Chemical similarity and biological activities

"Similar molecules exert similar biological activities". Since long, medicinal chemists use this concept to modify the structures of biologically active compounds. Bioisosteric replacements of atoms and groups pave the way from various lead structures to therapeutically useful enzyme inhibitors, receptor agonists and antagonists, and other active principles. However, similarity and diversity of chemical structures cannot be defined in an objective manner. Several surprising structure-activity relationships demonstrate that chemically similar compounds may have significantly different biological actions and activities. Some protein ligands exert unexpected new binding modes, after only minor chemical modification. Of course, even optical enantiomers most often have different biological activities.