Determination of free drug in protein binding equilibrium by high-performance frontal analysis using internal-surface reversed-phase silica support.

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Abstract
An internal-surface reversed-phase silica column was employed in the frontal analysis method to determine free drug concentration in warfarin (Wf)-bovine serum albumin (BSA) mixed solution and in indometacin(Im)-BSA mixed solution. When a 4-ml portion of aqueous solution containing 2-30 .mu.M Im and 28 .mu.M BSA and a 10-ml portion of aqueous solution containing 0.5-175 .mu.M Wf and 28 .mu.M BSA were applied, the elution curves reached a plateau level corresponding to both free drug and drug-BSA complex (.beta.-plateau), followed by another plateau due to free drug alone (.gamma.-plateau). The drug concentration at the .gamma.-plateau agreed well with the free drug concentration determined after ultrafiltration of the same solution. The .gamma.-plateau was observed even when the applied volume was reduced to a level which was insufficient to produce the .beta.-plateau. The injection of a 400-.mu.l portion of the 0.5-100 .mu.M Im and 28 .mu.M BSA mixed solution and a 90-.mu.l portion of 50 .mu.M Im and 550 .mu.M BSA mixed solution onto the ISRP silica column gave a clear .gamma.-plateau. Compared to the conventional frontal analysis, the present method can determine a wide range of drug levels with much smaller injection volume. This method is applicable to plasma. By a single frontal analysis, both free and total concentrations of carbamazepine in plasma were determined simultaneously.