Bivalent ligands have been developed for a variety of G protein-coupled receptor targets, including opioid, dopamine, serotonin and muscarinic receptors. The most successful application of the bivalent ligand approach has been in the development of selective opioid antagonists, such as norbinaltorphimine. Several important principles have emerged from the study of norbinaltorphimine and related compounds, including the utility of bivalent ligands for targeting particular receptor classes and serving as a scaffold for specific interactions with unique amino acid residues that render receptor subtype selectivity. In recent years, several novel bivalent compounds were synthesized and characterized for activity at muscarinic receptors. The compounds display an interesting profile of high binding affinity, strong agonist potency and receptor subtype selectivity. Bivalent ligands represent an important starting point for the development of selective muscarinic agonists with potential utility in treating a variety of neurological disorders, including Alzheimer's disease and schizophrenia. The bivalent ligand approach may be generally applicable to other G protein-coupled receptors.
