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An agonist is a compound that binds to a receptor and produces the biological response.

A drug receptor is a specialized target macromolecule that binds a drug and mediates its pharmacological action. These receptors may be enzymes, nucleic acids, or specialized membrane-bound proteins. The formation of the drug-receptor complex leads to a biological response. The magnitude of the response is proportional to the number of drug-receptor complexes. A common way to present the relationship between the drug concentration and the biological response is with a concentration- (or dose-) response curve (Figure 2–1). In many textbooks, you will see both dose-response curves and concentration-response curves. Because the biological effect is more closely related to the plasma concentration than to the dose, I will show concentration-response curves in this chapter.


In A, the concentration-response curve for a full agonist is presented. The drug can produce a maximal effect. In B, the concentration-response curve for a partial agonist is also shown. In this case, the partial agonist is able to produce only 60% of the maximal response.

An agonist can be a drug or the endogenous ligand for the receptor. Increasing concentrations of the agonist will increase the biological response until there are no more receptors for the agonist to bind or a maximal response has been reached.

A partial agonist produces the biological response but cannot produce 100% of the biological response even at very high doses.

Figure 2–1 shows concentration response curves and compares a partial agonist with a “full” agonist.


Efficacy and potency are terms used for comparisons between drugs.

Efficacy is the maximal response a drug can produce. Potency is a measure of the dose that is required to produce a response.

For example, one drug (drug A) produces complete eradication of premature ventricular contractions (PVCs) at a dose of 10 mg. A second drug (drug B) produces complete eradication of PVCs at a dose of 20 mg. Therefore, both drugs have the same efficacy (complete eradication of PVCs), but drug A is more potent than drug B. It takes less of drug A to produce the same effect. A third drug (drug C) can reduce the PVCs by only 60%, and it takes a dose of 50 mg to achieve that effect. Therefore, drug C has less efficacy and less potency in the reduction of PVCs compared with both drugs A and B.

Potency and efficacy are usually shown graphically (Figure 2–2).


Concentration-response curves for drugs A, B, and C are presented. Drugs A and B have equal efficacy, but drug A is more potent than drug B. Drug C is less efficacious and ...

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