title: Characterization of the major metabolites of verapamil as substrates and inhibitors of P-glycoprotein.
abstract: Verapamil is subject to extensive oxidative metabolism mediated by cytochrome P450 enzymes with less than 5% of an oral dose being excreted unchanged in urine. Furthermore, verapamil is known to be a potent inhibitor of P-glycoprotein function. There is evidence from in vivo investigations that some verapamil metabolites might be actively transported. The aim of the present study was to investigate P-glycoprotein-mediated transport and inhibition properties of verapamil and its metabolites norverapamil, D-620, D-617, and D-703. Polarized transport of these compounds was assessed in P-glycoprotein-expressing Caco-2 and L-MDR1 cells (LLC-PK1 cells stably transfected with human MDR1-P-glycoprotein). Inhibition of P-glycoprotein-mediated transport by these compounds was determined using digoxin as P-glycoprotein substrate. At concentrations of 5 microM, significant differences between basal-to-apical and apical-to-basal apparent permeability coefficients were observed for D-617 and D-620 in all P-glycoprotein-expressing cell monolayers, indicating that both are P-glycoprotein substrates. In contrast, no P-glycoprotein-dependent transport was found for verapamil, norverapamil, and D-703 in Caco-2 cells and for D-703 in L-MDR1 cells. Moreover, verapamil, norverapamil, and D-703 inhibited P-glycoprotein-mediated digoxin transport with IC(50) values of 1.1, 0.3, and 1.6 microM, respectively, whereas D-617 and D-620 did not (at concentrations up to 100 microM). We conclude that verapamil phase I metabolites exhibit different P-glycoprotein substrate and inhibition characteristics, with the N-dealkylated metabolites D-617 and D-620 being P-glycoprotein substrates and norverapamil and D-703 being inhibitors of P-glycoprotein function, which may influence P-glycoprotein-dependent drug disposition and elimination.
authors.affiliation: Dr. Margarete Fischer-Bosch-Institute of Clinical Pharmacology, Stuttgart, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Pauli-Magnus
authors.forename: O
authors.initials: O
authors.lastname: von Richter
authors.forename: O
authors.initials: O
authors.lastname: Burk
authors.forename: A
authors.initials: A
authors.lastname: Ziegler
authors.forename: T
authors.initials: T
authors.lastname: Mettang
authors.forename: M
authors.initials: M
authors.lastname: Eichelbaum
authors.forename: M F
authors.initials: MF
authors.lastname: Fromm
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active; D018938:Caco-2 Cells; D002121:Calcium Channel Blockers / Q000378:metabolism* / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000378:metabolism; D002460:Cell Line; D004077:Digoxin / Q000378:metabolism; D004847:Epithelial Cells / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D018374:LLC-PK1 Cells; D009570:Nitriles; D013552:Swine; D014700:Verapamil / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002121:Calcium Channel Blockers; D002316:Cardiotonic Agents; D009570:Nitriles; C482241:2-(3,4-dimethoxyphenyl)-5-amino-2-isopropylvaleronitrile; C409586:D 703; D004077:Digoxin; C016904:norverapamil; D014700:Verapamil