title: Omeprazole preferentially inhibits the metabolism of (+)-(S)-citalopram in healthy volunteers.
abstract: BACKGROUND
  Citalopram (CITA) pharmacokinetics are enantioselective in healthy volunteers and the metabolism of (+)-(S)-CITA to (+)-(S)-DCITA is dependent on CYP2C19. Omeprazole is a potent CYP2C19 inhibitor.
  
  
  CONCLUSIONS
  This study indicates that omeprazole induces a loss of enantioselectivity in the CITA pharmacokinetics because of the selective inhibition of (+)-(S)-CITA metabolism.
  
  
  OBJECTIVE
  The study assessed the influence of omeprazole on the kinetic disposition of the (+)-(S)-citalopram (CITA) and (-)-(R)-CITA enantiomers in healthy volunteers.
  
  
  METHODS
  In a cross-over study, healthy volunteers (n = 9) phenotyped as extensive metabolizers of CYP2C19 and CYP2D6 and with an oral midazolam clearance ranging from 10.9 to 149.3 ml min(-1) kg(-1) received a single dose of racemic CITA (20 mg orally) in combination or not with omeprazole (20 mg day(-1) for 18 days). Serial blood samples were collected up to 240 h after CITA administration. CITA and demethylcitalopram (DCITA) enantiomers were analyzed by LC-MS/MS using a Chiralcel OD-R column.
  
  
  RESULTS
  The kinetic disposition of CITA was enantioselective in the absence of treatment with omeprazole, with the observation of a greater proportion of plasma (-)-(R)-CITA [AUC S:R ratio of 0.53 (95% CI 0.41, 0.66) for CITA and 1.08 (95% CI 0.80, 1.76) for DCITA] than (+)-(S)-CITA. Racemic CITA administration to healthy volunteers in combination with omeprazole showed a loss of enantioselectivity in CITA pharmacokinetics with an increase of approximately 120% in plasma (+)-(S)-CITA concentrations [AUC S:R ratio of 0.95 (95% CI 0.72, 1.10) for CITA and 0.95 (95% CI 0.44, 1.72) for DCITA].
  
  
  CONCLUSIONS
  The administration of multiple doses of omeprazole preferentially inhibited (+)-(S)-CITA metabolism in healthy volunteers. Although omeprazole increased plasma concentrations of (+)-(S)-CITA by approximately 120%, it is difficult to evaluate the clinical outcome because the range of plasma CITA concentrations related to maximum efficacy and minimum risk of adverse effects has not been established.
authors.affiliation: Departamento de Análises Clínicas, Toxicológicas e Bromatológicas, Faculdade de Ciências Farmacêuticas de Ribeirão Preto-Universidade de São Paulo, Ribeirão Preto, SP, Brazil.
authors.forename: Adriana
authors.initials: A
authors.lastname: Rocha
authors.forename: Eduardo B
authors.initials: EB
authors.lastname: Coelho
authors.forename: Stefânia A
authors.initials: SA
authors.lastname: Sampaio
authors.forename: Vera L
authors.initials: VL
authors.lastname: Lanchote
mesh_terms: D000328:Adult; D001189:Aryl Hydrocarbon Hydroxylases; D015283:Citalopram / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism / Q000493:pharmacokinetics*; D018592:Cross-Over Studies; D065731:Cytochrome P-450 CYP2C19; D004347:Drug Interactions; D005260:Female; D006801:Humans; D008297:Male; D009853:Omeprazole / Q000008:administration & dosage / Q000493:pharmacokinetics*; D017367:Selective Serotonin Reuptake Inhibitors / Q000008:administration & dosage / Q000493:pharmacokinetics*; D055815:Young Adult
chemical_list: D001189:Aryl Hydrocarbon Hydroxylases; D015283:Citalopram; D065731:Cytochrome P-450 CYP2C19; D009853:Omeprazole; D017367:Selective Serotonin Reuptake Inhibitors; C045793:CYP2C19 protein, human