Quantifying Relative Potency in Dose-Response Studies

Abstract

Relative potency is an important concept in the comparative evaluation of chemicals via dose-response studies. For example, toxicologists use relative potency estimates to rank chemicals with respect to a given response endpoint, to convert doses of one chemical to equivalent doses of another chemical, and to combine information across studies and endpoints when calculating toxic equivalency factors. The conventional definition of relative potency, arising historically from dilution assays, is a ratio of equi-effective doses, that is, those doses that produce the same mean response. Specifically, the ratio is the dose of a reference chemical divided by the dose of a test chemical. In an analytical dilution assay, relative potency is constant regardless of the mean response used to select equi-effective doses. Nevertheless, researchers often observed data that were inconsistent with constant relative potency and desired ways to characterize non-constant relative potency. This article reviews various approaches for quantifying relative potency when it cannot be regarded as constant, including modifications to the usual definition. In particular, we focus on recent proposals that describe the relative potency of two chemicals as functions of dose or of response.