Determination of modafinil, modafinil acid and modafinil sulfone in human plasma utilizing liquid–liquid extraction and high-performance liquid chromatography

An assay was developed to determine concentrations of modafinil (dl-2-[(diphenylmethyl)sulfinyl]acetamide; Provigil®) and its two major circulating metabolites, modafinil acid and modafinil sulfone, in human plasma. The assay utilized liquid–liquid extraction of the analytes and an internal standard, (phenylthio)acetic acid, from plasma into a mixture of hexane–dichloromethane–glacial acetic acid (55:45:2, v/v). The analytes were resolved isocratically on a narrow-bore phenyl column at a mobile phase flow-rate of 0.3 ml/min and were monitored by UV detection at 235 nm. The method reported herein reduces the required sample volume of previously reported methods from 1.00 to 0.200 ml of plasma while lowering the limit of quantification (LOQ). The linear range of the assay was from 0.100 to 20.0 μg/ml for each of the three compounds.