text: Bivalent ligands consist of two drug-like molecules (pharmacophores or ligands) connected by an inert linker. There are various kinds of bivalent ligands and are often classified based on what the pharmacophores target. Homobivalent ligands target two of the same receptor types. Heterobivalent ligands target two different receptor types. Bitopic ligands target an orthosteric binding sites and allosteric binding sites on the same receptor.
  In scientific research, bivalent ligands have been used to study receptor dimers and to investigate their properties. This class of ligands was pioneered by Philip S. Portoghese and coworkers while studying the opioid receptor system. Bivalent ligands were also reported early on by Micheal Conn and coworkers for the gonadotropin-releasing hormone receptor. Since these early reports, there have been many bivalent ligands reported for various G protein-coupled receptor (GPCR) systems including cannabinoid, serotonin, oxytocin, and melanocortin receptor systems, and for GPCR-LIC systems (D2 and nACh receptors).
  Bivalent ligands usually tend to be larger than their monovalent counterparts, and therefore, not drug-like as in Lipinski's rule of five. Many believe this limits their applicability in clinical settings. In spite of these beliefs, there have been many ligands that have reported successful pre-clinical animal studies. Given that some bivalent ligands can have many advantages compared to their monovalent counterparts (such as tissue selectivity, increased binding affinity, and increased potency or efficacy), bivalents may offer some clinical advantages as well.