Read 11 answers by scientists with 31 recommendations from their colleagues to The EC50 indicates how much of a drug is needed to achieve 50% of the .. What is the difference between IC50, Ki and Kd of a given inhibitor in an assay?. Figure Selected structures of CDK5/p25 inhibitors. primarily p 15 (IC⁄ mM, substrate competitive), and 4-aminothiazole 16a (IC50 1⁄ mM. Extracts from 15 plants showed an IC50 of 5 mg/ml or less on either amastigote or including the species Polyalthia suaveolens (IC⁄ mg/ml; SI1⁄).
These receptors contain an extracellular domain that binds to a specific ligand, and a cytoplasmic domain that typically contains a protein tyrosine kinase Figure 9. However, other enzymes such as serine kinases, or a guanylyl cyclase may also be coupled to a receptor and work by the same mechanism.
EGF, Insulin, various growth factors Figure 9.
The binding of a ligand to receptors produces a change in receptor conformation that allows receptors to interact. The auto-phosphorylation typically results in a prolonged response to the agonist e. Noncompetitive Antagonists Antagonists are drugs that bind to receptors have affinitybut do not produce a substantial degree of receptor stimulation they have very low efficacy.
BASIC PRINCIPLES OF PHARMACOLOGY
Antagonists are typically classified as competitive or noncompetitive. Competitive antagonists bind reversibly to the same receptor site as the agonist. This effect produces a rightward parallel shift of the dose-response for the agonist towards higher concentrations.
- 50% of what? How exactly are IC50 and EC50 defined?
In the presence of a competitive antagonist, agonists can still produce the same e. The vast majority of clinically used drugs that act as receptor antagonists are competitive antagonists. Noncompetitive antagonists either bind irreversibly e. The primary effect of a noncompetitive antagonist is a reduction in the maximal effect produced by the agonist see Figure 10B. In some cases the slope may also be reduced.
In contrast to a competitive antagonist, the effect of a noncompetitive antagonist cannot be reversed by simply increasing the concentration of the agonist, since the law of mass action does not apply.
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Examples of Competitive and Noncompetitive Antagonism. In the presence of the competitive antagonist, the dose-response curve is shifted to the right in a parallel manner. This reduces the fraction of available receptors, and reduces the maximal effect that can be produced by the agonist. Under physiological conditions, the level of such spontaneous activity is relatively low, and is not easily observed unless the wild-type receptor is cloned and over-expressed e.
More recently, several naturally occurring mutant GPCRs with increased constitutive activity have been identified. Interestingly, recent research using a mouse model of heart failure indicates that mechanical stretch, such as that caused by heart failure, enhances the constitutive activity of cardiac angiotensin II receptors, resulting in the development of cardiac remodeling hypertrophyindependent of Angiotensin II stimulation.
Furthermore, this harmful effect contributing to cardiac remodeling can be reversed by treatment with the AT1 receptor inverse agonist candesartan Yasuda et al, Whether this mechanism contributes to the well documented harmful effects of angiotensin-II in patients with heart failure, as well as the beneficial effects of angiotensin receptor antagonists in heart failure including candesartanis yet to be clearly documented. Figure 12 illustrates proposed models of drug-receptor interaction for receptors exhibiting an absence of constitutive activity, and for receptors that are spontaneously active in the absence of ligand.
Drugs that selectively stabilize the inactive receptor conformation Di act as inverse agonists when they bind to constitutively active receptors, due to their ability to reduce the degree of basal activity. In the absence of basal activity e. Drugs that selectively stabilize the active receptor conformation e. Drugs that bind non-selectively equally to both receptor conformations behave as classical antagonists. Physiological antagonism involves drug activation of two different compensatory biological mechanisms that exist to maintain homeostasis by different mechanisms.
Acetylcholine and norepinephrine exert their effects through different receptors and signal transduction pathways, which when activated produced opposing effects e.
50% of what? How exactly are IC50 and EC50 defined? - FAQ - GraphPad
Chemical antagonism occurs when a drug reduces the concentration of an agonist by forming a chemical complex e. Pharmacokinetic antagonism occurs when one drug accelerates the metabolism or elimination of another e.
Drugs often work on multiple receptors Drugs often work on more than one receptor, and as a result produce more than one kind of biological response Figure One good example is norepinephrine NEthe sympathetic neurotransmitter which can relax bronchial smooth muscle, but constrict arterial smooth muscle.
A single drug can interact with multiple receptors. These receptors are coupled to different intracellular messenger systems, and produce different responses when stimulated. These receptor subtypes are not typically expressed in equal amounts in the same tissue e. Selectivity, and the Therapeutic Window If a drug has one effect, and only one effect on all biological systems it possesses the property of specificity.
In experience, the vast majority of drugs are selective rather than specific. This is the case because most drugs will act on more than one receptor site once they reach an appropriately high concentration. The concentration range over which a drug produces its therapeutic effect is known as its therapeutic window. Similar to most drugs, yohimbine lacks true specificity in its biological actions.
EC50 — used for in vitro studies only When investigating drug effects in a tissue bath setting, drug concentrations are typically precisely known. An example of an exception to this rule is when one is using an impure source of a drug e. In this case the total dose gram weight of St. John's wort used would be more easily defined vs the concentration of the active ingredient swhich may be unknown.
The concentration of drug achieved in the bloodstream e. As a result, in whole animal experiments, we talk of doses that produce a given magnitude of therapeutic effect, e.
One can also define how drug responses vary in a population of animals or patients. In this situation, one can define the minimum dose needed to produce the desired effect in each animal or patient. The results of this type of study can be plotted in the form of a quantal dose response curve Figure To summarize, ED50 is a value defined in whole animal or population studies.
In contrast, if the discussion concerns drug responses in a population, ED50 will most likely indicate the median dose producing the desired therapeutic effect e. Drug responses can also be defined as quantal.
An illustration of a quantal dose-response relationship is shown in Figure 14, which depicts the relationship between the dose of a drug vs. With a sufficiently large patient population, this type of relationship is often well-fit by a Gaussian distribution, in which statistical parameters can be used to predict the variability of drug response in the patient population.
This type of dose-response relationship is most useful for defining quantal events such as the prevention of convulsions, arrhythmia or death by a drug. A set of data obtained after administration of increasing doses of a drug to a group of patients, and observation of the minimum dose at which each patient responded with the desired outcome. The results have been plotted as a histogram, and fit with a gaussian curve. The dose-relationships for toxic and lethal effects may have different slopes compared to therapeutic dose-response relationship because they produce effects by different receptors or mechanisms e.
It is of value to know the relative difference between the average toxic dose and the average therapeutic dose. The TI is also sometimes referred to as the therapeutic ratio. A drug having a TI of 2 would be relatively unsafe, because doubling the dose e.
If this number is much greater than one, it is a relatively safe drug. The reason this index is less commonly used is because the LD1 value, at least for human populations, is typically unknown or poorly defined, for ethical reasons! The relationship between the dose-response relationships for producing therapeutic and toxic side effects.
This topic will be covered in more depth later during the course. Examples of physiological, chemical and pharmacokinetic antagonism were discussed above. When two drugs with similar mechanisms are given together, they typically produce additive effects. This is also referred to as summation.
However, if the effect of two drugs exceeds the sum of their individual effects, this is referred to as potentiation or synergism. Potentiation requires that the drugs act at different receptors or effector systems. An example of potentiation would be the increase in beneficial effects noted in the treatment of AIDS by combination therapy with AZT a nucleoside analog that inhibits HIV reverse transcriptase and a protease inhibitor protease activity is important for viral replicationor the combined effects of norepinephrine and cocaine on arterial blood pressure.
An additional graphical illustration of additive and synergistic effects is shown in Figure Illustration of drug combinations producing Summation vs. Drugs A and B produce equal effects, and their effects are additive when combined.
Desensitization Receptor-mediated responses to drugs, hormones and neurotransmitters commonly desensitize with time. I prefer the third method, as it analyzes all the data, but that is not a strong preference. 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. 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. 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. The distinction between relative and absolute IC50 doesn't really apply to these data.
Because the data don't define a bottom plateau, the IC50 must be defined relative to the NS control values. You can fit curves using data in their natural units. A common mistake is to assume that fitting dose-response curves requires that data first be normalized.