Background: The conventional thiopurines azathioprine and mercaptopurine are considered maintenance immunosuppressive drugs of choice in the treatment of inflammatory bowel disease (IBD). Unfortunately, treatment is often discontinued because of adverse events (AEs) or refractoriness, retrospectively associated with the high levels of the thiopurine metabolites 6-methylmercaptopurine ribonucleotides (6-MMPR). Patients with a clinically "skewed" thiopurine metabolism may be particularly at risk for therapy failure. We determined the predictive value of this pharmacological phenomenon in patients with IBD during regular thiopurine therapy.
Methods: Clinical effectiveness and tolerability of weight-based thiopurine therapy were determined in all patients with IBD displaying a skewed metabolism [ratio 6-MMPR/6-thioguanine nucleotide (6-TGN) >20]. All samples were routinely assessed between 2008 and 2012, as part of standard clinical follow-up after initiation of conventional thiopurine therapy.
Results: Forty-one (84%) of 49 included patients with IBD discontinued thiopurines (55% female, 53% with Crohn disease) with a median duration of 14 weeks (range, 7-155). The majority of patients with a skewed metabolism discontinued thiopurines because of adverse events (55%) or refractoriness (12%). The most commonly observed adverse event was hepatotoxicity (18 patients, 37%). Median 6-TGN level was 159 pmol/8 × 10 RBC (range, 46-419), median 6-MMPR level was 11,020 pmol/8 × 10 RBC (range, 3610-43,670), and the median 6-MMPR/6-TGN ratio was 72 (range, 29-367). Thiopurine therapy failure was associated with a ratio above 50 (P < 0.03). Hepatotoxicity occurred more frequently in patients with an extremely skewed metabolism (6-MMPR/6-TGN ratio >100) (P < 0.01).
Conclusions: This study demonstrates that a routinely established skewed metabolism is a major risk factor for future thiopurine failure in patients with IBD. These observations imply that routine thiopurine metabolite measurements may be used as a prognostic tool to identify those patients with an aberrant-skewed metabolism at an early stage, possibly benefitting from therapy adjustments.