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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.

Gabexate (GB), a serine protease inhibitor that is widely used for the treatment of acute pancreatitis and disseminated intravascular coagulation, has been reported to be partly hydrolyzed by human serum albumin. However, other enzymes responsible for GB hydrolysis in human blood remain unclear. In this study, we examined in vitro metabolism of GB with human blood samples. The hydrolytic activities of the plasma and erythrocytes at 100µM of GB were 167 ± 26 and 151 ± 9n mol/min/ml blood fraction (mean ± S.D., n = 8), respectively. Kinetic analysis indicated that Vmax and Km values were 1.75µmol/min/ml blood fraction and 529µ Mf or the plasma and 10.6µmol/min/ml blood fraction and 7360µ Mf or the erythrocytes, respectively. The activity of human plasma wa si nhibited by ethopropazine, a butyrylcholinesterase inhibitor (27% inhibition at 100µM). Furthermore, the plasma activity was inhibited by 5,5 � -dithiobis(2-nitrobenzoic acid) (DTNB) (40% inhibition at 5000µM), suggesting the involvement of albumin in the plasma GB hydrolysis. The erythrocyte activity was also decreased by DTNB (56% inhibition at 5000µM), implying that this activity was dependent on the presence of sulfhydryl group(s), while little or no inhibition of the activity was seen with phenylmethylsulfonyl fluoride, diisopropyl fluorophosphate, and BW284C51. Butyrylcholinesterase from human serum showed GB hydrolytic activity with Vmax of 363 nmol/min/m gp rotein and Km of 263µM. These results suggest that, in addition to albumin, butyrylcholinesterase and erythrocyte sulfhydryl-dependent enzyme are responsible for GB hydrolysis in human blood.

Metabolism of nafamostat, a clinically used serine protease inhibitor, was investigated with human blood and liver enzyme sources. All the enzyme sources examined (whole blood, erythrocytes, plasma and liver microsomes) showed nafamostat hydrolytic activity. V(max) and K(m) values for the nafamostat hydrolysis in erythrocytes were 278 nmol/min/mL blood fraction and 628 microM; those in plasma were 160 nmol/min/mL blood fraction and 8890 microM, respectively. Human liver microsomes exhibited a V(max) value of 26.9 nmol/min/mg protein and a K(m) value of 1790 microM. Hydrolytic activity of the erythrocytes and plasma was inhibited by 5, 5'-dithiobis(2-nitrobenzoic acid), an arylesterase inhibitor, in a concentration-dependent manner. In contrast, little or no suppression of these activities was seen with phenylmethylsulfonyl fluoride (PMSF), diisopropyl fluorophosphate (DFP), bis(p-nitrophenyl)phosphate (BNPP), BW284C51 and ethopropazine. The liver microsomal activity was markedly inhibited by PMSF, DFP and BNPP, indicating that carboxylesterase was involved in the nafamostat hydrolysis. Human carboxylesterase 2 expressed in COS-1 cells was capable of hydrolyzing nafamostat at 10 and 100 microM, whereas recombinant carboxylesterase 1 showed significant activity only at a higher substrate concentration (100 microM). The nafamostat hydrolysis in 18 human liver microsomes correlated with aspirin hydrolytic activity specific for carboxylesterase 2 (r=0.815, p

Our recent work has shown that cannabidiol (CBD) exhibits the most potent direct inhibition of human cytochrome P450 1A1 (CYP1A1) among the CYP enzymes examined. However, the mechanism underlying this CBD inhibition remains to be clarified. Thus, to elucidate the structural requirements for the potent inhibition by CBD, the effects of CBD and its structurally related compounds on CYP1A1 activity were investigated with recombinant human CYP1A1. Olivetol, which corresponds to the pentylresorcinol moiety of CBD, inhibited the 7-ethoxyresorufin O-deethylase activity of CYP1A1; its inhibitory effect (IC50=13.8 µM) was less potent than that of CBD (IC50=0.355 µM). In contrast, d-limonene, which corresponds to the terpene moiety of CBD, only slightly inhibited CYP1A1 activity. CBD-2'-monomethyl ether (CBDM) and CBD-2',6'-dimethyl ether inhibited CYP1A1 activity with IC50 values of 4.07 and 23.0 µM, respectively, indicating that their inhibitory effects attenuated depending on the level of methylation on the free phenolic hydroxyl groups in the pentylresorcinol moiety of CBD. Cannabidivarin inhibited CYP1A1 activity, although its inhibitory potency (IC50=1.85 µM) was lower than that of CBD. The inhibitory effects of Δ(9)-tetrahydrocannabinol and cannabielsoin (IC50s ≈10 µM), which contain a free phenolic hydroxyl group and are structurally constrained, were less potent than that of CBDM, which contains a free phenolic hydroxyl group and is rotatable between pentylresorcinol and terpene moieties. These results suggest that the pentylresorcinol structure in CBD may have structurally important roles in direct CYP1A1 inhibition, although the whole structure of CBD is required for overall inhibition.

Summary: Inhibitory effects of Δ 9 -tetrahydrocannabinol (Δ 9 -THC), cannabidiol (CBD), and cannabinol (CBN), the three major constituents in marijuana, and polycyclic aromatic hydrocarbons (PAHs) contained in marijuana smoke on catalytic activity of human cytochrome P450 (CYP) 2C9 were investigated. These phytocannabinoids concentration-dependently inhibited S-warfarin 7-hydroxylase and diclofenac 4′-hydroxylase activities of human liver microsomes (HLMs) and recombinant CYP2C9 (rCYP2C9). In contrast, none of the twelve PAHs including benz[a]anthracene and benzo[a]pyrene exerted substantial inhibition (IC 50 > 10 μΜ). The inhibitory potentials of Δ 9 -THC ( K i = 0.937–1.50 μΜ) and CBN ( K i = 0.8821.29 μΜ) were almost equivalent regardless of the enzyme sources used, whereas the inhibitory potency of CBD ( K i = 0.954–9.88 μΜ) varied depending on the enzyme sources and substrates used. Δ 9 -THC inhibited both S-warfarin 7-hydroxylase and diclofenac 4′-hydroxylase activities of HLMs and rCYP2C9 in a mixed manner. CBD and CBN competitively inhibited the activities of HLMs and rCYP2C9, with the only notable difference being that CBD and CBN exhibited mixed-type inhibitions against diclofenac 4′-hydroxylation and S-warfarin 7-hydroxylation, respectively, by rCYP2C9. None of Δ 9 -THC, CBD, and CBN exerted metabolism-dependent inhibition. These results indicated that the three major phytocannabinoids but not PAHs contained in marijuana smoke potently inhibited CYP2C9 activity and that these cannabinoids can be characterized as direct inhibitors for CYP2C9.

Inhibitory effects of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), cannabidiol (CBD), and cannabinol (CBN), the three major constituents in marijuana, on catalytic activities of human cytochrome P450 (CYP) 1 enzymes were investigated. These cannabinoids inhibited 7-ethoxyresorufin O-deethylase activity of recombinant CYP1A1, CYP1A2, and CYP1B1 in a competitive manner. CBD most potently inhibited the CYP1A1 activity; the apparent K(i) value (0.155microM) was at least one-seventeenth of the values for other CYP1 isoforms. On the other hand, CBN more effectively decreased the activity of CYP1A2 and CYP1B1 (K(i)=0.0790 and 0.148microM, respectively) compared with CYP1A1 (K(i)=0.541microM). Delta(9)-THC less potently inhibited the CYP1 activity than CBD and CBN, and showed low selectivity against the CYP1 inhibition (K(i)=2.47-7.54microM). The preincubation of CBD resulted in a time- and concentration-dependent decrease in catalytic activity of all the recombinant CYP1 enzymes and human liver microsomes. Similarly, the preincubation of Delta(9)-THC or CBN caused a time- and concentration-dependent inhibition of recombinant CYP1A1. The inactivation of CYP1A1 by CBD indicated the highest k(inact)/K(I) value (540l/mmol/min) among the CYP1 enzyme sources tested. The inactivation of recombinant CYP1A1 and human liver microsomes by CBD required NADPH, was not influenced by dialysis and by glutathione, N-acetylcysteine, and superoxide dismutase as trapping agents. These results indicated that CBD and CBN showed CYP1 isoform-selective direct inhibition and that CBD was characterized as a potent mechanism-based inhibitor of human CYP1 enzymes, especially CYP1A1.