Investigation of the enantioselective interaction between selected drug enantiomers and human serum albumin by mobility shift-affinity capillary electrophoresis

Mobility shift-affinity capillary electrophoresis was employed for enantioseparation and simultaneous binding constant determination. Human serum albumin was used as a chiral selector in the background electrolyte composed of 20 mM phosphate buffer, pH 7.4. The applied setup supports a high mobility shift since albumin and the drug-albumin complex hold negative net charges, while model compounds of amlodipine and verapamil are positively charged. In order to have an accurate effective mobility determination, the Haarhoff-van der Linde function was utilized. Subsequently, the association constant was determined by nonlinear regression analysis of the dependence of effective mobilities on the total protein concentration. Differences in the apparent binding status between the enantiomers lead to mobility shifts of different extends (alpha). This resulted in enantioresolutions ofRs = 1.05-3.63 for both drug models.R-(+)-Verapamil (K(A)1844 M-1) proved to bind stronger to human serum albumin compared toS-(-)-verapamil (K(A)6.6 M-1). The association constant ofS-(-)-amlodipine (K(A)25 073 M-1) was found to be slightly higher compared to its antipode (K(A)22 620 M-1) when applying the racemic mixture. The low measurement uncertainty of this approach was demonstrated by the close agreement of the association constant of the enantiopureS-(-)-form (K(A)25 101 M-1).