A graded dose-response curve is a plot that shows the continuous relationship between the dose of a drug (or its logarithm) and the magnitude of its effect on a biological system. These curves are typically used for agonists that activate receptors.

Purpose

Graded D-R curves provide insight into three important pharmacologic properties of a drug:

1. Affinity – How strongly the drug binds to its receptor.
2. Potency – The concentration (or dose) of a drug required to produce a given effect.
3. Efficacy – The maximal response a drug can produce, regardless of dose.

Axes of the Curve

• X-axis: Dose or log(dose) of the drug. Using log(dose) often produces a sigmoidal shape, making it easier to analyze potency.
• Y-axis: Response (effect) of the tissue or organism, measured quantitatively (e.g., contraction of muscle, enzyme activity, blood pressure change).

Key Features

1. Threshold dose: The lowest dose that produces a measurable effect.
2. Slope: Indicates how the response changes with increasing dose.
3. EC50 (Half-maximal effective concentration): Dose at which 50% of the maximal effect is observed; a measure of potency.
4. Maximum response (Emax): Reflects efficacy; full agonists reach higher Emax than partial agonists.

Interpretation

• Shifts along the X-axis: Indicate differences in potency (left shift = more potent).
• Differences in maximum response: Indicate differences in efficacy (higher Emax = greater efficacy).
• Curve steepness: Can suggest receptor cooperativity or multiple mechanisms of action.

Activity