Alternatively we could measure the IC50 or the Ki (inhibitory constant) for the perpetrator. The Ki of a perpetrator that is capable of inhibiting an enzyme (or. What exactly is Ki and how does it differ from IC50? I found two definitions of Ki, one describes the Ki value as "the dissociation equilibrium constant of the. Learn quiz drug receptor interactions with free interactive flashcards. Choose from different sets of quiz drug receptor interactions flashcards on Quizlet.
This results in a flow of channel permeant ions e. Na and K for nicotinic receptors down their electrochemical gradient with a resultant change in membrane potential Figure 5.
IC50 - Wikipedia
In skeletal muscle, this results in a depolarization of the membrane potential, the production of an action potential, and contraction the biological response. G-protein activation of an ion channel The drug receptor stimulates an ion channel via activation of a G protein Figure 6. As an example, this is the mechanism by which acetylcholine acts to slow the heart rate. G-protein activated ion channel. Binding of an agonist to the m2 receptor activates a G-protein Gi which in turn stimulates a K-selective channel to open.
BASIC PRINCIPLES OF PHARMACOLOGY
The increase in K permeability will hyperpolarize the membrane potential. G-protein activation of a second messenger cascade There are two well characterized second messenger cascade mechanisms. One involves the G-protein Gs mediated activation of adenylyl cyclase, with subsequent formation of camp and activation of protein kinase A PK-A Figure 7.
DAG acts as a second messenger to stimulate protein kinase C, and IP3 stimulates the release of Ca ions from intracellular stores. DAG acts as a second messenger to activate protein kinase C PK-Cwhich phosphorylates a variety of intracellular proteins. IP3 stimulates the release of Ca from intracellular stores. These mechanisms are believed to mediate the vasoconstrictive effects of Ang II on vascular smooth muscle. Receptors linked to Cytoplasmic Enzymes e. 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.
50% of what? How exactly are IC50 and EC50 defined?
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.
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. 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.
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.
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. 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.
50% of what? How exactly are IC50 and EC50 defined? - FAQ - GraphPad
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. This article has been cited by other articles in PMC. Abstract A new web-server tool estimates Ki values from experimentally determined IC50 values for inhibitors of enzymes and of binding reactions between macromolecules e.
This converter was developed to enable end users to help gauge the quality of the underlying assumptions used in these calculations which depend on the type of mechanism of inhibitor action and the concentrations of the interacting molecular species.
Additional calculations are performed for nonclassical, tightly bound inhibitors of enzyme-substrate or of macromolecule-ligand systems in which free, rather than total concentrations of the reacting species are required. Required user-defined input values include the total enzyme or another target molecule and substrate or ligand concentrations, the Km of the enzyme-substrate or the Kd of the target-ligand reaction, and the IC50 value.
Assumptions and caveats for these calculations are discussed along with examples taken from the literature. The host database for this converter contains kinetic constants and other data for inhibitors of the proteolytic clostridial neurotoxins http: Typically, high-throughput screening assays are initially used to compare and down-select potential inhibitors of enzymatic activity or macromolecule-ligand binding. However, the IC50 value depends on concentrations of the enzyme or target moleculethe inhibitor, and the substrate or ligand along with other experimental conditions.