title: Mechanism of ritonavir changes in methadone pharmacokinetics and pharmacodynamics: II. Ritonavir effects on CYP3A and P-glycoprotein activities.
abstract: Ritonavir diminishes methadone plasma concentrations, an effect attributed to CYP3A induction, but the actual mechanisms are unknown. We determined short-term (2-day) and steady-state (2-week) ritonavir effects on intestinal and hepatic CYP3A4/5 (probed with intravenous (IV) and oral alfentanil (ALF) and with miosis) and P-glycoprotein (P-gp) (fexofenadine), and on methadone pharmacokinetics and pharmacodynamics in healthy volunteers. Acute ritonavir increased the area under the concentration-time curve (AUC)(0-infinity)/dose ratio (ritonavir/control) for oral ALF 25-fold. Steady-state ritonavir increased the AUC(0-Infinity)/dose ratio for IV and oral ALF 4- and 10-fold, respectively; reduced hepatic extraction (from 0.26 to 0.07) and intestinal extraction (from 0.51 to 0); and increased bioavailability (from 37 to 95%). Acute ritonavir inhibits first-pass CYP3A > 96%. Chronic ritonavir inhibits hepatic CYP3A (> 70%) and first-pass CYP3A (> 90%). Acute and steady-state ritonavir increased the fexofenadine AUC(0-infinity) 2.8- and 1.4-fold, respectively, suggesting P-gp inhibition. Steady-state compared with acute ritonavir caused mild apparent induction of P-gp and hepatic CYP3A, but net inhibition still predominated. Ritonavir inhibited both intestinal and hepatic CYP3A and drug transport. ALF miosis noninvasively determined CYP3A inhibition by ritonavir.
authors.affiliation: Division of Clinical and Translational Research, Department of Anesthesiology, Washington University, St. Louis, Missouri, USA. kharasch@wustl.edu
authors.forename: E D
authors.initials: ED
authors.lastname: Kharasch
authors.forename: P S
authors.initials: PS
authors.lastname: Bedynek
authors.forename: A
authors.initials: A
authors.lastname: Walker
authors.forename: D
authors.initials: D
authors.lastname: Whittington
authors.forename: C
authors.initials: C
authors.lastname: Hoffer
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000328:Adult; D015760:Alfentanil / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics; D019540:Area Under Curve; D001682:Biological Availability; D018592:Cross-Over Studies; D051544:Cytochrome P-450 CYP3A / Q000502:physiology*; D065692:Cytochrome P-450 CYP3A Inhibitors; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D017320:HIV Protease Inhibitors / Q000493:pharmacokinetics / Q000494:pharmacology*; D006801:Humans; D007422:Intestines / Q000201:enzymology; D008099:Liver / Q000201:enzymology; D008297:Male; D008691:Methadone / Q000493:pharmacokinetics* / Q000494:pharmacology; D009294:Narcotics / Q000493:pharmacokinetics* / Q000494:pharmacology; D011680:Pupil / Q000187:drug effects; D019438:Ritonavir / Q000493:pharmacokinetics / Q000494:pharmacology*; D013237:Stereoisomerism; D016593:Terfenadine / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000097:blood / Q000493:pharmacokinetics
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D065692:Cytochrome P-450 CYP3A Inhibitors; D017320:HIV Protease Inhibitors; D009294:Narcotics; D015760:Alfentanil; D016593:Terfenadine; C093230:fexofenadine; D051544:Cytochrome P-450 CYP3A; D019438:Ritonavir; D008691:Methadone