Novel Dopamine D3/D2 Receptor Ligands

National Institute On Drug Abuse

Investigators

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Abstract

Our primary objective has been to develop novel and selective D3R antagonists and partial agonists with drug-like physicochemical properties. To this end we discovered very selective D3R antagonists and partial agonists with D3R/D2R-selectivites reaching >1000-fold. In addition, several of these analogues have been further screened for binding to receptors and ion channels and did not show significant binding affinities at any of these other (off) targets, highlighting that these agents are some of the most potent and selective D3R-antagonists and partial agonists reported to date. Moreover, the (+)- and (-)-enantiomers of several of our 3-OH analogues have been synthesized using enantioselective synthetic strategies or chiral chromatography. Further chimera studies, with these enantiomers, identified an extracellular loop (E1) region that appears to differ between D3R and D2R. We combined small molecule SAR with the D3R crystal structure solved with the D2-like antagonist eticlopride to design our newest generation of D3R-selective compounds. We hypothesized that the substituted-4-phenylpiperazine terminus, defined as the primary pharmacophore (PP), binds within the orthosteric binding site (OBS) of both the D2R and D3Rs, while the indole amide terminus termed as the secondary pharmacophore (SP), binds in a secondary binding pocket (SBP) at the interface of transmembrane domains (TMs) 1, 2, and 7 and the EL1, EL2, that significantly differ from the D2R. Site-directed mutagenesis studies have identified a single amino acid (Gly94) in the EL1 that differs between D2 and D3 receptors and is critically important for subtype selectivity. These studies have provided a structural basis for the contribution each component in these molecules to the binding and functional efficacy at D3R, and to the relative orientation of the primary and secondary pharmacophores for optimal D3R binding affinity, selectivity and efficacy. We have explored numerous substituted phenyl-piperazines as the PP as well as SP with different heteroaryl amides and have further investigated the 3-substitution on the butyl linking chain and separated enantiomers of both the 3-OH and 3-F analogues, identifying new lead molecules for investigation in vivo. We have identified two lead molecules: VK4-116 and VK4-40, which show promising behavioral results in rodent models of opioid abuse. Both compounds are metabolically stable and reduce acquisition to oxycodone self-administration suggesting that they might be useful as treatments for opioid dependence, but also may be useful in preventing addiction to prescription opioids. We discovered that the R-enantiomer for both lead molecules was the eutomer and in both cases, an antagonist. R-VK4-40 was also effective in reducing cocaine self-administration and reinstatement to cocaine seeking in rats. More recently, the enantiomers of the 3-F analogue, ABS1-113, have been separated and evaluated in rodent models of opioid use disorder. The relatively efficacious partial agonist, S-ABS1-113 was surprisingly effective in these models suggesting that D3R partial agonists may have potential for development as treatments for SUD and possibly dual diagnoses with other affective disorders, which are notoriously difficult to treat. Further development of these compounds as well as other lead molecules is underway. In addition to bitopic ligands directed toward antagonists and partial agonists, we have also focused on D3R full agonists, using PD128,907 and PF592,379 as parent molecules. We have recently discovered one of the most D3R-selective ligands to date which emphasized the critical role of chirality in both the PP and linker. Although the D2R had not been a focus for medication development, the possibility of building new bioconjugate tools to further study this receptor subtype inspired us to learn if this drug design approach might be directed toward the more ubiquitous D2R. Further, we hypothesized that structural modifications of a known D2-like agonist would provide clues toward functionally biased and potentially therapeutically useful new drug targets. We have now expanded the SAR of a series of sumanirole-inspired molecules toward novel and functionally biased D2 receptor selective compounds. By investigating drug-protein interactions at an atomistic level using small molecule SAR, computational modeling and cell-based functional assays, we have been able to uncover drug molecule poses in the orthosteric binding site (OBS) that result in a preference for D2R or D3R selectivity as well as functional selectivity at D2R, despite high homology in this region of these receptor subtypes. Further investigation using this approach will unveil structure-function information that can be utilized for future drug design of molecules with improved therapeutic efficacy. Finally, we have embarked on a dual target strategy to discover small molecules that bind both the mu opioid receptor (MOR) and D3R with partial agonist efficacy at MOR and a partial agonist/antagonist efficacy at D3R. We have synthesized a large series of loperamide analogues and have identified several lead molecules that have moderately high affinities at MOR, D2R and D3R, with a partial agonist profile at all three sites. These compounds are currently being evaluated in vivo while we embark on the next series, in order to improve drug-like properties.