Difference in epoxides formation and their further metabolism between Δ9- and Δ8-tetrahydrocannabinols by human liver microsomes

In vitro metabolism of Δ⁹- and Δ⁸-tetrahydrocannabinols (THCs) was studied using human liver microsomes. Δ⁹- or Δ⁸-THC was incubated with microsomes in the presence of an NADPH-generating system. The metabolites formed were extracted with ethyl acetate, separated by preparative thin-layer chromatography, and identified as trimethylsilyl derivatives by gas chromatography-mass spectrometry. 9α,10α-Epoxyhexahydrocannabinol (EHHC) together with four monohydroxylated metabolites was formed from Δ⁹-THC. The epoxide was found to resist the hydrolysis by epoxide hydrolase, and was further converted to several metabolites by the monooxygenase system involving cytochrome P-450. On the other hand, 8β,9α-dihydroxyhexahydrocannabinol (diOH-HHC) instead of epoxy metabolites was formed from δ⁸-THC under the conditions for monooxygenase. When 1,1,1-trichlorpropene-2,3-oxide was further added to the incubation mixture, both 8α,9α- and 8β,9β-EHHCs were found to be formed from Δ⁸-THC. These epoxides of Δ⁸-THC were preferentially hydrolyzed to 8β,9α-diOH-HHC by epoxide hydrolase. These results indicate that 9α,10α-EHHC formed from Δ⁹-THC is further metabolized nor by epoxide hydrolase but by monooxygenase system involving cytochrome P-450, and that, on the contrary, 8α,9α- and 8β,9β-EHHCs derived from Δ⁸-THC may be metabolized by epoxide hydrolase rather than cytochrome P-450 in the human liver, forming 8β,9α-diOH-HHC.