This ratio is also known as the dissociation constant or Kd . Thus the Kd could be calculated for any drug-receptor interaction by knowing the concentrations of free drug and bound and unbound receptor. This would produce a numerically small Kd. One last important point here. However, remember that comparisons of affinity are not the same as comparisons of potency.
Affinity plays a role in drug potency, but so does intrinsic efficacy.
Potency is compared among drugs by using the EC50 values from a functional assay. Drug Concentration and Receptor Occupancy Remember that receptor occupancy by the drug is needed to elicit a response. Thus it is the affinity of a drug that determines what concentration of a drug will be needed to produce a given receptor occupancy. The easiest way to measure this relationship is by using a radioland binding assay . Lets beat a dead horse and look at this graph illustrating the relationship between the log drug concentration and fractional receptor occupancy.