EC50，半最大效應濃度（concentration for 50% of maximal effect,EC50）是指能引起50%最大效應的濃度。EC50是藥物安全性指標。其含義是：引起50%個體有效的藥物濃度。LD50/ED50、TD50/ED50、TC50/EC50等統稱為治療指數，是一類藥物的安全指標，通常其值越大越安全。需注意這些指標只反映治療作用與急性毒性的關係，並不能反映慢性毒性、過敏性。
The term half maximal effective concentration (EC50) refers to the concentration of a drug, antibody or toxicant which induces a response halfway between the baseline and maximum after a specified exposure time. It is commonly used as a measure of drug's potency.
The EC50 of a graded dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed. The EC50 of a quantal dose response curve represents the concentration of a compound where 50% of the population exhibit a response, after a specified exposure duration.
It is also related to IC50 which is a measure of a compound's inhibition (50% inhibition). For competition binding assays and functional antagonist assays IC50 is the most common summary measure of the dose-response curve. For agonist/stimulator assays the most common summary measure is the EC50.
In biological systems, small changes in ligand concentration typically result in rapid changes in response, following a sigmoidal function.The inflection point at which the increase in response with increasing ligand concentration begins to slow is the EC50. This can be determined mathematically by derivation of the best-fit line. While relying on a graph for estimation is more convenient, this method yields less precise and less accurate results.
The effects of a stressor or drug generally depend on the exposure time. Therefore, the EC50 (and similar statistics) will be a function of exposure time. The exact shape of this time function will depend upon the stressor (e.g., the specific toxicant), its mechanism of action, the organism exposed, etc. This time dependency hampers the comparison of potency or toxicity between compounds and between different organisms.
EC50表達式及簡圖
