Easson and Stedman (1933) advanced a drug-receptor interaction model to account for the differential pharmacodynamic activity between enantiomeric pairs.  In this model the more active enantiomer (the eutomer) take part in a minimum of three simultaneous intermolecular interactions with the receptor surface (good fit), Figure. A., where as the less active enantiomer (distomer) interacts at two sites only (bad fit), Figure B. [Refer image for Figure: Easson-Stedman model]. Thus the "fit" of the individual enantiomers to the receptor site differs, as does the energy of interaction. This is a simplistic model but used to explain the biological discrimination between enantiomeric pairs. 
In reality the drug-receptor interaction is not that simple, but this view of such complex phenomenon has provided major insights into the mechanism of action of drugs.