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Encyclopedia > IC50

IC50, or the half maximal inhibitory concentration, represents the concentration of an inhibitor that is required for 50% inhibition of its target (i.e. an enzyme, cell, cell receptor or a microorganism). In simpler terms, it measures how much of a particular substance/molecule is needed to inhibit some biological process by 50%. IC50 is commonly used as a measure of drug potency of antagonist drugs 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 comparible to an EC50 for agonist drugs. EC50 also represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.[1] Image File history File links Broom_icon. ... HIV protease in a complex with the protease inhibitor ritonavir. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the... In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm that binds to a specific factor (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ... A cluster of Escherichia coli bacteria magnified 10,000 times. ... Potency may refer to: Virility Lack of the above: Erectile dysfunction, Infertility or Effeminacy. ... Antagonists In medicine and biology, a receptor antagonist is a ligand that inhibits the function of an agonist and inverse agonist for a specific receptor. ... “FDA” redirects here. ... The term half maximal effective concentration (EC50) refers to the concentration of a drug which induces a response halfway between the baseline and maximum. ... Agonists An agonist is a substance that binds to a receptor and triggers a response in the cell. ...

Contents

Determinting IC50 of a drug

Functional antagonist assay

The IC50 of a drug can be determined constructing a dose-resonse curve and examing 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] The Dose-response relationship describes the change in effect on an organism caused by differing levels of exposure (or doses) to a substance. ...


IC50 values are dependent on conditions under which they are measured. For example for enzymes, 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. Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide that is most important as a molecular currency of intracellular energy transfer. ... Competitive inhibition is a form of enzyme inhibition where binding of the inhibitor to the enzyme prevents binding of the substrate and vice versa. ...


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).[3][2]


IC50 and affinity

IC50 is not a direct indicator of affinity although the two can be related at least for comptetitive agonists and antagonists by the Cheng-Prusoff eqtn.

 Ki=IC50/1+(S/Km) 

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 affinity of the substrate for the enzyme.Whereas the IC50 value for a compound may vary between experiments depending on radioligand concentration, 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 radioligand were present.[3]


See also

  • Certain safety factor
  • Dose-response relationship
  • EC50
  • LD50

The Certain Safety Factor is the ratio of the lethal dose to 1% of population to the effective dose to 99% of the population (LD1/ED99). ... The Dose-response relationship describes the change in effect on an organism caused by differing levels of exposure (or doses) to a substance. ... The term EC50 represents the concentration of a compound where 50% of its effect is observed. ... An LD50 test being administered In toxicology, the LD50 or colloquially semilethal dose of a particular substance is a measure of how much constitutes a lethal dose. ...

References

External links


  Results from FactBites:
 
amfAR Clinical Research (479 words)
Ten of 14 patients had mutant virus at codon 215 detected by differential hybridization; AZT IC50 ratio significantly correlated with the proportion of mutant virus at codon 215 (r=0.68, p=0.0078).
Nine of 10 patients had mutant virus detected at codon 70; a significant correlation between the proportion of mutant virus at codon 70 and AZT IC50 ratio was not seen.
Overall, ddI IC50 was weakly correlated with the proportion of 215 mutant virus detected (r=0.61, p=0.02).
  More results at FactBites »

 
 

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