A drug exhibits a Kd of 10 nM and an intrinsic activity of 0.6. Which statement best characterizes this drug's receptor interaction?

Explanation

## Interpreting Kd and Intrinsic Activity ### Kd (Dissociation Constant) and Affinity **Key Point:** Kd is the concentration at which 50% of receptors are occupied. **Lower Kd = higher affinity** (drug binds tightly). A Kd of 10 nM indicates high affinity—the drug binds strongly to its receptor. $$\text{Affinity} = \frac{1}{K_d}$$ ### Intrinsic Activity (α) and Efficacy **High-Yield:** Intrinsic activity (α) ranges from 0 to 1: - α = 1.0 → full agonist (maximal response) - 0 < α < 1 → partial agonist (submaximal response) - α = 0 → antagonist (no response) An intrinsic activity of 0.6 means this drug produces only 60% of the maximal possible response, even at saturating concentrations. ### Combined Interpretation | Property | Value | Interpretation | |----------|-------|----------------| | Kd | 10 nM | High affinity (binds well) | | Intrinsic Activity | 0.6 | Partial agonist (submaximal efficacy) | | **Classification** | — | **High-affinity partial agonist** | **Clinical Pearl:** High affinity + partial agonism = a drug that binds strongly but produces a limited response. This combination is therapeutically valuable: buprenorphine (opioid partial agonist) has high affinity and produces a ceiling effect, reducing overdose risk compared to full opioid agonists. **Mnemonic:** **Kd = K down** (lower Kd = higher affinity, drug "sticks down" to receptor). **α = Activation level** (intrinsic activity determines the response ceiling).