Thomas M. Keck

Identifying Medication Targets for Psychostimulant Addiction: Unraveling the Dopamine D3 Receptor Hypothesis

The dopamine D3 receptor (D3R) is a target for developing medications to treat substance use disorders. D3R-selective compounds with high affinity and varying efficacies have been discovered, providing critical research tools for cell-based studies that have been translated to in vivo models of drug abuse. D3R antagonists and partial agonists have shown especially promising results in rodent models of relapse-like behavior, including stress-, drug-, and cue-induced reinstatement of drug seeking. However, to date, translation to human studies has been limited. Herein, we present an overview and illustrate some of the pitfalls and challenges of developing novel D3R-selective compounds toward clinical utility, especially for treatment of cocaine abuse. Future research and development of D3R-selective antagonists and partial agonists for substance abuse remains critically important but will also require further evaluation and development of translational animal models to determine the best time in the addiction cycle to target D3Rs for optimal therapeutic efficacy.

Tranylcypromine substituted cis -hydroxycyclobutylnaphthamides as potent and selective dopamine D3 receptor antagonists

We report a class of potent and selective dopamine D3 receptor antagonists based upon tranylcypromine. Although tranylcypromine has a low affinity for the rat D3 receptor (Ki = 12.8 μM), our efforts have yielded (1R,2S)-11 (CJ-1882), which has Ki values of 2.7 and 2.8 nM at the rat and human dopamine D3 receptors, respectively, and displays respective selectivities of >10000-fold and 223-fold over the rat and human D2 receptors. Evaluation in a β-arrestin functional assay showed that (1R,2S)-11 is a potent and competitive antagonist at the human D3 receptor.

High Affinity Dopamine D3 Receptor (D3R)-Selective Antagonists Attenuate Heroin Self-Administration in Wild-Type but not D3R Knockout Mice

The dopamine D3 receptor (D3R) is a promising target for the development of pharmacotherapeutics to treat substance use disorders. Several D3R-selective antagonists are effective in animal models of drug abuse, especially in models of relapse. Nevertheless, poor bioavailability, metabolic instability, and/or predicted toxicity have impeded success in translating these drug candidates to clinical use. Herein, we report a series of D3R-selective 4-phenylpiperazines with improved metabolic stability. A subset of these compounds was evaluated for D3R functional efficacy and off-target binding at selected 5-HT receptor subtypes, where significant overlap in SAR with D3R has been observed. Several high affinity D3R antagonists, including compounds 16 (Ki = 0.12 nM) and 32 (Ki = 0.35 nM), showed improved metabolic stability compared to the parent compound, PG648 (6). Notably, 16 and the classic D3R antagonist SB277011A (2) were effective in reducing self-administration of heroin in wild-type but not D3R knockout mice.

Highly Selective Dopamine D3 Receptor (D3R) Antagonists and Partial Agonists Based on Eticlopride and the D3R Crystal Structure: New Leads for Opioid Dependence Treatment

The recent and precipitous increase in opioid analgesic abuse and overdose has inspired investigation of the dopamine D3 receptor (D3R) as a target for therapeutic intervention. Metabolic instability or predicted toxicity has precluded successful translation of previously reported D3R-selective antagonists to clinical use for cocaine abuse. Herein, we report a series of novel and D3R crystal structure-guided 4-phenylpiperazines with exceptionally high D3R affinities and/or selectivities with varying efficacies. Lead compound 19 was selected based on its in vitro profile: D3R Ki = 6.84 nM, 1700-fold D3R versus D2R binding selectivity, and its metabolic stability in mouse microsomes. Compound 19 inhibited oxycodone-induced hyperlocomotion in mice and reduced oxycodone-induced locomotor sensitization. In addition, pretreatment with 19 also dose-dependently inhibited the acquisition of oxycodone-induced conditioned place preference (CPP) in rats. These findings support the D3R as a target for opioid dependence treatment and compound 19 as a new lead molecule for development.

Chiral Resolution and Serendipitous Fluorination Reaction for the Selective Dopamine D3 Receptor Antagonist BAK2-66.

The improved chiral synthesis of the selective dopamine D3 receptor (D3R) antagonist (R)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)-3-hydroxybutyl)1H-indole-2-carboxamide (( R )-PG648) is described. The same chiral secondary alcohol intermediate was used to prepare the enantiomers of a 3-F-benzofuranyl analogue, BAK 2-66. The absolute configurations of the 3-F enantiomers were assigned from their X-ray crystal structures that confirmed retention of configuration during fluorination with N,N-diethylaminosulfur trifluoride (DAST). ( R )-BAK2-66 showed higher D3R affinity and selectivity than its (S)-enantiomer; however, it had lower D3R affinity and enantioselectivity than ( R )-PG648. Further, importance of the 4-atom linker length between the aryl amide and 4-phenylpiperazine was demonstrated with the 4-fluorobutyl-product (8).

Synthesis and Pharmacological Characterization of Novel trans-Cyclopropylmethyl-Linked Bivalent Ligands That Exhibit Selectivity and Allosteric Pharmacology at the Dopamine D3 Receptor (D3R)

The development of bitopic ligands directed toward D2-like receptors has proven to be of particular interest to improve the selectivity and/or affinity of these ligands and as an approach to modulate and bias their efficacies. The structural similarities between dopamine D3 receptor (D3R)-selective molecules that display bitopic or allosteric pharmacology and those that are simply competitive antagonists are subtle and intriguing. Herein we synthesized a series of molecules in which the primary and secondary pharmacophores were derived from the D3R-selective antagonists SB269,652 (1) and SB277011A (2) whose structural similarity and pharmacological disparity provided the perfect templates for SAR investigation. Incorporating a trans-cyclopropylmethyl linker between pharmacophores and manipulating linker length resulted in the identification of two bivalent noncompetitive D3R-selective antagonists, 18a and 25a, which further delineates SAR associated with allosterism at D3R and provides leads toward novel drug development.

Toward Understanding the Structural Basis of Partial Agonism at the Dopamine D3 Receptor

Both dopamine D3 receptor (D3R) partial agonists and antagonists have been implicated as potential medications for substance use disorders. In contrast to antagonists, partial agonists may cause fewer side effects since they maintain some dopaminergic tone and may be less disruptive to normal neuronal functions. Here, we report three sets of 4-phenylpiperazine stereoisomers that differ considerably in efficacy: the (R)-enantiomers are antagonists/weak partial agonists, whereas the (S)-enantiomers are much more efficacious. To investigate the structural basis of partial agonism, we performed comparative microsecond-scale molecular dynamics simulations starting from the inactive state of D3R in complex with these enantiomers. Analysis of the simulation results reveals common structural rearrangements near the ligand binding site induced by the bound (S)-enantiomers, but not by the (R)-enantiomers, that are features of partially activated receptor conformations. These receptor models bound with partial agonists may be useful for structure-based design of compounds with tailored efficacy profiles.

Novel Analogues of (R)-5-(Methylamino)-5,6-dihydro-4H-imidazo[4,5,1-ij]quinolin-2(1H)-one (Sumanirole) Provide Clues to Dopamine D2/D3 Receptor Agonist Selectivity

Novel 1-, 5-, and 8-substituted analogues of sumanirole (1), a dopamine D2/D3 receptor (D2R/D3R) agonist, were synthesized. Binding affinities at both D2R and D3R were higher when determined in competition with the agonist radioligand [3H]7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT) than with the antagonist radioligand [3H]N-methylspiperone. Although 1 was confirmed as a D2R-preferential agonist, its selectivity in binding and functional studies was lower than previously reported. All analogues were determined to be D2R/D3R agonists in both GoBRET and mitogenesis functional assays. Loss of efficacy was detected for the N-1-substituted analogues at D3R. In contrast, the N-5-alkyl-substituted analogues, and notably the n-butyl-arylamides (22b and 22c), all showed improved affinity at D2R over 1 with neither a loss of efficacy nor an increase in selectivity. Computational modeling provided a structural basis for the D2R selectivity of 1, illustrating how subtle differences in the highly homologous orthosteric binding site (OBS) differentially affect D2R/D3R affinity and functional efficacy.

The replacement of the 2-methoxy substituent of N-((6,6-diphenyl-1,4-dioxan-2-yl)methyl)-2-(2-methoxyphenoxy)ethan-1-amine improves the selectivity for 5-HT1A receptor over α1-adrenoceptor and D2-like receptor subtypes

N-((6,6-diphenyl-1,4-dioxan-2-yl)methyl)-2-(2-methoxyphenoxy)ethan-1-amine (3) is a potent 5-HT1A receptor and α1d-adrenoceptor (α1d-AR) ligand. Analogues 5-10 were rationally designed and prepared to evaluate whether electronic and/or lipophilic properties of substituents in the ortho position of its phenoxy moiety exert any favorable effects on the affinity/activity at 5-HT1A receptor and improve selectivity over α1-ARs. To rationalize the experimental observations and derive information about receptor-ligand interactions of the reported ligands, docking studies, using 5-HT1A and α1d-AR models generated by homology techniques, and a retrospective computational study were performed. The results highlighted that proper substituents in position 2 of the phenoxy moiety of 3 selectively address the ligands toward 5-HT1A receptor with respect to α1-ARs and D2-like receptor subtypes. Methoxymethylenoxy derivative 9 showed the best 5-HT1A selectivity profile and the highest potency at 5-HT1A receptor, behaving as a partial agonist. Finally, 9, tested in light/dark exploration test in mice, significantly reduced anxiety-linked behaviors. Therefore, it may be considered a lead for the design of partial agonists potentially useful in the treatment of disorders in which 5-HT1A receptor is involved.

A Novel Class of Dopamine D4 Receptor Ligands Bearing an Imidazoline Nucleus

Over the years, the 2‐substituted imidazoline nucleus has been demonstrated to be a bioversatile structural motif. In this study, novel imidazoline derivatives bearing a 3‐ and/or 4‐hydroxy‐ or methoxy‐substituted phenyl ring, linked by an ethylene bridge to position 2 of an N‐benzyl‐ or N‐phenethyl‐substituted imidazoline nucleus, were prepared and studied against D2‐like receptor subtypes. Binding studies highlighted that a set of N‐phenethylimidazoline compounds are selective for D4 over D2 and D3 receptors. In functional assays, the 3‐methoxy‐substituted derivative, endowed with the highest D4 affinity value, and its 3‐hydroxy analogue behaved as partial agonists with low intrinsic efficacy and as competitive D4 antagonists when tested in the presence of the D2‐like receptor agonist quinpirole. Molecular docking analysis, performed using a homology model of the human D4 receptor developed using the X‐ray crystal structure of the antagonist‐bound human D3 receptor as a template, was in accordance with the binding results and provided useful information for the design of novel imidazoline D4 receptor ligands based on this new scaffold.