CYP2A6 Genotype, Phenotype, and the Use of Nicotine Metabolites as Biomarkers during Ad libitum Smoking

Abstract

CYP2A6 inactivates nicotine to cotinine and cotinine to 3-hydroxycotinine. We investigated which of plasma nicotine and metabolites were most related to CYP2A6 genotype and smoking levels. We assessed demographic and smoking histories in 152 Caucasian ad libitum smokers, measured breath carbon monoxide (CO) levels, and determined plasma nicotine, cotinine, and 3-hydroxycotinine by highperformance liquid chromatography and CYP2A6 genotypes by PCR. Cigarettes per day was most closely related to CO (r = 0.60, P < 0.001) followed by plasma cotinine (r = 0.53, P < 0.001), whereas plasma cotinine was most strongly correlated with CO levels (r = 0.74, P < 0.001), confirming that cotinine is a good indicator of smoking levels; this was not limited by CYP2A6 variants. 3-Hydroxycotinine/cotinine is reported to be a good marker of CYP2A6 activity, and we found that the 3-hydroxycotinine/(cotinine + nicotine) ratio was most correlated with CYP2A6 genotype (r = 0.38, P < 0.001). Inclusion of the CYP2A6*12A allele strengthened the correlation (r = 0.46, P < 0.001), suggesting that the identification of novel alleles will continue to improve this relationship. Nicotine metabolism is slower in smokers, and we have shown that CYP2A6 is reduced by nicotine treatment in monkeys. Here, we found that plasma nicotine levels were inversely correlated with CYP2A6 activity (3-hydroxycotinine/cotinine, r = À0.41, P < 0.001) among those without CYP2A6 variants, suggesting a reduction in metabolism with higher nicotine levels. Together, these findings (a) confirm the use of plasma cotinine and CO as indicators of Caucasians' smoking levels, and that this is not limited by CYP2A6 genetic variation; (b) indicate that 3-hydroxycotinine/cotinine and 3-hydroxycotinine/(cotinine + nicotine) are moderately good indicators of the CYP2A6 genotype; and (c) support that nicotine exposure may reduce its own metabolism. (Cancer Epidemiol Biomarkers Prev 2006;15(10):1812 -9)

Nicotine Metabolism in Young Adult Daily Menthol and Nonmenthol Smokers

“…18 Recent studies have examined the nicotine metabolite ratio (NMR), the ratio of 3HC to COT, as a measure of nicotine metabolic activity. 20 The NMR is strongly associated with the CYP2A6 genotype [21][22][23][24] and highly correlated with the oral clearance of nicotine in smokers (r = 0.90). 20 Studies suggest that the NMR can be used to phenotype CYP2A6 activity, 20,25,26 is stable over time, and reproducible using saliva or plasma samples.…”

Systematic review of the relationship between the 3-hydroxycotinine/cotinine ratio and cigarette dependence

“…Malaiyandi et al (2006) Plasma ratio from regular smokers, N=152 Individuals with at least one *2, *7, *9, or *12 CYP2A6 allelic variant had significantly slower ratios than wild types. CPD not associated with ratio Moolchan et al (2009) Plasma ratio obtained from ad libitum smoker prior to NRT smoking cessation trial, N=85 (adolescent sample)…”

A novel CYP2A6 allele, CYP2A6*23, impairs enzyme function in vitro and in vivo and decreases smoking in a population of Black-African descent

“…The 3HC/COT is a validated phenotypic measure of CYP2A6 activity and rates of nicotine metabolism [23], with previous studies finding a significant association between the ratio and CYP2A6 genotype [38][39][40]. Because the 3HC/COT ratio did not differ significantly from CYP2A6*1/*1 individuals in this population of Black-African descent, the wildtype group (*1/*1) included individuals with the CYP2A6*1B allele.…”