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Clin Pharmacol Ther. 1997 Jul;62(1):21-8.

The effect of omeprazole pretreatment on acetaminophen metabolism in rapid and slow metabolizers of S-mephenytoin.

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Department of Pharmacology, Faculty of Medicine, University of British Columbia, Vancouver, Canada.


Omeprazole, a widely used and potent gastric proton pump inhibitor, induces cytochrome P450 (CYP) 1A2 in humans. Induction is most pronounced in slow metabolizers of S-mephenytoin because CYP2C19 (S-mephenytoin hydroxylase) is responsible for the elimination of omeprazole. Acetaminophen (INN, paracetamol), a widely used and effective analgesic and antipyretic agent, causes serious hepatic and renal toxicity at high doses by conversion of acetaminophen to the toxic intermediate N-acetyl-p-benzoquinone imine (NAPQI) through CYP1A2, CYP2E1, and CYP3A4. This study evaluated whether omeprazole pretreatment in five rapid and five slow metabolizers of S-mephenytoin could increase thioether (an estimate of NAPQI production) metabolite formation from acetaminophen. The results of this study show that, despite induction of CYP1A2 activity in slow metabolizers (a 75% increase in plasma clearance of caffeine), the formation of NAPQI from acetaminophen was not increased after 7 days of omeprazole administration (40 mg/day). This suggests that induction of CYP1A2 activity by omeprazole is unlikely to increase the risk of acetaminophen hepatotoxicity.

[Indexed for MEDLINE]

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