Half-Maximal Effective Concentration (EC50)

EC50 is a fitted parameter, not a measured point: it falls out of the curve drawn through a full concentration series, along with the maximum and the slope. It is defined against the agonist's own maximum in that system, so a partial agonist has an EC50 even though it never reaches the response a full agonist can produce. The in-vivo counterpart, where the x-axis is dose and the outcome is a proportion of animals or patients responding, is conventionally called ED50 instead.

The number is a property of the compound and the assay jointly. In a cell line overexpressing a receptor, cyclic AMP accumulation frequently yields picomolar values, while arrestin recruitment measured in the very same cells yields nanomolar ones, because the two readouts have different amplification. Receptor reserve moves EC50 well below the binding constant, sometimes by one to two orders of magnitude, so a compound can look far more potent functionally than its affinity suggests.

Used properly, EC50 ranks compounds tested side by side in one assay on one day, which is exactly what medicinal chemistry needs. Set against the free concentration a dose actually achieves, it also answers whether an activity is reachable in a living organism or only in a well. That second comparison is what separates a pharmacological finding from a laboratory curiosity.

Two misreadings recur. The first is quoting an EC50 as evidence of clinical effect, when it establishes only that a receptor responds in a dish. The second is treating potency as efficacy: a compound with a picomolar EC50 and a low maximum is clinically weaker than one with a nanomolar EC50 that drives the system fully. Grey-market pages fond of single-digit picomolar values rarely mention that the number came from a receptor-overexpressing line built for sensitivity.