[Gas chromatographic separation of the diastereomers of labetalol as n-butylboronate]

Sensitive high-performance liquid chromatographic method for the determination of labetalol diastereoisomers in plasma samples without derivatization

A direct high-performance liquid chromatographic assay for the determination of labetalol diastereoisomers in plasma without derivatization was developed. Baseline resolution of diastereoisomers was accomplished on a C18 bonded reversed-phase polymeric column with a basic (pH 11.5) mobile phase and isocratic elution. Sample treatment was optimized in order to achieve a complete extraction of labetalol diastereoisomers and to avoid racemization during extraction. Fluorimetric detection improved the selectivity and afforded a detection limit of 3 ng/ml for each diastereoisomer. This method is suitable for routine quantification of labetalol diastereoisomers and has been applied to a pharmacokinetic study in small laboratory animals.

Reversed-phase high-performance liquid chromatographic separation of the stereoisomers of labetalol via derivatization with chiral and non-chiral isothiocyanate reagents

The antihypertensive agent labetalol is a mixture of two racemates. We report reversed-phase high-performance liquid chromatographic (HPLC) methodology for the separation of the four stereoisomers of labetalol via derivatization with the chiral reagent (4S-cis)-2,2-dimethyl-5-isothiocyanato-4-phenyl-1,3-dioxane. The derivatives were separated on octadecylsilane columns with a methanol-ammonium phosphate buffer mixture as mobile phase. Separations of the diastereomeric forms of labetalol were achieved with the non-chiral derivatizing reagents benzyl isothiocyanate and 1-naphthalenemethyl isothiocyanate. In all cases the derivatives of the R,S/S,R forms eluted before those of the R,R/S,S forms. Isothiocyanates may have general utility in stereoisomer separations of amines by HPLC.