Abstract

Δ⁹-Tetrahydrocannabinol, cannabidiol (CBD), and cannabinol are the three major cannabinoids contained in marijuana, which are devoid of nitrogen atom in their structures. In this study, we investigated inhibitory effects of the major phytocannabinoids on catalytic activity of human cytochrome P450 (CYP) 2D6. These major cannabinoids inhibited 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin (AMMC) and dextromethorphan O-demethylase activities of recombinant CYP2D6 and pooled human liver microsomes in a concentration-dependent manner (IC₅₀ = 4.01-24.9 μM), indicating the strongest inhibitory potency of CBD. However, these cannabinoids showed no or weak metabolism-dependent inhibition. CBD competitively inhibited the CYP2D6 activities with the apparent K_i values of 1.16-2.69 μM. To clarify the structural requirement for CBD-mediated CYP2D6 inhibition, effects of CBD-related compounds on the AMMC O-demethylase activity of recombinant CYP2D6 were next examined. Olivetol (IC₅₀ = 7.21 μM) inhibited CYP2D6 activity as potently as CBD did (IC₅₀ = 6.52 μM), whereas d-limonene did not show any inhibitory effect. Pentylbenzene failed to inhibit CYP2D6 activity. Furthermore, neither monomethyl nor dimethyl ethers of CBD inhibited the activity. Cannabidivarin having a propyl side chain inhibited CYP2D6 activity; its inhibitory effect (IC₅₀ = 10.2 μM) was less potent than that of CBD. On the other hand, orcinol and resorcinol showed lack of inhibition. The inhibitory effect of CBD on CYP2D6 activity was more potent as compared with those of 16 compounds without nitrogen atom tested, such as progesterone. These results indicated that CBD caused potent direct CYP2D6 inhibition, in which two phenolic hydroxyl groups and the pentyl side chain of CBD may play important roles.