Variable CYP2A6-mediated nicotine metabolism alters smoking behavior and risk.

Nicotine is the psychoactive substance responsible for tobacco dependence; smokers adjust their cigarette consumption to maintain brain nicotine levels. In humans, 70 to 80% of nicotine is metabolized to the inactive metabolite cotinine by the enzyme CYP2A6. CYP2A6 can also activate tobacco smoke procarcinogens [e.g., NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone]. In initial studies we found that there was an under-representation of individuals carrying defective CYP2A6 alleles in a tobacco-dependent population, and that among smokers, those with deficient nicotine metabolism smoked fewer cigarettes. We have since reproduced this data in a prospective smoking study (400 male and female, heavy and light smokers) examining the role of the CYP2A6 genotype on carbon monoxide levels, plasma and urine nicotine and cotinine levels, and cigarette counts. We have also recently identified deletion and duplication variants in the CYP2A6 gene locus and have examined their impact on smoking. These data provide the impetus to examine how inhibition of CYP2A6 activity might be useful in a therapeutic context. Both kinetic and behavioral experiments in human smokers demonstrated that inhibiting CYP2A6 in vivo decreased nicotine metabolism and smoking behavior. This article summarizes the preliminary results from our studies.