Stereospecificity (2)

Example of stereospecificity: verapamil (a calcium channel antagonist) is administered as a racemic mixture (50% S-enantiomer and 50% R-enantiomer). See the graphic below.S-Verapamil is more potent in blocking the AV node, but it undergoes greater first-pass metabolism.


The more active enantiomer also has a greater unbound fraction in the plasma. A patient who suffers from severe constipation due to verapamil therapy could benefit from administration of a single enantiomer verapamil.