title: Mdr1 limits CYP3A metabolism in vivo.
abstract: We determined whether the drug efflux protein P-glycoprotein (Pgp) could influence the extent of CYP3A-mediated metabolism of erythromycin, a widely used model substrate for CYP3A. We compared CYP3A metabolism of erythromycin (a Pgp substrate) using the erythromycin breath test in mice proficient and deficient of mdr1 drug transporters. We first injected mdr1(+/+) mice with [(14)C]N-methyl erythromycin and measured the rate of appearance of (14)CO(2) in the breath as a measure of hepatic CYP3A activity. Animals treated with CYP3A inducers or inhibitor showed accelerated or diminished (14)CO(2) in the breath, respectively. The erythromycin breath test was next administered to mdr1a(-/-) and mdr1a/1b(+/+) and (-/-) mice. These animals had equivalent levels of immunoreactive CYP3A and CYP3A activity as measured by erythromycin N-demethylase activity in liver microsomes. Nevertheless, the rate of (14)CO(2) appearance in the breath showed no relationship with these measurements of CYP3A, but changed proportionally to expression of mdr1. The average breath test (14)CO(2) area under the curves were 1.9- and 1.5-fold greater in mdr1a/1b(-/-) and mdr1a(-/-) mice, respectively, compared with (+/+) mice, and CER(max) was 2-fold greater in mdr1a/1b(-/-) compared with (+/+) mice. We conclude that Pgp, by limiting intracellular substrate availability can be an important determinant of CYP3A metabolism of numerous medications that are substrates for CYP3A and Pgp.
authors.affiliation: Department of Pharmaceutical Sciences, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA.
authors.forename: L B
authors.initials: LB
authors.lastname: Lan
authors.forename: J T
authors.initials: JT
authors.lastname: Dalton
authors.forename: E G
authors.initials: EG
authors.lastname: Schuetz
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D001944:Breath Tests; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D020536:Enzyme Activators / Q000494:pharmacology; D004790:Enzyme Induction; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004917:Erythromycin / Q000378:metabolism; D005838:Genotype; D008099:Liver / Q000201:enzymology / Q000378:metabolism; D008297:Male; D051379:Mice; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D065607:Cytochrome P-450 Enzyme Inhibitors; D020536:Enzyme Activators; D004791:Enzyme Inhibitors; D004917:Erythromycin; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating