Characterization of Cytochrome P450-Mediated Metabolism of Δ 8 -tetrahydrocannabinol in vitro.

Abstract

UNLABELLED: Δ 8 -tetrahydrocannabinol (Δ 8 -THC) is a pharmacologically active isomer of Δ 9 -THC, the main psychoactive component of cannabis. Δ 8 -THC use has increased rapidly, but the metabolism of Δ 8 -THC has been incompletely characterized. This study identified metabolites of Δ 8 -THC formed by cytochrome P450 (CYP) enzymes in human liver microsomes (HLM), assigned the main CYPs responsible for the metabolic pathways, and evaluated the impact of genetic polymorphisms on Δ 8 -THC metabolism in vitro . Five monohydroxylated Δ 8 -THC metabolites, identified as 11-OH-Δ 8 -THC, two aliphatic side-chain hydroxylated products (M1, M2), and two ring hydroxylated products (M3, M4) were detected in HLM. 11-OH-Δ 8 -THC was the most abundant metabolite. Unbound binding constants for metabolite formation in HLM ranged from 3.6 nM to >121 nM, suggesting multiple enzymes contribute to Δ 8 -THC metabolism. The contributing enzymes were identified using recombinant CYPs, inhibition studies, and correlation analyses. 11-OH-Δ 8 -THC was predominantly formed by CYP2C9 with a minor contribution from CYP2C19. CYP2C9 genotype-based activity scores across donors correlated with 11-OH-Δ 8 -THC formation (p = 0.0315). An 80% decrease in 11-OH-Δ 8 -THC formation clearance was predicted in CYP2C9*3/*3 carriers. The ring-hydroxylated M3 and M4 were mainly formed by CYP3A. CYP2D6, CYP1A2, and CYP3A4 activities were associated with formation of the pentyl side-chain oxidized metabolites M1 and M2. Although C11 hydroxylation appeared more important metabolic pathway for Δ 8 -THC than for Δ 9 -THC, the catalytic efficiency of CYP2C9 towards Δ 8 -THC was lower than towards Δ 9 -THC. In the presence of CYP2C9 inhibitors or genetic polymorphisms, Δ 8 -THC clearance is likely mediated by CYP3A4 and CYP2C19. SIGNIFICANCE STATEMENT: This study shows that 11-OH-Δ 8 -THC, a psychoactive metabolite, is the primary oxidation product of Δ 8 -THC in HLM and that CYP2C9 predominantly catalyzes its formation. This work expands the limited literature on Δ 8 -THC metabolism, and shows that reduced-function CYP2C9 variants substantially impair formation of 11-OH-Δ 8 -THC. The characterization of Δ 8 -THC metabolism in HLM provides the foundation for risk assessment of drug-drug interactions and establishes the metabolic profile of Δ 8 -THC in the human liver.

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