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Research ArticleArticle

Comparative Contribution to Dextromethorphan Metabolism by Cytochrome P450 Isoforms in Vitro: Can Dextromethorphan Be Used as a Dual Probe for Both CYP2D6 and CYP3A Activities?

Aiming Yu and Robert L. Haining
Drug Metabolism and Disposition November 2001, 29 (11) 1514-1520;
Aiming Yu
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Abstract

Dextromethorphan (DXM) is a widely used probe drug for human CYP2D6 activity both in vitro and in vivo. In humans, DXM is metabolized to dextrorphan (DXO), as well as 3-methoxymorphinan (MEM) and 3-hydroxymorphinan (HYM). The formation of MEM has been attributed primarily to CYP3A4, and the use of DXM has been debated as a simultaneous probe for CYP3A4 and CYP2D6 activities. Recently, we found that highly purified CYP2D6 has significant DXMN-demethylase activity in addition to its well known DXMO-demethylase activity. Therefore, we desired to further compare the contribution to DXM metabolism by individual human cDNA-expressed cytochromes P450, including 2C8, 2C9, 2C18, 2C19, 2D6, 2B6, and 3A4. Metabolites were quantified following separation by high-pressure liquid chromatography and apparent Michaelis-Menten constants determined for the appearance of DXO and MEM. Intrinsic clearance values were estimated for each P450 and normalized using the average percentage content and relative activity factor approaches for comparison. Simplified kinetic models (when [S] ≪Km,Vmax/Km =Vo/[S]) were used at fixed DXM concentrations of 20 (for DXM N-demethylation) and 0.2 μM (for DXM O-demethylation), as well as 2 μM to mimic plasma DXM concentrations in human extensive metabolizers. The results confirm that CYP2D6 contributes at least 80% to the formation of DXO, and CYP3A4 contributes more than 90% to the formation of MEM. All of our in vitro results are consistent and indicate that DXM as a marker for monitoring both CYP2D6 and CYP3A activities is practical in an average human or human liver microsomal preparation.

Footnotes

  • This work was supported by National Institute of Environmental Health Sciences Grant ES09894 and was presented in abstract form at the Gordon Conference on Drug Metabolism, July 2001 in Plymouth, NH.

  • Abbreviations used are::
    DXM
    dextromethorphan
    DXO
    dextrorphan
    MEM
    3-methoxymorphinan
    HYM
    3-hydroxymorphinan
    P450
    cytochrome P-450
    HPLC
    high-pressure liquid chromatography
    DLPC
    l-α-dilaurylphosphatidylcholine
    APC
    average percentage content
    RAF
    relative activity factor
    HLM
    human liver microsome
    • Received May 17, 2001.
    • Accepted August 15, 2001.
  • The American Society for Pharmacology and Experimental Therapeutics
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Drug Metabolism and Disposition: 29 (11)
Drug Metabolism and Disposition
Vol. 29, Issue 11
1 Nov 2001
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Research ArticleArticle

Comparative Contribution to Dextromethorphan Metabolism by Cytochrome P450 Isoforms in Vitro: Can Dextromethorphan Be Used as a Dual Probe for Both CYP2D6 and CYP3A Activities?

Aiming Yu and Robert L. Haining
Drug Metabolism and Disposition November 1, 2001, 29 (11) 1514-1520;

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Research ArticleArticle

Comparative Contribution to Dextromethorphan Metabolism by Cytochrome P450 Isoforms in Vitro: Can Dextromethorphan Be Used as a Dual Probe for Both CYP2D6 and CYP3A Activities?

Aiming Yu and Robert L. Haining
Drug Metabolism and Disposition November 1, 2001, 29 (11) 1514-1520;
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