An Intuitive Look at the Relationship of Ki and IC50: A More General Use for the Dixon Plot

Abstract

To compare the relative potencies of a variety of enzyme-inhibiting compounds, a quantitative measure must be employed. IC50 values and Ki values are often reported in the literature, but direct comparison of these values is not possible. The relationship of Ki and IC50 for a given compound varies depending on the assay conditions and the compound’s mechanism of inhibition. The Cheng–Prusoff equations describe this relationship mathematically, and an intuitive description of this relationship is presented. Although there are currently a number of means to graphically determine IC50 values, there is not a biochemically intuitive linear method. A novel use of the Dixon plot will aid students in the determination of IC50 values. While the Dixon plot has been traditionally used to determine Ki in limited cases, a more widely applicable use is in IC50 determination. A new convention for presenting IC50 values is suggested.