Study of interaction between drug enantiomers and serum albumin by capillary electrophoresis

The interaction between drugs and human serum albumin (HSA) was investigate d by capillary electrophoresis (CE). It involves stereoselectivity, drug di splacement and synergism effects. Under protein-drug binding equilibrium, t he unbound concentrations of drug enantiomers were measured by frontal anal ysis (FA). The stereoselectivity of verapamil (VER) binding to HSA was prov ed by the different free fractions of two enantiomers. In physiological pH (7.4, ionic strength 0.17 phosphate buffer) when 300 mu M (+/-) VER were eq uilibrated with 500 mu M HSA, the concentration of unbound S-VER was about 1.7 times its antipode. The binding constants of two enantiomers, KR-VER an d KS-VER, were 2670 and 850 M-1, respectively. However, no obvious stereose lective binding of propranolol (PRO) to HSA was observed. Trimethyl-beta-cy clodextrin (45 mM) was used as a chiral selector in pH 2.5 phosphate buffer . Several drug systems were studied by the method. When ibuprofen (IBU) was added into VER-HSA solution. R-VER was partially displaced while S-VER was not displaced at all. A binding synergism effect between bupivacaine (BUP) and verapamil was observed and further study suggested that verapamil and bupivacaine occupy different binding site of HSA (site II and site III, res pectively).