In this study, we examined the inhibitory effects of Δ9-tetrahydrocannabinol (Δ9-THC), cannabidiol (CBD), and cannabinol (CBN), the three major cannabinoids, on the activity of human cytochrome P450 (CYP) 3A enzymes. Furthermore, we investigated the kinetics and structural requirement for the inhibitory effect of CBD on the CYP3A activity.

Main methods: Diltiazem N-demethylase activity of recombinant CYP3A4, CYP3A5, CYP3A7, and human liver microsomes (HLMs) in the presence of cannabinoids was determined. Key findings: Among the three major cannabinoids, CBD most potently inhibited CYP3A4 and CYP3A5 (IC50=11.7 and 1.65 μM, respectively). The IC50 values of Δ9-THC and CBN for CYP3A4 and CYP3A5 were higher than 35 μM. For CYP3A7, Δ9-THC, CBD, and CBN inhibited the activity to a similar extent (IC50=23–31 μM). CBD competitively inhibited the activity of CYP3A4, CYP3A5, and HLMs (Ki=1.00, 0.195, and 6.14 μM, respectively).

On the other hand, CBD inhibited the CYP3A7 activity in a mixed manner (Ki=12.3 μM). Olivetol partially inhibited all the CYP3A isoforms tested, whereas d-limonene showed lack of inhibition. The lesser inhibitory effects of monomethyl and dimethyl ethers of CBD indicated that the ability of CYP3A inhibition by the cannabinoid attenuated with the number of methylation on the phenolic hydroxyl groups in the resorcinol moiety.

Significance: This study indicated that CBD most potently inhibited catalytic activity of human CYP3A enzymes, especially CYP3A4 and CYP3A5. These results suggest that two phenolic hydroxyl groups in the resorcinol moiety of CBD may play an important role in the CYP3A inhibition.