IC50

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The correct title of this article is IC₅₀. It features superscript or subscript characters that are substituted or omitted because of technical limitations.

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IC₅₀ is a measure of concentration used in pharmacological research. 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 effectiveness. Another measure of drug efficacy is EC₅₀ which represents the concentration of a compound that is required to obtain 50% of the maximum effect in vivo. (Ex. EC₁₀₀ would be the minimum concentration of a compound to obtain 100% of its effect). 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. (http://www.ncgc.nih.gov/guidance/section3.html)

According to the FDA, IC₅₀ represents the concentration of a drug that is required for 50% inhibition in vitro whereas EC₅₀ represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.(http://www.fda.gov/ohrms/dockets/ac/00/slides/3621s1d/sld036.htm)

IC₅₀ values are dependent on conditions under which they are measured. For example for enzymes, 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.