BACKGROUND: Single isomers of the selective serotonin reuptake inhibitors citalopram (escitalopram, S-citalopram) and fluoxetine (R-fluoxetine) are currently under development for the treatment of depression and other psychiatric disorders. Previous studies conducted in laboratory animals have revealed that the biological effects on serotonin reuptake for citalopram reside in the S enantiomer. In contrast, both enantiomers of fluoxetine contribute to its biological activity. METHODS: In the present study, the potency and selectivity of escitalopram, R-fluoxetine, and all of the other currently available selective serotonine reuptake ihibitors were compared for binding affinity at the human serotonin, norepinephrine, and dopamine transporters and several select neurotransmitter receptors using radioligand binding assays. RESULTS: Both escitalopram and R-fluoxetine were potent inhibitors of the serotonin transporter (Ki=1,1 and 1,4 nmol/L, respectively). escitalopram was the most serotonin transporter-selective compound tested and was approximately 30 fold more potent than R-citalopram. CONCLUSIONS: As noted previously, paroxetine and sertraline possess moderate affinity (<50 nmol/L) for the human norepinephrine transporter and dopamine transporter, respectively. R-fluoxetine, unlike the other selective serotonin reuptake inhibitors, possesses moderate affinity (Ki=64 nmol/L) for the serotonin 2C receptor. Potential clinical correlates of these unique attributes of escitalopram and R-fluoxetine are discussed. (Biol Psychiatry 2001; 50: 345-350 " 2001 Society of Biological Psychiatry).