Recent Advances in the Development of Dopamine D3 Receptor Antagonists: a Medicinal Chemistry Perspective

Dopamine D3 receptor ligands: a patent review (2014-2020)

INTRODUCTION

Compelling evidence identified D3 dopamine receptor (D3R) as a suitable target for therapeutic intervention on CNS-associated disorders, cancer and other conditions. Several efforts have been made toward developing potent and selective ligands for modulating signalling pathways operated by these GPCRs. The rational design of D3R ligands endowed with a pharmacologically relevant profile has traditionally not encountered much support from computational methods due to a very limited knowledge of the receptor structure and of its conformational dynamics. We believe that recent progress in structural biology will change this state of affairs in the next decade.

AREAS COVERED

This review provides an overview of the recent (2014-2020) patent literature on novel classes of D3R ligands developed within the framework of CNS-related diseases, cancer and additional conditions. When possible, an in-depth description of both in vitro and in vivo generated data is presented. New therapeutic applications of known molecules with activity at D3R are discussed.

EXPERT OPINION

Building on current knowledge, future D3R-focused drug discovery campaigns will be propelled by a combination of unprecedented availability of structural information with advanced computational and analytical methods. The design of D3R ligands with the sought activity, efficacy and selectivity profile will become increasingly more streamlined.

Chiral Cyclic Aliphatic Linkers as Building Blocks for Selective Dopamine D2 or D3 Receptor Agonists

Linkers are emerging as a key component in regulating the pharmacology of bitopic ligands directed toward G-protein coupled receptors (GPCRs). In this study, the role of regio- and stereochemistry in cyclic aliphatic linkers tethering well-characterized primary and secondary pharmacophores targeting dopamine D2 and D3 receptor subtypes (D2R and D3R, respectively) is described. We introduce several potent and selective D2R (rel-trans-16b; D2R Ki = 4.58 nM) and D3R (rel-cis-14a; D3R Ki = 5.72 nM) agonists while modulating subtype selectivity in a stereospecific fashion, transferring D2R selectivity toward D3R via inversion of the stereochemistry around these cyclic aliphatic linkers [e.g., (-)-(1S,2R)-43 and (+)-(1R,2S)-42]. Pharmacological observations were supported with extensive molecular docking studies. Thus, not only is it an innovative approach to modulate the pharmacology of dopaminergic ligands described, but a new class of optically active cyclic linkers are also introduced, which can be used to expand the bitopic drug design approach toward other GPCRs.

Scaffold Hybridization Strategy Leads to the Discovery of Dopamine D3 Receptor-Selective or Multitarget Bitopic Ligands Potentially Useful for Central Nervous System Disorders

In the search for novel bitopic compounds targeting the dopamine D₃ receptor (D₃R), the N-(2,3-dichlorophenyl)piperazine nucleus (primary pharmacophore) has been linked to the 6,6- or 5,5-diphenyl-1,4-dioxane-2-carboxamide or the 1,4-benzodioxane-2-carboxamide scaffold (secondary pharmacophore) by an unsubstituted or 3-F-/3-OH-substituted butyl chain. This scaffold hybridization strategy led to the discovery of potent D₃R-selective or multitarget ligands potentially useful for central nervous system disorders. In particular, the 6,6-diphenyl-1,4-dioxane derivative 3 showed a D₃R-preferential profile, while an interesting multitarget behavior has been highlighted for the 5,5-diphenyl-1,4-dioxane and 1,4-benzodioxane derivatives 6 and 9, respectively, which displayed potent D₂R antagonism, 5-HT_1AR and D₄R agonism, as well as potent D₃R partial agonism. They also behaved as low-potency 5-HT_2AR antagonists and 5-HT_2CR partial agonists. Such a profile might be a promising starting point for the discovery of novel antipsychotic agents.

New tetrahydroisoquinoline-based D3R ligands with an o-xylenyl linker motif

The effect of rigidification of the n-butyl linker region of tetrahydroisoquinoline-containing D₃R ligands via inclusion of an o-xylenyl motif was examined in this study. Generally, rigidification with an o-xylenyl linker group reduces D₃R affinity and negatively impacts selectivity versus D₂R for compounds possessing a 6-methoxy-1,2,3,4,-tetrahydroisoquinolin-7-ol primary pharmacophore group. However, D₃R affinity appears to be regulated by the primary pharmacophore group and high affinity D₃R ligands with 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline and 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline primary pharmacophore groups were identified. The results of this study also indicate that D₃R selectivity versus the σ₂R is dictated by the benzamide secondary pharmacophore group, this being facilitated with 4-substituted benzamides. Compounds 5s and 5t were identified as high affinity (K_i < 4 nM) D₃R ligands. Docking studies revealed that the added phenyl ring moiety interacts with the Cys181 in D₃R which partially accounts for the strong D₃R affinity of the ligands.

Exploring the structural basis and atomistic binding mechanistic of the selective antagonist blockade at D3 dopamine receptor over D2 dopamine receptor

More recently, there has been a paradigm shift toward selective drug targeting in the treatment of neurological disorders, including drug addiction, schizophrenia, and Parkinson's disease mediated by the different dopamine receptor subtypes. Antagonists with higher selectivity for D₃ dopamine receptor (D3DR) over D₂ dopamine receptor (D2DR) have been shown to attenuate drug-seeking behavior and associated side effects compared to non-subtype selective antagonists. However, high conservations among constituent residues of both proteins, particularly at the ligand-binding pockets, remain a challenge to therapeutic drug design. Recent studies have reported the discovery of two small-molecules R-VK4-40 and Y-QA31 which substantially inhibited D3DR with >180-fold selectivity over D2DR. Therefore, in this study, we seek to provide molecular and structural insights into these differential binding mechanistic using meta-analytic computational simulation methods. Findings revealed that R-VK4-40 and Y-QA31 adopted shallow binding modes and were more surface-exposed at D3DR while on the contrary, they exhibited deep hydrophobic pocket binding at D2DR. Also, two non-conserved residues; Tyr36^1.39 and Ser182^45.51 were identified in D3DR, based on their crucial roles and contributions to the selective binding of R-VK4-40 and Y-QA31. Importantly, both antagonists exhibited high affinities in complex with D3DR compared to D2DR, while van der Waals energies contributed majorly to their binding and stability. Structural analyses also revealed the distinct stabilizing effects of both compounds on D3DR secondary architecture relative to D2DR. Therefore, findings herein pinpointed the origin and mechanistic of selectivity of the compounds, which may assist in the rational design of potential small molecules of the D₂ -like dopamine family receptor subtype with improved potency and selectivity.

2016 Philip S. Portoghese Medicinal Chemistry Lectureship: Designing Bivalent or Bitopic Molecules for G-Protein Coupled Receptors. The Whole Is Greater Than the Sum of Its Parts

The genesis of designing bivalent or bitopic molecules that engender unique pharmacological properties began with Portoghese's work directed toward opioid receptors, in the early 1980s. This strategy has evolved as an attractive way to engineer highly selective compounds for targeted G-protein coupled receptors (GPCRs) with optimized efficacies and/or signaling bias. The emergence of X-ray crystal structures of many GPCRs and the identification of both orthosteric and allosteric binding sites have provided further guidance to ligand drug design that includes a primary pharmacophore (PP), a secondary pharmacophore (SP), and a linker between them. It is critical to note the synergistic relationship among all three of these components as they contribute to the overall interaction of these molecules with their receptor proteins and that strategically designed combinations have and will continue to provide the GPCR molecular tools of the future.

[O-methyl-11C]N-(4-(4-(3-Chloro-2-methoxyphenyl)-piperazin-1-yl)butyl)-1H-indole-2-carboxamide ([11C]BAK4-51) Is an Efflux Transporter Substrate and Ineffective for PET Imaging of Brain D₃ Receptors in Rodents and Monkey

Selective high-affinity antagonists for the dopamine D₃ receptor (D₃R) are sought for treating substance use disorders. Positron emission tomography (PET) with an effective D₃R radioligand could be a useful tool for the development of such therapeutics by elucidating pharmacological specificity and target engagement in vivo. Currently, a D₃R-selective radioligand does not exist. The D₃R ligand, N-(4-(4-(3-chloro-2-methoxyphenyl)piperazin-1-yl)butyl)-1H-indole-2-carboxamide (BAK4-51, 1), has attractive properties for PET radioligand development, including full antagonist activity, very high D₃R affinity, D₃R selectivity, and moderate lipophilicity. We labeled 1 with the positron-emitter carbon-11 (t_1/2 = 20.4 min) in the methoxy group for evaluation as a radioligand in animals with PET. However, [¹¹C]1 was found to be an avid substrate for brain efflux transporters and lacked D₃R-specific signal in rodent and monkey brain in vivo.

A perspective on multi-target drug discovery and design for complex diseases

Diseases of infection, of neurodegeneration (such as Alzheimer’s and Parkinson’s diseases), and of malignancy (cancers) have complex and varied causative factors. Modern drug discovery has the power to identify potential modulators for multiple targets from millions of compounds. Computational approaches allow the determination of the association of each compound with its target before chemical synthesis and biological testing is done. These approaches depend on the prior identification of clinically and biologically validated targets. This Perspective will focus on the molecular and computational approaches that underpin drug design by medicinal chemists to promote understanding and collaboration with clinical scientists.

Highly Selective Dopamine D3 Receptor Antagonists with Arylated Diazaspiro Alkane Cores

A series of potent and selective D3 receptor (D3R) analogues with diazaspiro alkane cores were synthesized. Radioligand binding of compounds 11, 14, 15a, and 15c revealed favorable D3R affinity (Ki = 12-25.6 nM) and were highly selective for D3R vs D3R (ranging from 264- to 905-fold). Variation of these novel ligand architectures can be achieved using our previously reported 10-20 min benchtop C-N cross-coupling methodology, affording a broad range of arylated diazaspiro precursors.

Novel Bivalent Ligands Based on the Sumanirole Pharmacophore Reveal Dopamine D2 Receptor (D2R) Biased Agonism

The development of bivalent ligands has attracted interest as a way to potentially improve the selectivity and/or affinity for a specific receptor subtype. The ability to bind two distinct receptor binding sites simultaneously can allow the selective activation of specific G-protein dependent or β-arrestin-mediated cascade pathways. Herein, we developed an extended SAR study using sumanirole (1) as the primary pharmacophore. We found that substitutions in the N-1- and/or N-5-positions, physiochemical properties of those substituents, and secondary aromatic pharmacophores can enhance agonist efficacy for the cAMP inhibition mediated by G_i/o-proteins, while reducing or suppressing potency and efficacy toward β-arrestin recruitment. Compound 19 was identified as a new lead for its selective D₂ G-protein biased agonism with an EC₅₀ in the subnanomolar range. Structure-activity correlations were observed between substitutions in positions N-1 and/or N-5 of 1 and the capacity of the new bivalent compounds to selectively activate G-proteins versus β-arrestin recruitment in D₂R-BRET functional assays.

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.

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.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

Polypharmacology of dopamine receptor ligands

Most neurological diseases have a multifactorial nature and the number of molecular mechanisms discovered as underpinning these diseases is continuously evolving. The old concept of developing selective agents for a single target does not fit with the medical need of most neurological diseases. The development of designed multiple ligands holds great promises and appears as the next step in drug development for the treatment of these multifactorial diseases. Dopamine and its five receptor subtypes are intimately involved in numerous neurological disorders. Dopamine receptor ligands display a high degree of cross interactions with many other targets including G-protein coupled receptors, transporters, enzymes and ion channels. For brain disorders like Parkinsońs disease, schizophrenia and depression the dopaminergic system, being intertwined with many other signaling systems, plays a key role in pathogenesis and therapy. The concept of designed multiple ligands and polypharmacology, which perfectly meets the therapeutic needs for these brain disorders, is herein discussed as a general ligand-based concept while focusing on dopaminergic agents and receptor subtypes in particular.

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 [(3)H]7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT) than with the antagonist radioligand [(3)H]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.

Aryl and heteroaryl N-[4-[4-(2,3-substituted-phenyl)piperazine-1-yl]alkyl]carbamates with improved physico-chemical properties as dual modulators of dopamine D3 receptor and fatty acid amide hydrolase

Improving the physico-chemical profile of a MTDL series.

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 (K_i = 0.12 nM) and 32 (K_i = 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.

Structure-activity study of N-((trans)-4-(2-(7-cyano-3,4-dihydroisoquinolin-2(1H)-yl)ethyl)cyclohexyl)-1H-indole-2-carboxamide (SB269652), a bitopic ligand that acts as a negative allosteric modulator of the dopamine D2 receptor

We recently demonstrated that SB269652 (1) engages one protomer of a dopamine D2 receptor (D2R) dimer in a bitopic mode to allosterically inhibit the binding of dopamine at the other protomer. Herein, we investigate structural determinants for allostery, focusing on modifications to three moieties within 1. We find that orthosteric "head" groups with small 7-substituents were important to maintain the limited negative cooperativity of analogues of 1, and replacement of the tetrahydroisoquinoline head group with other D2R "privileged structures" generated orthosteric antagonists. Additionally, replacement of the cyclohexylene linker with polymethylene chains conferred linker length dependency in allosteric pharmacology. We validated the importance of the indolic NH as a hydrogen bond donor moiety for maintaining allostery. Replacement of the indole ring with azaindole conferred a 30-fold increase in affinity while maintaining negative cooperativity. Combined, these results provide novel SAR insight for bitopic ligands that act as negative allosteric modulators of the D2R.

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.

Using click chemistry toward novel 1,2,3-triazole-linked dopamine D3 receptor ligands

The dopamine D3 receptor (D3R) is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, Parkinson's disease, restless leg syndrome, and drug addiction. A common molecular template used in the development of D3R-selective antagonists and partial agonists incorporates a butylamide linker between two pharmacophores, a phenylpiperazine moiety and an extended aryl ring system. The series of compounds described herein incorporates a change to that chemical template, replacing the amide functional group in the linker chain with a 1,2,3-triazole group. Although the amide linker in the 4-phenylpiperazine class of D3R ligands has been previously deemed critical for high D3R affinity and selectivity, the 1,2,3-triazole moiety serves as a suitable bioisosteric replacement and maintains desired D3R-binding functionality of the compounds. Additionally, using mouse liver microsomes to evaluate CYP450-mediated phase I metabolism, we determined that novel 1,2,3-triazole-containing compounds modestly improves metabolic stability compared to amide-containing analogues. The 1,2,3-triazole moiety allows for the modular attachment of chemical subunit libraries using copper-catalyzed azide-alkyne cycloaddition click chemistry, increasing the range of chemical entities that can be designed, synthesized, and developed toward D3R-selective therapeutic agents.

Beyond small-molecule SAR: using the dopamine D3 receptor crystal structure to guide drug design

The dopamine D3 receptor is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, restless leg syndrome, and drug addiction. The high protein sequence homology between the D3 and D2 receptors has posed a challenge to developing D3 receptor-selective ligands whose behavioral actions can be attributed to D3 receptor engagement, in vivo. However, through primarily small-molecule structure-activity relationship (SAR) studies, a variety of chemical scaffolds have been discovered over the past two decades that have resulted in several D3 receptor-selective ligands with high affinity and in vivo activity. Nevertheless, viable clinical candidates remain limited. The recent determination of the high-resolution crystal structure of the D3 receptor has invigorated structure-based drug design, providing refinements to the molecular dynamic models and testable predictions about receptor-ligand interactions. This chapter will highlight recent preclinical and clinical studies demonstrating potential utility of D3 receptor-selective ligands in the treatment of addiction. In addition, new structure-based rational drug design strategies for D3 receptor-selective ligands that complement traditional small-molecule SAR to improve the selectivity and directed efficacy profiles are examined.

Design, synthesis, and structure-activity relationship studies of a series of [4-(4-carboxamidobutyl)]-1-arylpiperazines: insights into structural features contributing to dopamine D3 versus D2 receptor subtype selectivity

Antagonist and partial agonist modulators of the dopamine D3 receptor (D3R) have emerged as promising therapeutics for the treatment of substance abuse and neuropsychiatric disorders. However, development of druglike lead compounds with selectivity for the D3 receptor has been challenging because of the high sequence homology between the D3R and the dopamine D2 receptor (D2R). In this effort, we synthesized a series of acylaminobutylpiperazines incorporating aza-aromatic units and evaluated their binding and functional activities at the D3 and D2 receptors. Docking studies and results from evaluations against a set of chimeric and mutant receptors suggest that interactions at the extracellular end of TM7 contribute to the D3R versus D2R selectivity of these ligands. Molecular insights from this study could potentially enable rational design of potent and selective D3R ligands.

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).

Applying a multitarget rational drug design strategy: the first set of modulators with potent and balanced activity toward dopamine D3 receptor and fatty acid amide hydrolase

Combining computer-assisted drug design and synthetic efforts, we generated compounds with potent and balanced activities toward both D3 dopamine receptor and fatty acid amide hydrolase (FAAH) enzyme. By concurrently modulating these targets, our compounds hold great potential toward exerting a disease-modifying effect on nicotine addiction and other forms of compulsive behavior.

Dopamine D3 receptor antagonists: a patent review (2007 - 2012)

INTRODUCTION

The synthesis and characterization of new highly potent and selective dopamine (DA) D3 receptor antagonists has permitted to characterize the role of the DA D3 receptor in the control of drug-seeking behavior and in the pathophysiology of impulse control disorders and schizophrenia.

AREAS COVERED

In the present review, the authors will first describe most recent classes of DA D3 receptor antagonists by reviewing about 43 patent applications during the 2007 - 2012 period; they will then outline the biological rationale in support of the use of selective DA D3 receptor antagonists in the treatment of drug addiction, impulse control disorders and schizophrenia.

EXPERT OPINION

The strongest clinical application and potential for selective DA D3 receptor antagonists lies in the reduction of drug-induced incentive motivation, the attenuation of drug's rewarding efficacy and the reduction in reinstatement of drug-seeking behavior triggered either by re-exposure to the drug itself, re-exposure to environmental cues that had been previously associated with drug-taking behavior or stress. The selectivity of these antagonists together with reduced lipophilicity (minimizing unspecific binding), increased brain penetration and improved physico-chemical profile are all key factors for clinical efficacy and safety.

Rationale in support of the use of selective dopamine D₃ receptor antagonists for the pharmacotherapeutic management of substance use disorders

Growing evidence indicates that dopamine (DA) D(3) receptors are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders. First, DA D(3) receptors are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse has been shown to produce neuroadaptations in the DA D(3) system. Third, the synthesis and characterization of highly potent and selective DA D(3) receptor antagonists has permitted to further define the role of the DA D(3) receptor in drug addiction. Provided that the available preclinical and preliminary clinical evidence can be translated into clinical proof of concept in human, selective DA D(3) receptor antagonists show promise for the treatment of substance use disorders as reflected by their potential to (1) regulate the motivation to self-administered drugs under schedules of reinforcement that require an increase in work demand and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in the reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, re-exposure to environmental cues that had been previously associated with drug-taking behavior, or stress.

Medication discovery for addiction: translating the dopamine D3 receptor hypothesis

The dopamine D3 receptor (D3R) has been investigated as a potential target for medication development to treat substance use disorders (SUDs) with a particular focus on cocaine and methamphetamine. Currently, there are no approved medications to treat cocaine and methamphetamine addiction and thus developing pharmacotherapeutics to complement existing behavioral strategies is a fundamental goal. Novel compounds with high affinity and D3R selectivity have been evaluated in numerous animal models of drug abuse and favorable outcomes in nonhuman primate models of self-administration and relapse have provided compelling evidence to advance these agents into the clinic. One approach is to repurpose drugs that share the D3R mechanism and already have clinical utility, and to this end buspirone has been identified as a viable candidate for clinical trials. A second, but substantially more resource intensive and risky approach involves the development of compounds that exclusively target D3R, such as GSK598809 and PG 619. Clinical investigation of these drugs or other novel D3R-selective agents will provide a better understanding of the role D3R plays in addiction and whether or not antagonists or partial agonists that are D3R selective are effective in achieving abstinence in this patient population.

A two-step target binding and selectivity support vector machines approach for virtual screening of dopamine receptor subtype-selective ligands

Target selective drugs, such as dopamine receptor (DR) subtype selective ligands, are developed for enhanced therapeutics and reduced side effects. In silico methods have been explored for searching DR selective ligands, but encountered difficulties associated with high subtype similarity and ligand structural diversity. Machine learning methods have shown promising potential in searching target selective compounds. Their target selective capability can be further enhanced. In this work, we introduced a new two-step support vector machines target-binding and selectivity screening method for searching DR subtype-selective ligands, which was tested together with three previously-used machine learning methods for searching D1, D2, D3 and D4 selective ligands. It correctly identified 50.6%–88.0% of the 21–408 subtype selective and 71.7%–81.0% of the 39–147 multi-subtype ligands. Its subtype selective ligand identification rates are significantly better than, and its multi-subtype ligand identification rates are comparable to the best rates of the previously used methods. Our method produced low false-hit rates in screening 13.56 M PubChem, 168,016 MDDR and 657,736 ChEMBLdb compounds. Molecular features important for subtype selectivity were extracted by using the recursive feature elimination feature selection method. These features are consistent with literature-reported features. Our method showed similar performance in searching estrogen receptor subtype selective ligands. Our study demonstrated the usefulness of the two-step target binding and selectivity screening method in searching subtype selective ligands from large compound libraries.

Molecular determinants of selectivity and efficacy at the dopamine D3 receptor

The dopamine D3 receptor (D3R) has been implicated in substance abuse and other neuropsychiatric disorders. The high sequence homology between the D3R and D2R, especially within the orthosteric binding site (OBS) that binds dopamine, has made the development of D3R-selective compounds challenging. Here, we deconstruct into pharmacophoric elements a series of D3R-selective substituted-4-phenylpiperazine compounds and use computational simulations and binding and activation studies to dissect the structural bases for D3R selectivity and efficacy. We find that selectivity arises from divergent interactions within a second binding pocket (SBP) separate from the OBS, whereas efficacy depends on the binding mode in the OBS. Our findings reveal structural features of the receptor that are critical to selectivity and efficacy that can be used to design highly D3R-selective ligands with targeted efficacies. These findings are generalizable to other GPCRs in which the SBP can be targeted by bitopic or allosteric ligands.

Structure-activity relationship studies of SYA 013, a homopiperazine analog of haloperidol

Structure-activity relationship studies on 4-(4-(4-chlorophenyl)-1,4-diazepan-1-yl)-1-(4-fluorophenyl)butan-1-one (SYA 013), a homopiperazine analog of haloperidol has resulted in an understanding of the effect of structural modifications on binding affinity at dopamine and serotonin receptor subtypes. Further exploration, using bioisosteric replacement strategies has led to the identification of several new agents including compounds 7, 8, 11 and 12 which satisfy the initial criteria for further exploration as new antipsychotic agents. In addition, compound 18, a D(3) selective tropanol, has been identified as having the potential for further optimization into a useful drug which may combat neuropsychiatric diseases.