Comparative bioavailability of fluoxetine after transdermal and oral administration to healthy cats

Objective-To determine bioavailability, pharmacokinetics, and safety for transdermal (TD) and oral administration of fluoxetine hydrochloride to healthy cats. Animals-12 healthy mixed-breed sexually intact 1- to 4-year-old purpose-bred cats. Procedure-A single-dose pharmacokinetic study involving 3 groups of 4 cats each was conducted in parallel. Fluoxetine in a formulation of pluronic lecithin organogel (PLO gel) was applied to the hairless portion of the pinnae of cats at 2 dosages (5 or 10 mg/kg), or it was administered orally in capsules at a dosage of I mg/kg. Plasma samples were obtained and submitted for liquid chromatography-mass spectrometry-mass spectrometry analysis of fluoxetine and its active metabolite, norfluoxetine. Results-Peak fluoxetine concentration (C-max) was lower and time to C-max longer for TD administration versus oral administration. Relative bioavailability of each dose administered via the TD route was 10% of the value for oral administration of the drug. Mean plasma elimination half-life after oral administration was 47 and 55 hours for fluoxetine and norfluoxetine, respectively. Conclusions and Clinical Relevance-This study provides evidence that fluoxetine in a 15% (wt:vol) PLO gel formulation can be absorbed through the skin of cats into the systemic circulation. However, the relative bioavailability for TD administration is approximately only 10% of that for the oral route of administration.