Atropisomeric derivatives of 2',6'-disubstituted (R)-11-phenylaporphine: selective serotonin 5-HT(7) receptor antagonists

Stereoselective Synthesis Axially Chiral Arylnitriles through Base-Induced Chirality-Relay β-Carbon Elimination of α-Hydroxyl Ketoxime Esters

We report herein a point-to-axial chirality transfer reaction of α-hydroxyl oxime esters for the synthesis of axially chiral arylnitriles. The reaction proceeds smoothly through a base-promoted retro-benzoin condensation reaction of α-hydroxyl oxime esters, where the axial chirality is created via the C-C bond cleavage based on a proper distorted conformation of the biaryl structure induced by its stereogenic carbon center.

Aporphines: A privileged scaffold in CNS drug discovery

Aporphine alkaloids embedded in 4H-dibenzo[de,g]quinoline four-ring structures belong to one of the largest subclasses of isoquinoline alkaloids. Aporphine is a privileged scaffold in the field of organic synthesis and medicinal chemistry for the discovery of new therapeutic agents for central nervous system (CNS) diseases, cancer, metabolic syndrome, and other diseases. In the past few decades, aporphine has attracted continuing interest to be widely used to develop selective or multitarget directed ligands (MTDLs) targeting the CNS (e.g., dopamine D_1/2/5, serotonin 5-HT_1A/2A/2C and 5-HT₇, adrenergic α/β receptors, and cholinesterase enzymes), thereby serving as valuable pharmacological probes for mechanism studies or as potential leads for CNS drug discovery. The aims of the present review are to highlight the diverse CNS activities of aporphines, discuss their SAR, and briefly summarize general synthetic routes, which will pave the way for the design and development of new aporphine derivatives as promising CNS active drugs in the future.

Structural manipulation of aporphines via C10 nitrogenation leads to the identification of new 5-HT7AR ligands

Aporphine alkaloids containing a C10 nitrogen motif were synthesized and evaluated for affinity at 5-HT_1AR, 5-HT_2AR, 5-HT₆R and 5-HT_7AR. Three series of racemic aporphines were investigated: 1,2,10-trisubstituted, C10 N-monosubstituted and compounds containing a C10 benzofused aminothiazole moiety. The 1,2,10-trisubstituted series of compounds as a group displayed modest selectivity for 5-HT_7AR and also had moderate 5-HT_7AR affinity. Compounds from the C10 N-monosubstituted series generally lacked affinity for 5-HT_2AR and 5-HT₆R and showed strong affinity for 5-HT_1A or 5-HT_7AR. Compounds in this series that contained an N6-methyl group were up to 27-fold selective for 5-HT_7AR over 5-HT_1AR, whereas compounds with an N6-propyl substituent showed a reversal in this selectivity. The C10 benzofused aminothiazole analogues showed a similar binding profile as the C10 N-monosubstituted series i.e. strong affinity for 5-HT_1AR or 5-HT_7AR, with selectivity between the two receptors being similarly influenced by N6-methyl or N6-propyl substituents. Compounds 29 and 34a exhibit high 5-HT_7AR affinity, excellent selectivity versus dopamine receptors and function as antagonists in 5-HT_7AR cAMP-based assays. Compounds 29 and 34a have been identified as new lead molecules for further tool and pharmaceutical optimization.

Structure-activity profiling of alkaloid natural product pharmacophores against a Schistosoma serotonin receptor

Serotonin (5-HT) is an important regulator of numerous aspects of flatworm biology, ranging from neuromuscular function to sexual maturation and egg laying. In the parasitic blood fluke Schistosoma mansoni, 5-HT targets several G-protein coupled receptors (GPCRs), one of which has been demonstrated to couple to cAMP and regulate parasite movement. This receptor, Sm.5HTR_L, has been successfully co-expressed in mammalian cells alongside a luminescent cAMP-biosensor, enabling pharmacological profiling for candidate anti-schistosomal drugs. Here, we have utilized this assay to perform structure-activity investigations of 143 compounds containing previously identified alkaloid natural product pharmacophores (tryptamines, aporphines and protoberberines) shown to regulate Sm.5HTR_L. These experiments mapped regions of the tryptamine pharmacophore amenable and intolerant to substitution, highlighting differences relative to orthologous mammalian 5-HT receptors. Potent Sm.5HTR_L antagonists were identified, and the efficacy of these compounds were evaluated against live adult parasites cultured ex vivo. Such structure-activity profiling, characterizing the effect of various modifications to these core ring systems on Sm.5HTR_L responses, provides greater understanding of pharmacophores selective for this target to aid future drug development efforts.

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Various alkaloids were screened against a schistosome serotonin receptor, Sm.5HTR_L.

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Compounds with a tryptamine core displayed agonist activity at Sm.5HTR_L.

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Aporphine and protoberberine compounds displayed antagonist activity at Sm.5HTR_L.

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Compound activity at Sm.5HTR_L is broadly mirrored by motility effects on adult worms.

Discovery of β-Arrestin Biased Ligands of 5-HT7R

Though many studies have been published about therapeutic potentials of selective 5-HT₇R ligands, there have been few biased ligands of 5-HT₇R. The development of potent and selective biased ligands of 5-HT₇R would be of great help in understanding the relationship between pharmacological effects and G protein/β-arrestin signaling pathways of 5-HT₇R. In order to identify 5-HT₇R ligands with biased agonism, we designed and synthesized a series of tetrahydroazepine derivatives 1 and 2 with arylpyrazolo moiety or arylisoxazolo moiety. Through several biological evaluations such as binding affinity, selectivity profile, and functions in G protein and β-arrestin signaling pathways, 3-(4-chlorophenyl)-1,4,5,6,7,8-hexahydropyrazolo[3,4- d]azepine 1g was discovered as the β-arrestin biased ligand of 5-HT₇R. In an electroencephalogram (EEG) test, 1g increased total non-rapid eye movement (NREM) sleep time and decreased total rapid eye movement (REM) sleep time.

Redundant Gs-coupled serotonin receptors regulate amyloid-β metabolism in vivo

BACKGROUND

The aggregation of amyloid-β (Aβ) into insoluble plaques is a hallmark pathology of Alzheimer's disease (AD). Previous work has shown increasing serotonin levels with selective serotonin re-uptake inhibitor (SSRI) compounds reduces Aβ in the brain interstitial fluid (ISF) in a mouse model of AD and in the cerebrospinal fluid of humans. We investigated which serotonin receptor (5-HTR) subtypes and downstream effectors were responsible for this reduction.

RESULTS

Agonists of 5-HT4R, 5-HT6R, and 5-HT7R significantly reduced ISF Aβ, but agonists of other receptor subtypes did not. Additionally, inhibition of Protein Kinase A (PKA) blocked the effects of citalopram, an SSRI, on ISF Aβ levels. Serotonin signaling does not appear to change gene expression to reduce Aβ levels in acute timeframes, but likely acts within the cytoplasm to increase α-secretase enzymatic activity. Broad pharmacological inhibition of putative α-secretases increased ISF Aβ and blocked the effects of citalopram.

CONCLUSIONS

In total, these studies map the major signaling components linking serotonin receptors to suppression of brain ISF Aβ. These results suggest the reduction in ISF Aβ is mediated by a select group of 5-HTRs and open future avenues for targeted therapy of AD.

Serotonin 5-HT7 receptor agents: Structure-activity relationships and potential therapeutic applications in central nervous system disorders

Since its discovery in the 1940s in serum, the mammalian intestinal mucosa, and in the central nervous system, serotonin (5-HT) has been shown to be involved in virtually all cognitive and behavioral human functions, and alterations in its neurochemistry have been implicated in the etiology of a plethora of neuropsychiatric disorders. The cloning of 5-HT receptor subtypes has been of importance in enabling them to be classified as specific protein molecules encoded by specific genes. The 5-HT(7) receptor is the most recently classified member of the serotonin receptor family. Since its identification, it has been the subject of intense research efforts driven by its presence in functionally relevant regions of the brain. The availability of some selective antagonists and agonists, in combination with genetically modified mice lacking the 5-HT(7) receptor, has allowed for a better understanding of the pathophysiological role of this receptor. This paper reviews data on localization and pharmacological properties of the 5-HT(7) receptor, and summarizes the results of structure-activity relationship studies aimed at the discovery of selective 5-HT(7) receptor ligands. Additionally, an overview of the potential therapeutic applications of 5-HT(7) receptor agonists and antagonists in central nervous system disorders is presented.

(Phenylpiperazinyl-butyl)oxindoles as selective 5-HT7 receptor antagonists

A series of potent 5-hydroxytryptamine 7 (5-HT 7) ligands has been synthesized that contain a 1,3-dihydro-2 H-indol-2-one (oxindole) skeleton. The binding of these compounds to the 5-HT 7 and 5-HT 1A receptors was measured. Despite the structural similarity of these two serotonin receptor subtypes, several derivatives exhibited a high selectivity to the 5-HT 7 receptor. According to the structure-activity relationship observations, compounds unsubstituted at the oxindole nitrogen atom and containing a tetramethylene spacer between the oxindole skeleton and the basic nitrogen atom are the most potent ligands. Concerning the basic group, besides the moieties of the 4-phenylpiperazine type, halophenyl-1,2,3,6-tetrahydropyridines also proved to be 5-HT 7 receptor-ligands. Because of halogen substitution on the aromatic rings, good metabolic stability could be achieved. A representative of the family, 3-{4-[4-(4-chlorophenyl)-piperazin-1-yl]-butyl}-3-ethyl-6-fluoro-1,3-dihydro-2 H-indol-2-one ( 9e') exhibited selective 5-HT 7 antagonist activity ( K i = 0.79 nM). The in vivo pharmacological potencies of these 5-HT 7 receptor-ligands were estimated by the conflict drinking (Vogel) and the light-dark anxiolytic tests.

Novel aminoethylbiphenyls as 5-HT7 receptor ligands

The synthesis of a series of aminoethylbiphenyls as novel 5-HT(7) receptor ligands is described. The novel derivatives exhibit high affinity for the 5-HT(7) receptor with selectivity toward 5-HT(1A) receptor.

Receptor-based pharmacophores for serotonin 5-HT7R antagonists-implications to selectivity

A set of 31 diversified 5-HT7 receptor antagonists was automatically docked to a conformational ensemble of rhodopsin-based 5-HT7R models (flexible docking). It was found that ergolines, aporphines, and tricyclic psychotropic agents were always docked in a pocket formed by TMHs 4-6, and besides the main ionic bond with Asp3.32, they had specific interactions with Phe6.52, Phe6.51, Trp6.48, and Ser5.42. The arylpiperidine, arylpiperazine, or beta-carboline fragment of the complex ligands occupied the same pocket, whereas the terminal amide/imide part of those compounds reached the second cavity formed by TMHs 7-3 and interacted with Phe3.28, Arg7.36, and Tyr7.43. A similar orientation of selective antagonists of the group of arylsulfonamidoalkylamines was observed, that is, the sulfonamide part was located in the second pocket. Coherent docking results allowed the generation of two receptor-based pharmacophores: first containing features necessary for high 5-HT7R affinity and the other defining selectivity for this receptor subtype. The latter model indicated the importance of specific interactions with the residues of the TMHs 7-3 pocket (especially nonconserved Arg7.36) for selectivity over other monoamine GPCRs.

Molecular modeling studies focused on 5-HT7 versus 5-HT1A selectivity. Discovery of novel phenylpyrrole derivatives with high affinity for 5-HT7 receptors

The present study discusses the well-known 5-HT7/5-HT1A selectivity issue through a new series of phenylpyrrole derivatives. The first hits emerged from a virtual screening performed on a chemolibrary. Further study led to an optimization of a preliminary 5-HT7 pharmacophore model. The importance of each pharmacophoric feature is confirmed, but these characteristics have to be coupled to geometric constraints in order to achieve a 5-HT7 selectivity. Indeed, 5-HT1A affinity probably arises from extended conformations, whereas a bent one appears to be best suited for 5-HT7 selectivity.

Aminoalkyl phenyl sulfones--a novel series of 5-HT7 receptor ligands

A novel series of 5-HT(7) receptor ligands has been identified and evaluated, providing compounds showing a broad spectrum of functional activities and good selectivity over selected receptors and ion channels.

Phenylpyrroles, a new chemolibrary virtual screening class of 5-HT7 receptor ligands

Virtual screening studies have identified a series of phenylpyrroles as novel 5-HT7 receptor ligands. The synthesis and the affinity for the 5-HT7 receptor of these phenylpyrroles are described. Some of these compounds exhibited high affinity for the 5-HT7 receptors.

Generalization of a targeted library design protocol: application to 5-HT7 receptor ligands

Herein a general concept for the design of targeted libraries for proteins with binding sites that are divided into subsites is laid out, including several practical aspects and their solutions. The design is based on a chemogenomic classification of the subsites followed by collection of bioactive molecular fragments and virtual library generation. The general process is outlined and applied to the assembly of a library of 500 molecules targeting the serotonin type 7 (5-HT7) receptor, a class A G-Protein Coupled Receptor (GPCR). Utilizing commercially available building blocks of similar size and composition, a reference library was created. Control sets of known ligands for the 5-HT7 receptor, other GPCRs, and nuclear receptors were collected from literature sources. Principal component analysis of molecular descriptors for the two libraries and the literature sets, displayed a focusing of the targeted library to the region in the chemical space defined by the literature actives, suggesting a denser coverage of the bioactive region than for the more diverse reference library. Additional computational validations, including PCA class predictions, 3D pharmacophore modeling, and docking calculations all indicated an enrichment factor of 5-HT7 ligand-like molecules in the range of 2-4 for the targeted library compared to the reference library.

Molecular design based on 3D pharmacophores. Applications to 5-HT7 receptors

A definition of a pharmacophore for the 5-HT7 antagonists was carried out by searching the common chemical features of selective antagonists from the literature. A molecular design is described by analyzing the differences between this new pharmacophore and three other 3D serotonin pharmacophores previously described. This comparison led to the synthesis of a new series of potent 5-HT7 antagonists.

Aminotriazine 5-HT7 antagonists

The present studies have identified a series of aminotriazines as novel 5-HT(7) receptor antagonists. Compounds 10 and 17 have high affinity for the 5-HT(7) receptor and do not bind to either the 5-HT(2C) or 5-HT(6) receptors. These compounds produce no agonist effects by themselves, and shift the dose-response curve of 5-CT to the right in the manner of an antagonist.

Novel 3-aminochromans as potential pharmacological tools for the serotonin 5-HT7 receptor

The synthesis of novel C6-aryl substituted derivatives of 3-(dimethylamino)chroman is described. The novel derivatives display 5-HT(7) receptor affinities that varies from nM to muM, indicating that this small set of derivatives constitute a novel and interesting starting point for further structure-serotonin 5-HT(7) activity relationship (SAR) studies.

Novel 5-HT7 receptor inverse agonists. Synthesis and molecular modeling of arylpiperazine- and 1,2,3,4-tetrahydroisoquinoline-based arylsulfonamides

A series of arylpiperazine- and 1,2,3,4-tetrahydroisoquinoline-based arylsulfonamides was synthesized and evaluated for their interactions with the constitutively active 5-HT7 receptor. Effects on basal adenylate cyclase activity were measured using HEK-293 cells expressing the rat 5-HT7. All ligands produced a decrease of adenylate cyclase activity, indicative of their inverse agonism. Additionally, computational studies with a set of 22 inverse agonists, including these novel inverse agonists and inverse agonists known from literature, resulted in a pharmacophore model and a CoMFA model (R2 = 0.97, SE = 0.18). Docking of inverse agonists at the binding site of a model of the helical parts of the 5-HT7 receptor, based on the alpha carbon template for 7-TM GPCRs, revealed interesting molecular interactions and a possible explanation for observed structure-activity relationships.

Novel 2-aminotetralin and 3-aminochroman derivatives as selective serotonin 5-HT7 receptor agonists and antagonists

The understanding of the physiological role of the G-protein coupled serotonin 5-HT(7) receptor is largely rudimentary. Therefore, selective and potent pharmacological tools will add to the understanding of serotonergic effects mediated through this receptor. In this report, we describe two compound classes, chromans and tetralins, encompassing compounds with nanomolar affinity for the 5-HT(7) receptor and with good selectivity. Within theses classes, we have discovered both agonists and antagonists that can be used for further understanding of the pharmacology of the 5-HT(7) receptor.

Optimization of the pharmacophore model for 5-HT7R antagonism. Design and synthesis of new naphtholactam and naphthosultam derivatives

We present in this study an optimization of a preliminary pharmacophore model for 5-HT(7)R antagonism, with the incorporation of recently reported ligands and using an efficient procedure with the CATALYST program. The model consists of five features: a positive ionizable atom (PI), a H-bonding acceptor group (HBA), and three hydrophobic regions (HYD). This model has been supported by the design, synthesis, and biological evaluation of new naphtholactam and naphthosultam derivatives of general structure I (39-72). A systematic structure-affinity relationship (SAFIR) study on these analogues has allowed us to confirm that the model incorporates the essential structural features for 5-HT(7)R antagonism. In addition, computational simulation of the complex between compound 56 and a rhodopsin-based 3D model of the 5-HT(7)R transmembrane domain has permitted us to define the molecular details of the ligand-receptor interaction and gives additional support to the proposed pharmacophore model for 5-HT(7)R antagonism: (i) the HBA feature of the pharmacophore model binds Ser(5.42) and Thr(5.43), (ii) the HYD1 feature interacts with Phe(6.52), (iii) the PI feature forms an ionic interaction with Asp(3.32), and (iv) the HYD3 (AR) feature interacts with a set of aromatic residues (Phe(3.28), Tyr(7.43)). These results provide the tools for the design and synthesis of new ligands with predetermined affinities and pharmacological properties.

Beta-phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids and compounds derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2001 to June 2002 is reviewed, with 581 references cited.

Synthesis and structure-affinity relationships of 1-[omega-(4-aryl-1-piperazinyl)alkyl]-1-aryl ketones as 5-HT(7) receptor ligands

Structural requirements for 5-HT(7) receptor affinity and selectivity over that for the 5-HT(1A) receptor were studied on a series of 1-[omega-(4-aryl-1-piperazinyl)alkyl]-1-aryl ketones. The presence of a hydroxy or methoxy substituent on aryl ketone moiety, alkyl chain length, and the nature of N-1-piperazine substituent were explored. 6-[4-(3-Benzisoxazolyl)-1-piperazinyl]-1-(2-hydroxyphenyl)-1-hexanone (40) and its methoxy analogue 43 exhibited high 5-HT(7) receptor affinities (Ki = 2.93 nM and 0.90 nM, respectively) and agonist properties when tested for 5-HT(7) receptor-mediated relaxation of substance P-induced guinea-pig ileum contraction.

New tetrahydrobenzindoles as potent and selective 5-HT(7) antagonists with increased In vitro metabolic stability

Chemical modifications of compound 1 (DR4004), a potent, selective antagonist of the 5-HT(7) receptor, were conducted with the aim of improving its metabolic stability. Halogenation of putative sites of oxidative metabolism afforded compounds 7-10, which retained high affinity and selectivity for the 5-HT(7) receptor, and showed increased in vitro metabolic stability. Compound 10 (DR4485) showed oral bioavailability, and should be a useful tool for evaluating the therapeutic potential of 5-HT(7) antagonists.

1-(Bicyclopiperazinyl)ethylindoles and 1-(homopiperazinyl)ethyl-indoles as highly selective and potent 5-HT(7) receptor ligands

A novel series of 1-(bicyclopiperazinyl)ethylindole and 1-(homopiperazinyl)ethyl-indole derivatives was synthesized and found to be potent and selective 5-HT(7) receptor ligands.

Therapeutic potential of serotonin antagonists in depressive disorders

Although the precise neurochemical imbalances in depression are still unknown, a role for the neurotransmitter 5-hydroxytryptamine (serotonin) has been implicated since the identification of the first effective antidepressants, imipramine and iproniazid. This led to the development of the selective serotonin re-uptake inhibitors which are widely used in the treatment of depression and depressive disorders, including generalised anxiety disorder, social phobia, obsessive compulsive disorder etc. Studies involving chronic administration in rats led to the hypothesis that the different classes of antidepressant treatment produce a common neuroadaptive change, namely an enhancement of serotonin neurotransmission, albeit via different pre and postsynaptic mechanisms. From this, it was suggested that serotonin antagonists should induce similar neuroadaptive changes, either directly or through a potentiation of other antidepressant agents. Here, the profiles of novel serotonin antagonists currently in preclinical development are reviewed and their therapeutic potential is assessed.