What are the current trends in G protein-coupled receptor targeted drug discovery?

G protein-coupled receptors (GPCRs) constitute a superfamily of membrane proteins which have in common the presence of 7 transmembrane α-helices linked by several intra- and extracellular loops. This particular structure is important for extracellular stimulus recognition through the amino acids forming the extracellular loops (class B, C and F of GPCRs) or by the transmembrane α-helices (class A and B of GPCRs). Their structure is also essential for signal transduction via intracellular proteins (G proteins, arrestins and kinases – GRKs-) [Citation1]. These proteins are attached to the intracellular loops and/or to the intracellular regions of the α-helices of the GPCRs, even when they are not activated by extracellular ligands or drugs.