The reciprocal EC50 value as a convenient measure of the potency of a compound in bioactivity-guided purification of natural products

Abstract

Identification of potent natural products is a challenging task in which sophisticated separation processes including HPLC are employed. The bioactivity of HPLC fractions is determined with a bioassay, and the most potent compounds are progressed to structural elucidation. In pharmacology, the potency of a compound is expressed as the half-maximal effective concentration (EC₅₀), which refers to the concentration of a drug that induces a response halfway between the baseline and maximum. While expressing the potency of a compound by its EC₅₀ value makes sense in a clinical context, it is counterintuitive in the context of bioactivity-guided purification, as the potency of a compound is inversely related to its EC₅₀ value, and the most potent compound is the one with the lowest EC₅₀. In natural products chemistry, it would be more logical if an increase in potency would be reflected by an increase of a parameter reflecting the potency. In this study, we introduce the term "effective dilution volume (EDV₅₀)" as the reciprocal of the EC₅₀ (1/EC₅₀). We show how the EDV₅₀ can be used to identify potent compounds in chromatographic separations, allowing to easily graph and identify anti-inflammatory compounds. We show two examples of this approach by overlaying an HPLC chromatogram with the EDV₅₀ to point out the most potent compounds. We hope that the EDV₅₀ will make the illustration of active fractions containing potent compounds in a chromatogram obvious for the reader and will become a useful graphic tool in the natural products literature in the future.

Keywords: Bioactivity; Effective dilution volume; Efficacy; Potency; Purification.