“IC50”的意思、由来-开放百科全书

The half maximal inhibitory concentration (IC₅₀) is a measure of the potency of a substance in inhibiting a specific 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. The values are typically expressed as molar concentration.

It is commonly used as a measure of antagonist drug potency in pharmacological research. 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]

Determining the 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 can be used to compare the potency of two antagonists.

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 agonist 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.

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 K_d 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 K_i using the Cheng-Prusoff equation formulated by Yung-Chi Cheng and William Prusoff (see K_i).^[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] For enzymatic reactions, this equation is:

where K_i is the binding affinity of the inhibitor, IC₅₀ is the functional strength of the inhibitor, [S] is fixed substrate concentration and K_m 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).

where [A] is the fixed concentration of agonist and EC₅₀ is the concentration of agonist that results in half maximal activation of the receptor. Whereas the IC₅₀ 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.^[3]

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.^[5]

pIC₅₀

Note the minus sign, which means that higher values of pIC₅₀ indicate exponentially greater potency.

pIC₅₀ is usually given in terms of molar concentration (mol/L, or M). Therefore, to obtain a pIC₅₀, an IC₅₀ should be specified in units of M. When IC₅₀ is expressed in μM or nM, it will need to be converted to M before conversion to pIC₅₀.^[6]

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.^[7]

See also

References

1. ^¹IC50 versus EC50
2. ^¹[https://www.ncbi.nlm.nih.gov/books/NBK91994/ NIH Chemical Genomics Center // Assay Guidance // Assay Guidance Manual // Assay Operations for SAR Support]
3. ^¹Glaxo Wellcome and Science - Global
4. ^{{cite journal|author=Cheng Y, Prusoff WH|title=Relationship between the inhibition constant (K_I) and the concentration of inhibitor which causes 50 per cent inhibition (I₅₀) of an enzymatic reaction|journal=Biochem Pharmacol|volume=22|issue=23|pages=3099–108|date=December 1973|pmid=4202581|doi=10.1016/0006-2952(73)90196-2}}
5. ^¹{{Cite journal | last1 = Lazareno | first1 = S. | last2 = Birdsall | first2 = N. J. | title = Estimation of competitive antagonist affinity from functional inhibition curves using the Gaddum, Schild and Cheng-Prusoff equations | journal = British Journal of Pharmacology | volume = 109 | issue = 4 | pages = 1110–1119 | year = 1993 | pmid = 8401922 | pmc = 2175764 | doi=10.1111/j.1476-5381.1993.tb13737.x}}
6. ^{{cite journal|author=Stewart MJ, Watson ID|title=Standard units for expressing drug concentrations in biological fluids|journal=British Journal of Clinical Pharmacology|volume=16|issue=1|pages=3–7|year=1983|doi=10.1111/j.1365-2125.1983.tb02136.x|pmc=1427960}}
7. ^{{cite journal|pmid=8739548|year=1996|last1=Robinson|first1=SF|last2=Marks|first2=MJ|last3=Collins|first3=AC|title=Inbred mouse strains vary in oral self-selection of nicotine|volume=124|issue=4|pages=332–9|journal=Psychopharmacology|doi=10.1007/bf02247438}}

Determining the IC50 of a drug