Case ReportsSevere hepatotoxicity with high 6-methylmercaptopurine nucleotide concentrations after thiopurine dose escalation due to low 6-thioguanine nucleotidesGardiner, Sharon J.b; Gearry, Richard B.a c; Burt, Michael J.c; Ding, Steven L.c; Barclay, Murray L.a b cAuthor Information Departments of aMedicine bClinical Pharmacology cGastroenterology, Christchurch Hospital and University of Otago, Christchurch, New Zealand Correspondence to Dr Richard Gearry, PhD, Department of Medicine, University of Otago, Christchurch, PO Box 4345, Christchurch, New Zealand E-mail: [email protected] Received 17 January 2008 Accepted 4 March 2008 European Journal of Gastroenterology & Hepatology: December 2008 - Volume 20 - Issue 12 - p 1238-1242 doi: 10.1097/MEG.0b013e3282ffda37 Buy Metrics Abstract Azathioprine and its initial metabolite, 6-mercaptopurine (6-MP), are associated with high rates of treatment cessation due to toxicity or inadequate response. Individualization of thiopurine dose based on concentrations of the active 6-thioguanine nucleotide (6-TGN) metabolites can help improve outcomes with this class. Some individuals, however, preferentially metabolize thiopurine drugs to the potentially hepatotoxic 6-methylmercaptopurine nucleotide (6-MMPN) metabolites rather than the 6-TGNs. For these patients, escalation in thiopurine dose is not likely to increase 6-TGN concentrations sufficiently but may lead to a disproportionate increase in exposure to the 6-MMPNs. We present three cases in whom thiopurine dose escalation based on clinical status and low 6-TGN concentrations (100–262 pmol/8×108 RBC) resulted in severe hepatotoxicity (liver failure in two cases) associated with unrecognized extremely high 6-MMPN concentrations of 26 000–40 000 pmol/8×108 RBC. These cases illustrate a risk with thiopurine dose adjustment based on monitoring of 6-TGN metabolites without also monitoring 6-MMPN. © 2008 Lippincott Williams & Wilkins, Inc.