Mechanisms of Drug Action

DYNAMICS OF DRUG–RECEPTOR BINDING፦ 

 The drug molecule, following its administration and passage to the area immediately adjacent to the receptor surface (sometimes called the biophase), must bond with the receptor before it can initiate a response. Resisting this bond formation is a random thermal agitation that is inherent in every molecule and tends to keep the molecule in constant motion. Under normal circumstances, the electrostatic attraction of the ionic bond, which can be exerted over longer distances than can the attraction of either the hydrogen or van der Waals bond,is the first force that draws the ionized molecule toward the oppositely charged receptor surface. This is a reasonably strong bond and will lend some stability to the drug–receptor complex. 

DOSE–RESPONSE RELATIONSHIP ፦

To understand drug–receptor interactions, it is necessary to quantify the relationship between the drug and the biological effect it produces.Since the degree of effect produced by a drug is generally a function of the amount administered, we can express this relationship in terms of a dose–response curve. Because we cannot always quantify the concentration of drug in the biophase in the intact individual, it is customary to correlate effect with dose administered.

The principles derived from dose–response curves are the same in animals and humans. However, obtaining the data for complete dose–response curves in humans is generally difficult or dangerous.We shall therefore use animal data to illustrate these principles.