S-fluoxetine is an enantiomer of fluoxetine, a potent and selective inhibitor of the neuronal serotonin-uptake carrier and a clinically effective antidepressant. R-fluoxetine and S-fluoxetine enantiomers are nearly equipotent at blocking serotonin reuptake. In all biochemical and pharmacological studies, the eudismic ratio for the fluoxetine enantiomers is also near unity. A study examining the relative contributions of CYP enzymes to the metabolism of S-fluoxetine, and R-fluoxetine found dramatic differences. These data led to discovery programs between Lilly and Sepracor for the individual enantiomers. S-fluoxetine was studied in a phase 2 clinical trial for the prophylaxis of migraine, however, development was discontinued.

The selective serotonin reuptake inhibitor fluoxetine consists of equal amounts of R and S stereoisomers (antidepressant Prozac (racemic fluoxetine). Binding to the transporter confirmed selectivity of R- and S-fluoxetine for the 5-HT transporter versus the dopamine (DA) and norepinephrine (NE) human transporters. In addition, receptor binding studies demonstrated significant affinity of R-fluoxetine, but not S-fluoxetine, for human 5-HT(2A) and 5-HT(2C) receptor subtypes. R-fluoxetine acts as an antagonist at 5-HT(2A) and 5-HT(2C) receptors. The attempt to develop a single-enantiomer formulation of fluoxetine for the treatment of depression was unsuccessful. Eli Lilly has terminated its licensing and development agreement with Marlborough, Mass.-based Sepracor for (R)-fluoxetine, the single-isomer version of Lilly's antidepressant Prozac.R-enantiomer of fluoxetine, at its highest administered dose, led to statistically significant prolongation of cardiac repolarization in phase II studies; the studies were subsequently stopped.