Assessing biological activities of polyphenols in the context of 4R - challenges of a ubiquitous compound class with complex modes of action.

Polyphenols are widespread in plants used for nutritional and medicinal purposes, and have been associated with multiple beneficial effects on health. From a drug development perspective, however, many polyphenols have been disapproved as lead structures and designated as nuisance compounds. Nevertheless, research on health promoting effects of polyphenols has continuously increased over the past decade. Given their ubiquitous presence in plants and their occurrence as mixtures, an application in form of herbal preparations could be more advantageous than the administration of single compounds with numerous, but often weak effects of unclear relevance. Currently, the most powerful technique attempting to capture the entirety of pleiotropic activities of herbal medicines is network pharmacology. However, the major challenges and limitations related to assessing the pharmacological effects of polyphenols, such as unspecific interactions in assays, pharmacokinetic properties and synergisms among extract ingredients largely remain, and can lead to uncertainties regarding the validity of the data input. Especially in light of the 4R principle for animal experimentation, attention to planning meaningful experiments continues to be of high importance when studying bioactivities of polyphenols. This mini review therefore revisits characteristics and challenges associated with pharmacological investigations of polyphenolic natural products to discuss them in the context of 4R.