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

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.