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The term half maximal effective concentration (EC₅₀) refers to the concentration of a drug or antibody which induces a response halfway between the baseline and maximum.^[1] It is commonly used as a measure of drug potency.

The EC₅₀ of a graded dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed^[2]. The EC₅₀ of a quantal dose response curve represents the concentration of a compound where 50% of the population exhibit a response^[3].

It is also related to IC₅₀ which is a measure of a compound's inhibition (50% inhibition). For competition binding assays and functional antagonist assays IC₅₀ is the most common summary measure of the dose-response curve. For agonist/stimulator assays the most common summary measure is the EC₅₀.^[4]

Concentration measures typically follow an S-shaped curve, increasing rapidly over a relatively small change in concentration. The point at which the effectiveness slows with increasing concentration is the IC₅₀. This can be determined mathematically by derivation of the best-fit line. However, it is more easily observed from a graph and estimated rather than through complex calculus equations.

Equation

The equation used to fit the EC₅₀ is:

Y = Bottom + (Top - Bottom)/(1 + (X/EC₅₀)^{(Hill slope)})

where Y is the observed value, Bottom is the lowest observed value, Top is the highest observed value, and the Hill coefficient gives the largest absolute value of the slope of the curve.

And an alternate form of the equation is:

Y = Bottom + (Top - Bottom)/(1+ 10^{((Log(EC₅₀)-X) * (Hill coefficient))})

See also

• Dose-response relationship
• Certain safety factor
• IC₅₀
• LD₅₀
• Therapeutic index

References

1. ^ Introducing doseresponse curves, Graphpad Software]
2. ^ EC50 definition
3. ^ definition of EC₅₀ for quantal dose response curve
4. ^ Assay Operations for SAR SupportNIH Chemical Genomics Center

External links

• Online IC50 Calculator
• S-shaped curves
• Determination of IC₅₀ values
• Neubig et al. International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on terms and symbols in quantitative pharmacology. Pharmacol Rev. 2003 Dec;55(4):597-606.

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• This page was last modified on 13 August 2008, at 01:43.

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