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 (n = 8) versus 33 ± 18 fmol/mL (p = 0.056) in “normal”
NMR (>0.3) smokers (n = 8). The total D4-hydroxy acid excreted by the low NMR group was half that of the
higher NMR group (29.1 ± 16.8 versus 59.7 ±
45.3 pmol/24h, p = 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.