Glossary of Pharmacology 

IC₅₀

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 antagonists’ affinity 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 (K_d) of the ligand to the receptor (the lower the K_d value, the more concentration of the antagonist will be needed to displace the ligand, and the IC₅₀ value will increase).  

Linked terms: K_i

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