3 Clinical psychopharmacology3.3 Pharmacodynamics
3.3.1.5.1 Intrinsic activity
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Written by Resident Doctor Committee · Published
Affinity, efficacy and potency
Concept | What it describes |
|---|---|
Affinity | How strongly the drug binds the receptor |
Intrinsic activity (efficacy) | How far the drug activates the receptor once bound |
Potency | The dose or concentration needed for a stated effect, usually the EC50, the concentration giving half the maximal response |
Clinical efficacy | Whether the patient gets better |
A lower maximal response at full occupancy means lower intrinsic activity, not lower affinity or lower potency.
Two drugs acting by the same mechanism that reach the same maximal effect are equally efficacious, even if they differ in potency and affinity.
High potency can coexist with low intrinsic activity.
A high-affinity antagonist can occupy receptors without activating them.
Types of ligand
Ligand | Intrinsic activity | Effect at full occupancy | Example |
|---|---|---|---|
Full agonist | Enough to reach the system maximum | Maximum response | Dopamine at D2 |
Partial agonist | Positive but submaximal | A ceiling that more drug cannot overcome | Aripiprazole at D2 |
Neutral antagonist | None | No effect alone, but blocks the agonist | |
Inverse agonist | Negative | Signalling falls below the constitutive (agonist-free) baseline | Many H1 antihistamines in laboratory assays |
Receptor reserve (spare receptors) amplifies signalling, so the same ligand can look partial in one tissue and full in another.
Aripiprazole's lower intrinsic activity at D2 than dopamine does not mean weaker clinical efficacy.