X-ray Structures of Target-Ligand Complexes Containing Compounds with Assay Interference Potential.

Pan assay interference compounds (PAINS) have become a paradigm for compound classes that might cause artifacts in biological assays. PAINS-defining substructures are typically contained in larger compounds. We have systematically examined X-ray structures of protein-ligand complexes for compounds containing PAINS motifs. In 2874 X-ray structures, 1107 unique ligands with PAINS substructures belonging to 70 different classes were identified. PAINS most frequently detected in crystallographic ligands included a number of prominent candidates such as quinones, catechols, or Mannich bases. However, on the basis of X-ray data, the presence of specific ligand-target interactions and reactivity under assay conditions were not mutually exclusive. In some instances, reactivity of ligands was likely responsible for complex formation. Different categories of PAINS-containing ligands were distinguished, which aided in the interpretation of specific interactions versus potential assay artifacts. Careful consideration of structural data adds another dimension to the analysis of interference compounds.