Pharmacodynamics-3: Drug-Receptor Interaction

Drug-Receptor Interaction

• The drug concentration that achieves 50% of the maximum effect is called EC50.

Types of Drugs

• A drug that binds to the receptor but does not cause a cellular response is called an antagonist.
• A partial agonist is a drug that binds to the receptor and causes a cellular response, but with lower efficacy than a full agonist.
• An example of a partial agonist acting as an antagonist in the presence of an agonist is the drug buspirone.

Affinity and Dissociation Constant

• The factor affecting affinity is the strength of binding between the drug and the receptor.
• The mathematical relationship that defines the dissociation constant (Kd) is Kd = [R] [D] / [RD], where [R] is the concentration of receptors, [D] is the concentration of drug, and [RD] is the concentration of drug-receptor complexes.

Receptor Binding

• The concentration of a drug around the receptor needed to form a certain number of drug-receptor complexes is determined by the law of mass action.
• The law of mass action states that the rate of a reaction is proportional to the concentrations of the reactants.

Receptor Activation

• The capacity to activate a receptor is called efficacy.
• The fraction of receptors at each state during equilibrium is determined by the affinity and efficacy of the drug.

Antagonists

• A competitive antagonist increases the EC50 of a full agonist, shifting the dose-response curve to the right.
• A noncompetitive antagonist decreases the efficacy of a full agonist, without changing the EC50.
• The main difference between a competitive and a noncompetitive antagonist is that competitive antagonists bind to the same receptor as the agonist, while noncompetitive antagonists bind to an allosteric site.