Partial Agonist

A partial agonist binds a receptor and switches it on, but even at saturating concentrations it produces less than the maximal response the system can give. The limit is intrinsic efficacy — how much signal each occupied receptor generates — not affinity, which only sets how many receptors are occupied. Intrinsic activity is reported as a fraction of a reference full agonist's Emax, so a compound rated at 40 percent plateaus near four tenths of that ceiling however much is added.

Buprenorphine is the textbook case: a partial agonist at the mu-opioid receptor whose ceiling on respiratory depression is why it can be given outside a monitored setting. Aripiprazole is a partial agonist at dopamine D2 receptors, varenicline at the alpha4beta2 nicotinic receptor. In peptides, partial agonism is more often an accident of engineering than a goal, arriving with a substitution made for protease resistance.

Partial agonism changes what a molecule does depending on what else is present. Alone it is a weak agonist; alongside a full agonist it competes for the same sites and pulls the response down, behaving as an antagonist. The classification is also system-dependent: in a tissue with large receptor reserve, a partial agonist can occupy enough spare receptors to reach the full ceiling.

The error worth naming is reading partial as weak. A partial agonist can be far more potent than a full one, binding at picomolar concentrations while topping out at half the effect. The second is claiming partial agonism from binding data, which cannot distinguish it from anything else. Only a functional assay run against a full-agonist reference in the same cell line, at comparable receptor density, supports the label.