EC50
EC50 is the concentration of an agonist that produces half of the maximum effect that agonist can achieve. It is the standard yardstick of potency for activators: the lower the EC50, the less drug is needed to reach the halfway response.
It is taken from the agonist's dose–response curve — the midpoint between baseline and the agonist's own ceiling (Emax). Note that EC50 is measured against that compound's own maximum, so a weak partial agonist can have a low (left-shifted) EC50 yet still produce only a modest overall effect; EC50 reports potency, Emax reports efficacy.
EC50 depends on the biological system as well as the molecule. Where there is receptor reserve (spare receptors), the EC50 of a full agonist can sit well below its binding Kd, because occupying only a fraction of receptors already triggers a half-maximal response. So EC50 and Kd, while related, need not coincide.
If an agonist's Emax is a 100-unit signal, its EC50 is the concentration at which it produces a 50-unit signal — regardless of how that ceiling compares to other agonists.
EC50 marks half-maximal effect on an agonist's own dose–response curve.
Potency (where the curve sits, EC50) and efficacy (how high the curve goes, Emax) are different things; a compound can be very potent yet only a partial agonist.