IC50
The IC50 is a measure of the effectiveness of a compound in inhibiting biological or biochemical function. Often, the compound in question is a drug candidate. 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 ud as a measure of antagonist drug potency in pharmacological rearch. Sometimes, it is also converted to the pIC50 scale (-log IC50), in which higher values indicate exponentially greater potency. According to the FDA, IC50 reprents the concentration of a drug that is required for 50% inhibition in vitro. It is comparable to an EC50 for agonist drugs. EC50 also reprents the plasma concentration required for obtaining 50% of a maximum effect in vivo.
Determination IC50 of a drug
Functional antagonist assay:
The IC50 of a drug can be determined constructing a do-respon 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 respon of the agonist.
IC50 values are dependent on conditions under which they are measured. In general, the higher the concentration of inhibitor, the more will agonist activity be lowered. IC50 value increas as enzyme concentration increas. 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 ud to compare the potency of two antagonists.
Competition binding assays:
In this type of assay, a single concentration of radioligand (usually an agonist) is ud in
every assay tube. The ligand is ud at a low concentration, usually at or below its KD value. The level of specific binding of the radioligand is then determined in the prence 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 (e Ki).
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 eqtn.
where Ki is the binding affinity of the inhibitor, IC50 is the functional strength of the inhibit
or, [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 prent.
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ECx Concentration having an effect on x% of the population compared to control
EC50 " Effect Concentration 50% " or concentration having 50% of effect compared to control
LCx concentration having caud the death of X% of the population tested compared to control
IC50 " Inhibition Concentration 50% " or concentration inhibiting of 50% a parameter like the growth or luminescence compared to control
LC50 concentration having caud the death of 50% of the population tested compared to control
LD50 " Lethal Proportions 50% " or proportions having caud the death of 50% of the population tested.
ED50 “50%median effective do”
LOEC " Low Obrved Effect Concentration " or concentration with a very weak obrvable effect. It is about the lowest concentration for which the effect is different from that of controls. It is the first concentration tested after the NOEC.
NOEC " No Obrved Effect Concentration " or concentration without obrvable effect. It is about the highest concentration tested for which the effect is significantly not different from controls
