Optical Isomerism and Pharmacodynamic Aspects

The biological response induced by a pair of enantiomers can differ in potency (quantitative difference) or in nature (qualitative difference). In the latter case, it is assumed that one enantiomer acts at one receptor site, whereas its antipode is recognized by other sites and possesses a different activity and toxicity profile. 

Two optical isomers are never antagonists, at least at comparable dosages. This comes from the space relationship required for the interaction with the receptor site which is only slightly altered by passing from S to R forms, or vice-versa. If one of the enantiomers achieves the optimal 

Racemate Levorotatory enantiomer Dextrorotatory enantiomer References 

Quinine/quinidine (racemate not in use) Quinine antipyretic, antimalarial Quinidine antiarrythmic antimalarial White et White et al., Alexander et 

Sotalol ( —)-Sotalol P-adrenoceptor blocker (-H)-Sotalol antiarrythmic agent Drayer  

---