title: Glutathione-dependent metabolism of cis-3-(9H-purin-6-ylthio)acrylic acid to yield the chemotherapeutic drug 6-mercaptopurine: evidence for two distinct mechanisms in rats.
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 glutathione S-transferases enhanced 6-MP formation from PTA by 1.8-fold, whereas human recombinant alpha, mu, and pi 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 gamma-glutamyltranspeptidase, dipeptidase, and beta-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.
authors.affiliation: Department of Comparative Biosciences and Environmental Toxicology Center, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: S
authors.initials: S
authors.lastname: Gunnarsdottir
authors.forename: A A
authors.initials: AA
authors.lastname: Elfarra
mesh_terms: D000179:Acrylates / Q000378:metabolism* / Q000493:pharmacokinetics; D000625:Aminooxyacetic Acid / Q000494:pharmacology; D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000378:metabolism* / Q000493:pharmacokinetics; D013437:Carbon-Sulfur Lyases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004150:Dipeptidases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005978:Glutathione / Q000378:metabolism* / Q000494:pharmacology; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D007555:Isoxazoles / Q000494:pharmacology; D007668:Kidney / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008297:Male; D015122:Mercaptopurine / Q000378:metabolism* / Q000493:pharmacokinetics; D011355:Prodrugs / Q000378:metabolism* / Q000493:pharmacokinetics; D011687:Purines / Q000378:metabolism* / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: C121224:3-(9H-purin-6-ylthio)acrylic acid; D000179:Acrylates; D000964:Antimetabolites, Antineoplastic; D004791:Enzyme Inhibitors; D007555:Isoxazoles; D011355:Prodrugs; D011687:Purines; D000625:Aminooxyacetic Acid; D015122:Mercaptopurine; D005982:Glutathione Transferase; D004150:Dipeptidases; D013437:Carbon-Sulfur Lyases; C019969:S-alkylcysteine lyase; D005978:Glutathione; C006303:acivicin