Stability of fluoxetine in stored plasma, aqueous, and methanolic solutions determined by HPLC with UV detection

The stability of fluoxetine was studied in plasma, and in aqueous and methanolic solutions at a concentration of 6 micrograms/ml under different storage temperatures and time intervals up to three months. Fluoxetine exhibited good stability at -20 and 5 degrees C, but was unstable at room temperature under the same conditions. A significant loss was observed at the second, third and fifth weeks in plasma, aqueous solution and methanolic solution respectively (P > 0.95). At the end of the experiment, the amount of fluoxetine-recovered was at least 55.25% regardless of the storage conditions. Chromatography was performed using a C8 column and the mobile phase consisted of methanol/acetonitrile/triethylamine solution (35:20:45) adjusted to pH 5.5. UV detection was at 230 nm. Fluoxetine was isolated from plasma by liquid-liquid extraction (LLE) with dichloromethane as extracting solvent after addition of 20% ammonia solution. The standard curve was linear over the range of 0.05-10 micrograms/ml. The inter-day coefficient of variation and the lower detectable limit were 6.92% and 0.05 microgram/ml respectively.