N-(4-(4-(2,3-dichloro- or 2-methoxyphenyl)piperazin-1-yl)butyl)heterobiarylcarboxamides with functionalized linking chains as high affinity and enantioselective D3 receptor antagonists

Quinoline- and isoquinoline-sulfonamide analogs of aripiprazole: novel antipsychotic agents?

The introduction of typical antipsychotics over six decades ago signaled an important milestone in psychiatry. However, second-generation antipsychotics ameliorated the positive symptoms of schizophrenia but displayed limited effectiveness for the negative and cognitive symptoms. In addition, while the newer antipsychotics produced fewer motor side effects, the atypical antipsychotics still induced weight gain and endocrinopathies. In recent years, a third generation of antipsychotics was identified. Aripiprazole was the first approved drug acting as a D2 partial agonist/functionally selective ligand. This review presents the state of the development of novel antipsychotic dopaminergic and non-dopaminergic agents, supported by an overview of the compounds evaluated under advanced preclinical and clinical development (e.g., cariprazine and brexpiprazole). In line with the recent trends in the development of modern atypical antipsychotics, we present our strategic development of long-chain arylpiperazine-derived quinoline- and isoquinoline-sulfonamide displaying a multireceptor binding profile and partial D2 receptor agonism.

Biodistribution studies of two 18F-labeled pyridinylphenyl amides as subtype selective radioligands for the dopamine D3 receptor

INTRODUCTION

Dopamine D3 receptors are implicated in various neuropsychiatric diseases, drug abuse and alcoholism, but specific agents for D3 molecular imaging are lacking. We evaluated two in vitro selective fluorine-18-labeled radioligand candidates ([(18)F]5 and [(18)F]6) for positron emission tomography (PET) imaging of D3 receptor availability in the brain.

METHODS

Biodistribution was evaluated in Sprague-Dawley rats using ex vivo autoradiography and small-animal PET. Protein binding studies were conducted in human plasma and cerebrospinal fluid.

RESULTS

[(18)F]5 showed rapid blood-brain barrier penetration and fast washout after intravenous injection, whereas the rat brain penetration of [(18)F]6 was lower. The total distribution volume (VT) of [(18)F]5 was 20-26 mL g(-1) throughout brain. Co-injection with the D3 antagonist BP897 resulted in globally increased cerebral washout of [(18)F]5 and [(18)F]6, but SUV analysis and parametric mapping of binding potential (BPND) relative to the cerebellum did not reveal specific binding of either ligand in D3-rich brain regions, i.e. the ventral striatum. However, there was substantial displaceable binding of [(18)F]5, and to a lesser extent [(18)F]6, in the pituitary.

CONCLUSION

These radioligands reveal dopamine D3 receptors in the pituitary, but are not suitable for PET imaging of in brain, possibly due to low specific signal relative to the globally high VT.

Design and synthesis of 1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile (citalopram) analogues as novel probes for the serotonin transporter S1 and S2 binding sites

The serotonin transporter (SERT) is the primary target for antidepressant drugs. The existence of a high affinity primary orthosteric binding site (S1) and a low affinity secondary site (S2) has been described, and their relation to antidepressant pharmacology has been debated. Herein, structural modifications to the N, 4, 5, and 4' positions of (±)citalopram (1) are reported. All of the analogues were SERT-selective and demonstrated that steric bulk was tolerated at the SERT S1 site, including two dimeric ligands (15 and 51). In addition, eight analogues were identified with similar potencies to S-1 for decreasing the dissociation of [(3)H]S-1 from the S1 site via allosteric modulation at S2. Both dimeric compounds had similar affinities for the SERT S1 site (Ki = 19.7 and 30.2 nM, respectively), whereas only the N-substituted analogue, 51, was as effective as S-1 in allosterically modulating the binding of [(3)H]S-1 via S2.

A single glycine in extracellular loop 1 is the critical determinant for pharmacological specificity of dopamine D2 and D3 receptors

Subtype-selective agents for the dopamine D3 receptor (D3R) have been considered as potential medications for drug addiction and other neuropsychiatric disorders. Medicinal chemistry efforts have led to the discovery of 4-phenylpiperazine derivatives that are >100-fold selective for the dopamine D3 receptor over dopamine D2 receptor (D2R), despite high sequence identity (78% in the transmembrane domain). Based on the recent crystal structure of D3R, we demonstrated that the 4-phenylpiperazine moiety in this class of D3R-selective compounds binds to the conserved orthosteric binding site, whereas the extended aryl amide moiety is oriented toward a divergent secondary binding pocket (SBP). In an effort to further characterize molecular determinants of the selectivity of these compounds, we modeled their binding modes in D3R and D2R by comparative ligand docking and molecular dynamics simulations. We found that the aryl amide moiety in the SBP differentially induces conformational changes in transmembrane segment 2 and extracellular loop 1 (EL1), which amplify the divergence of the SBP in D3R and D2R. Receptor chimera and site-directed mutagenesis studies were used to validate these binding modes and to identify a divergent glycine in EL1 as critical to D3R over D2R subtype selectivity. A better understanding of drug-dependent receptor conformations such as these is key to the rational design of compounds targeting a specific receptor among closely related homologs, and may also lead to discovery of novel chemotypes that exploit subtle differences in protein conformations.

Synthesis and in vitro pharmacological evaluation of indolyl carboxylic amide analogues as D3 dopamine receptor selective ligands

A series of substituted 1H-indolyl carboxylic acid amides that contain a N-(2-methoxyphenyl)piperazine or N-(2-fluoroethoxy)piperazine group were synthesized and their affinities for human dopamine D2, D3, and D4 receptors were determined. Two of these compounds, 14a and 14b, displayed high binding affinity at D3 (Ki = 0.18 and 0.4 nM, respectively), and selectivity for D3vs. D2 receptors (87-fold and 60-fold, respectively). These two compounds had low binding affinity at D4 receptors and σ receptor sites. The intrinsic activity of these compounds at D2 and D3 receptors was determined using a forskolin-dependent adenylyl cyclase inhibition assay; both 14a and 14b were found to be partial agonists. Furthermore, for compound 14a, the log D value of 2.85 suggested it has suitable lipophilicity for crossing the blood-brain-barrier.

Effects of chronic buspirone treatment on nicotine and concurrent nicotine+cocaine self-administration

Nicotine dependence and cocaine abuse are major public health problems, and most cocaine abusers also smoke cigarettes. An ideal pharmacotherapy would reduce both cigarette smoking and cocaine abuse. Buspirone (Buspar) is a clinically available, non-benzodiazepine anxiolytic medication that acts on serotonin and dopamine systems. In preclinical studies, it reduced cocaine self-administration following both acute and chronic treatment in rhesus monkeys. The present study evaluated the effectiveness of chronic buspirone treatment on self-administration of intravenous (IV) nicotine and IV nicotine+cocaine combinations. Five cocaine-experienced adult rhesus monkeys (Macaca mulatta) were trained to self-administer nicotine or nicotine+cocaine combinations, and food pellets (1 g) during four 1-h daily sessions under a second-order schedule of reinforcement (FR 2 (VR16:S)). Each nicotine+cocaine combination maintained significantly higher levels of drug self-administration than nicotine or cocaine alone (P<0.05-0.001). Buspirone (0.032-0.56 mg/kg/h) was administered IV through one lumen of a double-lumen catheter every 20 min for 23 h each day, for 7-10 consecutive days. Each 7-10-day sequence of buspirone treatment was followed by saline-control treatment for at least 3 days until food- and drug-maintained responding returned to baseline. Buspirone dose-dependently reduced responding maintained by nicotine alone (0.001-0.1 mg/kg/inj; P<0.01) and by nicotine (0.001 or 0.0032 mg/kg/inj)+cocaine combinations (0.0032 mg/kg/inj; P<0.05-0.001) with no significant effects on food-maintained responding. We conclude that buspirone selectively attenuates the reinforcing effects of nicotine alone and nicotine+cocaine polydrug combinations in a nonhuman primate model of drug self-administration.

Evaluation of N-phenyl homopiperazine analogs as potential dopamine D3 receptor selective ligands

A series of N-(2-methoxyphenyl)homopiperazine analogs was prepared and their affinities for dopamine D2, D3, and D4 receptors were measured using competitive radioligand binding assays. Several ligands exhibited high binding affinity and selectivity for the D3 dopamine receptor compared to the D2 receptor subtype. Compounds 11a, 11b, 11c, 11f, 11j and 11k had K(i) values ranging from 0.7 to 3.9 nM for the D3 receptor with 30- to 170-fold selectivity for the D3 versus D2 receptor. Calculated logP values (logP=2.6-3.6) are within the desired range for passive transport across the blood-brain barrier. When the binding and the intrinsic efficacy of these phenylhomopiperazines was compared to those of previously published phenylpiperazine analogues, it was found that (a) affinity at D2 and D3 dopamine receptors generally decreased, (b) the D3 receptor binding selectivity (D2:D3 K(i) value ratio) decreased and, (c) the intrinsic efficacy, measured using a forskolin-dependent adenylyl cyclase inhibition assay, generally increased.

High-affinity and selective dopamine D₃ receptor full agonists

We have designed, synthesized and evaluated a series of new compounds with the goal to identify potent and selective D(3) ligands. The two most potent and selective new D(3) ligands are compounds 38 and 52, which bind to the D(3) receptors with a K(i) value of <nM and display a selectivity of 450-494 times over the D(2) receptors and >10,000 times over the D(1) receptors. Both 38 and 52 are full agonists with high potency at the D(3) receptor in a D(3) functional assay.

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.

Ligand discovery from a dopamine D3 receptor homology model and crystal structure

G protein-coupled receptors (GPCRs) are intensely studied as drug targets and for their role in signaling. With the determination of the first crystal structures, interest in structure-based ligand discovery increased. Unfortunately, for most GPCRs no experimental structures are available. The determination of the D(3) receptor structure and the challenge to the community to predict it enabled a fully prospective comparison of ligand discovery from a modeled structure versus that of the subsequently released crystal structure. Over 3.3 million molecules were docked against a homology model, and 26 of the highest ranking were tested for binding. Six had affinities ranging from 0.2 to 3.1 μM. Subsequently, the crystal structure was released and the docking screen repeated. Of the 25 compounds selected, five had affinities ranging from 0.3 to 3.0 μM. One of the new ligands from the homology model screen was optimized for affinity to 81 nM. The feasibility of docking screens against modeled GPCRs more generally is considered.

CJ-1639: A Potent and Highly Selective Dopamine D3 Receptor Full Agonist

We have identified several ligands with high binding affinities to the dopamine D3 receptor and excellent selectivity over the D2 and D1 receptors. CJ-1639 (17) binds to the D3 receptor with a K(i) value of 0.50 nM and displays a selectivity of >5,000 times over D2 and D1 receptors in binding assays using dopamine receptors expressed in the native rat brain tissues. CJ-1639 binds to human D3 receptor with a K(i) value of 3.61 nM and displays over >1000-fold selectivity over human D1 and D2 receptors. CJ-1639 is active at 0.01 mg/kg at the dopamine D3 receptor in the rat and only starts to show a modest D2 activity at doses as high as 10 mg/kg. CJ-1639 is the most potent and selective D3 full agonist reported to date.

N-(3-fluoro-4-(4-(2-methoxy or 2,3-dichlorophenyl)piperazine-1-yl)butyl)arylcarboxamides as selective dopamine D3 receptor ligands: critical role of the carboxamide linker for D3 receptor selectivity

N-(3-fluoro-4-(4-(2,3-dichloro- or 2-methoxyphenyl)piperazine-1-yl)butyl)arylcarboxamides were prepared and evaluated for binding and function at dopamine D3 receptors (D3Rs) and dopamine D2 receptors (D2Rs). In this series, we discovered some of the most D3R selective compounds reported to date (e.g., 8d and 8j, >1000-fold D3R-selective over D2R). In addition, chimeric receptor studies further identified the second extracellular (E2) loop as an important contributor to D3R binding selectivity. Further, compounds lacking the carbonyl group in the amide linker were synthesized, and while these amine-linked analogues bound with similar affinities to the amides at D2R, this modification dramatically reduced binding affinities at D3R by >100-fold (e.g., D3R K(i) for 15b = 393 vs for 8j = 2.6 nM), resulting in compounds with significantly reduced D3R selectivity. This study supports a pivotal role for the D3R E2 loop and the carbonyl group in the 4-phenylpiperazine class of compounds and further reveals a point of separation between structure-activity relationships at D3R and D2R.

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.

Recent advances in the search for D3- and D4-selective drugs: probes, models and candidates

Dopamine D(2)-like receptors (including D(2), D(3) and D(4)) belong to the 'rhodopsin-like' family of G protein-coupled receptors (GPCRs), which represent the largest group of targets for bioactive molecules. Due to their high sequence similarity, the design of subtype-selective ligands requires rational and effective strategies. The general formula of 1,4-disubstituted aromatic piperidines and piperazines (1,4-DAPs) was extracted from classical dopaminergic drugs. The biological properties of this compound family are encoded by an aromatic head group that controls intrinsic activity, an amine moiety and a lipophilic appendage. D(3)- and D(4)-selective molecular probes and drug candidates were generated from the general formula of 1,4-DAP. Formal structural rearrangement led to investigational drugs beyond the 1,4-DAP structure. The very recent publication of the X-ray crystal structure of D(3) should facilitate efficient discovery of unprecedented chemotypes. However, the development of D(3)-selective agonists, functionally selective ligands and the exploitation of homo- and heteromers remain challenging.

Structure of the human dopamine D3 receptor in complex with a D2/D3 selective antagonist

Dopamine modulates movement, cognition, and emotion through activation of dopamine G protein-coupled receptors in the brain. The crystal structure of the human dopamine D3 receptor (D3R) in complex with the small molecule D2R/D3R-specific antagonist eticlopride reveals important features of the ligand binding pocket and extracellular loops. On the intracellular side of the receptor, a locked conformation of the ionic lock and two distinctly different conformations of intracellular loop 2 are observed. Docking of R-22, a D3R-selective antagonist, reveals an extracellular extension of the eticlopride binding site that comprises a second binding pocket for the aryl amide of R-22, which differs between the highly homologous D2R and D3R. This difference provides direction to the design of D3R-selective agents for treating drug abuse and other neuropsychiatric indications.

Synthesis, 3D-QSAR, and structural modeling of benzolactam derivatives with binding affinity for the D(2) and D(3) receptors

A series of 37 benzolactam derivatives were synthesized, and their respective affinities for the dopamine D(2) and D(3) receptors evaluated. The relationships between structures and binding affinities were investigated using both ligand-based (3D-QSAR) and receptor-based methods. The results revealed the importance of diverse structural features in explaining the differences in the observed affinities, such as the location of the benzolactam carbonyl oxygen, or the overall length of the compounds. The optimal values for such ligand properties are slightly different for the D(2) and D(3) receptors, even though the binding sites present a very high degree of homology. We explain these differences by the presence of a hydrogen bond network in the D(2) receptor which is absent in the D(3) receptor and limits the dimensions of the binding pocket, causing residues in helix 7 to become less accessible. The implications of these results for the design of more potent and selective benzolactam derivatives are presented and discussed.

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.

The dopamine D3 receptor partial agonist CJB090 and antagonist PG01037 decrease progressive ratio responding for methamphetamine in rats with extended-access

Previous work suggests a role for dopamine D3-like receptors in psychostimulant reinforcement. The development of new compounds acting selectively at dopamine D3 receptors has opened new possibilities to explore the role of these receptors in animal models of psychostimulant dependence. Here we investigated whether the dopamine D3 partial agonist CJB090 (1-10 mg/kg, i.v) and the D3 antagonist PG01037 (8-32 mg/kg, s.c.) modified methamphetamine (0.05 mg/kg/injection) intravenous self-administration under fixed- (FR) and progressive- (PR) ratio schedules in rats allowed limited (short access, ShA; 1-hour sessions 3 days/week) or extended access (long access, LgA; 6 hour sessions 6 days/week). Under a FR1 schedule, the highest dose of the D3 partial agonist CJB090 selectively reduced methamphetamine self-administration in LgA but not in ShA rats, whereas the full D3 antagonist PG01037 produced no effect in either group. Under a PR schedule of reinforcement, the D3 partial agonist CJB090 reduced the maximum number of responses performed ('breakpoint') for methamphetamine in LgA rats at the doses of 5 and 10 mg/kg, and also it produced a significant reduction in the ShA group at the highest dose. However, the D3 full antagonist PG01037 only reduced PR methamphetamine self-administration in LgA rats at the highest dose of 32 mg/kg with no effect in the ShA group. The results suggest that rats might be more sensitive to pharmacological modulation of dopamine D3 receptors following extended access to methamphetamine self-administration, opening the possibility that D3 receptors play a role in excessive methamphetamine intake.

Characterization of the transport, metabolism, and pharmacokinetics of the dopamine D3 receptor-selective fluorenyl- and 2-pyridylphenyl amides developed for treatment of psychostimulant abuse

The recent discovery of novel high-affinity and selective dopamine D3 receptor (DA D3R) antagonists and partial agonists has provided tools with which to further elucidate the role DA D3R plays in substance abuse. The present study was conducted to evaluate the transport, metabolism, pharmacokinetics, and brain uptake of the DA D3R-selective fluorenyl amides, NGB 2904 [N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-9H-fluorene-2-carboxamide] fumarate) and JJC 4-077 [N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)-3-hydroxybutyl)-9H-fluorene-2-carboxamide hydrochloride], and the 2-pyridylphenyl amides, CJB 090 [N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridine-2-yl)benzamide hydrochloride] and PG 01037 [N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)-trans-but-2-enyl)-4-(pyridine-2-yl)benzamide hydrochloride], all of which have been studied in animal models of psychostimulant abuse. Additional screening with a panel of human and rat Supersomes was performed for NGB 2904 and PG 01037. Drug-stimulated ATPase activation assays and bidirectional transport and efflux assays were used to test for substrate specificity of NGB 2904 and PG 01037 for human and rat efflux transporters. All compounds exhibited moderate elimination half-lives, ranging from 1.49 to 3.27 h, and large volumes of distribution (5.95-14.19 l/kg). The brain-to-plasma ratios ranged from 2.93 to 11.81 and were higher than those previously reported for cocaine. Brain exposure levels of NGB 2904 and PG 01037 were significantly reduced after intraperitoneal administration compared with intravenous administration. The metabolism of these compounds was mediated primarily by CYP3A subfamilies. PG 01037 was a P-glycoprotein-transported substrate. Higher doses of these compounds are often required for in vivo action, suggesting decreased bioavailability via extravascular administration that may be attributed to high drug efflux and hepatic metabolism. These studies provide important preclinical information for optimization of next-generation D3R selective agents for the treatment of drug addiction.

Development of (S)-N6-(2-(4-(isoquinolin-1-yl)piperazin-1-yl)ethyl)-N6-propyl-4,5,6,7-tetrahydrobenzo[d]-thiazole-2,6-diamine and its analogue as a D3 receptor preferring agonist: potent in vivo activity in Parkinson's disease animal models

Here we report structure-activity relationship study of a novel hybrid series of compounds where structural alteration of aromatic hydrophobic moieties connected to the piperazine ring and bioisosteric replacement of the aromatic tetralin moieties were carried out. Binding assays were carried out with HEK-293 cells expressing either D2 or D3 receptors with tritiated spiperone to evaluate inhibition constants (K(i)). Functional activity of selected compounds in stimulating GTPgammaS binding was assessed with CHO cells expressing human D2 receptors and AtT-20 cells expressing human D3 receptors. SAR results identified compound (-)-24c (D-301) as one of the lead molecules with preferential agonist activity for D3 receptor (EC(50) (GTP gamma S); D3 = 0.52 nM; D2/D3 (EC(50)): 223). Compounds (-)-24b and (-)-24c exhibited potent radical scavenging activity. The two lead compounds, (-)-24b and (-)-24c, exhibited high in vivo activity in two Parkinson's disease (PD) animal models, reserpinized rat model and 6-OHDA induced unilaterally lesioned rat model. Future studies will explore potential use of these compounds in the neuroprotective therapy for PD.

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.

The selective dopamine D3 receptor antagonist SB 277011-A, but not the partial agonist BP 897, blocks cue-induced reinstatement of nicotine-seeking

The dopamine D3 receptor (DRD3) has been suggested to be involved in the mechanisms underlying stimulus-controlled drug-seeking behaviour. Ligands acting as DRD3 antagonists (SB 277011-A) or DRD3 partial agonists (BP 897) have shown some promise for reducing the influence of drug-associated cues on motivational behaviour. Here, effects of SB 277011-A and BP 897 were evaluated on cue-induced reinstatement of nicotine-seeking in rats. The effects of BP 897 on nicotine self-administration under a fixed-ratio 5 (FR5) schedule of reinforcement were also evaluated. SB 277011-A (1-10 mg/kg) was able to block cue-induced reinstatement of nicotine-seeking, indicating that DRD3 selective antagonism may be an effective approach to prevent relapse for nicotine. In contrast, BP 897 did not block the cue-induced reinstatement of nicotine-seeking or nicotine-taking under the FR5 schedule. In a control study, rats did not respond to the light stimuli without nicotine delivery, indicating that the responding for the drug-associated cues was induced by the previous pairing of light stimuli with nicotine's effects. These findings validate the role of DRD3 on reactivity to drug-associated stimuli and suggest that the DRD3 antagonist, but perhaps not the DRD3 partial agonist, could be used to prevent relapse in tobacco smokers.

Current perspectives on selective dopamine D(3) receptor antagonists as pharmacotherapeutics for addictions and related disorders

Repeated exposure to drugs of abuse produces long-term molecular and neurochemical changes that may explain the core features of addiction, such as the compulsive seeking and taking of the drug, as well as the risk of relapse. A growing number of new molecular and cellular targets of addictive drugs have been identified, and rapid advances are being made in relating those targets to specific behavioral phenotypes in animal models of addiction. In this context, the pattern of expression of the dopamine (DA) D(3) receptor in the rodent and human brain and changes in this pattern in response to drugs of abuse have contributed primarily to direct research efforts toward the development of selective DA D(3) receptor antagonists. Growing preclinical evidence indicates that these compounds may actually regulate the motivation to self-administer drugs and disrupt drug-associated cue-induced craving. This report will be divided into three parts. First, preclinical evidence in support of the efficacy of selective DA D(3) receptor antagonists in animal models of drug addiction will be reviewed. The effects of mixed DA D(2)/D(3) receptor antagonists will not be discussed here because most of these compounds have low selectivity at the D(3) versus D(2) receptor, and their efficacy profile is related primarily to functional antagonism at D(2) receptors and possibly interactions with other neurotransmitter systems. Second, major advances in medicinal chemistry for the identification and optimization of selective DA D(3) receptor antagonists and partial agonists will be analyzed. Third, translational research from preclinical efficacy studies to so-called proof-of-concept studies for drug addiction indications will be discussed.