Dopamine D4 Ligands and Models of Receptor Activation: 2-(4-Pyridin-2-ylpiperazin-1-ylmethyl)-1H-benzimidazole and Related Heteroarylmethylarylpiperazines Exhibit a Substituent Effect Responsible for Additional Efficacy Tuning

Pharmacology and Therapeutic Potential of Benzothiazole Analogues for Cocaine Use Disorder

Pharmacological targeting of the dopamine D4 receptor (D4R)─expressed in brain regions that control cognition, attention, and decision-making─could be useful for several neuropsychiatric disorders including substance use disorders (SUDs). This study focused on the synthesis and evaluation of a novel series of benzothiazole analogues designed to target D4R. We identified several compounds with high D4R binding affinity (Ki ≤ 6.9 nM) and >91-fold selectivity over other D2-like receptors (D2R, D3R) with diverse partial agonist and antagonist profiles. Novel analogue 16f is a potent low-efficacy D4R partial agonist, metabolically stable in rat and human liver microsomes, and has excellent brain penetration in rats (AUCbrain/plasma > 3). 16f (5-30 mg/kg, i.p.) dose-dependently decreased iv cocaine self-administration in rats, consistent with previous results produced by D4R-selective antagonists. Off-target antagonism of 5-HT2A or 5-HT2B may also contribute to these effects. Results with 16f support further efforts to target D4R in SUD treatment.

Highly Potent and Selective Dopamine D4 Receptor Antagonists Potentially Useful for the Treatment of Glioblastoma

To better understand the role of dopamine D₄ receptor (D₄R) in glioblastoma (GBM), in the present paper, new ligands endowed with high affinity and selectivity for D₄R were discovered starting from the brain penetrant and D₄R selective lead compound 1-(3-(4-phenylpiperazin-1-yl)propyl)-3,4-dihydroquinolin-2(1H)-one (6). In particular, the D₄R antagonist 24, showing the highest affinity and selectivity over D₂R and D₃R within the series (D₂/D₄ = 8318, D₃/D₄ = 3715), and the biased ligand 29, partially activating D₄R G_i-/G_o-protein and blocking β-arrestin recruitment, emerged as the most interesting compounds. These compounds, evaluated for their GBM antitumor activity, induced a decreased viability of GBM cell lines and primary GBM stem cells (GSC#83), with the maximal efficacy being reached at a concentration of 10 μM. Interestingly, the treatment with both compounds 24 and 29 induced an increased effect in reducing the cell viability with respect to temozolomide, which is the first-choice chemotherapeutic drug in GBM.

Dopamine D4 Receptor-Selective Compounds Reveal Structure-Activity Relationships that Engender Agonist Efficacy

The dopamine D4 receptor (D4R) plays important roles in cognition, attention, and decision making. Novel D4R-selective ligands have promise in medication development for neuropsychiatric conditions, including Alzheimer's disease and substance use disorders. To identify new D4R-selective ligands, and to understand the molecular determinants of agonist efficacy at D4R, we report a series of eighteen novel ligands based on the classical D4R agonist A-412997 (1, 2-(4-(pyridin-2-yl)piperidin-1-yl)- N-( m-tolyl)acetamide). Compounds were profiled using radioligand binding displacement assays, β-arrestin recruitment assays, cyclic AMP inhibition assays, and molecular dynamics computational modeling. We identified several novel D4R-selective ( Ki ≤ 4.3 nM and >100-fold vs other D2-like receptors) compounds with diverse partial agonist and antagonist profiles, falling into three structural groups. These compounds highlight receptor-ligand interactions that control efficacy at D2-like receptors and may provide insights into targeted drug discovery, leading to a better understanding of the role of D4Rs in neuropsychiatric disorders.

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

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

A selective role for dopamine D₄ receptors in modulating reward expectancy in a rodent slot machine task

BACKGROUND

Cognitive distortions regarding gambling outcomes confer vulnerability to pathological gambling. Using a rat slot machine task (rSMT), we previously demonstrated that the nonspecific D₂ agonist quinpirole enhances erroneous expectations of reward on near-miss trials, suggesting a pivotal role for the D₂ receptor family in mediating the near-miss effect. Identifying which receptor subtype is involved could facilitate treatment development for compulsive slot machine play.

METHODS

Thirty-two male Long Evans rats learned the rSMT. Three flashing lights could be set to on or off. A win was signaled if all three lights were set to on, whereas any other light pattern indicated a loss. Rats then chose between responding on the collect lever, which delivered 10 sugar pellets on win trials but a 10-second time penalty on loss trials, or to start a new trial instead. Performance was assessed following systemic administration of selective D₂, D₃, and D₄ receptor ligands.

RESULTS

The selective D₂ antagonist L-741,626, the D₃ antagonist SB-277011-A, and the D₃ agonist PD128,907 had no effect. In contrast, the selective D₄ agonist PD168077 partially mimicked quinpirole's effects, increasing erroneous collect responses on nonwin trials, whereas the D₄ antagonist L-745,870 improved the error rate. L-745,870 was also the only antagonist that could attenuate the deleterious effects of quinpirole.

CONCLUSIONS

The dopamine D₄ receptor is critically involved in signaling reward expectancy in the rSMT. The ability of L-745,870 to reduce the classification of losses as wins suggests that D₄ antagonists could be effective in treating problematic slot machine play.

Drug induced increases in CNS dopamine alter monocyte, macrophage and T cell functions: implications for HAND

Central nervous system (CNS) complications resulting from HIV infection remain a major public health problem as individuals live longer due to the success of combined antiretroviral therapy (cART). As many as 70 % of HIV infected people have HIV associated neurocognitive disorders (HAND). Many HIV infected individuals abuse drugs, such as cocaine, heroin or methamphetamine, that may be important cofactors in the development of HIV CNS disease. Despite different mechanisms of action, all drugs of abuse increase extracellular dopamine in the CNS. The effects of dopamine on HIV neuropathogenesis are not well understood, and drug induced increases in CNS dopamine may be a common mechanism by which different types of drugs of abuse impact the development of HAND. Monocytes and macrophages are central to HIV infection of the CNS and to HAND. While T cells have not been shown to be a major factor in HIV-associated neuropathogenesis, studies indicate that T cells may play a larger role in the development of HAND in HIV infected drug abusers. Drug induced increases in CNS dopamine may dysregulate functions of, or increase HIV infection in, monocytes, macrophages and T cells in the brain. Thus, characterizing the effects of dopamine on these cells is important for understanding the mechanisms that mediate the development of HAND in drug abusers.

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.

The identification of structurally novel, selective, orally bioavailable positive modulators of mGluR2

The optimisation of an HTS hit series (1) leading to the identification of structurally novel, selective, orally bioavailable mGluR2 positive modulators GSK1331258 and GSK1331268 is described. Structure-activity relationships, attenuation of dopaminergic activity, and potentiation of mGluR2 responses in rat hippocampal MPP-DG synapses are also reported.

Design, synthesis and dopamine D4 receptor binding activities of new N-heteroaromatic 5/6-ring Mannich bases

A series of phenylpiperazine-methyl-substituted 1H-pyrrolo[2,3-c]pyridine, imidazo[1,2-c]-, pyrrolo[2,3-d]- and pyrrolo[3,2-d]pyrimidines were prepared as selective dopamine D4-ligands. The pyrrolo[2,3-d]pyrimidine derivatives 12d (K(i)=1,9 nM) and 34 d (K(i)=2,4 nM) as well as the pyrrolo[3,2-d]pyrimidine Mannich base 49f (K(i)=2,8 nM) showed high dopamine D4 receptor activity superior to the atypical antipsychotic agent clozapine.

Synthesis and evaluation of ligands for D2-like receptors: the role of common pharmacophoric groups

Arylcycloalkylamines, such as phenyl piperidines and piperazines and their arylalkyl substituents, constitute pharmacophoric groups exemplified in several antipsychotic agents. A review of previous reports indicates that arylalkyl substituents can improve the potency and selectivity of the binding affinity at D(2)-like receptors. In this paper, we explored the contributions of two key pharmacophoric groups, that is, 4'-fluorobutyrophenones and 3-methyl-7-azaindoles, to the potency and selectivity of synthesized agents at D(2)-like receptors. Preliminary observation of binding affinities indicates that there is little predictability of specific effects of the arylalkyl moieties but the composite structure is responsible for selectivity and potency at these receptors.

Structure-activity relationships of melanocortin agonists containing the benzimidazole scaffold

The melanocortin system has been implicated in regulating various physiological processes including pigmentation, energy homeostasis, obesity, steroidogenesis cardiovascular, and exocrine gland function. The five melanocortin receptors that belong to the super family of G protein-coupled receptors are stimulated by naturally occurring agonists. The aim of this research was focused on the design, synthesis, and pharmacological characterization of melanocortin ligands that contain the 1,2,5-trisubstituted benzimidazole scaffold. A series of benzimidazole analogues, with three points of diversity at positions 1, 2, and 5, were designed, synthesized, pharmacologically assayed at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R and resulted in ligands possessing a range of agonist activity from nm to no stimulation at up to 100 microM concentrations. This study demonstrates that the benzimidazole structure template can be appended with key melanocortin agonist amino acids for the design melanocortin receptor agonist ligands.

Discovery of 3-methyl-N-(1-oxy-3',4',5',6'-tetrahydro-2'H-[2,4'-bipyridine]-1'-ylmethyl)benzamide (ABT-670), an orally bioavailable dopamine D4 agonist for the treatment of erectile dysfunction

The goal of this study was to identify a structurally distinct D(4)-selective agonist with superior oral bioavailability to our first-generation clinical candidate 1a (ABT-724) for the potential treatment of erectile dysfunction. Arylpiperazines such as (heteroarylmethyl)piperazine 1a, benzamide 2, and acetamides such as 3a,b exhibit poor oral bioavailability. Structure-activity relationship (SAR) studies with the arylpiperidine template provided potent partial agonists such as 4d and 5k that demonstrated no improvement in oral bioavailability. Further optimization with the (N-oxy-2-pyridinyl)piperidine template led to the discovery of compound 6b (ABT-670), which exhibited excellent oral bioavailability in rat, dog, and monkey (68%, 85%, and 91%, respectively) with comparable efficacy, safety, and tolerability to 1a. The N-oxy-2-pyridinyl moiety not only provided the structural motif required for agonist function but also reduced metabolism rates. The SAR study leading to the discovery of 6b is described herein.

2-[(4-phenylpiperazin-1-yl)methyl]imidazo(di)azines as selective D4-ligands. Induction of penile erection by 2-[4-(2-methoxyphenyl)piperazin-1-ylmethyl]imidazo[1,2-a]pyridine (PIP3EA), a potent and selective D4 partial agonist

A series of novel 2-[(4-phenylpiperazin-1-yl)methyl]imidazoazines and aza-analogues were prepared and screened at selected dopamine, serotonin, and adrenergic receptor subtypes. 2-Substituted imidazopyridines and pyridazines presented high affinities and selectivities for D4 dopamine receptors. Whereas functional experiments indicated neutral antagonists or weak partial agonist effects for most of the target compounds, the 2-methoxyphenyl substituted 2-piperazinylmethylimidazopyridine 3c (PIP3EA) displayed substantial agonist efficacy in mitogenesis experiments and GTPgammaS binding tests, resulting in EC50 values of 3.0 (46%) and 4.5 nM (57%), respectively. Our D4 agonist 3c induced penile erection in vivo when administered to rats. This effect was inhibited by L-745,870 a D4 selective antagonist, confirming the mechanistic pathway.

1-Aryl-3-(4-pyridine-2-ylpiperazin-1-yl)propan-1-one Oximes as Potent Dopamine D4 Receptor Agonists for the Treatment of Erectile Dysfunction

A new series of dopamine D4 receptor agonists, 1-aryl-3-(4-pyridinepiperazin-1-yl)propanone oximes, was designed through the modification of known dopamine D4 receptor agonist PD 168077. Replacement of the amide group with a methylene-oxime moiety produced compounds with improved stability and efficacy. Structure-activity relationsips (SAR) of the aromatic ring linked to the N-4-piperazine ring confirmed the superiority of 2-pyridine as a core for D4 agonist activity. A two-methylene linker between the oxime group and the N-1-piperazine ring displayed the best profile. New dopamine D4 receptor agonists, exemplified by (E)-1-(4-chlorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (59a) and (E)-1-(3-chloro-4-fluorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (64a), exhibited favorable pharmacokinetic profiles and showed oral bioavailability in rat and dog. Subsequent evaluation of 59a in the rat penile erection model revealed in vivo activity, comparable in efficacy to apomorphine. Our results suggest that the oximes provide a novel structural linker for 4-arylpiperazine-based D4 agonists, possessing leadlike quality and with potential to develop a new class of potent and selective dopamine D4 receptor agonists.

Reciprocal mutations in TM2/TM3 in a D2 dopamine receptor background confirms the importance of this microdomain as a selective determinant of para-halogenated 1,4-disubstituted aromatic piperazines

We recently demonstrated that in the D4 dopamine receptor the aromatic microdomain that spans the interface of the second and third transmembrane (TM) domains influences the high affinity interactions of extremely D4-selective ligands possessing a 1,4-disubstituted aromatic piperazine/piperidine (1,4-DAP) structure. On the basis of their substructural features and patterns of sensitivity to mutations constructed in a D4 receptor background, the D4-selective 1,4-DAPs were categorized as having two distinct modes of binding that we named mode-1 and mode-3 [1]. Here we extend these findings of the ligand-receptor structure-affinity relationships for some of these highly D4-selective 1,4-DAPs by measuring the effect of the corresponding reciprocal TM2/TM3 mutations constructed in a D2 dopamine receptor background on the binding affinity of the para-halogenated mode-1 ligands L750,667 and FAUC213. The results indicate that the D2-V2.61F + FV3.28-3.29LM mutant binds L750,667 and FAUC213 with significantly increased affinity, i.e., its binding profile becomes more D4-like. These findings further support the assignment of the TM2/TM3 aromatic microdomain encompassing positions 2.61 and 3.28-3.29 as a 1,4-DAP D4-selectivity microdomain and highlights the importance of the precise emplacement of aromatics in this microdomain as key to the selective molecular recognition of L750,667 and FAUC213.

Synthesis and evaluation of 3-aryl piperidine analogs as potent and efficacious dopamine D4 receptor agonists

A series of 3-aryl piperidine analogs with 2-piperidinoalkylamino or 2-piperidinoalkyloxy fused bicyclic rings were prepared and found to be potent and efficacious human dopamine D4 agonists. The synthesis and structure-activity relationship (SAR) studies that led to the identification of these compounds are discussed.

2-[4-(3,4-Dimethylphenyl)piperazin-1-ylmethyl]-1H benzoimidazole (A-381393), a selective dopamine D4 receptor antagonist

2-[4-(3,4-Dimethylphenlyl)piperazin-1-ylmethyl]-1H benzoimidazole (A-381393) was identified as a potent dopamine D4 receptor antagonist with excellent receptor selectivity. [3H]-spiperone competition binding assays showed that A-381393 potently bound to membrane from cells expressing recombinant human dopamine D4.4 receptor (Ki=1.5 nM), which was 20-fold higher than that of clozapine (Ki=30.4 nM). A-381393 exhibited highly selective binding for the dopamine D4.4 receptor (>2700-fold) when compared to D1, D2, D3 and D5 dopamine receptors. Furthermore, in comparison to clozapine and L-745870, A-381393 exhibits better receptor selectivity, showing no affinity up to 10 microM for a panel of more than 70 receptors and channels, with the exception of moderate affinity for 5-HT2A (Ki=370 nM). A-381393 potently inhibited the functional activity of agonist-induced GTP-gamma-S binding assay and 1 microM dopamine induced-Ca2+ flux in human dopamine D4.4 receptor expressing cells, but not in human dopamine D2L or D3 receptor cells. In contrast to L-745870, A-381393 did not exhibit any significant intrinsic activity in a D4.4 receptor. In vivo, A-381393 has good brain penetration after subcutaneous administration. A-381393 inhibited penile erection induced by the selective D4 agonist PD168077 in conscious rats. Thus, A-381393 is a novel selective D4 antagonist that will enhance the ability to study dopamine D4 receptors both in vitro and in vivo.

Synthesis and activity of 2-[4-(4-[3H]-2-cyanophenyl)piperazinyl]-N-(2,4,6-[3H]3-3-methylphenyl)acetamide: a selective dopamine D4 receptor agonist and radioligand

The first selective dopamine D4 agonist radioligand is described. The synthesis of these piperazine radioligands relied on the transformation of brominated precursors 4a and 4b with tritium gas in the presence of a sensitive cyano functional group. The specific activity of these two radioligands was measured and [3H]6b found to be suitable for use in D4 saturation and competition binding studies. The synthesis, biological, and radioactivity of this new agonist radioligand as well as preliminary SAR will be discussed.

Certain 1,4-Disubstituted Aromatic Piperidines and Piperazines with Extreme Selectivity for the Dopamine D4 Receptor Interact with a Common Receptor Microdomain

We previously demonstrated that, in the D4 dopamine receptor, the aromatic microdomain that spans the interface of the second and third transmembrane segments influences the high-affinity interactions with the D4-selective ligand L750,667 [3-[[4-(4-iodophenyl) piperazin-1-yl]methyl]-1H-pyrrolo[2,3-b]pyridine] and the D2-selective ligands methylspiperone, aripiprazole, and its congener OPC4392 [7-[3-(4-(2,3-dimethylphenyl) piperazinyl) propoxy] 2-(1H)-quinolinone] (Schetz et al., 2000). Here we tested a variety of 1,4-disubstituted aromatic piperidines/piperazines (1,4-DAPs) with different subtype selectivities and functional properties against a panel of D4 receptor mutations in the aromatic microdomain to ascertain whether these ligands recognize this common site. Mutant D4 receptors were constructed by substituting the nonconserved amino acid(s) from the corresponding locations in the D2 receptor. The D4-L2.60W, D4-F2.61V, and D4-LM3.28-3.29FV substitutions result in alterations of the relative position of members of the aromatic microdomain. From these results and molecular models of the ligand-receptor complexes, we conclude that 9 of the 11 D4-selective 1,4-DAPs, including L750,667, have a common pattern of ligand-receptor recognition that depends upon favorable interactions with the phenylalanine at position 2.61 (D4-F2.61V, 20-96-fold decrease). Like methylspiperone, aripiprazole, and OPC4392, the two D4-selective 1,4-DAPs that are insensitive to the D4-F2.61V mutation are sensitive to aromatics at position 2.60 (D4-L2.60W, 7-20-fold increase), and they all have longer spacer arms that permit their tethered aromatics to adopt alternative orientations in the binding-site crevice. All 11 of the D4-selective 1,4-DAPs were sensitive to the D4-LM3.28-3.29FV mutation (13-494-fold decrease) but not the moderately D2-selective methylspiperone. The inferences suggest that subtype selectivity involves two different modes of interaction with the microdomain for the D4-selective 1,4-DAPs and a third mode for D2-selective 1,4-DAPs.

Comparative pharmacology of human dopamine D(2)-like receptor stable cell lines coupled to calcium flux through Galpha(qo5)

The goal of this study was to develop a new approach to study the pharmacology of the dopamine D(4) receptor that could be used in comparative studies with dopamine D(2) and D(3) receptors. Stable HEK-293 cell lines co-expressing recombinant human D(2L), D(3) or D(4) receptors along with Galpha(qo5) cDNA were prepared. Dopamine induced a robust, transient calcium signal in these cell lines with EC(50)s for D(2L), D(3) and D(4) of 18.0, 11.9 and 2.2 nM, respectively. Reported D(4)-selective agonists CP226269 and PD168077 were potent, partial D(4) agonists exhibiting 31-1700-fold selectivity for D(4) over D(3) or D(2). Non-selective D(2)-like agonists apomorphine and quinpirole showed full efficacy but did not discriminate across the three receptors. D(3)-selective agonists 7-hydroxy-DPAT and PD128907 were potent but non-selective D(2)-like agonists. The reported D(3) partial agonist BP-897 exhibited minimal agonist activity at D(3) but was a potent D(3) antagonist and a partial D(4) agonist. Other D(2)-like antagonists, haloperidol, clozapine, and domperidone showed concentration-dependent inhibition of dopamine responses at all three receptors with K(i) ranging from 0.05 to 48.3 nM. The D(3) selective antagonist S33084 and D(4)-selective antagonist L-745870 were highly selective for D(3) and D(4) receptors with K(b) of 0.7 and 0.1 nM, respectively. Stable co-expression of D(2)-like receptors with chimeric Galpha(qo5) proteins in HEK-293 cells is an efficient method to study receptor activation in a common cellular background and an efficient method for direct comparison of ligand affinity and efficacy across human D(2L), D(3) and D(4) receptors.

Discovery of 2-(4-Pyridin-2-ylpiperazin-1-ylmethyl)-1H-benzimidazole (ABT-724), a Dopaminergic Agent with a Novel Mode of Action for the Potential Treatment of Erectile Dysfunction

A new class of agents with potential utility for the treatment of erectile dysfunction has been discovered, guided by the hypothesis that selective D4 agonists are erectogenic but devoid of the side effects typically associated with dopaminergic agents. The lead agent 2-(4-pyridin-2-ylpiperazin-1-ylmethyl)-1H-benzimidazole (1, ABT-724) was discovered by optimization of a series of benzimidazole arylpiperazines. This highly selective D4 agonist was found to be very potent and efficacious in vivo, eliciting penile erections in rats at a dose of 0.03 micromol/kg, with a positive response rate of 77% erectile incidence. Even at high doses, it was devoid of side effects in animal models of central nervous system behaviors, emesis, or nausea. The structure-activity relationship of the parent benzimidazole series leading to 1 is described, with the detailed in vitro and in vivo profiles described. Distinctive structural features were discovered that are associated with D4 selective agonism in this series of analogues.

Synthesis and functional activity of (2-aryl-1-piperazinyl)-N-(3-methylphenyl)acetamides: selective dopamine D4 receptor agonists

Diaryl piperazine acetamides were identified as potent and selective dopamine D(4) receptor agonists. Our strategy is based on an amide bond reversal of an acid sensitive, dopamine D(4) receptor partial agonist, PD 168077. This reversal provided compounds with excellent potency and improved stability. Systematic evaluation of the substitution on the aryl piperazine portion revealed a significant effect on functional activity. The synthesis and biological activity of these new dopamine D(4) agonists is discussed.