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. 2017 Apr 27;23(6):946–955. doi: 10.1097/MIB.0000000000001131

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Copyright © 2017 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the Crohn's & Colitis Foundation.

This is an open access article distributed under the Creative Commons Attribution License 4.0 (CCBY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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FIGURE 1. — Schematic outline of thiopurine metabolism. Azathioprine is nonenzymatically cleaved to 6-MP which is metabolized by the enzymes (blue text) inosine-monophosphate dehydrogenase (IMPDH), thiopurine methyltransferase (TPMT), hypoxanthine-guanine phosphoribosyl transferase (HGPRT), inosine tri-phosphatase (ITPase) and guanosine monophosphate synthetase (GMPS). MeMP, methylmercaptopurine; TIMP, thioinosine monophosphate with metabolism to the di- and triphosphates TIDP and TITP, respectively; TXMP, thioxanthine monophosphate; TGMP, thioguanosine monophosphate with metabolism to the di- and triphosphates TGDP and TGTP, respectively, and incorporation into DNA. 6-MP is inactivated by metabolism to thioxanthine then thiouric acid via xanthine oxidase (XO), which is inhibited by allopurinol. Thioxanthine may inhibit TPMT. The molecular mechanism of TGN immunosuppression may include G-protein (rac-1) inhibition.