What is the difference between citalopram and escitalopram?

Composition

Citalopram is a racemic mixture, (i.e. a mixture of the S-enantiomer and the R-enantiomer in equal proportions), whereas escitalopram is the purified S-enantiomer.

Pharmacodynamics

Both citalopram and escitalopram are selective serotonin reuptake inhibitors (SSRIs). However, studies have shown that the S-enantiomer of citalopram is the active enantiomer, whilst the R-enantiomer virtually lacks 5-HT-reuptake blocking activity. Newer studies have shown that the R-enantiomer actually counteracts the effect of the S-enantiomer (Sanchez 2004). Following multiple doses of citalopram it has been demonstrated that the levels of the inhibitory R-enantiomer exceed those of the S-enantiomer.

Tanum et al. (2010) analysed the serum samples taken from 88 female patients receiving treatment with citalopram. They found that the mean levels of the R-enantiomer were significantly higher than those of the S-enantiomer in all dose groups. The R/S ratio increased from 1.99 to 2.45 with an increase in the dose (p < 0.05), and the inter-individual variance in this ratio was up to four-fold on the same dosage. These differences occur because the enantiomers are metabolised via different pathways (Rochat 1998).

The S-enantiomer (Cipralex) can be described as having a serotonin dual action – it binds to two sites on the serotonin transporter (both the primary site and the allosteric site) (Sanchez 2006).

References:

Rochat B et al. Biochem Pharmacol 1998;56:15-23

Sanchez C et al. Psychopharmacology 2004;174:163-176

Sanchez C. Basic Clin Pharmacol Toxicol 2006;99:91–95

Tanum L et al. Pharmacopsychiatry 2010;43:190-193.