Enantioselective tissue distribution of the basic drugs disopyramide, flecainide and verapamil in rats: Role of plasma protein and tissue phosphatidylserine binding

Purpose, The stereoselective distribution of three basic drugs, disopyramide (DP), flecainide (FLC) and verapamil (VP), was studied to clarify the relationships between the tissue-to-unbound plasma concentration ratio (Kpf) and drug lipophilicity and binding to phosphatidylserine (PhS), which are possible factors determining the tissue distribution of these drug enantiomers. Methods, The drug enantiomer or racemate was administered to rats by intravenous constant infusion. Their concentrations in plasma and tissues were determined using enantioselective high-performance liquid chromatography. Plasma protein binding, and buffer-octanol and buffer-hexane containing PhS partition coefficients were also determined. Results, The stereoselectivity of the tissue-to-plasma concentration ratio (Kp) was partly associated with that of serum protein binding. However, the Kpf value of R(+)-VP in the lung was significantly higher than that of S(-)-VP. A linear correlation was observed between the Kpf values of these drug enantiomers in brain, heart, lung and muscle, and their buffer-hexane containing PhS partition coefficients. The in vitro data for the binding of these drugs to PhS suggest that stereoselective binding of VP to PhS may correspond to its stereoselective tissue binding. Conclusions. Our findings provide some evidence for a role of tissue PhS in the tissue distribution of basic drugs with respect to stereoselectivity of drug enantiomers distribution.