Types of Drugs (according to reaction with receptor)

I. Agonist is a drug that binds to receptors and mimics the effects of endogenous compounds. It has ability to initiate a biological activity 

          Characteristics of Agonist

• Affinity which is the closeness or attraction to a receptor.
• Intrinsic activity or efficacy
  
  

Mechanism of Action of Agonist

• Agonist I: The drug binds to the same receptor site as to endogenous to produce a similar or greater response.

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• Agonist II: The drug binds to the different intracellular site and enhances the response when the endogenous substance binds to the receptor site.

Legend: D     -  Drug

               R     -  Receptor

               E.S. -  Endogenous Substance

3 Types of Agonist

A) Full Agonist or Strong Agonist (Potent)

• A drug has a high intrinsic activity and does not need to occupy all the receptors to produce a maximal response or produces 100% of the maximum possible biologic response.

B) Partial or Weak Agonist

• A drug ha a low intrinsic activity and needs to occupy all the available receptor to produce a maximal response or produces less than 100% of the maximum possible biologic response no matter how high their concentration.

C) Inverse Agonist

• A drug can cause an effect opposite to that of endogenous substance.
  
  
  
  
  
  
  
  

II. Antagonist is to bind or block receptor that prevents binding of agonist to the receptor to oppose an effect. It only possess affinity but not efficacy or intrinsic activity.

2 Types of Antagonist

A) Competitive Antagonist

• Antagonist with an affinity to the receptor site which is similar to that of agonist and inhibits response. It binds reversibly to the same active site of an enzyme as an agonist; however, can be overcome by increasing the concentration of the agonist. Examples are 1)Acetylcholine and atropine drug, 2) a-Beta blockers and H1 and H2 antihistamines.

B) Non-competitive Antagonist

• Combines with either different parts of receptors to inactivate it or binds in a covalent way to the receptor site resulting to a decrease in the agonist effect. Binding irreversibly to the different active site of an enzyme as an agonist; however, cannot be overcome by increasing the concentration of the agonist. Examples are 1)Acetylcholine and papaverine on the smooth muscle, 2) Acetylcholine and decamethorium at the neuromuscular junction.

Mechanism of Action of Antagonist

• Antagonist I: The drug binds to the same receptor sites used to be bound by endogenous substance to diminish or block the biologic response generated by the endogenous substance.

• Antagonist II: The drug binds to extracellular site producing diminished response. The drug permeates in the cell membrane or crosses the membrane to intercept the response generated by endogenous substance.

Legend: AD     -  Antagonistic Drug

               R         -  Receptor

               E.S.     -  Endogenous Substance