IC50

Half maximal inhibitory concentration (IC₅₀) is a measure of the potency of a substance in inhibiting a specific biological or biochemical function. IC₅₀ is a quantitative measure that indicates how much of a particular inhibitory substance (e.g. drug) is needed to inhibit, in vitro, a given biological process or biological component by 50%.^[1] The biological component could be an enzyme, cell, cell receptor or microbe. IC₅₀ values are typically expressed as molar concentration.

IC₅₀ is commonly used as a measure of antagonist drug potency in pharmacological research. IC₅₀ is comparable to other measures of potency, such as EC₅₀ for excitatory drugs. EC₅₀ represents the dose or plasma concentration required for obtaining 50% of a maximum effect in vivo.^[1]

IC₅₀ can be determined with functional assays or with competition binding assays.

Sometimes, IC₅₀ values are converted to the pIC₅₀ scale.

${\displaystyle {\ce {pIC_{50}}}=-\log _{10}{\ce {(IC_{50})}}}$

Due to the minus sign, higher values of pIC₅₀ indicate exponentially more potent inhibitors. 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. When animals decrease consumption from the drug-laced water bottle, the concentration of the drug that results in a 50% decrease in consumption is considered the IC₅₀ for fluid consumption of that drug.^[3]