A Ligand-Triggered Receptor Conformation Enables the Design of Selective Agonists for the Dopamine 3 Receptor (D₃R) Using a Bitopic Strategy

While G protein-coupled receptors (GPCRs) represent the largest drug target family, designing subtype-selective molecules is still a challenge, especially to distinguish among closely related subtypes. One of the most challenging cases is the distinction between dopamine D₂R and D₃R, pivotal receptors in motor functions and cognition, and targets of Parkinson’s disease treatments, schizophrenia, or substance use disorders. Attempts to design D₃R-selective molecules with ligands binding toward the first transmembrane helix (the most sequence-diverse and conformationally flexible segment in GPCRs but rarely participating in ligand binding) allowed us to discover a ligand-induced ordering of TM1 unique to D₃R, yielding an unexploited selectivity site for drug development. Using rational bitopic drug design and the ligand-triggered conformation of the D₃R we designed, synthesized, and characterized the most selective D₃R agonists to date, >100,000-fold more selective than available ligands. More specifically, we report D₃R partial agonists AB12-82 (6d) and AB13-73A (11), with >575,000- and >750,000-fold subtype selectivity, picomolar potency, and 85% and 49% efficacy, respectively. We also present the most selective full agonists reported to date, AB13-08 (4b) and AB13-46A (9), presenting low and subnanomolar potencies with >2,800- and 6,300-fold selectivity for D₃R. Overall, we introduce a first-in-class pharmacological toolbox to dissect the (patho)-physiology of D₃R, open new avenues for the design of improved neurotherapeutics, and show that using ligand-induced TM1 reorganizations might represent a promising strategy for the design of subtype-selective molecules in other GPCRs.