Molecular determinants of biased agonism at the dopamine D₂ receptor

The development of biased (functionally selective) ligands provides a formidable challenge in medicinal chemistry. In an effort to learn to design functionally selective molecular tools for the highly therapeutically relevant dopamine D2 receptor, we synthesized a collection of agonists based on structurally distinct head groups derived from canonical or atypical dopaminergic pharmacophores. The test compounds feature a long lipophilic appendage that was shown to mediate biased signaling. By employing functional assays and molecular dynamics simulations, we could show that atypical dopamine surrogates of type 1 and 2 promote biased signaling, while ligands built from classical dopaminergic head groups (type 3 and 4) typically elicit more balanced signaling profiles. Besides this, we found a strong influence of the stereochemistry of type 4 aminotetraline-derived agonists on functional selectivity at D2 receptors. Whereas the (S)-enantiomer behaved as a full agonist, the biased ligand (R)-4 induced poor G protein coupling but substantial β-arrestin recruitment.