HUMAN CYP2C19 IS A MAJOR OMEPRAZOLE 5-HYDROXYLASE, AS DEMONSTRATED WITH RECOMBINANT CYTOCHROME-P450 ENZYMES

Omeprazole (OP) is a potent antiulcer drug that is metabolized by live r cytochrome P450 (P450) enzymes, However, the identities of the P450 isoforms responsible for its metabolism have been controversial, 5-Hyd roxyomeprazole (5OH-OP) formation cosegregates with the polymorphism o f (S)-mephenytoin 4'-hydroxylation in humans, which is now known to be mediated by CYP2C19, Previous in vitro studies have indicated that li ver microsomal 5OH-OP formation correlates with both (S)-mephenytoin 4 '-hydroxylase and CYP3A content, Inhibitor and CYP2C antibody studies also suggested that both enzymes may be involved in the 5-hydroxylatio n of OP, whereas CYP3A appears to be the predominant enzyme involved i n OP sulfone (OP-S) formation. The present studies assessed the contri bution of various CYP2C and CYP3A4 enzymes to OP metabolism by using r ecombinant human enzymes, CYP2C19, CYP2C8, CYP2C18, and CYP2C9 formed a single metabolite with an HPLC retention time identical to that of 5 OH-OP, The turnover number for CYP2C19 was 13.4 +/- 1.4 nmol/min/nmol of P450, whereas those for CYP2C8, CYP2C18, and CYP2C9 were 2.2 +/- 0. 1, 1.5 +/- 0.1, and approximate to 0.5 nmol/min/nmol of P450, respecti vely, Recombinant human CYP3A4 formed 5OH-OP and OP-S with turnover nu mbers of 5.7 +/- 1.1 and 7.4 +/- 0.9 nmol/min/nmol of P450, respective ly, and formed a minor unidentified metabolite, CYP2C19 had a substant ially lower K-M for 5OH-OP formation than did CYP3A4, CYP2C8, or CYP2C 18, Antibody to CYP2C proteins inhibited approximate to 70% of OP 5-hy droxylation at low substrate concentrations, comparable to those that may be encountered at therapeutically relevant doses, whereas antibody to CYP3A4 inhibited approximate to 30% of the activity, At high subst rate concentrations, the contributions of the two enzymes to OP hydrox ylation were roughly comparable (40-50%). In contrast, OP-S formation was completely inhibited by antibody to CYP3A4 proteins. The present s tudy provides the first direct confirmation, using human recombinant P 450 enzymes and selective antibody inhibition, that CYP2C19 is a major high affinity OP 5-hydroxylase and CYP3A4 is a low affinity OP-hydrox ylating enzyme, The current work also shows, for the first time, that other CYP2C enzymes (CYP2C8, CYP2C9, and CYP2C18) may contribute to OP hydroxylation at high substrate concentrations. In contrast, OP-S was formed principally by CYP3A4.