Half maximal effective concentration

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The term half maximal effective concentration (EC₅₀) refers to the concentration of a drug 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.

1 Equation
2 See also
3 References
4 External links

[edit] 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))})