title: Methadone pharmacokinetics are independent of cytochrome P4503A (CYP3A) activity and gastrointestinal drug transport: insights from methadone interactions with ritonavir/indinavir.
abstract: BACKGROUND
  Methadone clearance is highly variable, and drug interactions are problematic. Both have been attributed to CYP3A, but actual mechanisms are unknown. Drug interactions can provide such mechanistic information. Ritonavir/indinavir, one of the earliest protease inhibitor combinations, may inhibit CYP3A. We assessed ritonavir/indinavir effects on methadone pharmacokinetics and pharmacodynamics, intestinal and hepatic CYP3A activity, and intestinal transporters (P-glycoprotein) activity. CYP3A and transporters were assessed with alfentanil and fexofenadine, respectively.
  
  
  METHODS
  Twelve healthy human immunodeficiency virus-negative volunteers underwent a sequential three-part crossover. On three consecutive days, they received oral alfentanil/fexofenadine, intravenous alfentanil, and intravenous plus oral (deuterium-labeled) methadone, repeated after acute (3 days) and steady-state (2 weeks) ritonavir/indinavir. Plasma and urine analytes were measured by mass spectrometry. Opioid effects were assessed by miosis.
  
  
  RESULTS
  Alfentanil apparent oral clearance was inhibited more than 97% by both acute and steady-state ritonavir/indinavir, and systemic clearance was inhibited more than 90% due to diminished hepatic and intestinal extraction. Ritonavir/indinavir increased fexofenadine area under the plasma concentration-time curve four- to five-fold, suggesting significant inhibition of gastrointestinal P-glycoprotein. Ritonavir/indinavir slightly increased methadone N-demethylation, but it had no significant effects on methadone plasma concentrations or on systemic or apparent oral clearance, renal clearance, hepatic extraction or clearance, or bioavailability. Ritonavir/indinavir had no significant effects on methadone plasma concentration-effect relationships.
  
  
  CONCLUSIONS
  Inhibition of both hepatic and intestinal CYP3A activity is responsible for ritonavir/indinavir drug interactions. Methadone disposition was unchanged, despite profound inhibition of CYP3A activity, suggesting little or no role for CYP3A in clinical methadone metabolism and clearance. Methadone bioavailability was unchanged, despite inhibition of gastrointestinal P-glycoprotein activity, suggesting that this transporter does not limit methadone intestinal absorption.
authors.affiliation: Division of Clinical and Translational Research, Department of Anesthesiology, Washington University in St. Louis, St. Louis, Missouri 63110-1093, USA. kharasch@wustl.edu
authors.forename: Evan D
authors.initials: ED
authors.lastname: Kharasch
authors.forename: Christine
authors.initials: C
authors.lastname: Hoffer
authors.forename: Dale
authors.initials: D
authors.lastname: Whittington
authors.forename: Alysa
authors.initials: A
authors.lastname: Walker
authors.forename: Pamela Sheffels
authors.initials: PS
authors.lastname: Bedynek
mesh_terms: D000293:Adolescent; D000328:Adult; D018592:Cross-Over Studies; D051544:Cytochrome P-450 CYP3A / Q000378:metabolism*; D004347:Drug Interactions / Q000502:physiology; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D005260:Female; D041981:Gastrointestinal Tract / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D019469:Indinavir / Q000378:metabolism*; D007408:Intestinal Absorption / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008691:Methadone / Q000493:pharmacokinetics*; D019438:Ritonavir / Q000378:metabolism*; D055815:Young Adult
chemical_list: D019469:Indinavir; D051544:Cytochrome P-450 CYP3A; D019438:Ritonavir; D008691:Methadone