ED50, EC50, and IC50 Concentrations

ED50, EC50 and IC50 are half-maximal parameters that locate a dose-response curve on the dose axis and serve as the standard numerical indices of potency. EC50 is the concentration producing half of the maximal effect, ED50 is the dose producing half of the maximal (or, for quantal data, half-population) effect, and IC50 is the concentration producing half-maximal inhibition of a process.

Definition

EC50 is the concentration of a drug that produces 50 percent of its maximal effect; ED50 is the dose producing 50 percent of the maximal graded effect or, in quantal analysis, the dose at which 50 percent of a population shows the defined response; IC50 is the concentration producing 50 percent inhibition of a measured activity.

Scope

This topic defines the three half-maximal indices, explains how each is read from a dose-response or inhibition curve, distinguishes their use in graded versus quantal contexts, and notes that these are empirical descriptors of a curve rather than direct measures of binding affinity. It is reference-educational and gives no dosing guidance.

Core questions

• What do EC50, ED50 and IC50 each measure, and how do they differ?
• How is a half-maximal value read off a dose-response or inhibition curve?
• Why are half-maximal indices used as the standard measure of potency?
• Why is IC50 not the same as a binding affinity constant?

Key concepts

• EC50 (half-maximal effective concentration)
• ED50 (median effective dose)
• IC50 (half-maximal inhibitory concentration)
• Potency index
• Graded versus quantal ED50
• Curve midpoint and inflection
• Assay-dependence of IC50

Mechanisms

Each half-maximal index marks the point on a dose-response curve where the response is halfway between its baseline and its maximum. For a graded sigmoid curve fitted by the Hill equation, EC50 is the concentration at the curve's midpoint and is the conventional index of potency: a lower EC50 means a more potent drug. ED50 plays the same role on a dose scale and, in quantal analysis, denotes the dose at which half the population reaches the defined endpoint. IC50 is the analogous midpoint for an inhibition curve, the concentration that halves a measured activity. These indices are empirical features of the observed curve and depend on the assay conditions; in particular, IC50 is influenced by the concentration of competing substrate or ligand and so is not, by itself, an affinity constant. The IUPHAR terminology standardises these symbols and their definitions.

Clinical relevance

Half-maximal values are the common currency for reporting and comparing the potency of drugs and inhibitors across studies. This entry presents them for educational reference; they describe where a curve sits on the dose axis and are not a basis for selecting doses in patients.

History

Half-maximal indices became standard descriptors as quantitative pharmacology adopted sigmoid curve-fitting. The relationship between an observed IC50 and the underlying inhibition constant was clarified by the Cheng-Prusoff treatment of competitive inhibition, and IUPHAR terminology later fixed the symbols EC50, ED50 and IC50 and their meanings.

Key figures

• Terry Kenakin
• David Colquhoun
• Yung-Chi Cheng
• William Prusoff

Seminal works

• neubig-2003

Frequently asked questions

What is the difference between EC50 and ED50?

EC50 is expressed in terms of concentration at the site of action and is read from a graded concentration-effect curve, while ED50 is expressed as a dose; in quantal analysis ED50 specifically means the dose at which half of a tested population shows the all-or-none response.

Is IC50 a measure of binding affinity?

Not directly. IC50 is an empirical concentration that halves a measured activity under particular assay conditions, and it depends on factors such as competing substrate concentration; converting it to an affinity constant requires additional assumptions, as in the Cheng-Prusoff relationship.