Quantitation of the diastereoisomers of l-buthionine-(R,S)-sulfoximine in human plasma: a validated assay by capillary electrophoresis

An assay for the diastereoisomers of the biochemical modifier $\text{l}\text{-buthionine}\text{-(R,S)-}\text{sulfoximine}$ (BSO) in human plasma has been developed using capillary electrophoresis (CE). Separation of the diastereoisomers is achieved by the micellar electrokinetic chromatography (MEKC) mode of CE. Plasma is injected directly onto the separation capillary without any extraction step, and BSO is detected directly by ultraviolet absorbance measurements at 190 nm without prior derivatization. The whole assay, including capillary conditioning, takes approximately 30 min. Intra- and inter-day R.S.D. values are approximately 7% at sample concentrations around 25 μg ml^?1, and approximately 3% at sample concentrations around 500 μg ml^?1. The limit of detection in plasma is 3.9 μg ml^?1 (S/N = 2). The assay has been used to quantitate the diastereoisomers of BSO in patient samples in a pharmacokinetic study.