Mechanisms of drug action

Drugs may act in a number of ways to exert their effect. These range from relatively simple non-specific actions that depend on the physicochemical properties of a drug to highly specific and stereoselective actions on proteins in the body, namely enzymes, voltage-gated ion channels and receptors.

Actions dependent on chemical properties

The antacids exert their effect by neutralizing gastric acid. The chelating agents are used to reduce the concentration of certain metallic ions within the body. Dicobalt edetate chelates cyanide ions and may be used in cyanide poisoning or following a potentially toxic dose of sodium nitroprusside. The new reversal agent, γ-cyclodextrin, selectively chelates rocuronium and reversal is possible from deeper levels of block than can be effected with the anticholinesterases.

Enzymes

Enzymes are biological catalysts, and most drugs that interact with enzymes are inhibitors. The results are twofold: the concentration of the substrate normally metabolized by the enzyme is increased and that of the product(s) of the reaction is decreased. Enzyme inhibition may be competitive (edrophonium for anticholinesterase), non-competitive or irreversible (aspirin for cyclo-oxygenase and omeprazole for the Na+/H+ATPase). Angiotensin-converting enzyme (ACE) inhibitors such as captopril prevent the conversion of angiotensin I to II and bradykinin to various inactive fragments. Although reduced levels of angiotensin II are responsible for the therapeutic effects when used in hypertension and heart failure, raised levels of bradykinin may cause an intractable cough.