Quantification of baclofen and its fluoro analogue in plasma and urine after fluorescent derivatisation with benoxaprofen chloride and thin-layer chromatographic separation.

An assay method for the quantification of the centrally acting muscle relaxant baclofen in human plasma and urine is described. Baclofen is separated from biological samples using Sep-Pak C18-cartridges. The liquid-solid extraction is followed by ion-pair extraction. The following procedure involves the formation of the baclofen butyl ester and an additional ion-pair extraction of the ester. Then a fluorescent derivatisation is performed, using the fluorescence marker benoxaprofen chloride. The resulting amide is separated from interfering compounds by TLC (silica gel) and quantified by directly measuring the fluorescence (313 nm/365 nm). The procedure described can also be applied for the determination of the fluoro analogue of baclofen. The lower limit of detection is 10 ng per 1 ml plasma and 20 ng per 0.1 ml urine. The applicability of the method was proved by investigating plasma and urine samples of 2 volunteers after oral administration of 20 mg baclofen as single dose.