BACKGROUND: The thiopurines, azathioprine (AZA) and mercaptopurine are extensively used in Crohn's discase (CD). Thiopurine bioactivation can be diverted by either thiopurine methyltransferase (TPMT), or by xanthine oxidase/dehydrogenase (XOD) which forms 6-thiouric acid (6TU). AIM: To investigate whether chronic inflammation could influence small intestinal XOD activity using urinary excretion of 6TU as a surrogate marker of XOD activity. METHODS: 6-Thiouric acid excretion was compared between 32 CD patients and nine dermatology patients (control group), on AZA. Six CD patients were interesting: five with low TPMT activity (one deficient, four intermediate), and one receiving AZA/allopurinol co-therapy. RESULTS: There was no statistical difference in 6TU excretion between the CD and control group. CD location, severity or surgery did not affect excretion. The TPMT-deficient patient excreted 89% of daily AZA dose as 6TU, but excretion by TPMT carriers was essentially normal. Concurrent 5-aminosalicylic acid therapy increased 6TU excretion significantly (median 32.9%), consistent with inhibiting TPMT. 6TU was undetectable in the patient on AZA/allopurinol co-therapy. CONCLUSIONS: The results refuted our hypothesis, but fitted a model where most of an oral thiopurine dose effectively escapes first-pass metabolism by gut XOD, but is heavily catabolized by TPMT. Bioavailability of thiopurines may be competitively inhibited by dietary purines.
BACKGROUND: The thiopurines, azathioprine (AZA) and mercaptopurine are extensively used in Crohn's discase (CD). Thiopurine bioactivation can be diverted by either thiopurine methyltransferase (TPMT), or by xanthine oxidase/dehydrogenase (XOD) which forms 6-thiouric acid (6TU). AIM: To investigate whether chronic inflammation could influence small intestinal XOD activity using urinary excretion of 6TU as a surrogate marker of XOD activity. METHODS:6-Thiouric acid excretion was compared between 32 CDpatients and nine dermatology patients (control group), on AZA. Six CDpatients were interesting: five with low TPMT activity (one deficient, four intermediate), and one receiving AZA/allopurinol co-therapy. RESULTS: There was no statistical difference in 6TU excretion between the CD and control group. CD location, severity or surgery did not affect excretion. The TPMT-deficientpatient excreted 89% of daily AZA dose as 6TU, but excretion by TPMT carriers was essentially normal. Concurrent 5-aminosalicylic acid therapy increased 6TU excretion significantly (median 32.9%), consistent with inhibiting TPMT. 6TU was undetectable in the patient on AZA/allopurinol co-therapy. CONCLUSIONS: The results refuted our hypothesis, but fitted a model where most of an oral thiopurine dose effectively escapes first-pass metabolism by gut XOD, but is heavily catabolized by TPMT. Bioavailability of thiopurines may be competitively inhibited by dietary purines.
Authors: Sophie Gibbings; Nancy D Elkins; Hillary Fitzgerald; Janice Tiao; Mari E Weyman; Gayle Shibao; Mehdi A Fini; Richard M Wright Journal: J Biol Chem Date: 2010-11-08 Impact factor: 5.157
Authors: B Jharap; Nkh de Boer; Rm Vos; K Smid; A Zwiers; Gj Peters; Cjj Mulder; Aj Wilhelm; Aa van Bodegraven Journal: Br J Pharmacol Date: 2011-06 Impact factor: 8.739