Enantioselective distribution of verapamil and norverapamil into human and rat erythrocytes: the role of plasma protein binding.

In this in vitro study, the distribution of the enantiomers of verapamil (VER) and its active metabolite, norverapamil (NOR), into the red blood cells (RBCs) of humans and rats was investigated using a chiral liquid chromatographic assay. When plasma was replaced with buffer, the distribution of VER and NOR enantiomers into both human and rat RBCs was substantial (RBC:blood concentration ratios, 1.39-1.79), non-stereoselective, concentration (125-1000 ng mL-1) linear, and species independent. However, in the presence of plasma, the RBC distribution of VER and NOR was stereoselective, with opposite stereoselectivity for human (S > R) and rat (R > S) blood. Additionally, the presence of plasma caused a reduction in the extent of RBC distribution for both VER and NOR enantiomers and in some cases resulted in nonlinearity in the RBC distribution of the enantiomers. Plasma protein binding studies revealed opposite stereoselectivity in the free fractions in human (S > R) and rat (R > S) plasma for both VER and NOR. These data suggest that the stereoselective protein binding is responsible for the apparent stereoselectivity in the RBC distribution of VER and NOR. The data are also in agreement with the opposite stereoselectivity in the plasma concentrations of VER observed in vivo in rats and humans.