Improved HPLC determination of fluoxetine and norfluoxetine in human plasma

Summary An HPLC method with fluorescence detection has been developed for the determination of fluoxetine and its main metabolite norfluoxetine in human plasma. Pretreatment of the biological samples by liquid-liquid extraction was used to improve the sensitivity of a previously published SPE procedure. The method uses 200 μL plasma and recovery is good for both analytes. On a C₈ column with a mixture of perchlorate buffer and acetonitrile as mobile phase fluoxetine, norfluoxetine and the internal standard (paroxetine) were eluted in less than 9 min, without interference from the biological matrix. Response for both analytes was linearly dependent on concentration over the range 2.5–500 ng mL⁻¹, and repeatability (RSD%) was <4%. The limit of detection was 1 ng mL⁻¹ for both fluoxetines. Application to plasma samples from depressed patients treated with fluoxetine gave good results. There was no interference from other common CNS drugs. This method seems to be a useful tool for clinical monitoring, because it requires small plasma samples and is highly sensitive and highly selective.