[Development of high-performance frontal analysis and application to drug-plasma protein binding study]

This review summarizes the principle and the features of high-performance frontal analysis (HPFA), a novel chromatographic method to determine the concentrations of unbound drugs under drug-protein binding equilibrium conditions, and its application to the study on the plasma protein binding. HPFA uses a "restricted-access" type HPLC column which retains a small molecule in the drugs in the micropores, while a large molecule in the plasma protein is size-excluded. After direct and continuous injection of a sample, the drug-protein binding equilibrium in the sample solution is regenerated in the column, and the constant concentration zone of the unbound drug appears from the equilibrium zone. The unbound drug is eluted as a trapezoidal peak with a plateau. The concentration in this plateau region is equal to that of the unbound drug in the sample solution, and the concentration of the unbound drug can be determined by subsequent on-line HPLC analysis. HPFA allows a simple analysis following direct injection of a sample, and does not cause undesirable drug adsorption on the membrane nor leakage of the bound drug from the membrane, which are often encountered in the conventional ultrafiltration or dialysis method. HPFA allows the simultaneous determination of the concentrations of total and unbound drugs in a single analysis. HPFA can be easily incorporated into the on-line HPLC system. By coupling HPFA with a chiral HPLC column, the concentration of an unbound racemic drug can be determined enantioselectively. The detection limit can be improved dramatically by coupling HPEA with a preconcentration column. Frontal analysis in capillary electrophoresis format (CE/FA) allows us an ultramicro binding assay. The concentration of the unbound racemic drug can be determined stereoselectively by coupling HPFA with a chiral CE technique.