Cytochromes P450, 1A2, and 2C9 are responsible for the human hepatic O'demethylation of R- and S-naproxen

John O. Miners, Sally Coulter, Robert H. Tukey, Maurice E. Veronese, Donald J. Birkett

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131 Citations (Scopus)


A preliminary report implicated cytochrome P450 (CYP) 2C9 in the human liver microsomal O'demethylation of S-naproxen, suggesting that this pathway may be suitable for investigation of human hepatic CYP2C9 in vitro. Kinetic and inhibitor studies with human liver microsomes and confirmatory investigations with cDNA-expressed enzymes were undertaken here to define the role of CYP2C9 and other isoforms in the O-demethylation of R- and S-naproxen. All studies utilised a newly developed sensitive and specific HPLC assay that measured the respective O-desmethyl metabolites of R- and S-naproxen in incubations of human liver microsomes and in COS cell lysates. Microsomal R- and S-naproxen O-demethylation kinetics followed Michaelis-Menten kinetics, with respective mean apparent Km values of 123 μM and 143 μM. Sulfaphenazole, a specific inhibitor of CYP2C9, reduced the microsomal O-demethylation of R- and S-naproxen by 43% and 47%, respectively, and the CYP1A2 inhibitor furafylline decreased R- and S-naproxen O-demethylation by 38% and 28%, respectively. R,S-Mephenytoin was a weak inhibitor of R- and S-naproxen O-demethylation, but other CYP isoform specific inhibitors (e.g., coumarin, diethyldithiocarbamate, quinidine, troleandomycin) had little or no effect on these reactions. cDNA-expressed CYP2C9 and CYP1A2 were both shown to O-demethylate R- and S-naproxen. Apparent Km values (92-156 μM) for the reactions catalysed by the recombinant enzymes were similar to those observed for human liver microsomal R- and S-naproxen O-demethylation. The data demonstrate that CYP2C9 and CYP1A2 together account for the majority of human liver R- and S-naproxen O-demethylation, precluding the use of either R-or S-naproxen as a CYP isoform-specific substrate in vitro and in vivo.

Original languageEnglish
Pages (from-to)1003-1008
Number of pages6
JournalBiochemical Pharmacology
Issue number8
Publication statusPublished - 26 Apr 1996
Externally publishedYes


  • CYP1A2
  • CYP2C9
  • Cytochrome P450
  • Drug metabolism
  • Human liver
  • Naproxen


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