What does EC50 describe in pharmacodynamics?

• A. The concentration producing 50% of the maximal effect.
• B. The maximum concentration before toxicity.
• C. The time to reach 50% receptor occupancy.
• D. The affinity of the ligand for the receptor.

Answer: (B)

EC50 is the concentration of a drug that produces half of its maximal pharmacodynamic effect. This makes it a measure of potency on the dose–response curve: a lower EC50 means the drug is more potent because less of it is needed to reach 50% of the maximum effect. This value relates to the amount of effect achieved, not to toxicity limits, how fast receptors are occupied, or how tightly the drug binds.

The description about the maximum concentration before toxicity refers to toxic or therapeutic ceiling, not the effective concentration on the response curve. Describing the time to reach 50% receptor occupancy covers how quickly receptors are engaged, which is about occupancy kinetics rather than the level of effect. Describing the affinity of the ligand for the receptor concerns binding strength (often KD), not the actual effect level at half-max. In summary, EC50 directly pinpoints the concentration needed to yield half of the maximum response, reflecting potency on the dose–response relationship.