IC50

The half maximal inhibitory concentration (IC50) is a measure of the effectiveness of a compound in inhibiting biological or biochemical function. This quantitative measure indicates how much of a particular drug or other substance (inhibitor) is needed to inhibit a given biological process (or component of a process, i.e. an enzyme, cell, cell receptor or microorganism) by half. In other words, it is the half maximal (50%) inhibitory concentration (IC) of a substance (50% IC, or IC50). It is commonly used as a measure of antagonist drug potency in pharmacological research. Sometimes, it is also converted to the pIC50 scale (-log IC50), in which higher values indicate exponentially greater potency. According to the FDA, IC50 represents the concentration of a drug that is required for 50% inhibition in vitro.[1] It is comparable to an EC50 for agonist drugs. EC50 also represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.[1]

Contents

Determination IC50 of a drug

Functional antagonist assay

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.[2]

IC50 values are very dependent on conditions under which they are measured. In general, the higher the concentration of inhibitor, the more agonist activity will 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).[2][3]

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 equation.[4]

K_i = \frac{IC_{50}}{1%2B\frac{[S]}{K_m}}

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 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). Whereas the IC50 value for a compound may vary between experiments depending on experimental conditions, 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 ligand were present.[3]

Behavioral Assays

The IC50 terminology is also used for some behavioral measures in vivo, such as a 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.[5]

See also

References

  1. ^ a b IC50 versus EC50
  2. ^ a b NIH Chemical Genomics Center // Assay Guidance // Assay Guidance Manual // Assay Operations for SAR Support
  3. ^ a b Glaxo Wellcome and Science - Global
  4. ^ Cheng Y, Prusoff WH (December 1973). "Relationship between the inhibition constant (K1) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction". Biochem Pharmacol 22 (23): 3099–108. doi:10.1016/0006-2952(73)90196-2. PMID 4202581. 
  5. ^ Robinson, SF; Marks, MJ; Collins, AC (1996). "Inbred mouse strains vary in oral self-selection of nicotine". Psychopharmacology 124 (4): 332–9. PMID 8739548. 

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