Pharmacokinetics and pharmacodynamics of norfluoxetine in rats: Increasing extracellular serotonin level in the frontal cortex.

Norfluoxetine is the most important active metabolite of the widely used antidepressant fluoxetine. Although the pharmacokinetics/pharmacodynamics (PK/PD) relationship and neurochemical profile of fluoxetine is well characterized in human and in animals, little is known about the effect of its metabolite. The aim of this study was to characterize extracellular level of serotonin (5-hydroxytryptamine, 5-HT)-time profile of norfluoxetine after acute administration over 18 h post dose and to establish the relationship between this pharmacodynamic (PD) profile and its pharmacokinetic (PK) properties. Following subcutaneous administration of fluoxetine in rats, plasma and brain PK of fluoxetine and norfluoxetine were monitored respectively by liquid chromatography/tandem mass spectrometry (LC/MS/MS). The extracellular level of 5-HT in the frontal cortex was measured by microdialysis as a PD endpoint. Norfluoxetine when directly administrated to rats caused a significant increase in extracellular level of 5-HT in the frontal cortex and maintained for 18 h. This result is correlated well with higher plasma and brain concentration and longer plasma and brain retention time of norfluoxetine. Our results showed that norfluoxetine contributes to 5-HT transporter inhibition and extends fluoxetine efficacy.