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 concentration-response curve, the concentration of the drug is plotted against the percent maximal effect. Notice that the drug concentration is plotted on a log scale. In this graph, the drug is a full agonist—the effect reaches 100% of the maximum possible.
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 2–2 compares partial agonists with “full” agonists.
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.
Efficacy and potency are terms that students sometimes confuse. These terms are 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 ...