5-HT6 Receptor Ligands as Antidementia Drugs

Indirect influence on the BDNF/TrkB receptor signaling pathway via GPCRs, an emerging strategy in the treatment of neurodegenerative disorders

Neuronal survival depends on neurotrophins and their receptors. There are two types of neurotrophin receptors: a nonenzymatic, trans-membrane protein of the tumor necrosis factor receptor (TNFR) family-p75 receptor and the tyrosine kinase receptors (TrkR) A, B, and C. Activation of the TrkBR by brain-derived neurotrophic factor (BDNF) or neurotrophin 4/5 (NT-4/5) promotes neuronal survival, differentiation, and synaptic function. It is shown that in the pathogenesis of several neurodegenerative conditions (Alzheimer's disease, Parkinson's disease, Huntington's disease) the BDNF/TrkBR signaling pathway is impaired. Since it is known that GPCRs and TrkR are regulating several cell functions by interacting with each other and generating a cross-communication in this review we have focused on the interaction between different GPCRs and their ligands on BDNF/TrkBR signaling pathway.

Progress in Investigational Agents Targeting Serotonin-6 Receptors for the Treatment of Brain Disorders

Serotonin (5-HT) plays an important role in the regulation of several basic functions of the central and peripheral nervous system. Among the 5-HT receptors, serotonin-6 (5-HT6) receptor has been an area of substantial research. 5-HT6 receptor is a G-protein-coupled receptor mediating its effects through diverse signaling pathways. Exceptional features of the receptors fueling drug discovery efforts include unique localization and specific distribution in the brain regions having a role in learning, memory, mood, and behavior, and the affinity of several clinically used psychotropic agents. Although non-clinical data suggest that both agonist and antagonist may have similar behavioral effects, most of the agents that entered clinical evaluation were antagonists. Schizophrenia was the initial target; more recently, cognitive deficits associated with Alzheimer's disease (AD) or other neurological disorders has been the target for clinically evaluated 5-HT6 receptor antagonists. Several 5-HT6 receptor antagonists (idalopirdine, intepirdine and latrepirdine) showed efficacy in alleviating cognitive deficits associated with AD in the proof-of-concept clinical studies; however, the outcomes of the subsequent phase 3 studies were largely disappointing. The observations from both non-clinical and clinical studies suggest that 5-HT6 receptor antagonists may have a role in the management of neuropsychiatric symptoms in dementia. Masupirdine, a selective 5-HT6 receptor antagonist, reduced agitation/aggression-like behaviors in animal models, and a post hoc analysis of a phase 2 trial suggested potential beneficial effects on agitation/aggression and psychosis in AD. This agent will be assessed in additional trials, and the outcome of the trials will inform the use of 5-HT6 receptor antagonists in the treatment of agitation in dementia of the Alzheimer's type.

The Phenoxyalkyltriazine Antagonists for 5-HT6 Receptor with Promising Procognitive and Pharmacokinetic Properties In Vivo in Search for a Novel Therapeutic Approach to Dementia Diseases

Among the serotonin receptors, one of the most recently discovered 5-HT₆ subtype is an important protein target and its ligands may play a key role in the innovative treatment of cognitive disorders. However, none of its selective ligands have reached the pharmaceutical market yet. Recently, a new chemical class of potent 5-HT₆ receptor agents, the 1,3,5-triazine-piperazine derivatives, has been synthesized. Three members, the ortho and meta dichloro- (1,2) and the unsubstituted phenyl (3) derivatives, proved to be of special interest due to their high affinities (1,2) and selectivity (3) toward 5-HT₆ receptor. Thus, a broader pharmacological profile for 1–3, including comprehensive screening of the receptor selectivity and drug-like parameters in vitro as well as both, pharmacokinetic and pharmacodynamic properties in vivo, have been investigated within this study. A comprehensive analysis of the obtained results indicated significant procognitive-like activity together with beneficial drug-likeness in vitro and pharmacokinetics in vivo profiles for both, (RS)-4-[1-(2,3-dichlorophenoxy)propyl]-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine (2) and (RS)-4-(4-methylpiperazin-1-yl)-6-(1-phenoxypropyl)-1,3,5-triazin-2-amine (3), but insensibly predominant for compound 2. Nevertheless, both compounds (2 and 3) seem to be good Central Nervous System drug candidates in search for novel therapeutic approach to dementia diseases, based on the 5-HT₆ receptor target.

Preclinical validation of [18F]2FNQ1P as a specific PET radiotracer of 5-HT6 receptors in rat, pig, non-human primate and human brain tissue

INTRODUCTION

The aim of this study was to perform in-vitro and in-vivo radiopharmacological characterizations of [¹⁸F]2FNQ1P, a new PET radiotracer of 5-HT₆ receptors, in rat, pig, non-human primate and human tissues. The 5-HT₆ receptor is one of the more recently identified serotonin receptors in central nervous system and, because of its role in memory and cognitive processes, is considered as a promising therapeutic target.

METHODS

In-vitro autoradiography and saturation binding assays were performed in postmortem brain tissues from rat, pig, non-human primate and human caudate nucleus, completed by serum stability assessment in all species and cerebral radiometabolite and biodistribution studies in rat.

RESULTS

In all species, autoradiography data revealed high binding levels of [¹⁸F]2FNQ1P in cerebral regions with high 5-HT₆ receptor density. Binding was blocked by addition of SB258585 as a specific antagonist. Binding assays provided K_D and B_max values of respectively 1.34 nM and 0.03 pmol·mg^-1 in rat, 0.60 nM and 0.04 pmol·mg^-1 in pig, 1.38 nM and 0.07 pmol·mg^-1 in non-human primate, and 1.39 nM and 0.15 pmol·mg^-1 in human caudate nucleus. In rat brain, the proportion of unmetabolized [¹⁸F]2FNQ1P was >99% 5 min after iv injection and 89% at 40 min. The biodistribution studies found maximal radioactivity in lungs and kidneys (3.5 ± 1.2% ID/g and 2.0 ± 0.7% ID/g, respectively, 15 min post-injection).

CONCLUSION

These radiopharmacological data confirm that [¹⁸F]2FNQ1P is a specific radiotracer for molecular imaging of 5-HT₆ receptors and suggest that it could be used as a radiopharmaceutical in humans.

Virtual screening-driven discovery of dual 5-HT6/5-HT2A receptor ligands with pro-cognitive properties

A virtual screening campaign aimed at finding structurally new compounds active at 5-HT₆R provided a set of candidates. Among those, one structure, 4-(5-{[(2-{5-fluoro-1H-pyrrolo[2,3-b]pyridin-3-yl}ethyl)amino]methyl}furan-2-yl)phenol (1, 5-HT₆R K_i = 91 nM), was selected as a hit for further optimization. As expected, the chemical scaffold of selected compound was significantly different from all the serotonin receptor ligands published to date. Synthetic efforts, supported by molecular modelling, provided 43 compounds representing different substitution patterns. The derivative 42, 4-(5-{[(2-{5-fluoro-1H-pyrrolo[2,3-b]pyridin-3-yl}ethyl)amino]methyl}furan-2-yl)phenol (5-HT₆R K_i = 25, 5-HT_2AR K_i = 32 nM), was selected as a lead and showed a good brain/plasma concentration profile, and it reversed phencyclidine-induced memory impairment. Considering the unique activity profile, the obtained series might be a good starting point for the development of a novel antipsychotic or antidepressant with pro-cognitive properties.

5-HT6 receptor agonist and antagonist improve memory impairments and hippocampal BDNF signaling alterations induced by MK-801

The aim of the present study was to investigate and compare the effects of acute and chronic (21-day) administration of agonist (WAY-181187) and antagonist (SB-742457) of the 5-hydroxytryptamine 6 receptor (5-HT₆R) on MK-801-induced memory impairments in novel object recognition (NORT) and Y-maze continuous spontaneous alternation tests (Y-CAT). Further, the expression of the brain-derived neurotrophic factor (BDNF) in rat hippocampus was measured after 21-day administration to investigate BDNF participation in the pro-cognitive effects of 5-HT₆R ligands. We found that acute administration of WAY-181187, as well as SB-742457, reversed the effects of MK-801 in NORT and Y-CAT, and that this influence persisted after prolonged application in NORT but not in Y-CAT. Both 5-HT₆R ligands increased hippocampal BDNF protein expression, but WAY-181187 was much more potent than SB-742457 and alleviated the MK-801-induced inhibition of BDNF signaling pathways better, which seems to translate into a stronger WAY-181187 effect in behavioral tests. Collectively, both the 5-HT₆R agonist and the antagonist, administered acutely and chronically, prevent memory impairments and alterations in BDNF signaling induced by MK-801 in rats. The present results confirm the pro-cognitive properties of both types of 5-HT₆R ligands and suggest that BDNF pathways may be involved in their mechanism of action.

Serotonin 5-HT6 Receptor Antagonists in Alzheimer's Disease: Therapeutic Rationale and Current Development Status

Alzheimer's disease (AD) is the most common cause of dementia in elderly people. Because of the lack of effective treatments for this illness, research focused on identifying compounds that restore cognition and functional impairments in patients with AD is a very active field. Since its discovery in 1993, the serotonin 5-HT₆ receptor has received increasing attention, and a growing number of studies supported 5-HT₆ receptor antagonism as a target for improving cognitive dysfunction in AD. This article reviews the rationale behind investigations into the targeting of 5-HT₆ receptors as a symptomatic treatment for cognitive and/or behavioral symptoms of AD. In addition to describing the available clinical evidence, this article also describes the purported biochemical and neurochemical mechanisms of action by which 5-HT₆ receptor antagonists could influence cognition, and the preclinical data supporting this therapeutic approach to AD. A large number of publications describing the development of ligands for this receptor have come to light and preclinical data indicate the procognitive efficacy of 5-HT₆ receptor antagonists. Subsequently, the number of patents protecting 5-HT₆ chemical entities has continuously grown. Some of these compounds have successfully undergone phase I clinical studies and have been further evaluated in clinical phase II trials with variable success. Phase II studies have also revealed the potential of combining 5-HT₆ receptor antagonism and cholinesterase inhibition. Two of these antagonists, idalopirdine and RVT-101, have been further developed into ongoing phase III clinical trials. Overall, 5-HT₆ receptor antagonists can reasonably be regarded as potential drug candidates for the treatment of AD.

The procognitive effects of 5-HT6 receptor ligands in animal models of schizophrenia

In addition to positive and negative symptoms, cognitive deficits are increasingly being recognized as a core feature of schizophrenia. Neurocognitive impairments are strongly associated with functional outcomes; thus, the treatment of cognitive impairments is of central importance. A large body of evidence suggests that the serotonin 6 (5-HT6) receptors may be potential targets for cognitive improvement. Clinical and preclinical studies have supported the notion that using 5-HT6 receptor antagonists is a promising component in the treatment of cognitive dysfunctions associated with aging and Alzheimer's disease. However, less is known about the efficacy of this strategy in the treatment of schizophrenia-like cognitive disturbances. The purpose of this review is to summarize existing data on the effects of 5-HT6 receptor antagonists in animal experiments, utilizing tasks that assess cognitive domains that are relevant to the cognitive deficits characterizing schizophrenia. This review focuses primarily on animal models of schizophrenia that are based on the blockade of N-methyl-d-aspartate receptors; however, when relevant, data obtained in other models are also discussed. The putative procognitive actions of 5-HT6 agonists are also reviewed. Finally, the mechanisms that are putatively responsible for the procognitive effects of 5-HT6 receptor ligands are briefly discussed.

5HT(6) receptor antagonists: a patent update. Part 1. Sulfonyl derivatives

INTRODUCTION

Among a variety of proteins included in a relatively wide GPCR family, serotonin 5HT receptors (5HT(6)Rs) are highly attractive as important biological targets with enormous clinical importance. Among this subclass, 5HT(6)R is the most recently discovered group. Available biological data clearly indicate that 5HT(6)R antagonists can be used as effective regulators in a variety of contexts, including memory formation, age-related cognitive impairments and memory deficits associated with conditions such as schizophrenia, Parkinson's disease and Alzheimer's disease. Therefore, this receptor has already attracted a considerable attention within the scientific community, due to its versatile therapeutic potential.

AREAS COVERED

The current paper is an update to the comprehensive review article published previously in Expert Opinion on Therapeutic Patents (see issue 20(7), 2010). Here, the main focus is on small-molecule compounds - 5HT(6) antagonists - which have been described in recent patent literature, since the end of 2009. To obtain a clear understanding of the situation and dynamic within the field of 5HT(6) ligands, having an obvious pharmaceutical potential in terms of related patents, a comprehensive search through several key patent collections have been provided. The authors describe the reported chemical classes and scaffolds in sufficient detail to provide a valuable insight in the 5HT(6)R chemistry and pharmacology. The review consists of two core parts with separate sections arranged in accordance with the main structural features of 5HT(6)R ligands.

EXPERT OPINION

Recent progress in the understanding of the 5HT(6) receptor function and structure includes a suggested constitutive activity for the receptor, development of a number of multimodal small molecule ligands and re-classification of many selective antagonists as pseudo-selective agents. Heterocycles with sulfonyl group and without any basic center provide sufficient supramolecular interactions and show high antagonistic activity against 5HT(6)R.