An experiment was carried out to determine the Kd (equilibrium dissociation constant) values of two newly developed ligands at the same receptor target. Drug M: Kd = 85nM Drug P: Kd = 2μM Based on this information, it could be concluded that at this receptor, Drug M:

Question

BlackTom AI · Accepted Answer

Kd is the equilibrium dissociation constant and is numerically equal to the ligand concentration at which half of the receptors are occupied; a lower Kd therefore reflects tighter binding and higher affinity.

1) Relevant knowledge points and concepts
- Definition: Kd = [D][R]/[DR] at equilibrium; lower Kd → higher affinity (ligand binds more tightly, less free ligand needed to occupy receptors).
- Affinity is distinct from potency (EC50) and efficacy (intrinsic maximal effect). Agonist vs antagonist classification and potency require functional assays (EC50/IC50) or information about intrinsic activity, not just Kd.

2) Step-by-step reasoning
- Compare the two Kd values directly: Drug M Kd = 85 nM; Drug P Kd = 2 μM.
- Convert units to the same scale for direct comparison.

3) Intermediate calculations
- 2 μM = 2 × 10^(-6) M = 2000 nM.
- Ratio of Drug P Kd to Drug M Kd = 2000 nM / 85 nM ≈ 23.529.
- This shows Drug M’s Kd is about 23.5 times lower than Drug P’s, so Drug M binds the receptor approximately 23.5-fold more tightly.

4) Why the correct answer is correct
- Because Kd is an inverse measure of affinity, Drug M’s significantly lower Kd (85 nM vs 2000 nM) means Drug M has substantially higher affinity for the receptor than Drug P.

5) Why the other options are incorrect
- “Is a more potent agonist than Drug P”: Potency as an agonist depends on EC50 and whether the drug is an agonist at all; Kd alone does not tell whether a ligand activates the receptor or its potency.
- “Is a more potent antagonist than Drug P”: Antagonist potency requires functional inhibition data (IC50/Ki in functional assays); Kd alone cannot establish antagonist potency or mode of action.
- “Shows higher efficacy than Drug P”: Efficacy refers to the maximal response a drug can produce (intrinsic activity); Kd gives no information about maximal effect.

6) Conclusion confirming the answer
- Therefore, the correct conclusion is that Drug M has higher affinity than Drug P, which matches the provided answer.

An experiment was carried out to determine the Kd (equilibrium dissociation constant) values of two newly developed ligands at the same receptor target. Drug M: Kd = 85nM Drug P: Kd = 2μM Based on this information, it could be concluded that at this receptor, Drug M:单项选择题

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