Which of the following best describes the relationship between receptor occupancy and drug effect in the context of partial agonists?

## Partial Agonist Pharmacodynamics **Key Point:** A partial agonist is defined by its intrinsic activity (α), which lies between 0 and 1 (where 1 = full agonist, 0 = antagonist). This means partial agonists produce a submaximal response even when they occupy 100% of available receptors. ### Intrinsic Activity and Response | Parameter | Full Agonist | Partial Agonist | Antagonist | |-----------|--------------|-----------------|------------| | Intrinsic Activity (α) | 1.0 | 0 < α < 1 | 0 | | Max Response (E_max) | 100% | 30–80% | 0% | | Receptor Occupancy at Max Response | ~70–80% | 100% | Variable | **High-Yield:** The critical distinction is that a partial agonist's *intrinsic activity* is lower than that of a full agonist. Even if a partial agonist binds to all receptors, the conformational change it induces is insufficient to produce a full pharmacological response. ### Clinical Examples - **Partial agonists:** Salbutamol (β₂-adrenergic; though often classified as full agonist in clinical practice), buprenorphine (opioid), aripiprazole (D₂ dopamine) - **Full agonists:** Epinephrine, isoproterenol, morphine **Clinical Pearl:** In the presence of both a full agonist and a partial agonist competing for the same receptor, the full agonist will displace the partial agonist and produce a greater response—a phenomenon exploited therapeutically (e.g., buprenorphine as an opioid maintenance agent with lower overdose risk). **Mnemonic:** **α = Ability to activate** — intrinsic activity (α) determines the ceiling effect; partial agonists have a lower ceiling regardless of occupancy.