Partial agonist

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Type of receptor ligand

A partial agonist is a receptor ligand or drug that binds to a receptor and activates it, but not to the same degree as a full agonist. Partial agonists can produce a sub-maximal response even when occupying all available receptors, which makes them distinct from full agonists that elicit a maximal response.

Mechanism of Action

Partial agonists bind to the same receptor sites as full agonists but induce a lesser response. This is due to their intrinsic activity, which is lower than that of full agonists. The intrinsic activity of a partial agonist is between 0 and 1, where 1 represents the activity of a full agonist and 0 represents no activity (as seen with antagonists).

Clinical Significance

Partial agonists are important in pharmacology and medicine because they can modulate receptor activity in a more controlled manner. They are often used in situations where a full agonist might produce excessive effects or side effects. For example, buprenorphine is a partial agonist at the mu-opioid receptor, providing pain relief with a lower risk of respiratory depression compared to full agonists like morphine.

Examples

• Buprenorphine - used in the treatment of opioid addiction and pain management.
• Aripiprazole - an antipsychotic used in the treatment of schizophrenia and bipolar disorder.
• Varenicline - used as a smoking cessation aid by acting as a partial agonist at the nicotinic acetylcholine receptor.

Advantages and Disadvantages

Advantages

• Reduced risk of side effects compared to full agonists.
• Can act as a functional antagonist in the presence of a full agonist, reducing the overall effect.

Disadvantages

• May not provide sufficient therapeutic effect in all patients.
• Complex pharmacodynamics can make dosing and titration challenging.

See Also

• Agonist
• Antagonist
• Inverse agonist
• Receptor (biochemistry)
• Pharmacodynamics