Half maximal effective concentration (EC50)

The EC50 defines the half maximal concentration of a drug. In a dose response curve, the EC50 represents the concentration of a drug where 50% of its maximal effect is observed (Figure 1). It is also the inflection point of the sigmoidal curve.

Figure 1. Determination of the EC50 from a dose response curve.

In vitro based HTS, e.g. using MTT assay as a measure for cell survival, are in particular prone to generating false negatives results. This has potential implications for screens such as the NCI 60 in that compounds with good clinical activity could be missed. On the face of it therefore, modifications to the in vitro screens should be considered to reduce the impact of false negative predictions and steps such as the inclusion of liver microsomes into the assay could circumvent the problem seen with non-direct acting drugs.