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. Thus, the higher the affinity, the lower the concentration of drug needed to occupy the receptor and vice versa. Affinity is determined by the interaction of the molecular structures of the receptor and the drug.
Affinity relates the drug concentration to the fractional receptor occupancy.