EC50ED50

EC50 and ED50 are pharmacological metrics used to describe potency in producing a half-maximal effect. EC50 refers to the concentration of a drug that elicits 50% of its maximal response at a target site, typically measured in units of molarity (for example, micromolar). ED50 refers to the dose required to achieve 50% of the drug’s maximal effect in a living organism, expressed in units such as milligrams per kilogram. Both are derived from dose–response or concentration–response curves.

In practice, these curves are often modeled with a sigmoidal (Hill) equation. For a concentration-response: E = Emax × [A]^n / (EC50^n + [A]^n). For a dose-response: E = Emax × D^n / (ED50^n + D^n). Here E is the effect, Emax the maximal effect, [A] or D the concentration or dose, n the Hill coefficient describing slope, and EC50 or ED50 the concentration or dose giving half-maximal effect.

Interpretation centers on potency and efficacy. A lower EC50 or ED50 indicates greater potency, meaning less drug is needed to achieve half of Emax. However, potency does not imply higher efficacy; Emax and the response slope are also important. These metrics depend on assay conditions, tissue type, species, exposure time, and measurement endpoints, so they are not universally transferable.

Applications and limitations: EC50 and ED50 are used to compare compounds, inform dosing strategies, and guide drug development. ED50, in particular, raises ethical considerations due to animal testing, leading to reliance on in vitro proxies when possible. Related concepts include IC50 for inhibitors and LD50 for lethality, each serving different aspects of pharmacology and toxicology.