Background: 6-Mercaptopurine (6-MP) and azathioprine (AZA) have been used in patients with Crohn's disease (CD) and ulcerative colitis (UC) for reducing the dose of steroids and maintaining remission. However, some patients treated with 6-MP/AZA develop bone marrow suppression, one of the most serious side effects. The aim of this study was to evaluate the efficacy and safety of low-dose AZA (0.6-1.2 mg/kg per day) for maintaining remission in patients with UC. We also investigated the relationship between bone marrow suppression and thiopurine methyltransferase ( TPMT) mutation in the Japanese population.
Methods: Study 1. To investigate the frequency of TPMT mutation, findings for 82 patients among 141 patients with UC or CD who were treated with AZA or 6-MP were analyzed retrospectively. Polymerase chain reaction (PCR) methods were used to analyze allele mutations of the TPMT gene. Study 2. A multicenter prospective trial was performed. The subjects were 22 patients with UC with presence of remission for 3 months or more. They were treated with 50 mg/day of AZA, and we evaluated the remission rate at 6 months, adverse side effects, and changes in prednisone doses after the initiation of AZA.
Results: Study 1. Seventy-four (91%) of the 82 patients analyzed had no TPMT mutation, 7 (8%) had one mutant allele, and 1 (1%) had two mutant alleles. Of the total of 141 patients, 4 (44%) of the 9 patients who were treated with 50 mg/day of 6-MP or 100 mg/day of AZA developed bone marrow suppression, although no mutation of TPMT was seen in any of these patients. On the other hand, 8 (6%) of the 132 patients who were treated with 30 mg/day of 6-MP or 50 mg/day of AZA developed bone marrow suppression. Seven of 8 patients (88%) who developed bone marrow suppression with a low dose of AZA had a mutant TPMT allele. Study 2. In the 17 patients who could continue taking low-dose AZA for 6 months, 15 (88%) maintained remission. Of 8 patients treated with low-dose prednisone (5-10 mg/day), 3 patients (38%) could discontinue oral prednisone and 4 (50%) could reduce its dose. Six of the 22 patients (27%) had some adverse side effects. These side effects were ameliorated, or disappeared spontaneously, or disappeared with the discontinuation of AZA.
Conclusions: A dose of 50 mg/day of AZA is effective and safe for maintenance of remission in the Japanese population. Investigation of the TPMT allele may be useful for predicting the appearance of bone marrow suppression, when low-dose 6-MP or AZA is given.