An agonist is a term used to describe a type of ligand or drug that binds and alters the activity of a receptor. The ability to alter the activity of a receptor, also known as the agonist's efficacy is a property that distinguishes it from antagonists, a type of receptor ligand which also bind a receptor but which do not alter the activity of the receptor. The efficacy of an agonist may be positive, causing an increase in the receptor's activity or negative causing a decrease in the receptor's activity.
bind (have affinity
for) and activate a receptor, displaying full efficacy
at that receptor.
One example of a drug that acts as a full agonist is isoproterenol
which mimics the action of adrenaline
at β adrenoreceptors
. Another example is morphine, which mimics the actions of endorphins
at μ-opioid receptors
throughout the central nervous system
Partial agonists (such as buspirone, aripiprazole, buprenorphine, or norclozapine) also bind and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist.
An inverse agonist is an agent which binds to the same receptor binding-site as an agonist for that receptor and reverses constitutive activity of receptors. Inverse agonists exert the opposite pharmacological effect of a receptor agonist.
A co-agonist works with other co-agonists to produce the desired effect together. NMDA receptor activation requires the binding of both of its glutamate and glycine co-agonists. An antagonist blocks a receptor from activation by agonists.
A selective agonist is selective for one certain type of receptor. It can additionally be of any of the aforementioned types.
A physiological agonist is a substance that creates the same bodily responses, but does not bind to the same receptor.
Receptors can be activated or inactivated either by endogenous (such as hormones and neurotransmitters) or exogenous (such as drugs) agonists and antagonists, resulting in stimulating or inhibiting a biological response. To see how an agonist may activate a receptor see this link
New findings that broaden the conventional definition of pharmacology demonstrate that ligands can concurrently behave as agonist and antagonists at the same receptor, depending on effector pathways. Terms that describe this phenomenon are "functional selectivity" or "protean agonism".
of an agonist is usually defined by its EC50
value. This can be calculated for a given agonist by determining the concentration of agonist needed to elicit half of the maximum biological response of the agonist. Elucidating an EC50
value is useful for comparing the potency of drugs with similar efficacies
producing physiologically similar effects. The lower the EC50
, the greater the potency of the agonist the lower the concentration of drug that is required to elicit the maximum biological response
When a drug is used therapeutically, it is important to understand the margin of safety that exists between the dose needed for the desired effect and the dose that produces unwanted and possibly dangerous side effects. This relationship, termed the therapeutic index
, is defined as the ratio LD50
. In general, the narrower this margin, the more likely it is that the drug will produce unwanted effects. The therapeutic index has many limitations, notably the fact that LD50 cannot be measured in humans and, when measured in animals, is a poor guide to the likelihood of unwanted effects in humans. Nevertheless, the therapeutic index emphasizes the importance of the margin of safety, as distinct from the potency, in determining the usefulness of a drug.
From the Greek αγωνιστής
(agōnistēs), contestant; champion; rival < αγων
(agōn), contest, combat; exertion, struggle < αγω
(agō), I lead, lead towards, conduct; drive