Vol. 28, Issue 11, 1291-1296, November 2000

CYP3A4 Is a Major Isoform Responsible for Oxidation of 7-Hydroxy-⁸-tetrahydrocannabinol to 7-Oxo-⁸-tetrahydrocannabinol in Human Liver Microsomes

Tamihide Matsunaga, Nobuyuki Kishi, Shinsuke Higuchi, Kazuhito Watanabe, Tohru Ohshima, and Ikuo Yamamoto

Department of Hygienic Chemistry, Faculty of Pharmaceutical Sciences, Hokuriku University (T.M., N.K., S.H., K.W., I.Y.), Hokuriku, Japan; and Department of Legal Medicine, Kanazawa University Faculty of Medicine, School of Medicine, Kanazawa, Japan (T.O.)

The human liver enzyme microsomal alcohol oxygenase was able to oxidize both 7- and 7-hydroxy-⁸-tetrahydrocannabinol (7- and 7-hydroxy-⁸-THC) to 7-oxo-⁸-THC. The oxidative activity was determined by using a panel of 12 individual cDNA-expressed human cytochrome P450s (CYPs) (1A1, 1A2, 2A6, 2B6, 2C8, 2C9-Arg, 2C9-Cys, 2C19, 2D6-Met, 2D6-Val, 2E1 and 3A4). Among the CYP isoforms examined, CYP3A4 showed the highest activity for both of substrates. The metabolism of 7- and 7-hydroxy-⁸-THC to 7-oxo-⁸-THC was also detected for CYPs 1A1 (4.8% of CYP3A4), 1A2 (4.7%), 2A6 (2.3%), 2C8 (16.6%), and 2C9-Cys (5.4%), and CYPs 1A1 (0.4%), 2C8 (1.3%), 2C9-Arg (4.3%), and 2C9-Cys (0.9%), respectively. The 7- and 7-hydroxy-⁸-THC microsomal alcohol oxygenase activities in human liver were significantly inhibited by addition of 100 µM troleandomycin, 1 µM ketoconazole, and anti-CYP3A antibody, although these activities were not inhibited by 1 µM 7,8-benzoflavone and 50 µM sulfaphenazole. When the substrates were incubated with the CYP3A4-expressed microsomes under oxygen-18 gas phase, atmospheric oxygen was incorporated into 35% of 7-oxo-⁸-THC formed from 7-OH-⁸-THC, but only 12% of 7-oxo-⁸-THC formed from 7-OH-⁸-THC. These results indicate that CYP3A4 is a major isoform responsible for the oxidation of 7- and 7-hydroxy-⁸-THC to 7-oxo-⁸-THC in liver microsomes of humans, although the oxidation mechanisms for 7- and 7-hydroxy-⁸-THC might be different.