IC50

Half maximal inhibitory concentration (IC 50 ) is a measure of the potency of a substance in inhibiting a specific biological or biochemical function. IC 50 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%. The biological component could be an enzyme, cell, cell receptor or microbe. IC 50 values are typically expressed as molar concentration. IC 50 is commonly used as a measure of antagonist drug potency in pharmacological research. IC 50 is comparable to other measures of potency, such as EC 50 for excitatory drugs. EC 50 represents the dose or plasma concentration required for obtaining 50% of a maximum effect in vivo. IC 50 can be determined with functional assays or with competition binding assays. Sometimes, IC 50 values are converted to the pIC 50 scale. : \ce{pIC_{50}} = -\log_{10} \ce{(IC_{50})} Due to the minus sign, higher values of pIC 50 indicate exponentially more potent inhibitors. pIC 50 is usually given in terms of molar concentration (mol/L, or M), thus requiring IC 50 in units of M. The IC 50 terminology is also used for some behavioral measures in vivo, such as the two bottle fluid consumption test.…

Half maximal inhibitory concentration (IC 50 ) is a measure of the potency of a substance in inhibiting a specific biological or biochemical function. IC 50 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%. The biological component could be an enzyme, cell, cell receptor or microbe. IC 50 values are typically expressed as molar concentration. IC 50 is commonly used as a measure of antagonist drug potency in pharmacological research. IC 50 is comparable to other measures of potency, such as EC 50 for excitatory drugs. EC 50 represents the dose or plasma concentration required for obtaining 50% of a maximum effect in vivo. IC 50 can be determined with functional assays or with competition binding assays. Sometimes, IC 50 values are converted to the pIC 50 scale. \ce{pIC_{50}} = -\log_{10} \ce{(IC_{50})} Due to the minus sign, higher values of pIC 50 indicate exponentially more potent inhibitors. pIC 50 is usually given in terms of molar concentration (mol/L, or M), thus requiring IC 50 in units of M. The IC 50 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 IC 50 for fluid consumption of that drug. K_i = \frac\ce{IC50}{1+\frac{[S]}{K_m}} where K i is the binding affinity of the inhibitor, IC 50 is the functional strength of the inhibitor, [S] is fixed substrate concentration and K m is the Michaelis constant i.e. concentration of substrate at which enzyme activity is at half maximal (but is frequently confused with substrate affinity for the enzyme, which it is not). Alternatively, for inhibition constants at cellular receptors: K_i = \frac\ce{IC50}{\frac{[A]}\ce{EC50}+1} where [A] is the fixed concentration of agonist and EC 50 is the concentration of agonist that results in half maximal activation of the receptor. Whereas the IC 50 value for a compound may vary between experiments depending on experimental conditions, (e.g. substrate and enzyme concentrations) the K i is an absolute value. K i 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 ligand were present. The Cheng–Prusoff equation produces good estimates at high agonist concentrations, but over- or under-estimates K i at low agonist concentrations. In these conditions, other analyses have been recommended.

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