### Half maximal effective concentration

The term **half maximal effective concentration** (**EC _{50}**) refers to the concentration of a drug, antibody or toxicant which induces a response halfway between the baseline and maximum after a specified exposure time.

^{[1]}It is commonly used as a measure of drug's potency.

The EC_{50} of a *graded* dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed.^{[2]}
The EC_{50} of a *quantal* dose response curve represents the concentration of a compound where 50% of the population exhibit a response,^{[3]} after a specified exposure duration.

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

In biological systems, small changes in ligand concentration typically result in rapid changes in response, following a sigmoidal function.The inflection point at which the increase in response with increasing ligand concentration begins to slow is the EC_{50}. This can be determined mathematically by derivation of the best-fit line. While relying on a graph for estimation is more convenient, this method yields less precise and less accurate results**.
**

## Equation

Many different equations can be used to derive an EC_{50}. One possible function is:

$Y\; =\; Bottom\; +\; \backslash frac\{Top\; -\; Bottom\}\{1\; +\; (\backslash frac\{X\}\{EC\_\{50\}\})^\{\backslash mathrm\{-Hill\; coefficient\}\}\; \}$

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.^{[5]}

## Limitations

The effects of a stressor or drug generally depend on the exposure time. Therefore, the EC_{50} (and similar statistics) will be a function of exposure time. The exact shape of this time function will depend upon the stressor (e.g., the specific toxicant), its mechanism of action, the organism exposed, etc. This time dependency hampers the comparison of potency or toxicity between compounds and between different organisms.

## See also

- Measures of pollutant concentration
- Dose-response relationship
- Certain safety factor
- IC
_{50}(half maximal inhibitory concentration) - LD
_{50}(median lethal dose) - Therapeutic index

## References

## External links

- Online IC50 Calculator
- 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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