Stereoselective protein binding of verapamil enantiomers

The binding of the (+)- and (-)-enantiomers of verapamil (V) to purified albumin (40 g/L), alpha 1-acid glycoprotein (0.55 g/L) and fresh serum has been studied over a wide range of verapamil concentrations (0.055 to 22 microM). The free fraction of the pharmacologically more potent (-)-V was always greater than that of (+)-V. Similar free fractions were observed in solutions of alpha 1-acid glycoprotein ((+)-V 0.079 +/- 0.016; (-)-V 0.142 +/- 0.020) and fresh serum ((+)-V 0.096 +/- 0.009; (-)-V 0.136 +/- 0.006), however the free fraction was higher in a solution of albumin ((+)-V 0.400 +/- 0.030; (-)-V 0.572 +/- 0.029). Saturation of verapamil binding sites was observed for alpha 1-acid glycoprotein only. Enantioselective verapamil serum binding was also noted in samples collected from five healthy volunteers following oral and intravenous verapamil administration. The free fraction of the individual isomers in vitro when added to predose serum as the pseudoracemic drug ((+)-V 0.06 +/- 0.01, (-)-V 0.12 +/- 0.02) was similar to that observed for the enantiomers when studied separately in vitro, indicating that the binding of each enantiomer is independent of the other optical isomer. The free fraction ex vivo after intravenous therapy ((+)-V 0.06 +/- 0.01, (-)-V 0.12 +/- 0.02) was similar to that observed in vitro in that subjects pre-dose serum. The free fraction of both enantiomers, however, was higher after oral drug therapy ((+)-V 0.13 +/- 0.02, (-)-V 0.23 +/- 0.03). The lower binding noted may be a result of competition for serum binding sites by verapamil metabolites, which attain higher concentrations following oral dosing.