title: Escitalopram versus citalopram: the surprising role of the R-enantiomer.
abstract: BACKGROUND
  Citalopram is a racemate consisting of a 1:1 mixture of the R(-)- and S(+)-enantiomers. Non-clinical studies show that the serotonin reuptake inhibitory activity of citalopram is attributable to the S-enantiomer, escitalopram. A series of recent non-clinical and clinical studies comparing escitalopram and citalopram to placebo found that equivalent doses of these two drugs, i.e. containing the same amount of the S-enantiomer, showed better effect for escitalopram. These results suggested that the R-citalopram in citalopram inhibits the effect of the S-enantiomer.
  
  
  OBJECTIVE
  To review the pharmacological and non-clinical literature that describes the inhibition of escitalopram by R-citalopram, as well as the implications of this inhibition for the clinical efficacy of escitalopram compared to citalopram.
  
  
  METHODS
  The information in this review was gathered from published articles and abstracts.
  
  
  RESULTS
  In appropriate neurochemical, functional, and behavioural non-clinical experiments, escitalopram shows greater efficacy and faster onset of action than comparable doses of citalopram. The lower efficacy of citalopram in these studies is apparently due to the inhibition of the effect of the S-enantiomer by the R-enantiomer, possibly via an allosteric interaction with the serotonin transporter. Data from randomised clinical trials consistently show better efficacy with escitalopram than with citalopram, including higher rates of response and remission, and faster time to symptom relief.
  
  
  CONCLUSIONS
  The R-enantiomer present in citalopram counteracts the activity of the S-enantiomer, thereby providing a possible basis for the pharmacological and clinical differences observed between citalopram and escitalopram.
authors.affiliation: Research and Development, H. Lundbeck A/S, Ottiliavej 9, 2500 Valby Copenhagen, Denmark. cs@lundbeck.com
authors.forename: Connie
authors.initials: C
authors.lastname: Sánchez
authors.forename: Klaus P
authors.initials: KP
authors.lastname: Bøgesø
authors.forename: Bjarke
authors.initials: B
authors.lastname: Ebert
authors.forename: Elin Heldbo
authors.initials: EH
authors.lastname: Reines
authors.forename: Claus
authors.initials: C
authors.lastname: Braestrup
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D015283:Citalopram / Q000737:chemistry / Q000627:therapeutic use*; D003865:Major Depressive Disorder / Q000188:drug therapy*; D004347:Drug Interactions; D006801:Humans; D051379:Mice; D016584:Panic Disorder / Q000188:drug therapy*; D016032:Randomized Controlled Trials as Topic; D051381:Rats; D017367:Selective Serotonin Reuptake Inhibitors / Q000737:chemistry / Q000627:therapeutic use*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship
chemical_list: D015283:Citalopram; D017367:Selective Serotonin Reuptake Inhibitors