Abstract
cis-3-(9H-Purin-6-ylthio)acrylic acid (PTA) is a structural analog of azathioprine, a prodrug of the antitumor and immunosuppressive drug 6-mercaptopurine (6-MP). In this study, we examined the in vitro and in vivo metabolism of PTA in rats. Two metabolites of PTA, 6-MP and the major metabolite,S-(9H-purin-6-yl)glutathione (PG), were formed in a time- and GSH-dependent manner in vitro. Formation of 6-MP and PG occurred nonenzymatically, but 6-MP formation was enhanced 2- and 7-fold by the addition of liver and kidney homogenates, respectively. Purified rat liver glutathioneS-transferases enhanced 6-MP formation from PTA by 1.8-fold, whereas human recombinant α, μ, and π isozymes enhanced 6-MP formation by 1.7-, 1.3-, and 1.3-fold, respectively. In kidney homogenate incubations, PG accumulation was only observed during the first 15 min because of further metabolism by γ-glutamyltranspeptidase, dipeptidase, and β-lyase to yield 6-MP, as indicated by the use of the inhibitors acivicin and aminooxyacetic acid. Based on these results and other lines of evidence, two different GSH-dependent pathways are proposed for 6-MP formation: an indirect pathway involving PG formation and further metabolism to 6-MP, and a direct pathway in which PTA acts as a Michael acceptor. HPLC analyses of urine of rats treated i.p. with PTA (100 mg/kg) showed that 6-MP was formed in vivo and excreted in urine without apparent liver or kidney toxicity. Collectively, these studies show that PTA is metabolized to 6-MP both in vitro and in vivo and may therefore be a useful prodrug of 6-MP.
Footnotes
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Send reprint requests to: Adnan A. Elfarra, Department of Comparative Biosciences, University of Wisconsin School of Veterinary Medicine, 2015 Linden Dr. West, Madison, WI 53706. E-mail:elfarraa{at}svm.vetmed.wisc.edu
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↵1 This research was supported in part by Grant DK44295 from the National Institute of Diabetes, Digestive and Kidney Diseases.
- Abbreviations:
- 6-MP
- 6-mercaptopurine
- AOAA
- aminooxyacetic acid
- γ-GT
- γ-glutamyltranspeptidase
- GST
- glutathioneS-transferase
- PG
- S-(9H-purin-6-yl)glutathione
- PTA
- cis-3-(9H-purin-6-ylthio)acrylic acid
- TU
- thiouric acid
- Received December 29, 1998.
- Accepted April 20, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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