Controlling the selectivity of aminergic GPCR ligands from the extracellular vestibule

In addition to the orthosteric binding pocket (OBP) of GPCRs, recent structural studies have revealed that there are several allosteric sites available for pharmacological intervention. The secondary binding pocket (SBP) of aminergic GPCRs is located in the extracellular vestibule of these receptors, and it has been suggested to be a potential selectivity pocket for bitopic ligands. Here, we applied a virtual screening protocol based on fragment docking to the SBP of the orthosteric ligand-receptor complex. This strategy was employed for a number of aminergic receptors. First, we designed dopamine D₃ preferring bitopic compounds from a D₂ selective orthosteric ligand. Next, we designed 5-HT_2B selective bitopic compounds starting from the 5-HT_1B preferring ergoline core of LSD. Comparing the serotonergic profiles of the new derivatives to that of LSD, we found that these derivatives became significantly biased towards the desired 5-HT_2B receptor target. Finally, addressing the known limitations of H₁ antihistamines, our protocol was successfully used to eliminate the well-known side effects related to the muscarinic M₁ activity of amitriptyline while preserving H₁ potency in some of the designed bitopic compounds. These applications highlight the usefulness of our new virtual screening protocol and offer a powerful strategy towards bitopic GPCR ligands with designed receptor profiles.