PT - JOURNAL ARTICLE AU - Xue-Qing Li AU - Lars Weidolf AU - Roger Simonsson AU - Tommy B. Andersson TI - Enantiomer/Enantiomer Interactions between the <em>S</em>- and <em>R</em>- Isomers of Omeprazole in Human Cytochrome P450 Enzymes: Major Role of CYP2C19 and CYP3A4 AID - 10.1124/jpet.105.090928 DP - 2005 Nov 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 777--787 VI - 315 IP - 2 4099 - http://jpet.aspetjournals.org/content/315/2/777.short 4100 - http://jpet.aspetjournals.org/content/315/2/777.full SO - J Pharmacol Exp Ther2005 Nov 01; 315 AB - We investigated the enzyme kinetic basis for the stereoselective disposition of R- and S-omeprazole (OME) and racemic OME in human liver microsomes. OME is primarily metabolized by the hepatic cytochrome P450 enzyme system (CYP2C19 and 3A4). The metabolism of each enantiomer and pseudoracemic OME was studied using unlabeled and 13C7-labeled enantiomers. The enantiomers inhibited each other's metabolism competitively in human liver microsomes and in recombinant CYP2C19 and 3A4. The results obtained with the individual enantiomers allowed successful prediction of the enzyme kinetics for the pseudoracemate. The intrinsic clearance of each enantiomer in a pseudoracemic mixture remained the same as those of the individually incubated enantiomers, although Km and Vmax decreased. In the pseudoracemate, the relative contribution of CYP2C19 and 3A4 to 5-hydroxylation and 5′-O-demethylation of R-OME was comparable to that obtained for the incubation of R-OME alone. For S-OME, however, the presence of its antipode greatly increased the contribution of CYP3A4, with increasing concentrations, compared with that obtained when incubating S-OME alone. The results of our in vitro study clearly show metabolic interactions between the OME enantiomers, which may also occur in vivo. Because the enantiomers of OME produce similar pharmacological effects, the enantiomer interactions should not significantly affect the pharmacodynamics. On the other hand, the use of the S-enantiomer results in less complex enzyme kinetics than those of the racemate; thus, the outcome of its clinical use is more predictable. The American Society for Pharmacology and Experimental Therapeutics