Limitations of dextromethorphan N-demethylation as a measure of CYP3A activity

Pharmacogenetics. 1999 Aug;9(4):453-62.


We evaluated the utility of the 3-methoxymorphinan/dextromethorphan (3MM/DM) urinary ratio to reflect baseline CYP3A activity, and its ability to discriminate moderate CYP3A inhibition during fluvoxamine therapy. For 4 months, oral dextromethorphan 30 mg and intravenous midazolam 0.025 mg/kg were administered to nine men every 14 days, and to 10 premenopausal women during the follicular and luteal phases of their menstrual cycles. Phenotyping during the first 3 months or cycles established baseline CYP3A activity. During the fourth month, individuals were given fluvoxamine 150 mg/day. CYP3A activity was expressed as both the urinary 3MM/DM molar ratio and midazolam plasma clearance (MDZ CL). 3MM/DM ratios were independent of dextromethorphan CYP2D6 phenotype (r = 0.13, P = 0.6). Intraindividual variability in baseline CYP3A activity (median, 25-75th percentile), as determined by coefficients of variation, was 48.3% (36.8-68.8%) for 3MM/DM and 10.3% (8.3-11.8%) for MDZ CL. No significant correlation between 3MM/DM and MDZ CL either at baseline (r = -0.22, P = 0.4) or during fluvoxamine therapy (r = -0.15, P = 0.6) was noted. With fluvoxamine 150 mg/day, median percentage change in the 3MM/DM ratios was -50.0% (-105.6-6.0%; P = 0.7), and median percentage change in MDZ CL was -33.7% (-27.0-39.3%; P < 0.0001). Only MDZ CL consistently indicated moderate inhibition of hepatic CYP3A activity. In addition, there was a lack of correlation between the magnitudes of fluvoxamine-induced change in 3MM/DM and MDZ CL (r = 0.41, P = 0.1). The large intraindividual variability of the 3MM/DM urinary ratio, as well as the inability to discriminate moderate CYP3A inhibition, makes this a suboptimal method for accurately assessing CYP3A activity.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Aryl Hydrocarbon Hydroxylases*
  • Cytochrome P-450 CYP2D6 / genetics
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme Inhibitors
  • Cytochrome P-450 Enzyme System / metabolism*
  • Dextromethorphan / analogs & derivatives
  • Dextromethorphan / metabolism*
  • Dextromethorphan / urine
  • Enzyme Inhibitors / pharmacology
  • Female
  • Genotype
  • Humans
  • Male
  • Methylation
  • Oxidoreductases, N-Demethylating / antagonists & inhibitors
  • Oxidoreductases, N-Demethylating / metabolism*
  • Phenotype


  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • 3-methoxymorphinan
  • Dextromethorphan
  • Cytochrome P-450 Enzyme System
  • Aryl Hydrocarbon Hydroxylases
  • Cytochrome P-450 CYP2D6
  • Cytochrome P-450 CYP3A
  • Oxidoreductases, N-Demethylating