
The 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
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology and ...
'', a given biological process or biological component by 50%.
The biological component could be an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
,
cell,
cell receptor
In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a recepto ...
or
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
. IC
50 values are typically expressed as
molar concentration
Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...
.
IC
50 is commonly used as a measure of
antagonist drug potency in
pharmacological
Pharmacology is a branch of medicine, biology and pharmaceutical sciences concerned with drug or medication action, where a drug may be defined as any artificial, natural, or endogenous (from within the body) molecule which exerts a biochemic ...
research. IC
50 is comparable to other measures of potency, such as
EC50 for
excitatory drugs. EC
50 represents the dose or plasma concentration required for obtaining 50% of a maximum effect ''
in vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
''.
IC
50 can be determined with functional assays or with competition binding assays.
Sometimes, IC
50 values are converted to the pIC
50 scale.
:
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
Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...
(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 a
two bottle fluid consumption test
2 (two) is a number, numeral and digit. It is the natural number following 1 and preceding 3. It is the smallest and only even prime number. Because it forms the basis of a duality, it has religious and spiritual significance in many cultu ...
. 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.
Functional antagonist assay
The IC
50 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. IC
50 values can be calculated for a given antagonist by determining the concentration needed to inhibit half of the maximum biological response of the agonist.
IC
50 values can be used to compare the potency of two antagonists.
IC
50 values are very dependent on conditions under which they are measured. In general, ''a higher concentration of inhibitor leads to lowered agonist activity.'' IC
50 value increases as agonist concentration increases. Furthermore, depending on the type of inhibition, other factors may influence IC
50 value; for
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
dependent enzymes, IC
50 value has an interdependency with concentration of ATP, especially if inhibition is
competitive
Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...
.
IC50 and affinity
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 IC
50 is the concentration of competing ligand which displaces 50% of the specific binding of the radioligand. The IC
50 value is converted to an absolute
inhibition constant
In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from ''ligare'', which means 'to bind'. In protein-ligand binding, the ligand is usually a mole ...
K
i using the Cheng-Prusoff equation formulated by Yung-Chi Cheng and
William Prusoff
William Herman Prusoff (June 25, 1920 – April 3, 2011) was a pharmacologist who was an early innovator in antiviral drugs, developing idoxuridine, the first antiviral agent approved by the FDA, in the 1950s, and co-developing (with Tai-shun ...
(see K
i).
Cheng Prusoff equation
IC
50 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.
For enzymatic reactions, this equation is:
:
where K
i is the binding affinity of the inhibitor, IC
50 is the functional strength of the inhibitor,
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:
:
where
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.
See also
*
Certain safety factor
The therapeutic index (TI; also referred to as therapeutic ratio) is a quantitative measurement of the relative safety of a drug. It is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes ...
*
EC50 (half maximal effective concentration)
*
LD50 (median lethal dose)
*
Ki (equilibrium constant)
References
External links
AAT Bioquest Online IC50 CalculatorOnline IC50 calculator(www.ic50.tk) based on the
C programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
and
gnuplot
gnuplot is a command-line and GUI program that can generate two- and three-dimensional plots of functions, data, and data fits. The program runs on all major computers and operating systems (Linux, Unix, Microsoft Windows, macOS, FreeDOS ...
Alternative online IC50 calculator(www.ic50.org) based on
Python,
NumPy,
SciPy
SciPy (pronounced "sigh pie") is a free and open-source Python library used for scientific computing and technical computing.
SciPy contains modules for optimization, linear algebra, integration, interpolation, special functions, FFT, signal ...
and
Matplotlib
Matplotlib is a plotting library for the Python programming language and its numerical mathematics extension NumPy. It provides an object-oriented API for embedding plots into applications using general-purpose GUI toolkits like Tkinter, wx ...
ELISA IC50/EC50 Online Tool(link seems broken)
Online tool for analysis of in vitro resistance to antimalarial drugsIC50-to-Ki converterof an inhibitor and enzyme that obey classic
Michaelis-Menten kinetics.
{{pharmacology, state=collapsed
Concentration indicators
Pharmacodynamics