Mechanisms Of Drug Action

a. Receptor Site Theory. A drug that finally enters a cell may produce an effect. It is able to produce this effect by a variety of complex biochemical processes. Most of the processes can be simplified into one explanation of the mechanism of drug action known as the receptor-site or "lock and key" theory. This theory states that a drug (the key) combines with a receptor-site (the lock) to produce a pharmacological effect. Drugs that will fit into the receptor-site are said to have an "affinity" for that receptor-site. Only drugs that fit into the receptor-site will produce a pharmacological response. Figure 3-5 visually represents the receptor-site theory.

b. Chemical Structure Activity Relationship. As a review, drug molecules have specific chemical structures. The chemical structure of a drug will determine if a drug molecule (the key) will fit into the receptor-site (the lock) and produce a pharmacological effect. For example, whether the hydroxyl (OH-) group is on the left or right side of the molecule or is at a 520 angle with the molecule can determine whether the "key" will fit the "lock." This is referred to as chemical structure activity relationship. From this, we can say that drugs that are similar in composition and chemical structure may have similar effects. The chemical structure of a drug can be altered with no effect upon the pharmacological effect the drug produces. However, the change in the chemical structure of the drug molecule can increase or decrease its side effects. Further, the modification of a drug molecule can influence its pharmacological actions. Therefore, the modification of a drug's molecular structure can result in the formation of a drug that can produce a desired pharmacological effect with a significantly lower dose and with an accompanying decrease in undesired side effects.

Figure 3-5. The receptor-site theory.

c. Antagonists. Antagonists are drugs that will or reverse block the action of other drugs. There are two types of antagonists: competitive and physiological.

(1) Competitive antagonists. Competitive antagonists combine with the receptor-site and prevent another drug from combining with the receptor-site. A competitive antagonist does not displace a drug at the receptor-site. Figure 3-6 illustrates the concept of a competitive antagonist.

Figure 3-6. A competitive antagonist.

(2) Physiological antagonists. Physiological antagonists reverse the action of the drug by acting on a different receptor-site to cause a different physiological effect.

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