[Thiopurine methyltransferase activity in inflammatory bowel disease. A study on 7046 Spanish patients].

Servicio de Aparato Digestivo, Hospital Universitario de la Princesa, Universidad Autónoma, Madrid, Spain.
Medicina Clínica (Impact Factor: 1.42). 10/2005; 125(8):281-5.
Source: PubMed


Our objective was to assess the activity of thiopurine methyltransferase (TPMT) in a very large number of Spanish patients with inflammatory bowel disease (IBD), to evaluate the influence of several variables (including azathioprine or 6-mercaptopurine) on that activity, and to know the proportion of patients with low TPMT activity and therefore high risk of myelotoxicity when treated with these drugs.
TPMT activity in red blood cells (RBCs) was measured by a radiochemical method. The association between several variables and TPMT values was assessed by multiple lineal regression.
7046 patients were included (mean age: 37 years; 53% males): 70% with Crohn's disease, 22% with ulcerative colitis, and 8% with indeterminate colitis. Mean TPMT value was 20 (6) U/ml RBCs (minimum 0 and maximum 46). TPMT activity distribution was as follows: low levels (< 5 U/ml), 0.5%; intermediate (5-13.7), 11.1%; and high (> or = 13.8), 88.4%. TPMT values did not follow a normal distribution (p < 0.001). In the univariate study, statistically significant differences (p < 0.001), yet of doubtly clinical significance because its minimal magnitude, were demonstrated in TPMT values depending on age, sex, type of disease, and treatment with azathioprine/6-mercaptopurine. In the multivariate study, the variables associated with TPMT activity were: sex, treatment with 5-aminosalicylates, steroids and azathioprine/6-mercaptopurine.
This study shows that 0.5% of the Spanish patients with IBD have low TPMT activity (< 5 U/ml RBCs), a figure similar to that reported in other countries, these patients being at higher risk of myelotoxicity when treated with azathioprine or 6-mercaptopurine. The drugs usually prescribed for the treatment of IBD, including 5-aminosalicylates and azathioprine/6-mercaptopurine, do not seem to modify in a clinically relevant manner TPMT activity.

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    ABSTRACT: To prospectively evaluate whether a relationship between thiopurine methyltransferase (TPMT) activity and incidence of adverse effects (especially myelotoxicity) exists, in a long-term follow-up study of a large group of patients with inflammatory bowel disease treated with azathioprine. TPMT activity in red blood cells (RBC) was measured by a radiochemical method in 394 consecutive patients with Crohn's disease (238) or ulcerative colitis (156) starting treatment with azathioprine. The relationship among several variables and TPMT values was assessed, and the correlation between such levels and the incidence of adverse effects was evaluated. Mean TPMT value was 18.6 +/- 4 U/mL RBCs (range 9.4-33.7). No patient had low levels (<5), 7.1% had intermediate levels (5-13.7), and 92.9% had high levels (>13.8). Differences (P < 0.001) were demonstrated in TPMT activity depending on the type of inflammatory bowel disease, but not on the remaining variables (including treatment with 5-aminosalycilates). Adverse effects were reported in 74 patients (18.8%), the most frequent being gastrointestinal intolerance (9.1%) and myelotoxicity (4.3%). No patient having adverse effects had low TPMT levels. However, mean TPMT activity was lower in those with adverse effects (16.6 +/- 3 vs 19.1 +/- 4 U/mL, P < 0.001). Moreover, the probability of suffering myelotoxicity in the high TPMT group was only 3.5%, compared with 14.3% in the TPMT intermediate group (95% CI = 1.37-14.9; OR = 4.5). The strategy of determining TPMT activity in all patients prior to initiating treatment with azathioprine could help to minimize the risk of myelotoxicity, as patients with intermediate TPMT activity had fourfold more risk than high TPMT activity patients.
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    ABSTRACT: A therapeutic level of 6-thioguanine nucleotides (6-TGN) has been reported in inflammatory bowel disease (IBD) patients under azathioprine (AZA). We investigated the threshold value of 6-TGN that may be predictive of AZA refractoriness and its impact on safety profile. Patients with normal thiopurine methyltransferase (TPMT) activity (7.5-14 U/mL erythrocytes), suffering from steroid-dependent or active IBD despite AZA use for at least 6 months, were prospectively included. Clinical efficacy, adverse events, and thiopurine metabolite levels were recorded at baseline, 1 month after each dose escalation, and thereafter every 3 months. Fifty-five patients were included (43 with Crohn's disease, 12 with ulcerative colitis). After a mean follow-up of 12 months, 31 patients (56.3%) did not reach clinical remission despite a gradual increase in AZA dose and 6-TGN level of >400 pmol/8 x 10(8) erythrocytes, and were considered refractory to AZA (sensitivity 45%, specificity 100%). Adverse events occurred more frequently in these patients than in responders (42%vs 25%, respectively, P= 0.02). Among 55 patients, 15 cases of myelotoxicity associated with elevated levels of total methylated metabolites (14,500 pmol/8 x 10(8) erythrocytes vs 5,230 pmol/8 x 10(8) erythrocytes in patients without myelotoxicity, P= 0.03) were observed. Patients with total methylated metabolites of >11,100 pmol/8 x 10(8) erythrocytes had an increased risk of developing myelotoxicity (odds ratio [OR] 11.0, 95% confidence interval [CI] 1.1-250, P= 0.05). A 6-TGN level of >400 pmol/8 x 10(8) erythrocytes in IBD patients with normal TPMT activity and steroid-dependent or active disease despite an optimal AZA regimen may predict refractoriness to this drug. Furthermore, high levels of methylated derivatives are associated with an increased risk of myelotoxicity.
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