Is Citalopram / Escitalopram sold as a single enantiomer or racemic mixture?

Racemic citalopram (Celexa) and pure S-citalopram (Lexapro) both available

Citalopram / Escitalopram Enantiomers - Chirality in Pharmacology

What happens with Citalopram / Escitalopram enantiomers?

Citalopram / Escitalopram is a SSRI (Antidepressant). The S-enantiomer is Highly selective serotonin reuptake inhibitor, while the R-enantiomer is Weakly active, may partially antagonize S-form. Escitalopram (S-citalopram) is a "chiral switch" success story. It is more selective and causes fewer side effects than racemic citalopram. The R-enantiomer may actually interfere with the S-form's binding.

Drug Name	Citalopram / Escitalopram
Drug Class	SSRI (Antidepressant)
R-Enantiomer Activity	Weakly active, may partially antagonize S-form
S-Enantiomer Activity	Highly selective serotonin reuptake inhibitor
FDA Status	Racemic citalopram (Celexa) and pure S-citalopram (Lexapro) both available
Year of Discovery	1989
Clinical Significance	Escitalopram (S-citalopram) is a "chiral switch" success story. It is more selective and causes fewer side effects than racemic citalopram. The R-enantiomer may actually interfere with the S-form's binding.

Introduction

Citalopram is a selective serotonin reuptake inhibitor (SSRI) used to treat depression and anxiety. The racemic form (Celexa) contains both enantiomers, but the pure S-form was developed as a superior drug.

Structure Comparison

Both enantiomers share a bicyclic ring system with a fluorine substituent, a cyano group, and a dimethylaminopropyl side chain. The chiral center is at the junction connecting the cyano group and side chain to the ring system.

Chiral Center

The chiral center bears four different groups: the fluorinated bicyclic ring system, a cyano group (-CN), the dimethylaminopropyl chain, and the oxygen-containing ring fragment. This quaternary-like center creates significant 3D differences.

Pharmacological Differences

S-citalopram binds with high affinity and selectivity to the serotonin transporter (SERT). R-citalopram binds weakly and may actually partially block the S-form's binding site, reducing its effectiveness in the racemic mixture.

Clinical Impact

Forest Laboratories developed escitalopram (Lexapro) as a "chiral switch" from racemic citalopram (Celexa). Lexapro showed improved efficacy at lower doses and became one of the most prescribed antidepressants worldwide.

Key Takeaway

The citalopram case is unique because the R-enantiomer is not just inactive - it may actively interfere with the therapeutic S-form. Removing it creates a cleaner, more effective drug with fewer side effects.

Visualize R and S enantiomers side by side with activity indicators and pharmacological profiles.

Compare Citalopram / Escitalopram Enantiomers in 3D