Bitopic ligands for beta-adrenergic receptors

G protein-coupled receptors (GPCRs) are highly important pharmaceutical targets with multiple binding sites modulating their function. Apart from the orthosteric site, binding the native hormone/ neurotransmitter and an intracellular site recognizing a G protein or β-arrestin, allosteric binding sites located at the extracellular vestibule and the membrane-facing side of the receptor have been identified. Bitopic ligands are drugs or lead compounds comprising a pharmacophore for the orthosteric site and a second moiety specifically addressing an allosteric site. -Adrenergic receptors (ARs) induce a beneficial increase in inotropy of the heart in acute heart failure, strengthening myocardial contraction and, thus, heart function. Structural elucidation of our bitopic ligand SX164 bound to the 2AR in its inactive state showed an analogous but not identical binding pose when compared to the partial agonist salmeterol bound to the active state-like 2AR-Nb80 complex. Interestingly, SX164 showed a very high affinity with picomolar Ki values for both 2AR and 1AR whereas the allosterically binding appendage of salmeterol is largely responsible for its more than 1000-fold 2AR selectivity. Hence, the selectivity induced by an individual allosteric module appears to be dependent on the functional state of the receptor and the exact binding pose, which is controlled by the orthosteric module. In continuation of our preliminary studies, we synthesized LM189, an enantiopure hybrid of norepinephrine and the allosteric appendage of salmeterol. Interestingly, the bitopic ligand LM189 showed not only high affinity and subtype selectivity for the 2AR but also remarkable Gi protein bias. This presentation focusses on the design, synthesis and biologically investigations of bitopic -adrenergic ligands based on an understanding of their structural and functional properties. Our final goal is to identify the structural determinants that control the ligand’s subtype and functional selectivity.