CYP2A6 Activity and Deuterated 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) Metabolism in Cigarette Smokers

Smoking is the leading cause of lung cancer. Differences in CYP2A6-catalyzed nicotine metabolism affect smoking dose and intensity, which, in turn, can affect lung cancer risk. CYP2A6 also catalyzes the bioactivation of the tobacco-specific lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). To determine the contribution of CYP2A6 to the metabolic activation of NNK, a group of Japanese American and Native Hawaiian smokers with little or no CYP2A6 activity was recruited to smoke [pyridyl-D₄]-NNK-containing cigarettes for a week. [Pyridyl-D₄]-4-hydroxy-4-(3-pyridyl)butanoic acid (D₄-hydroxy acid), the urinary product of NNK α-hydroxylation, the major bioactivation pathway, was quantified in these individuals and in an equal number of smokers with "normal" CYP2A6 activity. In expectation of low D₄-hydroxy acid levels, a sensitive nanoflow LC-MS/MS assay was developed. CYP2A6 activity was measured as the plasma ratio of 3'-hydroxycotinine to cotinine, which is the nicotine metabolite ratio (NMR). The average concentration of D₄-hydroxy acid in 24 h urine samples over 3 days was 20 ± 14 fmol/mL in low NMR (<0.05) smokers (<i>n</i> = 8) <i>versus</i> 33 ± 18 fmol/mL (<i>p</i> = 0.056) in "normal" NMR (>0.3) smokers (<i>n</i> = 8). The total D₄-hydroxy acid excreted by the low NMR group was half that of the higher NMR group (29.1 ± 16.8 <i>versus</i> 59.7 ± 45.3 pmol/24h, <i>p</i> = 0.048). These data support the role of CYP2A6 in the metabolic activation of NNK. However, it is unlikely that more modest differences in CYP2A6 activity, for example, as might be seen across smokers of European ancestry, would significantly impact NNK bioactivation. The influence of CYP2A6 activity on nicotine metabolism and the associated carcinogen uptake is likely the primary influence of CYP2A6 activity on a smoker's risk of lung cancer, not a modest effect on the metabolic activation of NNK, one of several lung carcinogens in tobacco smoke.