Inhibitory effects of Δ8-tetrahydrocannabinol on nicotine metabolism and implications as a smoking cessation agent

Tobacco use remains the leading cause of preventable death worldwide. The major metabolic pathway for nicotine, the addictive component in tobacco, is via cytochrome P450 (CYP) 2A6-mediated metabolism to cotinine. Cannabidiol has been shown to reduce cigarette consumption in vivo and inhibit CYP2A6-mediated nicotine metabolism in vitro. In the present study, Δ-8-tetrahydrocannabinol (Δ8-THC), an isomer of Δ-9-tetrahydrocannabinol, was examined as a potential inhibitor of CYP2A6-mediated nicotine metabolism. While Δ-9-tetrahydrocannabinol showed no significant inhibition of nicotine metabolism to cotinine, Δ8-THC demonstrated unbound IC₅₀ values of 0.57 ± 0.04 μM in microsomes from recombinant wild-type CYP2A6 overexpressing human embryonic kidney 293 cells and 0.70 ± 0.16 μM in human liver microsomes (HLMs). A similar unbound IC₅₀ value was observed for recombinant CYP2A6∗5 microsomes (0.52 ± 0.17 μM) and was modestly elevated in recombinant CYP2A6∗2 microsomes (1.00 ± 0.12 μM). IC₅₀ shift experiments were consistent across pooled HLM (5.3-fold) and microsomes from liver specimens exhibiting the CYP2A6 (∗2/∗2) and (∗9/∗9) genotypes (6.1- and 4.0-fold, respectively) but were reduced in CYP2A6 (∗35/∗35) microsomes (1.0-fold). Irreversible inhibition kinetics in pooled HLMs by Δ8-THC yielded a k_inact value of 0.022 ± 0.001 min^-1 and an unbound K_I value of 0.232 ± 0.062 μM. Static modeling predicted that oral dosing with 10 mg Δ8-THC increased the nicotine plasma area under the curve by 189%, with further increases observed at 20 mg and 40 mg; interactions were also observed with inhalation doses ≥70 mg. These findings suggest that, based on CYP2A6 genotype, Δ8-THC could be a candidate for smoking cessation therapy. SIGNIFICANCE STATEMENT: This study is the first, to the best of our knowledge, to identify Δ-8-tetrahydrocannabinol as a potent and irreversible inhibitor of nicotine metabolism to cotinine. The extent of inhibition is modulated by genetic variation in cytochrome P450 2A6. These findings suggest that further investigations focusing on Δ-8-tetrahydrocannabinol and its potential as a candidate for smoking cessation therapy are warranted.