A researcher compares two drugs acting on the same G-protein-coupled receptor (GPCR). Drug A produces a dose–response curve with an EC50 of 10 nM and Emax of 100%, while Drug B produces an EC50 of 50 nM and Emax of 60%. Which feature best distinguishes Drug A from Drug B in terms of receptor pharmacodynamics?

Explanation

## Interpreting Dose–Response Curves: Affinity vs. Intrinsic Activity ### Analysis of the Given Data **Key Point:** EC50 reflects **affinity** (lower EC50 = higher affinity), while Emax reflects **intrinsic activity** (α). These are independent properties. - **Drug A**: EC50 = 10 nM (higher affinity), Emax = 100% (full agonist, α = 1.0) - **Drug B**: EC50 = 50 nM (lower affinity), Emax = 60% (partial agonist, α ≈ 0.6) ### The Discriminating Feature **High-Yield:** The **maximum effect (Emax)** is the best discriminator between a full and partial agonist. Drug A achieves 100% of the maximal possible response; Drug B achieves only 60% even at saturating concentrations. This difference in intrinsic activity (not affinity) defines their agonist classification. ### Dose–Response Curve Interpretation ```mermaid flowchart TD A["Two drugs on same GPCR"]:::outcome A --> B{"Compare EC50 and Emax"}:::decision B -->|"EC50: Drug A 10 nM vs B 50 nM"|C["Drug A has higher affinity"]:::outcome B -->|"Emax: Drug A 100% vs B 60%"|D["Drug A is full agonist<br/>Drug B is partial agonist"]:::outcome C --> E["Affinity alone does NOT<br/>distinguish agonist type"]:::action D --> F["Intrinsic activity (α)<br/>distinguishes agonist type"]:::action ``` ### Comparison Table | Parameter | Drug A | Drug B | Meaning | |---|---|---|---| | **EC50** | 10 nM | 50 nM | A has 5-fold higher affinity | | **Emax** | 100% | 60% | A is full agonist; B is partial agonist | | **Intrinsic Activity (α)** | 1.0 | ~0.6 | Defines agonist classification | | **Receptor Occupancy at EC50** | 50% | 50% | EC50 is the dose producing 50% Emax | **Clinical Pearl:** A partial agonist with high affinity (low EC50) can still produce submaximal response. Conversely, a full agonist with low affinity (high EC50) still achieves 100% Emax. Affinity and intrinsic activity are orthogonal properties. ### Why Other Options Fail - **Option A ("Drug A has lower affinity")**: False. Drug A's EC50 of 10 nM is *lower* than Drug B's 50 nM, meaning Drug A has *higher* affinity. This is the opposite claim. - **Option C ("Drug A has faster onset")**: EC50 and Emax are equilibrium pharmacodynamic parameters; they do not reflect kinetics (kon, koff). Onset speed requires kinetic data, not dose–response curves. - **Option D ("Drug A is irreversible")**: Reversibility cannot be inferred from EC50 and Emax alone. Both reversible and irreversible agonists can produce identical dose–response curves if they reach the same steady-state occupancy. [cite:KD Tripathi 8e Ch 1]