Theoretical and Experimental Relationships between %-Inhibition and IC50 Data observed in HTS

The 4-parameter logistic Hill equation models the theoretical relationship between inhibitor concentration and %-inhibition in response, and is used to derive IC50 values as a measure of compound potency. This relationship is the basis for screening strategies that first measure %-Inhibition at a single, uniform concentration, and then determine IC50 values for compounds above a threshold. In screening practice, however, a “good” correlation between %-inhibition values and IC50 values is not always observed, and in literature there seems confusion about what correlation even to expect. We examined the relationship between %-inhibition data and IC50 data in a HDAC4 HTS data set and compared our findings to a series of numerical simulations that allowed the investigation of the influence of various parameters including uncertainties in the screening concentration and in Hill model parameters as well as influences of assay data quality parameters.