Inhibition of Human Liver Microsomal (S)-Nicotine Oxidation by (−)-Menthol and Analogues

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Abstract
(−)-Menthol is a widely used flavoring ingredient present in mouthwash, foods, toothpaste, and cigarettes; yet, the pharmacological effects of menthol have not been widely studied. Mentholated cigarette smoking may increase the risk for lung cancer. Many African American smokers smoke mentholated cigarettes, and African Americans have a significantly higher incidence of lung cancer as compared with whites. There may be a relationship between the incidence of lung cancer and the type of cigarette smoked because the use of mentholated cigarettes by white smokers is significantly less and the incidence of lung cancer is less. The mechanism whereby (−)-menthol could increase the health risk of smoking is not known. The results of our in vitro studies herein show that (−)-menthol and synthetic congeners inhibit the microsomal oxidation of nicotine to cotinine and the P450 2A6-mediated 7-hydroxylation of coumarin. Replacement of the alcohol oxygen atom of menthol with other heteroatoms increased the potency of P450 2A6 inhibition. Thus, the Ki value of (−)-menthol for inhibition of microsomal nicotine oxidation was 69.7 μM but neomenthyl thiol possesses a Ki value of 13.8 μM. Menthylamine inhibited nicotine oxidation with a Ki value of 49.8 μM, but its hydroxylamine derivative gave an IC50 value of 2.2 μM. A series of 16 menthol derivatives and putative metabolites were procured or chemically synthesized and tested as inhibitors of P450 2A6. While highly potent inhibition of P450 2A6 was not observed for the menthol analogues examined, it is nevertheless possible that smoking mentholated cigarettes leads to inhibition of nicotine metabolism and allows the smoker to achieve a certain elevated dose of nicotine each day. This may be another example of self-medication to obtain the desired effect of nicotine.