Title:The IC50 Concept Revisited
VOLUME: 12 ISSUE: 11
Author(s):Gary W. Caldwell, Zhengyin Yan, Wensheng Lang and John A. Masucci
Affiliation:Janssen Research and Development, Welsh and McKean Roads, Spring House, PA 19477-0776, USA.
Keywords:IC50, Ki, EC50, kinetic parameters, assay conditions, data fitting strategies, mathematical modeling techniques, antagonists, cyclooxygenase (COX) enzymes, invitro cell assays, insect cell assay
Abstract:A major strategy used in drug design is the inhibition of enzyme activity. The ability to accurately measure the
concentration of the inhibitor which is required to inhibit a given biological or biochemical function by half is extremely
important in ranking compounds. Since the concept of the half maximal inhibitory concentration (IC50) is used extensively
for studying reversible inhibition enzymatic reactions, it is important to clearly understand the experimental design and
the mathematical modeling techniques used to generate IC50 values. The most important part of the experimental design is
to measure the rate of production of [P] during the linear phase of the time course of the reaction and to prove that the enzyme-
catalyzed reaction is reversible. The most important part of the mathematical modeling is to select the correct model
and to have a firm understanding on how to handle outliers in the data. These topics are discussed in greater detail along
with a discussion on how much quantitative and mechanistic information can be reasonably deduced from an experiment.
