title: Association of tamoxifen biliary excretion rate with prior tamoxifen exposure and increased mdr1b expression.
abstract: ATPase transporter proteins are commonly found in the hepatocyte canalicular membrane. Some of these, in particular the multidrug resistance (mdr1b) gene, have been previously demonstrated to be inducible genes. In this study, we found that tamoxifen induced expression of the mdr1b gene in the liver up to 40-fold after 14 days' exposure to tamoxifen in the diet at a concentration of 420 ppm. As tamoxifen and its metabolites are primarily excreted into the bile, we investigated if the increased expression of mdr1b in the liver following tamoxifen exposure had any effect on its excretion in rats. We found that the excretion of tamoxifen and its metabolites into bile was increased from 8 +/- 1% to 51 +/- 18% (mean +/- SD) of an administered dose of 180 nmol/kg over a collection period of 3 hr in rats that had received tamoxifen (35 mg/kg) orally for 12 days (plus a 3-day rest) prior to the experiment. These data suggest that prolonged treatment with tamoxifen may result in lower serum and tumour concentrations, due to a self-mediated enhancement of excretion via mdr1b gene-encoded P-glycoprotein. This may have implications for other drugs sharing the same route of excretion and co-administered with tamoxifen.
authors.affiliation: MRC Toxicology Unit, University of Leicester, Hodgkin Building, P.O. Box 138, Lancaster Road, LE1 9HN, Leicester, UK.
authors.forename: J
authors.initials: J
authors.lastname: Riley
authors.forename: J
authors.initials: J
authors.lastname: Styles
authors.forename: R D
authors.initials: RD
authors.lastname: Verschoyle
authors.forename: L A
authors.initials: LA
authors.lastname: Stanley
authors.forename: I N
authors.initials: IN
authors.lastname: White
authors.forename: T W
authors.initials: TW
authors.lastname: Gant
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000493:pharmacokinetics / Q000494:pharmacology; D001648:Bile Canaliculi / Q000378:metabolism*; D015870:Gene Expression / Q000187:drug effects; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008657:Metabolic Clearance Rate; D051381:Rats; D011917:Rats, Inbred Lew; D013629:Tamoxifen / Q000493:pharmacokinetics* / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D018931:Antineoplastic Agents, Hormonal; D013629:Tamoxifen