title: Nicotine metabolite ratio as an index of cytochrome P450 2A6 metabolic activity.
abstract: 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).
authors.affiliation: Department of Medicine, Division of Clinical Pharmacology and Experimental Therapeutics, Medical Service, San Francisco General Hospital Medical Center, University of California, 94143-1220, USA.
authors.forename: Delia
authors.initials: D
authors.lastname: Dempsey
authors.forename: Piotr
authors.initials: P
authors.lastname: Tutka
authors.forename: Peyton
authors.initials: P
authors.lastname: Jacob
authors.forename: Faith
authors.initials: F
authors.lastname: Allen
authors.forename: Kerri
authors.initials: K
authors.lastname: Schoedel
authors.forename: Rachel F
authors.initials: RF
authors.lastname: Tyndale
authors.forename: Neal L
authors.initials: NL
authors.lastname: Benowitz
mesh_terms: D000328:Adult; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics / Q000378:metabolism*; D003367:Cotinine / Q000031:analogs & derivatives* / Q000097:blood*; D065637:Cytochrome P-450 CYP2A6; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism*; D009538:Nicotine / Q000378:metabolism*; D012463:Saliva / Q000378:metabolism
chemical_list: C001381:hydroxycotinine; D009538:Nicotine; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; D065637:Cytochrome P-450 CYP2A6; D003367:Cotinine