Determination of the unbound concentration of hydrophobic drugs in albumin solutions by high-performance frontal analysis using a diol-silica column.

High-performance frontal analysis (HPFA) using diol-silica column allows the determination of the unbound concentration of hydrophobic drugs under protein binding equilibrium, which is often difficult by the conventional methods. After the direct injection of a drug-protein mixed solution onto a diol-silica column, the drug is eluted as a zonal peak containing a plateau region under a mild mobile phase condition (phosphate buffer, pH 7.4, ionic strength = 0.17). The unbound drug concentration was determined as the drug concentration in the plateau region. The reliability of this method was confirmed by comparison with the results obtained by the conventional ultrafiltration-HPLC method. The versatility of the HPFA method using a diol-silica column was demonstrated by developing a novel on-line HPFA-chiral HPLC system for a simple and easy determination of the unbound concentration of nilvadipine (NV) enantiomers. The direct injection of 1.33 mL of a sample solution containing 20 nM-1 microM NV and 550 microM HSA gave the plateau region due to unbound NV, which was heart-cut and transferred on-line into a preconcentration and chiral HPLC separation system. As low as a few hundred picomolar level of unbound NV enantiomers was determined by use of UV detection (at 244 nm). The binding between NV and HSA is enantioselective; (R)-NV binds more strongly than (S)-NV.