## Dose-Response Curves: Potency vs. Efficacy **Key Point:** Potency and efficacy are distinct pharmacodynamic properties. Conflating them is a common and high-yield exam trap. ### Definitions and Distinctions | Property | Definition | Measure | Clinical Relevance | | --- | --- | --- | --- | | **Potency** | Concentration (or dose) required to produce a given effect | EC₅₀ (lower = more potent) | Determines how much drug is needed; affects dosing frequency | | **Efficacy** | Maximum response (E~max~) a drug can achieve, regardless of dose | Intrinsic activity (0–1) | Determines whether the drug can produce the desired therapeutic outcome | **High-Yield:** A drug can be **potent but have low efficacy** (e.g., partial agonist) or **weak but have high efficacy** (e.g., full agonist requiring higher doses). These are independent properties. ### Why Option 4 Is Wrong Option 4 incorrectly states that potency and efficacy are synonymous. They are **not**: - **Potency** = EC₅₀ (position of curve on x-axis) - **Efficacy** = E~max~ (height of curve on y-axis) **Clinical Pearl:** Morphine (high potency, high efficacy) vs. aspirin (low potency, moderate efficacy) — morphine works at lower doses but both can produce strong effects. Conversely, a partial agonist may have high potency (low EC₅₀) but low efficacy (cannot reach 100% response). ### Correct Statements Explained - **Option 1:** EC₅₀ is the concentration producing 50% E~max~; it inversely reflects potency (lower EC₅₀ = higher potency). ✓ - **Option 2:** Lower EC₅₀ = higher potency; the drug achieves the same response at a lower concentration. ✓ - **Option 3:** Hill coefficient (slope) quantifies cooperativity; slope > 1 = positive cooperativity (sigmoidal curve); slope < 1 = negative cooperativity (hyperbolic curve). ✓ **Mnemonic:** **"Potency is Position, Efficacy is Peak"** — EC₅₀ shifts left (potency), E~max~ goes higher (efficacy).
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