Half-Maximal Inhibitory Concentration (IC50)

Molar concentration of an inhibitor, antagonist, or drug required to reduce a specific biochemical or cellular response by 50 percent.

Specifications & Metrics

type: Pharmacodynamic Potency Metric
symbol: IC50 / pIC50 (−log10 IC50)
units: mol/L (M, μM, nM, pM) or dimensionless log scale (pIC50)
equation: Ki = IC50 / (1 + [L] / Kd) (Cheng–Prusoff equation for competitive inhibition)
pioneer: Yung-Chi Cheng and William H. Prusoff (derived the IC50-to-Ki conversion relationship, 1973)

IC50 quantifies functional antagonist or enzyme-inhibitor potency as the molar concentration that reduces a target enzymatic reaction rate, ion current, radioligand binding, or tumor cell proliferation halfway between the uninhibited baseline and maximal inhibition.
Unlike the thermodynamic inhibition constant (Ki), an operational IC50 value depends on the substrate or radioligand concentration [L] present in the assay; Yung-Chi Cheng and William Prusoff formulated the Cheng–Prusoff equation (1973) to convert competitive IC50 values into absolute Ki affinities.
Medicinal chemists routinely report potency on the logarithmic pIC50 scale (−log10 of the molar IC50, where an IC50 of 1 nM equals pIC50 = 9.0) because error distributions from serial dilution curves are log-normal.