Simultaneous determination of fluoxetine and norfluoxetine in dried blood spots using high-performance liquid chromatography-tandem mass spectrometry

Background: Therapeutic drug monitoring (TDM) of the widely prescribed antidepressant fluoxetine (FLU) is recommended in certain situations, such as occurrence of toxicity, inadequate response or suspect of poor adherence. Dried blood spot (DBS) sampling is an increasingly studied alternative for TDM, particularly for outpatients, due to its ease of collection and inherent stability. Objectives: The aim of this study was to develop and validate an LC-MS/MS assay for the simultaneous quantification of FLU and norfluoxetine (NFLU) in DBS. Design and methods: The assay is based on a liquid extraction of single DBS with 8 mm of diameter, using FLU-D6 as the internal standard, followed by reversed phase separation in an Accucore (R) C18 column (100 x 2.1 mm, 2.6 mu m). Mobile phase was composed of water and acetonitrile (gradient from 80: 20 to 50: 50, v/v), both containing formic acid 0.1%. The assay was validated and applied to 30 patients under FLU pharmacotherapy. Results: The assay was linear in the range 10-750 ng mL(-1). Precision assays presented CV% of 3.13-9.61 and 3.54-7.99 for FLU and NFLU, respectively, and accuracy in the range of 97.98-110.44% and 100.25-105.8%. FLU and NFLU were stable at 25 and 45 degrees C for 7 days. The assay was evaluated in 30 patients under FLU treatment. Concentrations of both compounds were higher in DBS than in plasma, and the use of the multiplying factors 0.71 and 0.68 for FLU and NFLU, respectively, allowed acceptable estimation of plasma concentrations, with median prediction bias of -0.55 to 0.55% and mean differences of 0.4 to 2.2 ng mL(-1). Conclusions: The presented data support the clinical use of DBS for therapeutic drug monitoring of FLU.