Effect of cigarette smoking on coumarin metabolism in humans

Extant data, mostly from studies in vitro, suggest that coumarin and nicotine are both metabolized by CYP2A6, a cytochrome P450 isozyme. In order to investigate this issue further, the activity of this enzyme in vivo was measured in 37 non-smokers and 37 smokers using coumarin (2.0 mg, PO) as the metabolic probe. The percentage of coumarin metabolized to 7-hydroxycoumarin in 8 h was measured in urine by high-pressure liquid chromatography. There was more than 10-fold variability in coumarin metabolism in both groups. Coumarin metabolism was significantly reduced in smokers (46.6 +/- 4.4%) as compared to non-smokers (66.4 +/- 3.5%; p < or = .001). The results support previous in vitro findings that both coumarin and nicotine are metabolized, at least in part, by a common pathway, which most likely is CYP2A6.