Identification of a new selective dopamine D4 receptor ligand

Design, synthesis, and cytotoxicity of ibuprofen-appended benzoxazole analogues against human breast adenocarcinoma

A library of novel ibuprofen-appended benzoxazole analogues (7a-l) was synthesized via a series of nitration, reduction, and condensation-cyclization reactions and screened for their in vitro anticancer activity against human breast cancer MCF-7 and MDA-MB-231 cell lines using doxorubicin as a standard reference. Compounds 7h and 7j displayed outstanding activity against the MCF-7 cell line with an IC50 value of 8.92 ± 0.91 μM and 9.14 ± 8.22 μM, respectively, compared to the doxorubicin IC50 value of 9.29 ± 1.02 μM. Compound 7h also exhibited outstanding activity against the MDA-MB-231 cell line with an IC50 value of 7.54 ± 0.95 μM compared to the doxorubicin IC50 value of 7.68 ± 5.36 μM. Compounds 7h, 7i, 7j, and 7g showed identical morphological changes to those showed by doxorubicin. The molecular docking study against ERα unveiled their best docking scores and binding interactions in agreement to experimental results. Pharmacokinetics prediction envisaged their drug-like properties suitable for therapeutic applications.

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.

Dopamine Receptor Ligand Selectivity-An In Silico/In Vitro Insight

Different dopamine receptor (DR) subtypes are involved in pathophysiological conditions such as Parkinson's Disease (PD), schizophrenia and depression. While many DR-targeting drugs have been approved by the U.S. Food and Drug Administration (FDA), only a very small number are truly selective for one of the DR subtypes. Additionally, most of them show promiscuous activity at related G-protein coupled receptors, thus suffering from diverse side-effect profiles. Multiple studies have shown that combined in silico/in vitro approaches are a valuable contribution to drug discovery processes. They can also be applied to divulge the mechanisms behind ligand selectivity. In this study, novel DR ligands were investigated in vitro to assess binding affinities at different DR subtypes. Thus, nine D2R/D3R-selective ligands (micro- to nanomolar binding affinities, D3R-selective profile) were successfully identified. The most promising ligand exerted nanomolar D3R activity (Ki = 2.3 nM) with 263.7-fold D2R/D3R selectivity. Subsequently, ligand selectivity was rationalized in silico based on ligand interaction with a secondary binding pocket, supporting the selectivity data determined in vitro. The developed workflow and identified ligands could aid in the further understanding of the structural motifs responsible for DR subtype selectivity, thus benefitting drug development in D2R/D3R-associated pathologies such as PD.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

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.

New dual 5-HT1A and 5-HT7 receptor ligands derived from SYA16263

We have previously reported that dual 5-HT_1A and 5-HT₇ receptor ligands might find utility as treatment options for various CNS related conditions including cognitive and anxiolytic impairments. We have also more recently reported that SYA16263 has antipsychotic-like properties with an absence of catalepsy in animal models ascribed to its ability to recruit β-arrestin to the D₂ receptor. However, SYA16263 also binds with very high affinity to 5-HT_1AR (Ki = 1.1 nM) and a moderate affinity at 5-HT₇R (Ki = 90 nM). Thus, it was of interest to exploit its pharmacophore elements in designing new dual receptor ligands. Using SYA16263 as the lead molecule, we have conducted a limited structure-affinity relationship (SAFIR) study by modifying various structural elements in the arylalkyl moiety, resulting in the identification of a new dual 5-HT_1AR and 5-HT₇R ligand, 6-chloro-2-methyl-2-(3-(4-(pyridin-2-yl)piperazin-1-yl)propyl)-2,3-dihydro-1H-inden-1-one (21), which unlike SYA16263, has a sub-nanomolar (5-HT_1AR, Ki = 0.74 nM) and a low nanomolar (5-HT₇R, Ki = 8.4 nM) affinity for these receptors. Interestingly, 21 is a full agonist at 5-HT_1AR and antagonist at the 5-HT₇R, functional characteristics which point to its potential as an antidepressant agent.

A study of the structure-affinity relationship in SYA16263; is a D2 receptor interaction essential for inhibition of apormorphine-induced climbing behavior in mice?

Dopamine (DA) and serotonin (5-HT) receptors are prime targets for the development of antipsychotics. The specific role of each receptor subtype to the pharmacological effects of antipsychotic drugs remains unclear. Understanding the relationship between antipsychotic drugs and their binding affinities at DA and 5-HT receptor subtypes is very important for antipsychotic drug discovery and could lead to new drugs with enhanced efficacies. We have previously disclosed SYA16263 (5) as an interesting compound with moderate radioligand binding affinity at the D₂ & D₃ receptors (Ki = 124 nM & 86 nM respectively) and high binding affinities towards D₄ and 5-HT_1A receptors (Ki = 3.5 nM & 1.1 nM respectively). Furthermore, we have demonstrated SYA16263 (5) is functionally selective and produces antipsychotic-like behavior but without inducing catalepsy in rats. Based on its pharmacological profile, we selected SYA16263 (5) to study its structure-affinity relationship with a view to obtaining new analogs that display receptor subtype selectivity. In this study, we present the synthesis of structurally modified SYA16263 (5) analogs and their receptor binding affinities at the DA and 5-HT receptor subtypes associated with antipsychotic action. Furthermore, we have identified compound 21 with no significant binding affinity at the D₂ receptor subtype but with moderate binding affinity at the D₃ and D₄ receptors subtypes. However, because 21 is able to demonstrate antipsychotic-like activity in a preliminary test, using the reversal of apomorphine-induced climbing behavior experiment in mice with SYA16263 and haloperidol as positive controls, we question the essential need of the D₂ receptor subtype in reversing apomorphine-induced climbing behavior.

Recent findings leading to the discovery of selective dopamine D4 receptor ligands for the treatment of widespread diseases

Since its discovery, the dopamine D₄ receptor (D₄R) has been suggested to be an attractive target for the treatment of neuropsychiatric diseases. Novel findings have renewed the interest in such a receptor as an emerging target for the management of different diseases, including cancer, Parkinson's disease, alcohol or substance use disorders, eating disorders, erectile dysfunction and cognitive deficits. The recently resolved crystal structures of D₄R in complexes with the potent ligands nemonapride and L-745870 strongly improved the knowledge on the molecular mechanisms involving the D₄R functions and may help medicinal chemists in drug design. This review is focused on the recent development of the subtype selective D₄R ligands belonging to classical or new chemotypes. Moreover, ligands showing functional selectivity toward G protein activation or β-arrestin recruitment and the effects of selective D₄R ligands on the above-mentioned diseases are discussed.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

New analogs of SYA013 as sigma-2 ligands with anticancer activity

Our previous study has revealed 4-(4-(4-chlorophenyl)-1,4-diazepan-1-yl)-1-(4-fluorophenyl)butan-1-one·2HCl (SYA013) 1 as a sigma ligand with moderate selectivity for the sigma-2 receptor. Given the overexpression of sigma receptors in solid tumors and reports of sigma ligands with anticancer activities, we selected 1 for evaluation in several solid tumor cell lines. In addition, we have synthesized new analogs of 1 and now report that several of them bind preferentially at the sigma-2 receptor and have shown inhibition of several cancer cell lines including MDA-MB-231, MDA-MB-486, A549, PC-3, MIA PaCa-2 and Panc-1 cells. In particular, compounds 1 and 12 have demonstrated sub-micromolar activity against the Panc-1 cell line. It has also been observed that several of these compounds demonstrate selective toxicity toward cancer cells, when compared to normal cells.

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 Complete Assessment of Dopamine Receptor- Ligand Interactions through Computational Methods

Background: Selectively targeting dopamine receptors (DRs) has been a persistent challenge in the last years for the development of new treatments to combat the large variety of diseases involving these receptors. Although, several drugs have been successfully brought to market, the subtype-specific binding mode on a molecular basis has not been fully elucidated. Methods: Homology modeling and molecular dynamics were applied to construct robust conformational models of all dopamine receptor subtypes (D₁-like and D₂-like). Fifteen structurally diverse ligands were docked. Contacts at the binding pocket were fully described in order to reveal new structural findings responsible for selective binding to DR subtypes. Results: Residues of the aromatic microdomain were shown to be responsible for the majority of ligand interactions established to all DRs. Hydrophobic contacts involved a huge network of conserved and non-conserved residues between three transmembrane domains (TMs), TM2-TM3-TM7. Hydrogen bonds were mostly mediated by the serine microdomain. TM1 and TM2 residues were main contributors for the coupling of large ligands. Some amino acid groups form electrostatic interactions of particular importance for D₁R-like selective ligands binding. Conclusions: This in silico approach was successful in showing known receptor-ligand interactions as well as in determining unique combinations of interactions, which will support mutagenesis studies to improve the design of subtype-specific ligands.

SYA 013 analogs as moderately selective sigma-2 (σ2) ligands: Structure-affinity relationship studies

Several lines of evidence suggest that selective sigma-2 (σ₂) ligands might be useful for the treatment of solid tumors. However, very few selective σ₂ ligands have been identified. This study was aimed at identifying new selective σ₂ receptor ligands using a previously identified agent, SYA 013 as a lead. Four groups, homopiperazine, piperazine, tropane and selected oxime analogs of the homopiperazines were identified, synthesized and subsequently screened at the σ₁ and σ₂ receptors. The results demonstrate that these scaffolds can be modified to obtain selective σ₂ receptor ligands. 1-(5-Chloropyridin-2-yl)-4-(3-((4-fluorophenyl)thio)propyl)-1,4-diazepane, 7 and 3-(4-chlorophenyl)-8-(3-((2-fluorophenyl)thio)propyl)-8-azabicyclo[3.2.1]octan-3-ol, 21 were identified as the highest binding affinity ligands (σ₂Ki = 2.2 nM) and (4-(4-(5-chloropyridin-2-yl)-1,4-diazepan-1-yl)-1-(4-fluorophenyl)-butan-1-one oxime, 22 as a high affinity and the most selective ligand for the σ₂ receptor (σ₁Ki/σ₂Ki = 41.8).

Synthesis and evaluation of the structural elements in alkylated tetrahydroisoquinolines for binding to CNS receptors

Diseases of the CNS are often complex and involve multiple receptor systems and thus, the treatment options for these diseases must focus on targeting the multiple receptors implicated in the various disorders. Schizophrenia and depression are examples of such diseases and their pharmacotherapy thus depends on agents which target multiple receptors including the dopamine, serotonin and even cholinergic receptors at the same time. In our previous campaign to find multi-receptor ligands, we have identified the benzothiazole 1a as an initial lead molecule. In the current work, we have expanded the structure affinity relationship (SAFIR) of 1a resulting in the identification of a partially restrained butyrophenone 3j as a potent and selective dual 5-HT1A and 5-HT7 receptor ligand. It is expected that compound 3j may serve as a new lead for further development in our search for newer and novel ligands with the potential to treat diseases of CNS origin.

Synthesis and in vitro evaluation of no-carrier-added 2-(3-(4-(4-[18F]fluorobenzyl)piperazin-1-yl)propyl)benzo[d]thiazole, a potential dopamine D4 receptor radioligand

The dopamine D4 receptor has been shown to play important roles in some central nervous system pathologies. Specific radioligands for the D4 receptor may be useful to understand the function of the D4 receptor and its correlations with various disorders. 2-(3-(4-(4-[18F]Fluorobenzyl)piperazin-1-yl)propyl)benzo[d]thiazole ([18F]4) was synthesized through a one-pot two-step procedure with total yield 18.6% (decay corrected). The specific activity of the radioligand was 112 GBq/μmol and its radiochemical purity was >95.0%. Its affinity and selectivity for dopamine D2-like receptors were measured through in vitro receptor binding evaluation and the K i value for the D4 receptor was determined to be 2.9±0.2 nM, and its selectivity for the dopamine D4 receptor is 709-fold versus D2long receptor, 823-fold versus D3 receptor. The partition coefficient (Log D) of it was determined to be 2.6±0.1 through octanol-water partition experiment. The ligand presents desirable combination of lipophilicity, affinity and selectivity for the dopamine D4 receptor. The results suggested that the radioligand shows promises for the in vivo study of the dopamine D4 receptor.

Development of CNS multi-receptor ligands: Modification of known D2 pharmacophores

Several known D2 pharmacophores have been explored as templates for identifying ligands with multiple binding affinities at dopamine and serotonin receptors considered as clinically relevant receptors in the treatment of neuropsychiatric diseases. This approach has resulted in the identification of ligands that target multiple CNS receptors while avoiding others associated with deleterious effects. In particular, compounds 11, 15 and 22 may have potential for further development as antipsychotic agents as they favorably interact with the clinically relevant receptors including D2R, 5-HT1AR, and 5-HT7R. We have also identified the pair of compounds 11 and 10 as high affinity D2R ligands with and without SERT binding affinities, respectively. These differential binding profiles endow the pair with the potential for evaluating SERT contributions to antipsychotic drug activity in animal behavioral models. In addition, compound 11 has no significant affinity for 5-HT2CR and binds only moderately to the H1R, suggesting it may not induce weight gain or sedation when used clinically. Taken together, compound 11 displays an interesting pharmacological profile that necessitates the evaluation of its functional and in vivo effects in animal models which are currently ongoing.

Expression of a Novel D4 Dopamine Receptor in the Lamprey Brain. Evolutionary Considerations about Dopamine Receptors

Numerous data reported in lampreys, which belong to the phylogenetically oldest branch of vertebrates, show that the dopaminergic system was already well developed at the dawn of vertebrate evolution. The expression of dopamine in the lamprey brain is well conserved when compared to other vertebrates, and this is also true for the D2 receptor. Additionally, the key role of dopamine in the striatum, modulating the excitability in the direct and indirect pathways through the D1 and D2 receptors, has also been recently reported in these animals. The moment of divergence regarding the two whole genome duplications occurred in vertebrates suggests that additional receptors, apart from the D1 and D2 previously reported, could be present in lampreys. We used in situ hybridization to characterize the expression of a novel dopamine receptor, which we have identified as a D4 receptor according to the phylogenetic analysis. The D4 receptor shows in the sea lamprey a more restricted expression pattern than the D2 subtype, as reported in mammals. Its main expression areas are the striatum, lateral and ventral pallial sectors, several hypothalamic regions, habenula, and mesencephalic and rhombencephalic motoneurons. Some expression areas are well conserved through vertebrate evolution, as is the case of the striatum or the habenula, but the controversies regarding the D4 receptor expression in other vertebrates hampers for a complete comparison, especially in rhombencephalic regions. Our results further support that the dopaminergic system in vertebrates is well conserved and suggest that at least some functions of the D4 receptor were already present before the divergence of lampreys.

Thiazole: a promising heterocycle for the development of potent CNS active agents

Thiazole is a valuable scaffold in the field of medicinal chemistry and has accounted to display a variety of biological activities. Thiazole and its derivatives have attracted continuing interest to design various novel CNS active agents. In the past few decades, thiazoles have been widely used to develop a variety of therapeutic agents against numerous CNS targets. Thiazole containing drug molecules are currently being used in treatment of various CNS disorders and a number of thiazole derivatives are also presently in clinical trials. A lot of research has been carried out on thiazole and their analogues, which has proved their efficacy to overcome several CNS disorders in rodent as well as primate models. The aim of present review is to highlights diverse CNS activities displayed by thiazole and their derivatives. SAR of this nucleus has also been well discussed. This review covers the recent updates present in literature and will surely provide a greater insight for the designing and development of potent thiazole based CNS active agents in future.