Frontal analysis of drug-plasma lipoprotein binding using capillary electrophoresis

High performance frontal analysis coupled with capillary electrophoresis (HPFA/CE) was applied to the ultramicroanalysis of enantioselective binding of drug to plasma lipoproteins. A small volume (ca. 80 nl) of (R)- or (S)-propranolol (PRO, 25-150 microM) and human high-density lipoprotein (HDL, 2.63 g/l) or human low-density lipoprotein (LDL, 4.37 g/l) mixed solution, which was in the state of binding equilibrium, was introduced hydrodynamically into a non-coated fused silica capillary. Positively charged unbound PRO enantiomers migrated toward cathodic end much faster than negatively charged lipoproteins and the bound form. Once unbound PRO migrated apart from lipoprotein, the bound PRO was quickly released from the lipoprotein to maintain the binding equilibrium. Thus, PRO migrated as a zone in the capillary, giving a peak with a plateau region, where the concentration is the same as the unbound PRO concentration in the original sample solution. The unbound PRO concentration calculated form the plateau height agreed with that determined by a conventional ultrafiltration method used as a reference method. It was found that the bindings of PRO to HDL and PRO to LDL were not enantioselective, while the total binding affinity of PRO to LDL (4.01 x 10(5) per M) was 17 times higher than that of PRO-HDL binding (2.38 x 10(4) per M).