Introduction to Agonism and Antagonism

Salbutamol and beclomethasone are agonists at different receptor types.  Drugs that act at receptors can be either agonists or antagonists.  Two properties are used to characterize agonists and antagonists.  These are affinity and efficacy.  Affinity is the attraction of the drug for the receptor.  If a drug has high affinity, low concentrations of the drug bind to the receptor.  If a drug has low affinity, high concentrations of the drug are required to get any binding to the receptor.  If the drug has no affinity, no concentrations of the drug bind to the receptor.  The best drugs are the ones that have high affinity for their target receptor to initiate a beneficial effect, and little or no affinity for other receptors, so that they have as few unwanted effects as possible.  Both agonists and antagonists have affinity.

Efficacy is also known as intrinsic activity or effect.   If a drug binds to receptor and has no effect, it has an efficacy of zero.  If a drug binds to receptor and, at the higher concentrations, can have the same maximum effect as the endogenous compound, it has an efficacy of 1.  Agonists have efficacy, antagonists do not.

To reiterate, agonists have affinity and efficacy (i.e. binds to receptor and have an effect) whereas antagonist has affinity but no efficacy (i.e. binds to receptor but does not have an effect alone).  Although receptor antagonists have no effects, they are important drugs as their binding prevents the binding of the endogenous compound and its effects.