Thiopurine methyltransferase activity and the use of azathioprine in inflammatory bowel disease.

BACKGROUND: Azathioprine therapy is discontinued in one-third of patients with inflammatory bowel disease because of toxicity or a lack of clinical response. Patients with thiopurine methyltransferase (TPMT) deficiency are intolerant to azathioprine, whilst carriers are at increased risk of side-effects. AIM: To evaluate the importance of TPMT activity in the management of azathioprine therapy in inflammatory bowel disease. METHODS: Clinical response, adverse effects and haematological parameters were determined and correlated with TPMT enzyme activity and genotype in 106 patients with inflammatory bowel disease. RESULTS: Ninety-six patients had high TPMT activity, and 10 had intermediate activity. Nineteen patients (18%) were intolerant to azathioprine. Fifteen (16%) of those with high TPMT activity were intolerant, compared with five (50%) with intermediate activity [odds ratio (OR), 5.4; 95% confidence interval (CI), 1.5-19.8]. Complete remission was achieved in 63% of cases, and complete or partial remission in 79%. Interestingly, very high TPMT activity (> 14 units/mL red blood cells) was significantly associated with non-response, irrespective of the time on azathioprine (OR, 0.21; 95% CI, 0.07-0.68). TPMT gene mutations correlated with TPMT activity. CONCLUSIONS: Inflammatory bowel disease patients with intermediate TPMT activity have an increased risk of azathioprine toxicity. Conversely, very high TPMT activity predicts treatment failure. TPMT genotype predicted TPMT phenotype in this study.

Demonstration of thiopurine methyltransferase activity in the erythrocytes of cats.

Azathioprine is a purine analogue used as an immunosuppressive and immunomodulator agent in various mammals, including cats. Several adverse reactions have been reported and have limited the use of the drug in the cat. Adverse reactions to azathioprine in humans have been correlated with reduced activity of thiopurine methyltransferase (TPMT) in erythrocytes. The purpose of this preliminary study was to determine if cats have TPMT activity in their erythrocytes and to compare the values obtained with the normal range for humans and the normal range for dogs in a preliminary report. Activity of the enzyme was measured in blood samples drawn from 41 cats. Blood also was taken from 5 dogs. The mean erythrocyte TPMT activity in the cats was 2.4 +/- 0.4 nmol (range, 1.2-3.9 nmol) per hour per milliliter of red blood cells (U/mL RBC) or 2-8 nmol per hour per gram of hemoglobin (U/g Hb). This range was far lower than the normal human range (8-15 U/mL RBC; 16-33 U/g Hb) and was of monopolar distribution. This observation apparently precludes any diagnostic purpose in assaying erythrocyte TPMT in this species. Erythrocyte TPMT activity in the 5 dogs ranged from 5.5 to 13.1 U/mL RBC (11-27 U/g Hb), which was comparable with normal and carrier ranges for humans, but proof of TPMT genetic polymorphism in either species will require genotyping studies.