Fluoxetine hydrochloride (fluoxetine) was developed in 1973 by Bryan Molly, David Wong and Roy Fuller of the Eli Lilly corporation and is today sold under the brand name Prozac (Fieve, 1994). The scientific name for fluoxetine HCL is even longer: (+)-N-methyl-3-phenyl-3-[(a,a,a-trifluoro-p-toyl)oxy] propylamine hydrochloride. Fluoxetine was designed specifically to block the reuptake of serotonin back into the pre-synaptic neuron. Structurally, fluoxetine is an enantiomer in racemic mixture (Arky, 1994). This means that there are two compounds with the same name that have the same chemical properties but a different physical structure as the result of a chiral carbon atom. A carbon atom can have up to four possible bonds, and with four different groups bonded, can have two different configurations. The condition of having four different groups makes the carbon atom chiral. If the chiral carbon is pictured with the smallest base (usually hydrogen) facing the observer, the three other bases can be seen as points on a circular plane. The highest atomic number on each remaining base is used to determine order. If the bases descend in a clockwise order, the compound is designated as the R enantiomer. If the bases descend counterclockwise, it is the S (for "Sinister") enantiomer.
Both enantiomers of fluoxetine are equally potent in function. However the S-enantiomer is eliminated more slowly from the body, and therefore present in greater quantity at a steady state. Fluoxetine is metabolized in the liver into nor-fluoxetine and inactive components. The S-enantiomer of nor-fluoxetine is essentially equivalent in action to R or S-Fluoxetine, however the R-enantiomer is less potent. Fluoxetine has a relatively slow elimination, with a half-life of one to three days for acute administration and four to six days under chronic administration. The half-life of nor-fluoxetine is four to sixteen days, with a average of 8.6 to 9.3 days for complete elimination (Arky, 1994). This means that a good amount of the re-uptake inhibition of serotonin is actually performed by S-nor-fluoxetine.
The metabolism of fluoxetine does not appear to depend upon concentration of the drug or the age of an individual. Plasma concentrations are stable over time, a patient who has taken fluoxetine for three years will have about the same plasma concentration as someone medicated for four weeks. The long elimination time ensures that there will be residual active drug substance in the patient for weeks after dosage is discontinued (Arky, 1994).