1.8: Drug-Receptor Interactions

Drug-receptor interaction describes the binding of receptors by drugs, but not all drug-receptor interactions result in activation and tissue response. For instance, the binding of agonists activates the receptor to generate a cellular reaction, while antagonists bind to receptors without causing their activation.

Several parameters, such as the drug's affinity for its receptor and its efficacy, which is its ability to activate the receptor, determine the drug's effect on the tissue. High-potency drugs have high affinity, even at low concentrations. Agonists have high affinity and efficacy, leading to maximal tissue response. Partial agonists have intermediate efficacy and produce submaximal responses, even with full receptor occupancy. Antagonists compete with endogenous ligands for receptor binding and have negligible efficacy.

Drugs can interact with receptors by activating the agonist binding site, competing with the agonist, or acting at separate allosteric sites. Pharmacologic antagonists compete with other molecules for receptor binding and stabilize the receptor in an inactive state. Others, like allosteric antagonists, change the affinity and efficacy of agonists. Understanding drug-receptor interactions, including binding, competitive antagonism, and efficacy, is essential for understanding drug effects. While qualitative understanding is often enough, a quantitative formulation is required for detailed analysis.

A drug activates its receptor by inducing conformational changes or recruiting effector molecules to generate a cellular response. However, not all drugs activate their target or receptors upon binding.

Drug binding depends on the drug's affinity for the receptor. Even at low concentrations, high-affinity drugs compete with endogenous ligands for maximum receptor occupancy.

A drug's efficacy determines its ability to elicit a cellular response. Drugs are categorized as agonists, partial agonists, or antagonists based on their affinity and efficacy.

An agonist activates the receptor to elicit a maximal response. Agonists can bind to an endogenous ligand binding site or a separate allosteric site to increase a receptor's activity. Even without total occupancy of receptors, the efficacy of an agonist remains nearly 100%.

A partial agonist has intermediate efficacy and can only produce a submaximal response even with 100% receptor occupancy.

Finally, an antagonist competes with the endogenous ligand to bind and inactivate the receptor. So, the antagonist has negligible efficacy and does not produce a response.