The primary pathway of nicotine metabolism is CYP2A6-catalyzed 5′-oxidation [7]. The product of this reaction is further oxidized, either by CYP2A6 or aldehyde oxidase, to cotinine [7, 8]. In most smokers, the conversion of nicotine to cotinine accounts for greater than 75% of nicotine metabolism. Cotinine is the most abundant nicotine metabolite in smokers’ plasma; nicotine is typically present at concentrations 10 to 20 times lower than those of cotinine due to its relatively short half-life [7]. The major metabolite of cotinine is trans 3′-hydroxycotinine, and CYP2A6 is also the primary catalyst of this reaction [9, 10]. Individuals who are homozygous for CYP2A6 deletion alleles (*4/*4), excrete much reduced levels of cotinine and trans 3′-hydroxycotinine [11, 12]. The ratio of trans 3′-hydroxycotinine to cotinine has been used as a proxy for CYP2A6 activity [13, 14]and is correlated with oral nicotine clearance [15].
