Significance of CYP2C9 genetic polymorphism in inhibitory effect of Δ9-tetrahydrocannabinol on CYP2C9 activity

We recently reported that Δ9-tetrahydrocannabinol (Δ9-THC), the primary psychoactive constituent in marijuana, potently inhibits the catalytic activity of cytochrome P450 (CYP) 2C9.1, a wild-type variant. However, effects of CYP2C9 genetic polymorphisms on the inhibitory potency of Δ9-THC have not been investigated. In this study, in vitro inhibitory effects of Δ9-THC on catalytic activities of two major allelic variants, CYP2C9.2 and CYP2C9.3, were examined with the recombinant enzymes. Δ9-THC inhibited S-warfarin 7-hydroxylation, diclofenac 4′-hydroxylation, and tolbutamide 4-hydroxylation by CYP2C9.2 and CYP2C9.3 in a concentration-dependent manner. The IC50 values for the catalytic activities toward S-warfarin, diclofenac, and tolbutamide were 2.39, 1.04, and 1.12 μM, respectively, for CYP2C9.2, and 2.91, 1.38, and 1.09 μM, respectively, for CYP2C9.3. The inhibitory potencies against these mutant variants were similar to those against the wild-type variant. In contrast, Δ9-THC inhibited 7-methoxy-4-(trifluoromethyl)coumarin O-demethylase activity of CYP2C9.1, CYP2C9.2, and CYP2C9.3 to various extents, showing the IC50 values of 1.02, 4.20 and >10 μM, respectively. Our recent study showed that a preincubation of Δ9-THC in the presence of NADPH decreased the inhibitory effect on CYP2C9.1 activity. Then the effect of preincubation on Δ9-THC-mediated inhibition of S-warfarin 7-hydroxylation was investigated with CYP2C9.2 and CYP2C9.3. The preincubation of Δ9-THC with CYP2C9.2 attenuated the inhibitory effect of the phytocannabinoid, whereas preincubation with CYP2C9.3 did not. These results indicate that the inhibitory effects of Δ9-THC on CYP2C9 variants depend on the substrates used and the preincubation time.