IC50

The IC50, or the half maximal inhibitory concentration, represents the concentration of an inhibitor that is required for 50% inhibition of its target (i.e. an enzyme, cell, cell receptor or a microorganism). In simpler terms, it measures how much of a particular substance/molecule is needed to inhibit some biological process by 50%. IC50 is commonly used as a measure of drug potency of antagonist drugs in pharmacological research. According to the FDA, IC50 represents the concentration of a drug that is required for 50% inhibition in vitro. It is comparible to an EC50 for agonist drugs. EC50 also represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.

Functional antagonist assay
The IC50 of a drug can be determined constructing a dose-response curve and examing 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.

IC50 values are dependent on conditions under which they are measured. In general, the higher the concentration of inhibitor, the more will agonist activity be lowered. IC50 value increases as enzyme concentration increases. Furthermore depending on the type of inhibition other factors may influence IC50 value; for ATP dependent enzymes IC50 value has an interdependency with concentration of ATP, especially so if inhibition is all of it competitive. IC50 values can be used to compare the potency of two antagonists.

Competition binding assays
In this type of assay, a single concentration of radioligand (usually an agonist) is used in every assay tube. The ligand is used at a low concentration, usually at or below its KD value. The level of specific binding of the radioligand is then determined in the presence of a range of concentrations of other competing non-radioactive compounds (usually antagonists), in order to measure the potency with which they compete for the binding of the radioligand. Competition curves may also be computer-fitted to a logistic function as described under direct fit.

In this situation the IC50 is the concentration of competing ligand which displaces 50% of the specific binding of the radioligand. The IC50 value is converted to an absolute inhibition constant Ki) using the Cheng-Prusoff equation (see Ki).

IC50 and affinity
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 eqtn.

Ki=IC50/(1+(S/Km))

where Ki is the binding affinity of the inhibitor, IC50 is the functional strength of the inhibitor, S is substrate concentration and Km is the affinity of the substrate for the enzyme.Whereas the IC50 value for a compound may vary between experiments depending on radioligand concentration, the Ki is an absolute value. Ki is the inhibition constant for a drug; the concentration of competing ligand in a competition assay which would occupy 50% of the receptors if no radioligand were present.