β-Arrestin biased dopamine D2 receptor partial agonists: Synthesis and pharmacological evaluation

β-Arrestin biased G protein-coupled receptor ligands represent important molecular probes and may increase favorable drug action and safety as novel therapeutics. Starting from recently discovered hydroxy-substituted heterocyclic piperazine scaffolds, we have developed a series of dopamine D₂ receptor ligands with a pyrazolo[1,5-a]pyridine as secondary pharmacophore that is functionalized in position 3 by a formyl or hydroxyiminomethyl substituent. The ligands, especially the benzoxazinone 9d, were found to stimulate substantial β-arrestin-2 recruitment, while being nearly devoid of activity in a GTPγS binding assay. Investigating a new series of truncated analogs lacking a secondary pharmacophore, considerable β-arrestin-2 recruitment in the absence of G protein activation was found, when a 5-hydroxy-2H-benzo[b][1,4]oxazin-3(4H)-one was combined with an N-propyl-substituted 1,4-diazepane (15c). Although 15c displayed reduced potency compared to 9d, the dose-response curves indicate that a hydroxy-substituted heterocyclic primary pharmacophore is sufficient for the functionally selective activation of D₂R.