EC50

The EC₅₀ of a graded dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed. The EC₅₀ of a quantal dose response curve represents the concentration of a compound where 50% of the population exhibit a response, after a specified exposure duration.

For clarification, a graded dose response curve shows the graded effect of the drug (y axis) over the dose of the drug (x axis) in one or an average of subjects. A quantal dose response curve shows the percentage of subjects where a response is noted in an all-or-none manner (y axis) over the dose of the drug (x axis).

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.

The EC₅₀ is also related to IC₅₀ which is a measure of a compound's inhibition (50% inhibition).

Calculation of EC50

Biological responses to ligand concentrations typically follow a sigmoidal function. The inflection point at which the increase in response with increasing ligand concentration begins to slow is the EC₅₀, which can be mathematically determined by derivation of the best-fit line. While relying on a graph for estimation is more convenient, this typical method yields less accurate and precise results.

The response or effect, E, is dependent on both the binding of the drug and the drug-bound receptor. The agonist that binds to the receptor and initiates the response is usually abbreviated A or D. At low agonist concentrations, [A], the response, E is immeasurably low but at higher [A], E becomes measurable. E increases with [A] until at sufficiently high [A], when E plateaus towards an asymptotic maximum attainable response, E_max. The [A] at which E is 50% of E_max is termed the half maximal effective concentration and is abbreviated EC₅₀, or rarely [A]₅₀. The term "potency" refers to the EC₅₀ value. The lower the EC₅₀, the less the concentration of a drug is required to produce 50% of maximum effect and the higher the potency. The EC₁₀ and EC₉₀ concentrations to induce 10% and 90% maximal responses are defined similarly.

There is a wide range of EC₅₀ values of drugs; they are typically anywhere from nM to mM. Hence, it is often more practical to refer to the logarithmically transformed pEC₅₀ values instead of EC₅₀, where

${\displaystyle {\text{pEC}}_{50}^{}=-{\log }_{10}({\text{EC}}_{50}^{})}$.