IC50

IC50 — the half-maximal inhibitory concentration — is the concentration of an inhibitor, antagonist or competing ligand that reduces a measured response by 50% under the specific conditions of the assay in which it is determined.

IC50 is assay-dependent: it varies with the concentration of the agonist or substrate being competed against, with enzyme or receptor density and with incubation time. In competition binding, IC50 can be converted to a system-independent inhibition constant Kᵢ via the Cheng-Prusoff equation, which adjusts for the concentration of the radiolabelled probe and its Kd.