(3) include three types of classic inhibitor mechanisms in which different relations may exist between S and Km. For tightly bound inhibitors, the. Is EC50/IC50 of a drug always directly proportional to the concentration of the concentration tested which means EC50/IC50 value should remain the same if then it is being driven by the apparent Ki, and not by the enzyme concentration. based on the relationship that, for competitive inhibition, Ki = IC50/2 when . concentration of inhibitor in the test system (typically human liver.
Thus, comparisons can be more readily made among different laboratories to characterize the inhibitors. While these more time-consuming assays are usually done with the most promising candidates, accurate, initial estimates of Ki values for more of the candidates would be beneficial. A much discussed problem in the literature 1—8 is converting IC50 to Ki values because even the simplest types of inhibitory mechanisms e.
To help address this problem, our web-server tool calculates Ki values from IC50 values using equations for enzyme-substrate and target-ligand interactions by different inhibitory mechanisms http: Additional calculations are performed for tightly bound inhibitors of enzyme-substrate reactions in which free, rather than total, concentrations of the molecular species are calculated for nonclassic Michaelis—Menten kinetics.
Similar calculations can be performed for target molecule-ligand systems. User-defined input values include total concentrations of the enzyme or target molecule and substrate or ligandthe Km of the enzyme-substrate or the Kd of the target-ligand reaction and the IC50 value.
The outputs include tabulations of the Ki values under different kinetic schemes, extensive tabulations of the results, summary histograms and the corresponding equations. Help buttons are available for Background, Assumptions, Literature, Links and Equations along with examples taken from the host database-server that contains kinetic information on neurotoxin inhibitors.
An example calculation is included here for a tight-binding inhibitor of an enzyme—substrate reaction, while other enzyme inhibitor and protein—ligand—inhibitor examples are also provided. Our rationale for creating this converter is to enable end users to judge the quality of the underlying assumptions for these calculations and to help facilitate research and the development of potential therapeutic products.
Equations 1—4 were adapted from refs. The analytic expressions for Ki that are shown below were verified numerically by methods used in a previous kinetic analysis The derivations for converting IC50 to Ki values published by Brandt et al. Enter the blank values as if they were part of the dose-response curve. Simply enter a low dose, perhaps or You can't enter zero, because zero is not defined on a log scale.
The results will be very similar with any of these methods, because the data form a complete dose-response curve with a clear top plateau that is indistinguishable from the blank. I prefer the third method, as it analyzes all the data, but that is not a strong preference.
IC50 - Wikipedia
Similarly, there are three ways to deal with the bottom plateau: That is the ideal situation. There is no ambiguity about what IC50 means. A situation where IC50 can be defined in two ways This figure shows an unusual situation where the inhibition curve plateaus well above the control values NS defined by a high concentration of a standard drug.
This leads to alternative definitions of IC Clearly, a single value cannot summarize such a curve.
50% of what? How exactly are IC50 and EC50 defined?
You'd need at least two values, one to quantify the middle of the curve the drug's potency and one to quantify how low it gets the drug's maximum effect. The graph above shows two definitions of the IC The relative IC50 is by far the most common definition, and the adjective relative is usually omitted.
The NS values are totally ignored with this definition of IC This definition is the one upon which classical pharmacological analysis of agonist and antagonist interactions is based.
With appropriate consideration of the biological system and concentrations of interacting ligands, estimated Kd values can often be derived from the IC50 value defined this way not so for the "so-called absolute IC50" mentioned below. This term is not entirely standard. The concept but not the term "absolute IC50" is used to quantify drugs that slow cell growth.
50% of what? How exactly are IC50 and EC50 defined? - FAQ - GraphPad
The abbreviation GI50 is used for what we call here the absolute IC They don't use the terms relative and absolute. Incomplete dose-response curves Any attempt to determine an IC50 by fitting a curve to the data in the graph above will be useless. A curve fitting program might, or might not, be able to fit a dose-response curve to the data.
But if the curve fits, the value of the IC50 is likely to be meaningless and have a very wide confidence interval. The data simply don't form a top plateau which would define or a bottom plateau which would define 0. If data haven't defined or 0, then 50 is undefined too, as is the IC If you also have control values that define and 0, then the curve can be easily fit. The curve below was created by fitting a dose response curve, but constraining the Top plateau to be a constant value equal to the mean of the Blanks values, and the Bottom plateau equal to the mean of the NS values.
The value of the IC50 fit this way only makes sense if you assume that higher concentrations of the inhibitor would eventually inhibit down to the NS values. That is an assumption that can't be tested with the data at hand.