Thiopurine methyltransferase activity combined with 6-thioguanine metabolite levels predicts clinical response to thiopurines in patients with inflammatory bowel disease

Background/aims: 6-Mercaptopurine and its prodrug azathioprine are effective for the treatment of inflammatory bowel disease. Thiopurine methyltransferase is important for the metabolism of thiopurines. However, there is controversy as to the clinical utility of measuring thiopurine methyltransferase enzyme activity and 6-thioguanine nucleotide levels. Our aim was to determine if thiopurine methyltransferase enzyme activity and 6-thioguanine nucleotide level monitoring would predict response to therapy with thiopurines in patients with inflammatory bowel disease. Methods: Baseline thiopurine methyltransferase enzyme activity prior to initiation of therapy with either 6-mercaptopurine or azathioprine was determined in 39 patients with inflammatory bowel disease. The association between clinical response and thiopurine methyltransferase activity and 6-thioguanine nucleotide levels singly or in combination were analysed. Results: Seventeen of 39 patients (44%) responded to 6-mercaptopurine or azathioprine therapy. Thiopurine methyltransferase enzyme activity below the mean of 30.5 U was significantly associated with clinical response. The thiopurine methyltransferase low phenotype was associated with response in 65% vs. 29% in individuals with thiopurine methyltransferase enzyme activity above 30.5 U (p = 0.05). There was no correlation between thiopurine methyltransferase activity and 6-thioguanine nucleotide levels. The maximal 6-thioguanine nucleotide levels did not predict clinical response. When combining thiopurine methyltransferase enzyme activity and 6-thioguanine nucleotide levels, the combination of thiopurine methyltransferase low/6-thioguanine nucleotide high was associated with response in 7/7 (100%) vs. only 2/8 (25%) with the combination of thiopurine methyltransferase high/6-thioguanine nucleotide low (p = 0.01). Conclusions: Thiopurine methyltransferase activity inversely correlated with clinical response to thiopurine treatment in inflammatory bowel disease. Thiopurine methyltransferase enzyme activity below 30.5 U combined with a post-treatment 6-thioguanine nucleotide level >230 pmol/8 × 10⁸ erythrocytes was the best predictor of response.