Principles of Drug Action

A drug is a chemical substance that triggers a change in a cellular process. How a drug acts on the body is described by pharmacodynamics.

Typically, a drug binds to targets such as receptors, ion channels, transporters, or enzymes to modify the target's function.

The drug-target interaction initiates a cascade of molecular reactions, generating second messengers that interact with downstream effectors.

Some drugs termed agonists mimic endogenous ligands to bind and activate the target.

For example, in cardiac muscle cells, the agonist isoprenaline, which is structurally similar to adrenaline, activates adrenergic receptors and increases the heart rate.

On the contrary, antagonist  drugs compete with endogenous ligands to bind the target and inhibit the biological response. The binding of atropine blocks the activity of the acetylcholine receptor to bring a slow-beating heart back to the normal heart rate.

Lastly, some drugs are selectively cytotoxic and can target cancer cells or pathogens. Chloroquine, an antimalarial drug, specifically accumulates inside malarial parasites and kills them.