Identification of a novel negative allosteric modulator of the D 3 dopamine receptor

Dopamine receptors (DARs) are G‐protein coupled receptors (GPCRs) that regulate several physiological functions including cognition, mood, movement, and reward‐related behaviors. They are composed of two classes based on structural homology and pharmacology: D1‐like (D1R and D5R) and D2‐like (D2R, D3R, and D4R). DARs are involved in the treatment/etiology of many neuropsychiatric disorders such as schizophrenia and Parkinson's disease. Due to the extremely high sequence homology shared by D2R and D3R within their orthosteric binding sites, known D2‐like ligands exhibit high degrees of cross reactivity, both with other D2‐like DAR subtypes, and with closely related GPCRs, leading to the potential for off‐target side effects due to simultaneous modulations. However, it has recently been appreciated that, in addition to highly conserved orthosteric sites, many GPCRs, including DARs, may possess distinct and non‐conserved allosteric sites. Compounds that modulate receptors through interaction with an allosteric site have the potential to be exceptionally selective. The D3R is enriched in the ventral striatum, associated with control of mood and emotion, while the D2R is enriched in the dorsal striatum, associated with control of movement. Therefore, D3R‐selective antagonists may be useful schizophrenia therapeutics as they, in theory, could attenuate psychotic symptoms without motor side effects frequently induced by current D2R‐preferring antagonists. Studies also suggest D3R‐preferring antagonists attenuate drug‐seeking behaviors and relapse without causing motor deficits associated with non‐selective antagonists. 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. Confirmation and counter‐screens were performed to obtain an initial assessment of D3R selectivity, and mechanisms of action, and identified 57 potential negative allosteric modulators (NAMs). Further triage of these compounds based on absolute D3R vs. D2R selectivity in the β‐arrestin recruitment assay yielded 3 lead NAMs, the most potent of which was selected for further characterization. This compound was found to be D3R vs. D2R selective in a separate BRET‐based β‐arrestin recruitment assay, and radioligand inhibition binding and Schild‐type functional assays indicate that this compound acts in a purely noncompetitive manner at the D3R. In addition, a radioligand inhibition binding screen of this compound on ~50 closely related GPCRs indicates this compound shows limited cross‐reactivity at their orthosteric sites. We will use medicinal chemistry efforts on this chemical scaffold in order to further improve its potency, and we ultimately hope that this compound or its analogs will prove useful as in vitro and in vivo pharmacological tools or leads for therapeutic drugs. Support or Funding Information NINDS Intramural Research Program