Synthesis and characterization of selective dopamine D2 receptor antagonists

In vitro characterization of [125I]HY-3-24, a selective ligand for the dopamine D3 receptor

Introduction

Dopamine D3 receptor (D3R) ligands have been studied for the possible treatment of neurological and neuropsychiatric disorders. However, selective D3R radioligands for in vitro binding studies have been challenging to identify due to the high structural similarity between the D2R and D3R. In a prior study, we reported a new conformationally-flexible benzamide scaffold having a high affinity for D3R and excellent selectivity vs. D2R. In the current study, we characterized the in vitro binding properties of a new radioiodinated ligand, [125I]HY-3-24.

Methods

In vitro binding studies were conducted in cell lines expressing D3 receptors, rat striatal homogenates, and rat and non-human primate (NHP) brain tissues to measure regional brain distribution of this radioligand.

Results

HY-3-24 showed high potency at D3R (Ki = 0.67 ± 0.11 nM, IC50 = 1.5 ± 0.58 nM) compared to other D2-like dopamine receptor subtypes (D2R Ki = 86.7 ± 11.9 nM and D4R Ki > 1,000). The Kd (0.34 ± 0.22 nM) and Bmax (38.91 ± 2.39 fmol/mg) values of [125I]HY-3-24 were determined. In vitro binding studies in rat striatal homogenates using selective D2R and D3R antagonists confirmed the D3R selectivity of [125I]HY-3-24. Autoradiography results demonstrated that [125I]HY-3-24 specifically binds to D3Rs in the nucleus accumbens, islands of Calleja, and caudate putamen in rat and NHP brain sections.

Conclusion

These results suggest that [125I]HY-3-24 appears to be a novel radioligand that exhibits high affinity binding at D3R, with low binding to other D2-like dopamine receptors. It is anticipated that [125I]HY-3-24 can be used as the specific D3R radioligand.

Discovery of new chemotypes of dual 5-HT2A/D2 receptor antagonists with a strategy of drug design methodologies

Aim: Through the application of structure- and ligand-based methods, the authors aimed to create an integrative approach to developing a computational protocol for the rational drug design of potent dual 5-HT_2A/D₂ receptor antagonists without off-target activities on H₁ receptors. Materials & methods: Molecular dynamics and virtual docking methods were used to identify key interactions of the structurally diverse antagonists in the binding sites of the studied targets, and to generate their bioactive conformations for further 3D-quantitative structure-activity relationship modeling. Results & conclusion: Toward the goal of finding multi-potent drugs with a more effective and safer profile, the obtained results led to the design of a new set of dual antagonists and opened a new perspective on the therapy for complex brain diseases.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

Current computer-aided drug design methodologies in discovery of novel drug candidates for neuropsychiatric and inflammatory diseases

Drug discovery and development is a very challenging, expensive and time-consuming process. Impressive technological advances in computer sciences and molecular biology have made it possible to use computer-aided drug design (CADD) methods in various stages of the drug discovery and development pipeline. Nowadays, CADD presents an efficacious and indispensable tool, widely used in medicinal chemistry, to lead rational drug design and synthesis of novel compounds. In this article, an overview of commonly used CADD approaches from hit identification to lead optimization was presented. Moreover, different aspects of design of multitarget ligands for neuropsychiatric and anti-inflammatory diseases were summarized. Apparently, designing multi-target directed ligands for treatment of various complex diseases may offer better efficacy, and fewer side effects. Antipsychotics that act through aminergic G protein-coupled receptors (GPCRs), especially Dopamine D2 and serotonin 5-HT2A receptors, are the best option for treatment of various symptoms associated with neuropsychiatric disorders. Furthermore, multi-target directed cyclooxygenase-2 (COX-2) and 5-lipoxygenase (5-LOX) inhibitors are also a successful approach to aid the discovery of new anti-inflammatory drugs with fewer side effects. Overall, employing CADD approaches in the process of rational drug design provides a great opportunity for future development, allowing rapid identification of compounds with the optimal polypharmacological profile.

Disruption of dopamine D1/D2 receptor complex is involved in the function of haloperidol in cardiac H9c2 cells

AIMS

Haloperidol is an antipsychotic agent and acts as dopamine D2 receptor (D2R) antagonist, as a prototypical ligand of sigma1 receptors (Sig1R) and it increases expression of type 1 IP₃ receptors (IP₃R1). However, precise mechanism of haloperidol action on cardiomyocytes through dopaminergic signaling was not described yet. This study investigated a role of dopamine receptors in haloperidol-induced increase in IP₃R1 and Sig1R, and compared physiological effect of melperone and haloperidol on basic heart parameters in rats.

MATERIALS AND METHODS

We used differentiated NG-108 cells and H9c2 cells. Gene expression, Western blot and immunofluorescence were used to evaluate haloperidol-induced differences; proximity ligation assay (PLA) and immunoprecipitation to determine interactions of D1/D2 receptors. To evaluate cardiac parameters, Wistar albino male rats were used.

KEY FINDINGS

We have shown that antagonism of D2R with either haloperidol or melperone results in upregulation of both, IP₃R1 and Sig1R, which is associated with increased D2R, but reduced D1R expression. Immunofluorescence, immunoprecipitation and PLA support formation of heteromeric D1/D2 complexes in H9c2 cells. Treatment with haloperidol (but not melperone) caused decrease in systolic and diastolic blood pressure and significant increase in heart rate.

SIGNIFICANCE

Because D1R/D2R complexes can engage Gq-like signaling in other experimental systems, these results are consistent with the possibility that disruption of D1R/D2R complex in H9c2 cells might cause a decrease in IP₃R1 activity, which in turn may account for the increase expression of IP₃R and Sig1R. D2R is probably not responsible for changes in cardiac parameters, since melperone did not have any effect.

Advances and challenges in the search for D2 and D3 dopamine receptor-selective compounds

Compounds that target D2-like dopamine receptors (DRs) are currently used as therapeutics for several neuropsychiatric disorders including schizophrenia (antagonists) and Parkinson's disease (agonists). However, as the D₂R and D₃R subtypes are highly homologous, creating compounds with sufficient subtype-selectivity as well as drug-like properties for therapeutic use has proved challenging. This review summarizes the progress that has been made in developing D₂R- or D₃R-selective antagonists and agonists, and also describes the experimental conditions that need to be considered when determining the selectivity of a given compound, as apparent selectivity can vary widely depending on assay conditions. Future advances in this field may take advantage of currently available structural data to target alternative secondary binding sites through creating bivalent or bitopic chemical structures. Alternatively, the use of high-throughput screening techniques to identify novel scaffolds that might bind to the D₂R or D₃R in areas other than the highly conserved orthosteric site, such as allosteric sites, followed by iterative medicinal chemistry will likely lead to exceptionally selective compounds in the future. More selective compounds will provide a better understanding of the normal and pathological functioning of each receptor subtype, as well as offer the potential for improved therapeutics.

Pharmacological modulation of abnormal involuntary DOI-induced head twitch response movements in male DBA/2J mice: II. Effects of D3 dopamine receptor selective compounds

We recently reported on the characterization of the hallucinogen 2,5-dimethoxy-4-methylamphetamine's (DOI) ability to elicit a head twitch response (HTR) in DBA/2J mice and the ability of D2 vs. D3 dopamine receptor selective compounds to modulate that response. For these studies, the ability of D3 vs. D2 dopamine receptor selective compounds to attenuate the DOI-dependent HTR was examined. WC 10, a D3 dopamine receptor weak partial agonist with 40-fold binding selectivity for D3 vs. D2 dopamine receptors, produced a dose-dependent decrease in the DOI-induced HTR (IC50 = 3.7 mg/kg). WC 44, a D3 receptor selective full agonist, also inhibited the DOI-induced HTR (IC50 = 5.1 mg/kg). The effect of two D3 receptor selective partial agonists, LAX-4-136 and WW-III-55, were also evaluated. These analogs exhibit 150-fold and 800-fold D3 vs. D2 binding selectivity, respectively. Both compounds inhibited the HTR with similar potency but with different maximum efficacies. At 10 mg/kg WW-III-55 inhibited the HTR by 95%, while LAX-4-136 administration resulted in a 50% reduction. In addition, DOI (5 mg/kg) was administered at various times after LAX-4-136 or WW-III-55 administration to compare the duration of action. The homopiperazine analog LAX-4-136 exhibited greater stability. An assessment of our test compounds on motor performance and coordination was performed using a rotarod test. None of the D3 dopamine receptor selective compounds significantly altered latency to fall, suggesting that these compounds a) did not attenuate the DOI-dependent HTR due to sedative or adverse motor effects and b) may have antipsychotic/antihallucinogenic activity.

Characterization of [(3) H]LS-3-134, a novel arylamide phenylpiperazine D3 dopamine receptor selective radioligand

LS-3-134 is a substituted N-phenylpiperazine derivative that has been reported to exhibit: (i) high-affinity binding (Ki value 0.2 nM) at human D3 dopamine receptors, (ii) > 100-fold D3 versus D2 dopamine receptor subtype binding selectivity, and (iii) low-affinity binding (Ki  > 5000 nM) at sigma 1 and sigma 2 receptors. Based upon a forskolin-dependent activation of the adenylyl cyclase inhibition assay, LS-3-134 is a weak partial agonist at both D2 and D3 dopamine receptor subtypes (29% and 35% of full agonist activity, respectively). In this study, [(3) H]-labeled LS-3-134 was prepared and evaluated to further characterize its use as a D3 dopamine receptor selective radioligand. Kinetic and equilibrium radioligand binding studies were performed. This radioligand rapidly reaches equilibrium (10-15 min at 37°C) and binds with high affinity to both human (Kd  = 0.06 ± 0.01 nM) and rat (Kd  = 0.2 ± 0.02 nM) D3 receptors expressed in HEK293 cells. Direct and competitive radioligand binding studies using rat caudate and nucleus accumbens tissue indicate that [(3) H]LS-3-134 selectively binds a homogeneous population of binding sites with a dopamine D3 receptor pharmacological profile. Based upon these studies, we propose that [(3) H]LS-3-134 represents a novel D3 dopamine receptor selective radioligand that can be used for studying the expression and regulation of the D3 dopamine receptor subtype.

Discovery, optimization, and characterization of novel D2 dopamine receptor selective antagonists

The D2 dopamine receptor (D2 DAR) is one of the most validated drug targets for neuropsychiatric and endocrine disorders. However, clinically approved drugs targeting D2 DAR display poor selectivity between the D2 and other receptors, especially the D3 DAR. This lack of selectivity may lead to undesirable side effects. Here we describe the chemical and pharmacological characterization of a novel D2 DAR antagonist series with excellent D2 versus D1, D3, D4, and D5 receptor selectivity. The final probe 65 was obtained through a quantitative high-throughput screening campaign, followed by medicinal chemistry optimization, to yield a selective molecule with good in vitro physical properties, metabolic stability, and in vivo pharmacokinetics. The optimized molecule may be a useful in vivo probe for studying D2 DAR signal modulation and could also serve as a lead compound for the development of D2 DAR-selective druglike molecules for the treatment of multiple neuropsychiatric and endocrine disorders.

Pharmacological modulation of abnormal involuntary DOI-induced head twitch response in male DBA/2J mice: I. Effects of D2/D3 and D2 dopamine receptor selective compounds

Because of the complexity and heterogeneity of human neuropsychiatric disorders, it has been difficult to identify animal models that mimic the symptoms of these neuropathologies and can be used to screen for antipsychotic agents. For this study we selected the murine 5HT2A/2C receptor agonist-induced head twitch response (HTR) induced by the administration of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), which has been proposed as an animal model of symptoms associated with a variety of behavioral and psychiatric conditions. We investigated the DOI-induced HTR in male DBA/2J mice using a panel of D2-like (D2, D3 and D4) and D2 dopamine receptor selective compounds. When DBA/2J mice were administered a daily dose of DOI (5 mg/kg), tolerance to the DOI occurs. However, administrations of the same dose of DOI every other day (48 h) or on a weekly basis did not lead to tolerance and the ability to induce tolerance after daily administration of DOI remains intact after repeated weekly administration of DOI. Subsequently, a panel of D2-like dopamine receptor antagonists was found to effectively inhibit the DOI-induced HTR in DBA/2J mice. However, the benzamide eticlopride, which is a high affinity D2-like antagonist, was a notable exception. SV 293, SV-III-130s and N-methylbenperidol, which exhibit a high affinity for D2 versus the D3 dopamine receptor subtypes (60- to 100-fold binding selectivity), were also found to inhibit the HTR in DBA/2J mice. This observation suggests a functional interaction between dopaminergic and serotonergic systems through D2 dopamine receptors and the 5-HT2A serotonin receptors in vivo.

The effect of SV 293, a D2 dopamine receptor-selective antagonist, on D2 receptor-mediated GIRK channel activation and adenylyl cyclase inhibition

SV 293 [1-([5-methoxy-1H-indol-3-yl]methyl)-4-(4-[methylthio]​phenyl)piperidin-4-ol] binds with 100-fold higher affinity to human D2 receptors compared to the human D3 and D4 dopamine receptor subtypes. We investigated the intrinsic efficacy of this compound at the D2 dopamine receptor subtype using both: (1) a forskolin-dependent adenylyl cyclase inhibition assay and (2) an electrophysiological assay for evaluating coupling to G-protein-coupled inwardly rectifying potassium channels. In both assays SV 293 was found to be a neutral antagonist capable of blocking the effects of the full D2-like receptor agonist quinpirole. Based upon these results we propose that SV 293 is a useful pharmacological tool that can be used for both in vitro and in vivo studies to investigate the role of D2-like dopamine receptor subtypes in neurological, neuropsychiatric and movement disorders where dopaminergic pathways have been implicated.

Comparison of the binding and functional properties of two structurally different D2 dopamine receptor subtype selective compounds

We previously reported on the synthesis of substituted phenyl-4-hydroxy-1-piperidyl indole analogues with nanomolar affinity at D2 dopamine receptors, ranging from 10- to 100-fold selective for D2 compared to the D3 dopamine receptor subtype. More recently, we evaluated a panel of aripiprazole analogues, identifying several analogues that also exhibit D2 vs D3 dopamine receptor binding selectivity. These studies further characterize the intrinsic efficacy of the compound with the greatest binding selectivity from each chemical class, 1-((5-methoxy-1H-indol-3-yl)methyl)-4-(4-(methylthio)phenyl)piperidin-4-ol (SV 293) and 7-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one (SV-III-130s), using an adenylyl cyclase inhibition assay, a G-protein-coupled inward-rectifying potassium (GIRK) channel activation assay, and a cell based phospho-MAPK (pERK1/2) assay. SV 293 was found to be a neutral antagonist at D2 dopamine receptors using all three assays. SV-III-130s is a partial agonist using an adenylyl cyclase inhibition assay but an antagonist in the GIRK and phospho ERK1/2 assays. To define the molecular basis for the binding selectivity, the affinity of these two compounds was evaluated using (a) wild type human D2 and D3 receptors and (b) a panel of chimeric D2/D3 dopamine receptors. Computer-assisted modeling techniques were used to dock these compounds to the human D2 and D3 dopamine receptor subtypes. It is hoped that these studies on D2 receptor selective ligands will be useful in the future design of (a) receptor selective ligands used to define the function of D2-like receptor subtypes, (b) novel pharmacotherapeutic agents, and/or (c) in vitro and in vivo imaging agents.

Neuroprotective effects of high affinity Σ1 receptor selective compounds

We previously reported that the antipsychotic drug haloperidol, a multifunctional D2-like dopamine and sigma receptor subtype antagonist, has neuroprotective properties. In this study we further examined the association between neuroprotection and receptor antagonism by evaluating a panel of novel compounds with varying affinity at sigma and D2-like dopamine receptors. These compounds were evaluated using an in vitro cytotoxicity assay that utilizes a hippocampal-derived cell line, HT-22, in the presence or absence of varying concentrations (5 to 20 mM) of glutamate. While haloperidol was found to be a potent neuroprotective agent in this in vitro cell assay, the prototypic sigma 1 receptor agonist (+)-pentazocine was found not to be neuroprotective. Subsequently, the potency for the neuroprotection of HT-22 cells was evaluated for a) three SV series indoles which have nMolar affinity at D2-like receptors but varying affinity at sigma 1 receptor and b) two benzyl phenylacetamides sigma 1 receptor selective compounds which bind with low affinity at D2-like receptors but have nMolar affinity for the sigma 1 receptor. We observed that cytoprotection correlated with the affinity of the compounds for sigma 1 receptors. Based upon results from the HT-22 cell-based in vitro assay, two phenylacetamides, LS-127 and LS-137, were further evaluated in vivo using a transient middle cerebral artery occlusion (t-MCAO) model of stroke. At a dose of 100 μg/kg, both LS-127 and LS-137 attenuated infarct volume by approximately 50%. These studies provide further evidence that sigma 1 receptor selective compounds can provide neuroprotection in cytotoxic situations. These results also demonstrate that sigma 1 receptor selective benzyl phenylacetamides are candidate pharmacotherapeutic agents that could be used to minimize neuronal death after a stroke or head trauma.

Synthesis and characterization of selective dopamine D₂ receptor ligands using aripiprazole as the lead compound

A series of compounds structurally related to aripiprazole (1), an atypical antipsychotic and antidepressant used clinically for the treatment of schizophrenia, bipolar disorder, and depression, have been prepared and evaluated for affinity at D(₂-like) dopamine receptors. These compounds also share structural elements with the classical D(₂-like) dopamine receptor antagonists, haloperidol, N-methylspiperone, domperidone and benperidol. Two new compounds, 7-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one oxalate (6) and 7-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one oxalate (7) were found to (a) bind to the D₂ receptor subtype with high affinity (K(i) values < 0.3 nM), (b) exhibit >50-fold D₂ versus D₃ receptor binding selectivity and (c) be partial agonists at both the D₂ and D₃ receptor subtype.

Evaluation of the D3 dopamine receptor selective agonist/partial agonist PG01042 on L-dopa dependent animal involuntary movements in rats

The substituted 4-phenylpiperazine D3 dopamine receptor selective antagonist PG01037 ((E)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)but-2-enyl)-4-(pyridin-2-yl)benzamide) was reported to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned rats, a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease (Kumar et al., 2009a). We now report that PG01042 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-3-yl)benzamide), which is a D3 dopamine receptor selective agonist for adenylyl cyclase inhibition and a partial agonist for mitogenesis, is also capable of attenuating AIMs scores. The intrinsic activity of PG01037 and PG01042 were determined using a) a forskolin-dependent adenylyl cyclase inhibition assay and b) an assay for agonist-associated mitogenesis. It was observed that the in vivo efficacy of PG01042 increased when administered by intraperitoneal (i.p.) injection simultaneously with L-dopa/benserazide (8 mg/kg each), as compared to a 60 min or 30 min pretreatment. PG01042 was found to attenuate AIM scores in these animals in a dose dependent manner. While PG01042 did not effectively inhibit SKF 81297-dependent AIMs, it inhibited apomorphine-dependent AIM scores. Rotarod studies indicate that PG01042 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01042 did not dramatically attenuate the beneficial effects of L-dopa. These studies and previously published studies suggest that both D3 dopamine receptor selective antagonists, partial agonists and agonists, as defined by an adenylyl cyclase inhibition assay and a mitogenic assay, are pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Efficient Pd-catalyzed amination reactions for heterocycle functionalization

The Pd-catalyzed amination of unprotected benzo-fused heterocycles is reported, which allows for greater flexibility and efficiency in the modification of this important class of molecules. The generality of these simple and efficient procedures is demonstrated through the synthesis of a wide variety of structural types.

Subtype selectivity of dopamine receptor ligands: insights from structure and ligand-based methods

Subtype selective dopamine receptor ligands have long been sought after as therapeutic and/or imaging agents for the treatment and monitoring of neurologic disorders. We report herein on a combined structure- and ligand-based approach to explore the molecular mechanism of the subtype selectivity for a large class of D₂-like dopamine receptor ligands (163 ligands in total). Homology models were built for both human D(₂L) and D₃ receptors in complex with haloperidol. Other ligands, which included multiple examples of substituted phenylpiperazines, were aligned against the binding conformations of haloperidol, and three-dimensional quantitative structure activity relationship (3D-QSAR) analyses were carried out. The receptor models show that although D₂ and D₃ share highly similar folds and 3D conformations, the slight sequence differences at their extracellular loop regions result in the binding cavity in D₂ being comparably shallower than in D₃, which may explain why some larger ligands bind with greater affinity at D₃ compared to D₂ receptors. The QSAR models show excellent correlation and high predictive power even when evaluated by the most stringent criteria. They confirm that the origins of subtype selectivity for the ligands arise primarily due to differences in the contours of the two binding sites. The predictive models suggest that while both steric and electrostatic interactions contribute to the compounds' binding affinity, the major contribution arises from hydrophobic interactions, with hydrogen bonding conferring binding specificity. The current work provides clues for the development of more subtype selective dopamine receptor ligands. Furthermore, it demonstrates the possibility of being able to apply similar modeling methods to other subtypes or classes of receptors to study GPCR receptor-ligand interactions at a molecular level.

Synthesis and characterization of selective dopamine D2 receptor antagonists. 2. Azaindole, benzofuran, and benzothiophene analogs of L-741,626

A series of indole, 7-azaindole, benzofuran, and benzothiophene compounds have been prepared and evaluated for affinity at D2-like dopamine receptors. These compounds share structural elements with the classical D2-like dopamine receptor antagonists haloperidol, N-methylspiperone and benperidol. Two new compounds, 4-(4-iodophenyl)-1-((4-methoxy-1H-indol-3-yl)methyl)piperidin-4-ol (6) and 4-(4-iodophenyl)-1-((5-methoxy-1H-indol-3-yl)methyl)piperidin-4-ol (7), were found to have high affinity to and selectivity for D2 versus D3 receptors. Changing the aromatic ring system from an indole to other heteroaromatic ring systems reduced the D2 binding affinity and the D2 versus D3 selectivity.

Dopamine D3 receptor selective ligands with varying intrinsic efficacies at adenylyl cyclase inhibition and mitogenic signaling pathways

A panel of structurally related substituted 4-phenylpiperazines with nanomolar affinity and selectivity at D3 dopamine receptors has been synthesized. Compounds in which a heterocyclic (2-phenyl pyridyl, 3-phenyl pyridyl, benzothiophene, or benzofuran) moiety is adjacent to the amide was varied and/or a double bond (trans-butenyl) replaced the four-carbon aliphatic chain linking the arylamide with the 4-phenylpiperazine moiety were compared for (a) affinity at human D2 and D3 dopamine receptors, (b) intrinsic efficacy using an adenylyl cyclase inhibition assay, and (c) intrinsic efficacy using a mitogenic assay. All 16 compounds were (a) more efficacious for the D3 receptor cyclase inhibition assay than for the D3 receptor mitogenic assay and (b) exhibited the same or greater efficacy at D3 compared to D2 receptor (with the exception of one compound). Although the heterocyclic amide moiety appears to be the pivotal structural element determining the intrinsic efficacy of our D3 receptor selective compounds, the magnitude of the efficacy is modulated by the (a) substituent(s) on the phenyl piperazine and (b) the saturation of the four-carbon chain that links the arylamide and the phenylpiperazine. In addition, our ligands are functionally selective, because they can have differing intrinsic efficacies for the cyclase inhibition and the mitogenic activation signaling pathways. Compounds that are essentially full agonists at the cyclase assay appear to be only partial agonists in the mitogenic assay and compounds that are partial agonists in our cyclase assay are partial agonists or antagonists in the mitogenic assay.

In vivo quantification of regional dopamine-D3 receptor binding potential of (+)-PHNO: Studies in non-human primates and transgenic mice

Examination of dopamine-D3 (D3) receptors with positron emission tomography (PET) have been hampered in the past by the lack of a PET ligand with sufficient selectivity for D3 over dopamine-D2 (D2) receptors. The two types co-localize in the brain, with D2 density significantly higher than D3, hence nonselective PET ligands inform on D2, rather than D3 status. [(11)C]-(+)-PHNO is a novel PET ligand with a preferential affinity for D3 over D2. We used the selective D3 antagonist, SB-277011 to dissect regional fractions of the [(11)C]-(+)-PHNO signal attributable to D3 and D2 in primate brain. The results were compared with quantitative autoradiography with (3)H-(+)-PHNO in wild-type, D2-knock-out, and D3-knock-out mice examined at baseline and following administration of SB-277011. Both sets of results converged to indicate a predominant D3-related component to (+)-PHNO binding in extra-striatal regions, with binding in the midbrain being entirely attributable to D3. The midbrain is thus an excellent target region to examine D3 receptor occupancy with [(11)C]-(+)-PHNO PET in vivo.

[(3)H]4-(Dimethylamino)-N-[4-(4-(2-methoxyphenyl)piperazin- 1-yl)butyl]benzamide, a selective radioligand for dopamine D(3) receptors. I. In vitro characterization

4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzamide (WC-10), a N-phenyl piperazine analog, has been shown to have high affinity and selectivity for dopamine D(3) receptors versus dopamine D(2) receptors (Chu et al. [2005] Bioorg Med Chem 13:77-87). In this study, WC-10 was radiolabeled with tritium (specific activity = 80 Ci/mmol) and [(3)H]WC-10 binding to genetically cloned dopamine D(2L) and D(3) receptors was evaluated in vitro. [(3)H]WC-10 binds with a 66-fold higher affinity to human HEK D(3) than HEK D(2L) receptors, with a dissociation constant (K(d)) of 1.2 nM at HEK D(3) receptors. However, [(3)H]WC-10 binds to rat Sf9 rD(3) receptors with a K(d) of 3.9 nM, a value that is 3-fold lower than binding to human HEK D(3) receptors and 40-fold value higher than binding to rat Sf9 rD(2L) receptors. The K(d) values obtained from saturation binding experiments were consistent with the results determined from kinetic (k(on) and k(off)) studies. The pharmacologic profiles of a series of dopaminergic drugs for inhibiting the binding of [(3)H]WC-10 to D(3) receptors was in agreement with previously reported data. In vitro autoradiography studies of rat and monkey brains show that [(3)H]WC-10 labeled D(3) sites in the striatal region.

Evaluation of D2 and D3 dopamine receptor selective compounds on L-dopa-dependent abnormal involuntary movements in rats

A panel of novel D2 and D3 dopamine receptor selective antagonists, partial agonists and full agonists have been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA) unilaterally lesioned male Sprague Dawley rats, which is an animal model of L-dopa-induced dyskinesia (LID). LID is often observed in patients with Parkinson's Disease following chronic treatment with L-dopa. The intrinsic activity of these dopaminergic compounds was determined using a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype. For the initial experiments the 5-HT1A receptor selective partial agonist buspirone was used to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Two D2 dopamine receptor selective antagonists, SV 156 and SV 293, were evaluated and found to minimally attenuate AIM scores in these animals. Four members of our WC series of D3 dopamine receptor selective compounds of varying intrinsic activity at the D3 dopamine receptor subtype, WC 10, WC 21, WC 26 and WC 44, were also evaluated and found to attenuate AIM scores in a dose dependent manner. The in vivo efficacy of the compounds increased when they were administered simultaneously with L-dopa, as compared to when the compounds were administered 60 min prior to the L-dopa/benserazide. It was also found that the D3 receptor antagonist WC 10 could inhibit the involuntary movements after they had achieved maximum intensity. Unlike the D1-like dopamine receptor selective agonist SKF 81297 and the D2-like dopamine receptor agonist bromocriptine which can precipitate abnormal involuntary movements in these unilaterally lesioned animals, abnormal involuntary movements were not observed after administration of our D3 receptor selective agonist WC 44. In addition, we evaluated the effect of these four D3 dopamine receptor selective compounds for their effect on a) spontaneous locomotion and b) coordination and agility using a rotarod apparatus. We also used a cylinder test to assess the effect of L-dopa on spontaneous and independent use of each of the rat's forelimbs in the presence or absence of test compound. The results of these studies suggest that substituted phenylpiperazine D3 dopamine receptor selective compounds are potential pharmacotherapeutic agents for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Structure-activity relationships for a novel series of dopamine D2-like receptor ligands based on N-substituted 3-aryl-8-azabicyclo[3.2.1]octan-3-ol

Discovering dopamine D2-like receptor subtype-selective ligands has been a focus of significant investigation. The D2R-selective antagonist 3-[4-(4-chlorophenyl)-4-hydroxypiperidinyl]methylindole (1, L741,626; K(i)(D2R/D3R) = 11.2:163 nM) has previously provided a lead template for chemical modification. Herein, analogues have been synthesized where the piperidine was replaced by a tropane ring that reversed the selectivity seen in the parent compound, in human hD2(L)R- or hD3R-transfected HEK 293 cells (31, K(i)(D2R/D3R) = 33.4:15.5 nM). Further exploration of both N-substituted and aryl ring-substituted analogues resulted in the discovery of several high affinity D2R/D3R ligands with 3-benzofurylmethyl-substituents (e.g., 45, K(i)(D2R/D3R) = 1.7:0.34 nM) that induced high affinity not achieved in similarly N-substituted piperidine analogues and significantly (470-fold) improved D3R binding affinity compared to the parent ligand 1. X-ray crystallographic data revealed a distinctive spatial arrangement of pharmacophoric elements in the piperidinol vs tropine analogues, providing clues for the diversity in SAR at the D2 and D3 receptor subtypes.

Molecular modeling of the second extracellular loop of G-protein coupled receptors and its implication on structure-based virtual screening

The current study describes the validation of high-throughput modeling procedures for the construction of the second extracellular loop (ecl2) of all nonolfactory human G Protein-coupled receptors. Our modeling flowchart is based on the alignment of essential residues determining the particular ecl2 fold observed in the bovine rhodopsin (bRho) crystal structure. For a set of GPCR targets, the dopamine D2 receptor (DRD2), adenosine A3 receptor (AA3R), and the thromboxane A2 receptor (TA2R), the implications of including ecl2 atomic coordinates is evaluated in terms of structure-based virtual screening accuracy: the suitability of the 3D models to distinguish between known antagonists and randomly chosen decoys using automated docking approaches. The virtual screening results of different models describing increasingly exhaustive receptor representations (seven helices only, seven helices and ecl2 loop, full model) have been compared. Explicit modeling of the ecl2 loop was found to be important in only one of three test cases whereas a loopless model was shown to be accurate enough in the two other receptors. An exhaustive comparison of ecl2 loops of 365 receptors to that of bRho suggests that explicit ecl2 loop modeling should be reserved to receptors where loop building can be guided by experimental restraints.

Exploring structure-selectivity relationships of biogenic amine GPCR antagonists using similarity searching and dynamic compound mapping

We design and analyze compound selectivity sets of antagonists with differential selectivity against seven biogenic amine G-protein coupled receptors. The selectivity sets consist of a total of 267 antagonists and contain a spectrum of in part closely related molecular scaffolds. Each set represents a different selectivity profile. Using these com- pound sets, a systematic computational analysis of structure-selectivity relationships is carried out with different 2D similarity methods including fingerprints, recursive partitioning, clustering, and dynamic compound mapping. Screening calculations are performed in a background database containing nearly four million molecules. Fingerprint searching and compound mapping are found to enrich target-selective antagonists over family-selective ones. Dynamic compound mapping effectively discriminates database compounds from GPCR antagonists and consistently retains target-selective antagonists during the final dimension extension levels. Furthermore, the widely used MACCS key fingerprint displays a strong tendency to distinguish between target- and family-selective GPCR antagonists. Taken together, the results indicate that different types of 2D similarity methods are capable of distinguishing closely related molecules having different selectivity. The reported compound benchmark system is made freely available in order to enable selectivity-oriented analyses using other computational approaches.

Evidence for the involvement of the monoaminergic system in the antidepressant-like action of two 4-amine derivatives of 10,11-dihydro-5H-dibenzo [a,d] cycloheptane in mice evaluated in the tail suspension test

Our previous study described the synthesis of 4-amine derivatives of 10,11-dihydro-5H-dibenzo-alkylamine-cycloheptane, 4-amine (3-N,N-dimethylpropylamine)-10,11-dihydro-5H-dibenzo[a,d] cycloheptane-5-one (ADDCH1), and 1,2,3,4,8,9-hexahydro-dibenzocycloheptane[4,4a,5-ef]1,4-diazepin (ADDCH2), and the characterization of their antidepressant-like effect in the forced swimming test in mice. This study investigated the involvement of monoaminergic pathways in the antidepressant-like effect of these compounds in mice evaluated in the tail suspension test (TST), another animal model to screen antidepressant drugs. Our results show that the immobility time in the TST was significantly reduced by ADDCH1 (15 to 50 mg/kg, i.p.) or ADDCH2 (30 and 50 mg/kg, i.p.). The antidepressant-like effect of ADDCH1 (30 mg/kg, i.p.) in the TST was prevented by pre-treatment of mice with methysergide (2 mg/kg, i.p.), a non-selective serotonin receptor antagonist, p-chlorophenylalanine methylester (pCPA, 100 mg/kg, i.p.), an inhibitor of serotonin synthesis, prazosin (62.5 microg/kg, i.p.), an alpha1-adrenoceptor antagonist, or yohimbine (1 mg/kg, i.p.), an alpha2-adrenoceptor antagonist. In contrast, the antidepressant-like effect of ADDCH2 was antagonized only by yohimbine (1 mg/kg) or haloperidol (50 microg/kg, i.p.), a dopamine D2/D3/D4 receptor antagonist, and was not affected by methysergide, pCPA or prazosin. Altogether, the present results strongly suggest the differential involvement of monoaminergic systems, serotonin/noradrenaline (ADDCH1) and noradrenaline/dopamine (ADDCH2) pathways, respectively, in the antidepressant-like effect of dibenzosuberone compounds.

Yawning and hypothermia in rats: effects of dopamine D3 and D2 agonists and antagonists

RATIONALE

Identification of behaviors specifically mediated by the dopamine D2 and D3 receptors would allow for the determination of in vivo receptor selectivity and aide the development of novel therapeutics for dopamine-related diseases.

OBJECTIVES

These studies were aimed at evaluating the specific receptors involved in the mediation of D2/D3 agonist-induced yawning and hypothermia.

MATERIALS AND METHODS

The relative potencies of a series of D2-like agonists to produce yawning and hypothermia were determined. The ability of D3-selective and D2-selective antagonists to inhibit the induction of yawning and hypothermia were assessed and a series of D2/D3 antagonists were characterized with respect to their ability to alter yawning induced by a low and high dose of PD-128,907 and sumanirole-induced hypothermia.

RESULTS

D3-preferring agonists induced yawning at lower doses than those required to induce hypothermia and the D2-preferring agonist, sumanirole, induced hypothermia at lower doses than were necessary to induce yawning. The rank order of D3 selectivity was pramipexole > PD-128,907 = 7-OH-DPAT = quinpirole = quinelorane > apomorphine = U91,356A. Sumanirole had only D2 agonist effects. PG01,037, SB-277,011A, and U99,194 were all D3-selective antagonists, whereas haloperidol and L-741,626 were D2-selective antagonists and nafadotride's profile of action was more similar to the D2 antagonists than to the D3 antagonists.

CONCLUSIONS

D3 and D2 receptors have specific roles in the mediation of yawning and hypothermia, respectively, and the analysis of these effects allow inferences to be made regarding the selectivity of D2/D3 agonists and antagonists with respect to their actions at D2 and D3 receptors.

N-oleoyl-dopamine increases locomotor activity in the rat

N-oleoyl-dopamine (OLDA), a condensation product of oleic acid and dopamine (DA), is a bioactive lipid whose biological functions are not yet fully explored. The compound crosses the blood-brain barrier and might be considered as a carrier of DA into the brain. In this study we sought to determine whether OLDA would influence locomotor behavior and whether the central DA system would be involved in such influence. We addressed this issue by investigating horizontal locomotor activity in male Wistar rats after intraperitoneal administration of OLDA, 5-20 mg/kg, before and after pre-treatment with haloperidol, a D2 receptor antagonist. We found that OLDA caused a prompt stimulation of locomotor activity, with a bell-shaped dose-response. The maximum stimulatory effect was observed after 10 mg/kg of OLDA where the mean distance traveled by rats during a 2-hour test increased to 1213+/-196(SE) cm from the 403+/-89 cm in the vehicle-treated rats (P<0.05). This effect was dose-dependently antagonized by haloperidol (0.1-0.2 mg/kg). The results support the hypothesis that the OLDA-induced hyperlocomotion was mediated by the stimulation of DA systems. Using in vitro assays, we further demonstrated that OLDA is a stable compound that resists hydrolysis over a 2-hour period and thus the integral OLDA compound exerted DA-like effects. We conclude that OLDA is a potential brain modifier of motor behavior, the biological consequences of which remain to be explored.

Analogues of the dopamine D2 receptor antagonist L741,626: Binding, function, and SAR

A series of analogues of the dopamine D2 receptor antagonist L741,626 were synthesized and evaluated for binding and function at D2 family receptor subtypes. Several analogues showed comparable binding profiles to the parent ligand, however, in general, chemical modification served to reduce D2 binding affinity and selectivity.