Internal standard capillary electrophoresis as a high-throughput method for pKa determination in drug discovery and development

A novel high-throughput method for determining acidity constants (pK_a) by capillary electrophoresis (CE) is developed. The method, based on the use of an internal standard (IS-CE), is implemented as a routine method for accurate experimental pK_a determination of drugs undergoing physicochemical measurements in drug discovery laboratories. Just two electropherograms at 2 different pH values are needed to calculate an acidity constant. Several ISs can be used in the same buffer and run to enhance precision. With 3 ISs, for example, the pK_a of the test compound (TC) can be obtained in triplicate in less than 3 min of electrophoresis. It has been demonstrated that the IS-CE method eliminates some systematic errors, maintaining, or even increasing the precision of the results compared with other methods. Furthermore, pH buffer instability during electrophoretic runs is not a problem in the IS-CE method. It is also proved that after 16 h of electroseparation using the same buffer vial, pH may change by around one unit; but the pK_a calculated by the IS-CE method remains constant. Thus, IS-CE is a powerful high-throughput method for pK_a determination in drug discovery and development.