CAPILLARY ELECTROPHORESIS FRONTAL ANALYSIS FOR MICROANALYSIS OF ENANTIOSELECTIVE PROTEIN-BINDING OF A BASIC DRUG

A new HPCE method was developed for the enantioselective determination of the unbound concentration of a basic drug under plasma protein bin ding equilibrium, The racemic basic drug verapamil (VER) and human ser um albumin mixed solution was used as a model sample solution. The sam ple solution was introduced into a fused-silica capillary hydrodynamic ally or electrokinetically. During the electrophoresis following hydro dynamic injection, the unbound drug zone migrated apart from the sampl e zone and was separated into two zonal peaks with a plateau due to en antiomers by a chiral selector (trimethyl-beta-cyclodextrin) dissolved in the acidic running buffer solution (pH 2.5). By the electrokinetic introduction of the same sample solution from the anodic end, only th e unbound drug entered the capillary and was separated into the enanti omers, which also gave the zonal peaks with plateau. The unbound conce ntration of each enantiomer was determined from the plateau peak heigh t. The results obtained by the different methods for sample introducti on agreed wed with those determined by conventional ultrafiltration-ch iral HPLC, which was employed as a reference method. The unbound conce ntration of (S)-VER was 1.7 times higher than that of the antipode. Th e sample size used in the present method was similar to 200 nL, which is about one-thousandth of that in the reference method. The electroki netic introduction gave a better peak shape than the hydrodynamic intr oduction.