The Thin Line between Promiscuous and Privileged Structures in Medicinal Chemistry

The concept of privileged structures in medicinal chemistry refers to commonly found substructures in approved drugs and lead small molecules, presenting a broad profile of pharmacological action, i.e., participating in the recognition by several classes of pharmacological targets or by modulating physicochemical properties. Privileged structures are also related to nontoxic effects, which make their use in the design of new drug candidates very attractive. In contrast, another concept also refers to structures or substructures capable of presenting a pleiotropic profile for several pharmacological targets, referred to as promiscuous compounds. Worth mentioning, more recently, a great majority of promiscuous compounds have been classified as Pan Assay Interference Compounds (PAINS). In its great majority, PAINS are electrophilic in nature, capable of covalently reacting indiscriminately with several pharmacological targets, and are associated with specific substructures, which are, in turn, used as an exclusion filter during screening campaigns. This work aims to critically discuss the thin line that separates the two concepts, clarifying their differences from a molecular and pharmacological point of view. Moreover, special considerations regarding PAINS and exclusion filters will be made.