Identification of a Novel Negative Allosteric Modulator of the D3 Dopamine Receptor

Abstract

Dopamine receptors (DARs) are G‐protein coupled receptors (GPCRs) that regulate diverse physiological functions including cognition, mood, movement, and reward‐related behaviors, and are involved in the treatment or etiology of many neuropsychiatric disorders including schizophrenia and Parkinson's disease. Based on structural homology and pharmacology, DARs are classified as either D1‐like (D1R and D5R) or D2‐like (D2R, D3R, and D4R). Due to the limited distribution of the D3R in limbic regions of the brain, D3R‐selective antagonists may be useful as schizophrenia or substance use disorder therapeutics as they could attenuate psychotic or drug craving symptoms without the motor side effects frequently incurred by D2R‐preferring antagonists. However, high sequence homology shared by the D2R and D3R within their orthosteric binding sites has made the discovery of individual subtype‐selective compounds difficult, leading to the potential for off‐target side effects due to simultaneous antagonism by existing agents. It has recently been appreciated that, in addition to highly conserved orthosteric sites, many GPCRs, including DARs, possess distinct and non‐conserved allosteric sites. Thus, compounds that modulate receptors through the interaction with an allosteric site have the potential to be exceptionally selective. In an effort to discover highly selective allosteric antagonists for the D3R, our lab employed a high‐throughput screen of the NIH Molecular Libraries Program 400,000+ small molecule library. The library was initially screened using a D3R‐mediated β‐arrestin recruitment assay. We advanced one compound for medicinal chemistry efforts after further triaging of the hits with confirmation and counter‐screens. This compound, MLS6357, was found to be D3R vs. D2R selective in several functional outputs including β‐arrestin recruitment and G‐protein activation. Furthermore, Schild‐type functional assays indicate that this compound acts in a purely non‐competitive manner at the D3R. While this compound does not inhibit the binding of orthosteric antagonists to the D3R, it does potentiate the binding of orthosteric agonists. Thus, the compound acts as a positive allosteric modulator (PAM) for agonist binding, but a NAM for agonist signaling. In addition, binding and functional screens of closely related GPCRs indicate that this compound has limited cross‐reactivity with other receptors. 30 analogs of this scaffold have been tested for activity and D3R selectivity, yielding SAR information for further refinement of the scaffold, and one analog was identified that was 15‐fold more potent than the parent. Further medicinal chemistry and determination of the binding site of this scaffold is ongoing. This pharmacophore may prove useful as a pharmacological probe or therapeutic lead for D3R‐related pathophysiology.Support or Funding InformationNINDS Intramural Research Program Eshelman Institute for InnovationThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.