SENSITIVE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC METHOD FOR DIRECT SEPARATION OF LABETALOL STEREOISOMERS IN BIOLOGICAL-FLUIDS USING AN ALPHA-1-ACID GLYCOPROTEIN STATIONARY-PHASE

A chiral high-performance liquid chromatographic assay for the separat ion of the four stereoisomers of labetalol, an antihypertensive, in bi ological fluids has been developed. Baseline separation of the isomers was achieved using an alpha1-acid glycoprotein stationary phase. No i nterference from endogenous substances was observed following extracti on from various biological fluids obtained from pregnant (ewe and fetu s) and non-pregnant sheep. The concentration of the individual isomers of labetalol was determined by first measuring the total concentratio n of racemic labetalol obtained from an achiral assay followed by reas say of each sample by the chiral method after which, by using the esti mate of the percentage of each individual isomer, the individual conce ntration of each of the four isomers was determined. The mobile phase was 0.02 M phosphate buffer containing 0.015 M tetrabutylammonium phos phate. The pH of the mobile phase was adjusted to 7.10. The detector w as set at an excitation wavelength of 230 nm and emission wavelength o f 400 nm to monitor the nascent fluorescence intensity of the isomers of labetalol. The limit of detection of the individual isomers was 0.1 5 ng (0.6 ng of injected racemic labetalol). The assay was linear over the range 0.6-15.0 ng of labetalol (injected) with the intra- and int er-day mean coefficients of variation being less than 9.0 and 6.0%, re spectively. Application of the assay in the study of pharmacokinetics of the stereoisomers of labetalol in sheep following administration of racemic labetalol has been demonstrated.