# Half maximal effective concentration

The term **half maximal effective concentration** (**EC _{50}**) 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 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]}.

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]}

According to the FDA, **IC _{50}** represents the concentration of a drug that is required for 50% inhibition in vitro whereas

**EC**represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.

_{50}^{[5]}

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 IC_{50}. 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 EC_{50} is:

Y = Bottom + (Top - Bottom)/(1 + (X/EC_{50})^{(Hill slope)})

where Y is the observed value, Bottom is the lowest observed value, Top is the highest observed value, and the Hill slope 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 slope))})

## See also

## References

- ↑ Introducing doseresponse curves, Graphpad Software]
- ↑ EC50 definition
- ↑ definition of EC
_{50}for quantal dose response curve - ↑ Assay Operations for SAR SupportNIH Chemical Genomics Center
- ↑ [1],FDA

## External links

40px | This biochemistry article is a stub. You can help Wikipedia by expanding it. |

.