EC50

EC50 (half-maximal Effective Concentration) designates the ligand concentration producing 50% of the maximum measurable biological effect on a given system. This parameter is the standard descriptor of agonist pharmacological potency: the lower the EC50, the more potent the compound, meaning capable of producing its effect at very low concentrations.

EC50 is determined experimentally by constructing a dose-response curve: a cellular or tissue preparation is exposed to increasing concentrations of the compound, the response is measured (intracellular cAMP, calcium flux, reporter protein activation, etc.), then data points are fitted to a four-parameter logistic equation (bottom, top, EC50, Hill coefficient). The curve typically takes a sigmoidal shape when concentrations are plotted on a logarithmic scale.

Several precautions apply when interpreting published EC50 values. First, the experimental system strongly conditions the value: an EC50 measured on HEK293 cells transfected with the human receptor often differs by 5- to 50-fold from that obtained on native tissue, due to receptor density and available receptor reserve. Second, EC50 does not predict efficacy (Emax): a partial agonist may have a lower EC50 than a full agonist while producing a lower ceiling effect. Third, in vitro measured EC50 does not translate directly to an in vivo therapeutic dose, because pharmacokinetic parameters modulate actual exposure at the site of action.

For peptides listed on lab-peptides-france.com, EC50 values enable researchers to rank analogues within the same family. Take GLP-1 agonists for example: semaglutide's EC50 on the human GLP-1 receptor sits in the low nanomolar range, comparable to liraglutide, while tirzepatide exhibits a bi-specific profile with distinct EC50 values on GLP-1 and GIP receptors. These fine pharmacological differences partly explain effect divergences observed in preclinical studies.

In rigorous exploratory research, EC50 is never used in isolation. It is accompanied by maximum efficacy, selectivity against other related receptor subtypes, and pharmacokinetic data to build a complete picture of the pharmacological profile.