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Antagonists

In pharmacology an antagonist is a binding partner (ligand) of a receptor and inhibits the function of an agonist or inverse agonist.

Most drugs exert effects, both beneficial and detrimental, by interacting with specific receptors that are present on or within cells. Historically, receptors were thought to act much like light switches. Agonists were thought to turn "on" a single cellular response by binding to the receptor, thus initiating a biochemical mechanism for change within a cell. Antagonists were thought to turn "off" that response by 'blocking' the receptor from the agonist.

Recently, the discovery of inverse agonists and other concepts such as functional selectivity have broadened traditional definitions. Functional selectivity means that a ligand can concurrently behave as an agonist and antagonist at the same receptor, depending upon the effector system.

Today's current theories involving receptor antagonists center around an antagonist's ability to counteract the effects of agonists and inverse agonists (if one exists for the specific receptor). Antagonists work by binding to receptors and stopping cascades put in place by an agonist. On their own, "silent" antagonists produce no effect by themselves to a cell, and have zero intrinsic activity and zero efficacy.

There are three kinds of receptor antagonists:

• Competitive antagonists reversibly bind to receptors and compete with other agonists and antagonists for a specific binding site. An example is the interleukin-1 receptor antagonist, IL-1Ra. High concentrations of an competitive agonist will displace the antagonist from the receptor.
• Reversible non-competitive antagonists bind to a different binding-site from the agonists, exerting their action to that receptor via the other binding site. They are called non-competitive antagonists because they do not compete for the same binding site as the agonists.
• Irreversible antagonist bind covalently to the receptor binding site. They do not compete with agonists since due to the covalent nature of the bond they can not be displaced from the receptor by raising the concentration of an agonist.

Antagonists may be naturally occurring as is the case with physiological antagonists, or they may be synthetic and made to mimic the body's physiological antagonists.

A further important characteristic is affinity − the tendency to bind to a receptor.

• Agonist
• Enzyme inhibitor
• Inverse agonist

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