Inhibition of cytochrome P450 2A6 increases nicotine's oral bioavailability and decreases smoking

Background Nicotine establishes and maintains tobacco dependence. Individuals with genetically deficient CYP2A6 nicotine metabolism are at lower risk to become smokers and, if dependent, will smoke fewer cigarettes. Hepatic CYP2A6 accounts for nicotine's low systemic bioavailability, precluding oral nicotine replacement to treat dependence. Objective We sought to determine whether CYP2A6 inhibition via oral methoxsalen decreases nicotine clearance, increases nicotine bioavailability, and decreases smoking. Methods Two within‐subject designs in healthy tobacco‐dependent volunteers were conducted: a single‐blind kinetic study (n = 17) of methoxsalen 30, 10, or 3.5 mg or placebo given with nicotine 4 mg orally to abstinent smokers; and a double‐blind randomized crossover study (n = 11) of methoxsalen 30 mg or placebo crossed with nicotine 4 mg given orally or placebo before 60 minutes’ abstinence and 90 minutes’ free smoking. Results Placebo plus nicotine 4 mg orally increased the mean 3‐hour plasma nicotine level by 4 ng/mL over residual baseline nicotine level, whereas methoxsalen 10 or 30 mg plus nicotine increased it by 9 ng/mL (P < .01), demonstrating in vivo inhibition of CYP2A6 nicotine metabolism. Methoxsalen 30 mg plus nicotine 4 mg given orally decreased breath carbon monoxide concentration at the end of free smoking by 47% (4.6 versus 8.7 ppm; P < .01) and cigarettes smoked by 24% (3.1 versus 4.1, P < .01) compared with placebo plus placebo. Conclusions Methoxsalen inhibits nicotine first‐pass metabolism of orally administered nicotine, and the combination directly reduces smoking in a laboratory setting. CYP2A6 inhibitors may have an important role in smoking cessation and tobacco exposure reduction. Clinical Pharmacology & Therapeutics (2000) 68, 35–43; doi: 10.1067/mcp.2000.107651