title: Meclizine, a pregnane X receptor agonist, is a direct inhibitor and mechanism-based inactivator of human cytochrome P450 3A.
abstract: Meclizine is an agonist of human pregnane X receptor (PXR). It increases CYP3A4 mRNA expression, but decreases CYP3A-catalyzed testosterone 6β-hydroxylation in primary cultures of human hepatocytes, as assessed at 24h after the last dose of meclizine. Therefore, the hypothesis to be tested is that meclizine inactivates human CYP3A enzymes. Our findings indicated that meclizine directly inhibited testosterone 6β-hydroxylation catalyzed by human liver microsomes, recombinant CYP3A4, and recombinant CYP3A5. The inhibition of human liver microsomal testosterone 6β-hydroxylation by meclizine occurred by a mixed mode and with an apparent Ki of 31±6μM. Preincubation of meclizine with human liver microsomes and NADPH resulted in a time- and concentration-dependent decrease in testosterone 6β-hydroxylation. The extent of inactivation required the presence of NADPH, was unaffected by nucleophilic trapping agents or reactive oxygen species scavengers, attenuated by a CYP3A substrate, and not reversed by dialysis. Meclizine selectively inactivated CYP3A4, but not CYP3A5. In contrast to meclizine, which has a di-substituted piperazine ring, norchlorcyclizine, which is a N-debenzylated meclizine metabolite with a mono-substituted piperazine ring, did not inactivate but directly inhibited hepatic microsomal CYP3A activity. In conclusion, meclizine inhibited human CYP3A enzymes by both direct inhibition and mechanism-based inactivation. In contrast, norchlorcyclizine is a direct inhibitor but not a mechanism-based inactivator. Furthermore, a PXR agonist may also be an inhibitor of a PXR-regulated enzyme, thereby giving rise to opposing effects on the functional activity of the enzyme and indicating the importance of measuring the catalytic activity of nuclear receptor-regulated enzymes.
authors.affiliation: Department of Pharmacy, Faculty of Science, National University of Singapore, Singapore. Electronic address: winnie_foo@u.nus.edu.
authors.forename: Winnie Yin Bing
authors.initials: WY
authors.lastname: Foo
authors.affiliation: Department of Pharmacy, Faculty of Science, National University of Singapore, Singapore. Electronic address: a0083608@u.nus.edu.
authors.forename: Hwee Ying
authors.initials: HY
authors.lastname: Tay
authors.affiliation: Department of Pharmacy, Faculty of Science, National University of Singapore, Singapore. Electronic address: phaccye@nus.edu.sg.
authors.forename: Eric Chun Yong
authors.initials: EC
authors.lastname: Chan
authors.affiliation: Department of Pharmacy, Faculty of Science, National University of Singapore, Singapore; Department of Pharmacology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore. Electronic address: aikjiang.lau@nus.edu.sg.
authors.forename: Aik Jiang
authors.initials: AJ
authors.lastname: Lau
mesh_terms: D051544:Cytochrome P-450 CYP3A / Q000378:metabolism*; D065692:Cytochrome P-450 CYP3A Inhibitors / Q000737:chemistry / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008468:Meclizine / Q000737:chemistry / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D008954:Models, Biological; D015394:Molecular Structure; D010879:Piperazines / Q000737:chemistry / Q000494:pharmacology; D000077297:Pregnane X Receptor; D011987:Receptors, Steroid / Q000819:agonists*; D011994:Recombinant Proteins
chemical_list: D065692:Cytochrome P-450 CYP3A Inhibitors; D010879:Piperazines; D000077297:Pregnane X Receptor; D011987:Receptors, Steroid; D011994:Recombinant Proteins; D008468:Meclizine; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; C008670:norchlorcyclizine
keywords: 6β-Hydroxytestosterone (PubChem CID: 65543); CYP3A; Erythromycin (PubChem CID: 12560); Glutathione (PubChem CID: 124886); Inhibition; Ketoconazole (PubChem CID: 456201); Mechanism-based inactivation; Meclizine; Meclizine dihydrochloride monohydrate (PubChem CID: 173612); N-acetylcysteine (PubChem CID: 12035); Norchlorcyclizine; Norchlorcyclizine (PubChem CID: 9340); Prednisolone (PubChem CID: 5755); Pregnane X receptor; Testosterone (PubChem CID: 6013)