Nicotine metabolite ratio as an index of cytochrome P450 2A6 metabolic activity

Clin Pharmacol Ther. 2004 Jul;76(1):64-72. doi: 10.1016/j.clpt.2004.02.011.


Background: Nicotine and a variety of other drugs and toxins are metabolized by cytochrome P450 (CYP) 2A6. Our objective was to evaluate the use of oral nicotine with measurement of the trans-3'-hydroxycotinine (3HC)/cotinine (COT) metabolite ratio as a noninvasive probe of CYP2A6 activity.

Methods: Sixty-two healthy volunteers received an oral solution of deuterium-labeled nicotine (2 mg) and its metabolite cotinine (10 mg). Plasma nicotine and plasma and saliva cotinine and 3HC concentrations were measured over time.

Results: The 3HC/COT ratio derived from deuterium-labeled cotinine, measured in either plasma (2-8 hours after administration) or saliva (at 6 hours), was strongly correlated with the oral clearance of nicotine (r = 0.76-0.83, depending on the time of measurement). The 6-hour 3HC/COT ratio from nicotine derived from tobacco in 14 smokers was highly correlated with the ratio derived from deuterium-labeled nicotine (r = 0.88) and was also highly correlated with the oral clearance of nicotine (r = 0.90). Two subjects homozygous for inactive CYP2A6 alleles produced no 3HC, confirming the specificity of the metabolite ratio. The 3HC/COT ratio was also highly correlated with the clearance and half-life of cotinine, consistent with the fact that cotinine is also primarily metabolized by CYP2A6.

Conclusions: The 3HC/COT ratio derived from nicotine either administered as a probe drug or from tobacco use, measured in either plasma or saliva, is highly correlated with the oral clearance of nicotine. The ratio appears to be a useful noninvasive marker of the rate of nicotine metabolism (which is important in studying nicotine addiction and smoking behavior), as well as a general marker of CYP2A6 activity (which is important in studying drug and toxin metabolism).

Publication types

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

MeSH terms

  • Adult
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Aryl Hydrocarbon Hydroxylases / metabolism*
  • Cotinine / analogs & derivatives*
  • Cotinine / blood*
  • Cytochrome P-450 CYP2A6
  • Female
  • Humans
  • Male
  • Middle Aged
  • Mixed Function Oxygenases / genetics
  • Mixed Function Oxygenases / metabolism*
  • Nicotine / metabolism*
  • Saliva / metabolism


  • hydroxycotinine
  • Nicotine
  • Mixed Function Oxygenases
  • Aryl Hydrocarbon Hydroxylases
  • CYP2A6 protein, human
  • Cytochrome P-450 CYP2A6
  • Cotinine