title: Tariquidar, a selective P-glycoprotein inhibitor, does not potentiate loperamide's opioid brain effects in humans despite full inhibition of lymphocyte P-glycoprotein.
abstract: BACKGROUND
  Loperamide, a potent opioid, has been used as an in vivo probe to assess P-glycoprotein activity at the blood-brain barrier, because P-glycoprotein inhibition allows loperamide to cross the blood-brain barrier and exert its central opioid effects. In humans, studies with nonselective and moderately potent inhibitors resulted in mild opioid effects but were confounded by the concurrent inhibition of loperamide's metabolism. The authors studied the effect of the highly selective, potent P-glycoprotein inhibitor tariquidar on loperamide's central opioid effects.
  
  
  METHODS
  In a randomized, double-blind, crossover study, nine healthy subjects received on 2 study days oral loperamide (32 mg) followed by an intravenous infusion of either tariquidar (150 mg) or placebo. Central opioid effects (pupil diameter, sedation) were measured for 12 h, and blood samples were drawn up to 48 h after drug administration to determine plasma loperamide concentrations and ex vivo P-glycoprotein activity in T lymphocytes. Values for pupil diameter and loperamide concentrations were plotted over time, and the areas under the curves on the tariquidar and placebo study day were compared within each subject.
  
  
  RESULTS
  Tariquidar did not significantly affect loperamide's central effects (median reduction in pupil diameter area under the curve, 6.9% [interquartile range, -1.4 to 12.1%]; P = 0.11) or plasma loperamide concentrations (P = 0.12) but profoundly inhibited P-glycoprotein in lymphocytes by 93.7% (95% confidence interval, 92.0-95.3%).
  
  
  CONCLUSIONS
  These results suggest that despite full inhibition of lymphocyte P-glycoprotein, the selective P-glycoprotein inhibitor tariquidar does not potentiate loperamide's opioid brain effects in humans.
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Kurnik
authors.forename: Gbenga G
authors.initials: GG
authors.lastname: Sofowora
authors.forename: John P
authors.initials: JP
authors.lastname: Donahue
authors.forename: Usha B
authors.initials: UB
authors.lastname: Nair
authors.forename: Grant R
authors.initials: GR
authors.lastname: Wilkinson
authors.forename: Alastair J J
authors.initials: AJ
authors.lastname: Wood
authors.forename: Mordechai
authors.initials: M
authors.lastname: Muszkat
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors* / Q000097:blood; D000328:Adult; D000701:Analgesics, Opioid / Q000008:administration & dosage* / Q000097:blood; D001812:Blood-Brain Barrier / Q000187:drug effects / Q000378:metabolism; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D018592:Cross-Over Studies; D004311:Double-Blind Method; D004357:Drug Synergism; D005260:Female; D006801:Humans; D008139:Loperamide / Q000008:administration & dosage* / Q000097:blood; D008214:Lymphocytes / Q000097:blood / Q000187:drug effects*; D008297:Male; D011680:Pupil / Q000187:drug effects / Q000502:physiology; D011804:Quinolines / Q000008:administration & dosage* / Q000097:blood; D055815:Young Adult
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000701:Analgesics, Opioid; D011804:Quinolines; D008139:Loperamide; C402343:tariquidar