IC50

Half maximal inhibitory concentration (IC50) is a measure of the potency of a substance in inhibiting a specific biological or biochemical function.

EC50 represents the dose or plasma concentration required for obtaining 50% of a maximum effect in vivo.

pIC50 is usually given in terms of molar concentration (mol/L, or M), thus requiring IC50 in units of M.[2] The IC50 terminology is also used for some behavioral measures in vivo, such as the two bottle fluid consumption test.

[3] The IC50 of a drug can be determined by constructing a dose-response curve and examining the effect of different concentrations of antagonist on reversing agonist activity.

IC50 values can be calculated for a given antagonist by determining the concentration needed to inhibit half of the maximum biological response of the agonist.

[4][5] IC50 is not a direct indicator of affinity, although the two can be related at least for competitive agonists and antagonists by the Cheng-Prusoff equation.

Graphical representation of the IC50 determination of the inhibition of an enzyme's activity by a small molecule ("drug"). Four different concentrations of the small molecule (ranging from 30 to 300 μM) were tested.