Thiopurine methyltransferase activity in the inflammatory bowel disease.: A study on 7046 Spanish patients

Background and Objectives: Our objective was to access 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. Patients and Methods: TPMT activity in red blood cells (RBCs) was measured by a radiochemical methods. The association between several variables and TPMT values was assessed by multiple lineal regression. Results: 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 (>= 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 azathloprine/6-mercaptopurine. In the multivariate study, the variables associated with TPMT activity were: sex, treatment with 5-aminosallcylates, steroids and azathioprine/6-mercaptopurine. Conclusions: 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 meyelotoxicity when treated with azathioprine or 6-mercaptopurine. The drugs usually prescribed for the treatment of IBD, including 5-aminosalicylates and azathioprine/6-ercaptopurine, do not seem to modify in a clinically relevant manners TPMT activity.