the radioligand is then determined in the presence of a range of concentrations of other competing non-radioactive compounds (usually antagonists), in order to measure the potency with which they compete for the binding of the radioligand. Competition curves may also be computer-fitted to a logistic function as described under direct fit. In this situation the IC50 is the concentration of competing ligand which displaces 50% of the specific binding of the radioligand. The IC50 value is converted to an absolute inhibition constant Ki using the Cheng-Prusoff equation formulated by Yung-Chi Cheng and William Prusoff (see Ki). Cheng Prusoff equation IC50 is not a direct indicator of affinity, although the two can be related at least for competitive agonists and antagonists by the Cheng-Prusoff equation. For enzymatic reactions, this equation is: where Ki is the binding affinity of the inhibitor, IC50 is the functional strength of the inhibitor, [S] is fixed substrate concentration and Km is the Michaelis constant i.e. concentration of substrate at which enzyme activity is at half maximal (but is frequently confused with substrate affinity for the enzyme, which it is not). Alternatively, for inhibition constants at cellular receptors: where [A] is the fixed concentration of agonist and EC50 is the concentration of agonist that results in half maximal activation of the receptor. Whereas the IC50 value for a compound may vary between experiments depending on experimental conditions, (e.g. substrate and enzyme concentrations) the Ki is an absolute value. Ki is the inhibition constant for a drug; the concentration of competing ligand in a competition assay which would occupy 50% of the receptors if no ligand were present. The Cheng-Prusoff equation produces good estimates at high agonist concentrations, but over- or under-estimates Ki at low agonist concentrations. In these conditions, other analyses have been recommended. See also Certain safety factor EC50 (half maximal effective concentration) LD50 (median lethal dose) Ki (equilibrium constant) References External links AAT Bioquest Online IC50 Calculator Online IC50 calculator (www.ic50.tk) based on the C programming language and gnuplot Alternative online IC50 calculator (www.ic50.org) based on Python, NumPy, SciPy and Matplotlib ELISA IC50/EC50 Online Tool (link seems broken) IC50 to pIC50 calculator Online tool for analysis of in vitro resistance to antimalarial drugs IC50-to-Ki converter of an inhibitor and enzyme that obey classic Michaelis-Menten kinetics. Concentration indicators Pharmacodynamics