The parameter EC₅₀ is abbreviated for 'half maximal effective concentration'. In a pharmacological context, this can be the concentration of a drug necessary to cause half of the maximum possible effect. In a binding experiment, the EC₅₀ is the ligand concentration at which half of the Target is present in the bound state.

The EC₅₀ and the Kd models quantify interactions and are usually used to compare the Binding Affinity of different ligands. Using the Kd Fit Model for data evaluation will yield a K_d while using the Hill Model will yield an EC₅₀ value.

However, there is an important difference: the K_d is a physical property of the interaction and independent of the target concentration. K_ds can generally be compared across technologies as long as the general interaction conditions are the same (for example, buffer conditions, protein construct, etc).

The EC₅₀, on the other hand, by definition, always depends on the target concentration. It is often measured via the effect it induces, for example, in cell-based assays, and allows only the comparison of ligands measured in the same experimental setup. In situations where the target concentration is lower than the K_d, K_d and EC₅₀ will have the same value, provided the interaction follows a 1:1 stoichiometry, is non-cooperative, and is determined by the law of mass action. In situations where the target concentration is in the range of or above the K_d, K_d and EC₅₀ can differ significantly.

Example

If a given interaction has a K_d value of 2 nM (a quite high affinity) and the target is present at a concentration of 2 µM, a ligand concentration of 1 µM is necessary to reach a state where half of all target molecules are in the bound state. By definition, the EC₅₀ amounts to 1 µM (500*K_d) in this case.