4.2: Targets for Drug Action: Overview
Drugs target macromolecules to modify ongoing cellular processes. Primary drug targets include receptors, ion channels, transporters, and enzymes.
Receptors are either membrane-spanning or intracellular proteins, which upon binding a ligand, get activated and transmit the signal downstream to elicit a response. Drugs bind receptors, either mimicking the action of endogenous ligands or blocking the receptor activity to bring about a modified response. Nearly 35% of approved drugs target the G protein–coupled receptors, a class of cell surface receptors.
A second class of drug targets includes the ion channels that facilitate the ion movement across the cell and help in neurotransmission or muscle contraction. Ion channels can be ligand-gated, whose opening and closing are controlled by an external ligand, or voltage-gated, controlled by changing membrane potential. Drugs target ion channels either by binding to the ligand binding site or the allosteric site, or by directly blocking the channel pore. Sedatives such as benzodiazepines bind at the allosteric site of the GABA receptor, a ligand-gated ion channel, and enhance the binding of GABA to the ligand-binding site. In contrast, vasodilators such as dihydropyridine block L-type calcium channels in cardiac muscle cells.
A third class of drug targets includes transporters. Transporters help move small lipid-insoluble molecules and other metabolites across the membrane. Many available drugs block the functioning of these transporters. For example, diuretics such as furosemide and hydrochlorothiazide, block kidney tubule transporters to prevent sodium reabsorption.
Several other drugs target enzymes, or other biological catalysts, that carry out biochemical reactions inside a cell. Some drugs act as substrate analogs that may bind reversibly or irreversibly to inhibit enzyme activity. Others work as false substrates, which bind the enzyme to allow the biochemical reaction but form an abnormal end product. Examples include fluorouracil, an anticancer drug that replaces uracil during purine biosynthesis, blocking DNA biosynthesis and inhibiting cell division.
In addition to these targets, certain drugs, such as colchicine, interact with tubulin, a structural protein, and are often used to treat arthritic gout. Others prevent bacterial invasion or cancer cell proliferation by targeting bacterial cell wall proteins, DNA, and other proteins.
