotoxicity or digestive intolerance. Kennedy et al., in a cohort of 149 patients, showed that the MP was tolerated by 58% of patients previously intolerant to AZA. The authors conducted a meta-analysis of published studies that included 455 patients intolerant to AZA of which 68% tolerated MP, especially in cases with digestive intolerance to AZA (62%) or hepatotoxicity (81%) (23). In a previous study by our group, 87% of patients with liver toxicity with AZA eventually tolerated MP, most of them at full doses, without further toxicity after prolonged mean follow-up (close to three years) (6,24). Hindorff et al. described a series of 17 patients with AZA-induced liver toxicity, 71% of whom tolerated MP at an equivalent dose with recurrent toxicity observed in only two cases. Several studies have been conducted to assess the clinical tolerance of MP in patients with prior azathioprine digestive intolerance, and they showed that the MP may finally be tolerated in more than half of patients (7,18,24,25). Domenech et al. showed that about three-quarters of patients with digestive intolerance to AZA such as nausea, vomiting or abdominal pain, usually of early onset, can tolerate MP and reach the therapeutic target (7).

The existence of genetic polymorphisms of the TPMT enzyme that determine the activity level, which can be measured, has focused interest on this enzyme. Approximately 11% of patients have intermediate activity (heterozygous) and 0.3% have negligible-low TPMT activity (deficient homozygotes) (26). In our study there were no patients with low TPMT since we do not prescribe thiopurines in such patients. We found no relationship between TPMT activity and the occurrence of adverse effects to MP in the group of patients in whom we performed this analysis, a similar finding to that described by Kennedy et al. (23), but contradictory to the results of Hindorff et al. (24), who reported that only 18% of cases with intermediate TPMT tolerated treatment with MP.

Patients with low TPMT have a greater risk of myelotoxicity (27). However, myelotoxicity has many causes, and is not only associated with TPMT enzyme activity. In fact, most patients with myelotoxicity induced by thiopurines have a normal TPMT activity or genotype (28). In our series, myelotoxicity with MP was more common in patients with intermediate TPMT activity than in those with normal activity. Ansari et al. reported in a prospective study that myelosuppression was a more common side effect in patients with the heterozygous TPMT genotype. They also reported the existence of an association between heterozygous status for TPMT and other adverse effects such as digestive intolerance (29), which was not observed in our series.

The use of MP as a rescue therapy due to adverse effects with AZA in most of the patients in our series (MP was the first-line thiopurine in only 15% of the cases) was the basis of the results we obtained: a high percentage of patients with adverse effects due to MP (53%) and a low frequency of idiosyncratic reactions to the drug. The replacement of AZA by MP was conducted in patients with digestive intolerance and liver toxicity caused by AZA in most cases in our study. This was performed in only a small percentage of cases with other adverse effects such as myelotoxicity, a dose dependent adverse effect but often manageable with dose adjustment of AZA, or idiosyncratic reactions due to AZA (fever, myalgia, pancreatitis, etc.). The appearance of these effects usually prompts the clinician to try other treatment options such as methotrexate or biologics.

In our experience, in approximately one-quarter of patients with digestive intolerance to AZA and in a third of cases with AZA-induced hepatotoxicity, these adverse effects reappeared with the shift to MP. On the other hand, most of the adverse effects of idiosyncratic origin reappear after replacement of AZA by MP, and indicate the definite withdrawal of thiopurines. Whenever the adverse effect reappeared, it had no serious consequences and was completely reversible. In the study by Kennedy et al. among patients who discontinued MP due to adverse effects, 59% experienced the same side effects they had with AZA (23). In any case the use of MP avoids the rise of therapeutic step-up in a high percentage of cases of hepatotoxicity and digestive intolerance due to AZA. The failure of thiopurine therapy in clinical scenarios in which it is used raises the possibility of a therapeutic step-up to biological agents, whose economic cost per patient is much higher. Some patients can be managed with methotrexate; however, as this is a teratogenic drug, its use is limited in young patients with IBD who wish to have children. It is therefore important to optimize the therapeutic option with thiopurines before completely removing them for a particular patient (30). The dose adjustment of AZA or switching to MP can obtain a therapeutic effect, without the need to step-up the treatment schedule.

Our study has some limitations, such as the retrospective nature of the data collection. However, the number of patients included is greater when compared to previous studies analyzing the adverse effects of MP treatment. It provides information on a scenario unlikely to occur when performing controlled clinical trials. Moreover, we did not use thiopurine metabolites to analyze adverse events or to optimize clinical efficacy (31). Monitoring the levels of metabolites may be useful in some cases of dose-dependent toxicity (32). Unfortunately, we did not analyze metabolites, although this determination can never replace regular clinical and biochemical controls.

We can conclude that, in our setting, MP is mainly used as a rescue therapy in IBD patients with adverse effects to AZA, which explains the modest efficacy and a high rate of adverse effects. However, this drug is still an alternative in this group of patients before considering therapeutic step-up to biological agents. Practice patterns in many countries have led to the use of AZA as the first thiopurine in the majority of cases, but MP is better tolerated in some groups of patients.



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Fernando Bermejo.
Department of Gastroenterology.
Hospital Universitario de Fuenlabrada.
C/ Camino del Molino, 2.
28942 Fuenlabrada, Madrid. Spain

Received: 25-07-2016
Accepted: 25-09-2016