Identification and characterization of a novel dopamine receptor antagonist with high selectivity for the D2 dopamine receptor
Authors: Free R Benjamin , Dulcey Andrés E. , Moritz Amy E. , Doyle Trevor B. , Javitch Jonathan A. , Southall Noel T. , Ferrer Marc , Marugan Juan J. , Sibley David R. 
Published: November 2023

Despite its clinical importance in the treatment of a number of neuropsychiatric disorders, such as Parkinson's disease and schizophrenia, there are few compounds, either agonists or antagonists that are highly selective for the D2 dopamine receptor (D2R). Most compounds with activity at the D2R also exhibit significant affinity for the D3 or D4 dopamine receptors (D3R or D4R), or other G protein‐coupled receptors (GPCRs). Development of D2R‐selective ligands has proven especially problematic due to the high structural similarities of the orthosteric binding sites of the D2‐like receptors (D2R, D3R and D4R). Importantly, highly selective D2R antagonists could represent improved therapeutics for treating the positive symptoms of schizophrenia with potentially fewer side effects due to off‐target activities. Thus, in order to identify selective antagonists of the D2R, we implemented a high‐throughput screening (HTS) campaign to interrogate over 400,000 unique compounds in the Molecular Libraries Screening Center Network library. Not surprisingly, most of the compounds that were identified as antagonists of the D2R were also antagonists of the closely related D3R. However, a small pool of antagonist compounds that exhibited significant D2R>D3R selectivity was identified. These compounds were characterized using orthogonal D2R and D3R functional assays (including several G protein‐mediated assays and b‐arrestin recruitment) as well as radioligand binding assays. One compound, MLS1946, was found to exhibit high affinity for the D2R (Ki ~100 nM) without measurable affinity for the D3R (Ki > 40 mM). This compound was further characterized using a number of D2R and D3R signaling assays and found to exert no activity an the D3R, while exhibiting potent antagonism of the D2R. Follow‐up studies were performed using an analog of the hit compound and a functional screen (b‐arrestin recruitment) of a panel of 168 different GPCRs. We found that the hit compound analog was completely devoid of agonist activity within this panel of GPCRs. In contrast, at the dose tested, the compound was found to completely antagonize dopamine stimulation of the D2R while exhibiting partial antagonism of the D4R. Surprisingly, the compound was devoid of significant antagonist activity at the remaining 166 GPCRs. These results suggest that the MLS1946 scaffold displays a highly selective D2R antagonist profile. We are currently engaged in a chemical optimization campaign to synthesize analogs around this scaffold to both interrogate structure‐activity relationships and optimize compounds for in vivo testing. It is anticipated that further development of the MLS1946 scaffold may lead to an optimized D2R‐selective antagonist with drug‐like properties for the treatment and understanding of dopamine‐related pathologies.Support or Funding InformationNINDS Intramural Research Program