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Glossary of Pharmacology 



IC50 is the concentration of the competitive antagonist required to reduce the activity/binding of an agonist to a specific enzyme, receptor or transporter by 50%. The term can be estimated from in-vitro or ex-vivo binding assay plots (fig) and is used to describe antagonistsaffinity to enzyme/receptor. IC50 value depends on three factors; affinity of the antagonist to the receptor (the higher the affinity, the lower the IC50 value), concentration of the ligand being displaced (higher concentrations of the ligand will require more antagonist to reach 50% inhibition and the IC50 values will increase), and the dissociation constant (Kd) of the ligand to the receptor (the lower the Kd value, the more concentration of the antagonist will be needed to displace the ligand, and the IC50 value will increase).  

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Linked terms: Ki

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