Optimization of a Novel D2 Dopamine Receptor Antagonist Scaffold Reveals Exceptionally Selective and Potent Lead Molecules

Abstract

FDA‐approved antipsychotics are primarily D2 dopamine receptor (D2R) antagonists, however they generally lack receptor selectivity and interact with other GPCRs, including D3Rs. Despite its clear clinical importance, there are few compounds that selectively modulate the D2R. This is due to structural similarities in the orthosteric binding sites of the D2‐like receptors (D2R, D3R, and D4R), as well as other biogenic amine receptors. To find better therapeutic candidates, as well as probe compounds to investigate D2R pathophysiology, we sought to identify highly selective D2R antagonists by implementing a high‐throughput screening (HTS) campaign using more than 400,000 unique compounds in the Molecular Libraries Screening Center Network library. Most of the hit compounds that were identified as antagonists of the D2R were also antagonists of the closely related D3R. However, several compounds were found to exhibit high affinity for the D2R with low affinity for the D3R. One such compound (MLS6916) served as our lead scaffold for optimization. MLS6916 is ~700 fold selective for the D2R vs. D3R as determined in functional β‐arrestin recruitment assays and radioligand binding competition assays. Importantly, MLS6916 was also examined for functional activity at 168 different GPCRs and showed no agonist activity on any GPCR tested, and only exhibited antagonist activity at the D2‐like receptors. Preliminary investigations, however, revealed that this initial hit compound was metabolically unstable. To optimize this hit scaffold, we conducted an iterative medicinal chemistry campaign with the goals of increasing selectivity, potency, and engendering metabolic stability. Over 70 analogs of the MLS6916 parent compound were synthesized to dissect the influence of various constituents of the molecule and to construct structure‐activity relationships. Analogs were analyzed for activity on D2R, D3R, and D4R using both radioligand binding and functional assays to determine affinities and potencies for the D2‐like receptors. In addition, analogs were tested for solubility, permeability, and stability. Several lead compounds were identified with Ki values of < 50 nM for the D2R with > 1,000‐fold selectivity vs. the D3R. These selectivity trends were confirmed in functional β‐arrestin recruitment assays. Importantly, lead compounds with enhanced metabolic stability and permeability were identified. Further analysis and optimization of these lead compounds will provide new insight into highly selective D2R antagonists for the treatment and understanding of dopamine‐related disorders.Support or Funding InformationNINDS Intramural Research Program