Antagonism of the 5-HT 6 receptor – Preclinical rationale for the treatment of Alzheimer's disease

Procognitive Potential of Neuroprotective Triazine 5-HT6 Receptor Antagonists Tested on Chronic Activity In Vivo in Rats: Computer-Aided Insight into the Role of Chalcogen-Differences on the Pharmacological Profile

Among serotonin receptors, the 5-HT6 subtype is an important protein target and its ligands may play a key role in the innovative treatment of cognitive disorders. This study aimed to extend the body of preclinical research on two naphthyl-derived methylpiperazine-1,3,5-triazine analogues with thioether (WA-22) or Se-ether (PPK-32) linkers, the newly described compounds having high affinity and selectivity for 5-HT6 receptors and drug-like parameters in vitro. Thus, crystallography-supported deeper insight into their chemical properties, the comparison of their neuroprotective and pharmacokinetic profiles, and especially their impact on memory disturbances after chronic administration to rats were investigated. As a result, the chronic administration of WA-22 completely reversed (+)MK-801-induced memory disturbances evaluated in the novel object recognition test (NORT) in rats. The pharmacokinetic and biochemical results support the notion that this 1,3,5-triazine 5-HT6 receptor ligand could offer a promising therapeutic tool in CNS-related disorders. The selenium compound PPK-32, with a similar range of activity at acute administration, has shown even broader neuroprotective profiles, especially at the genetic level. However, for therapeutic use, its weaker pharmacokinetics (stability), which is a probable limit for action upon chronic administration, would require improvement, e.g., by an appropriate formulation.

Advancements in autophagy perturbations in Alzheimer's disease: Molecular aspects and therapeutics

Emerging evidences suggest that autophagy, a key cellular process responsible for degrading and recycling damaged organelles and proteins, plays a crucial role in maintaining neuronal health. Dysfunctional autophagy has been linked to the pathogenesis of Alzheimer's disease (AD), contributing to the accumulation of misfolded proteins and cellular debris. Molecular mechanisms underlying autophagy dysfunction in AD involve amyloid-beta (Aβ) and tau accumulation, neuroinflammation, mitochondrial dysfunction, oxidative stress and endoplasmic reticulum stress. Disrupted signaling pathways such as TRIB3, Nmnat and BAG3 that regulate key processes like autophagosome initiation, lysosome function, and protein homeostasis also play a crucial role in the pathogenesis. Restoration of autophagy by modulating these molecular and signaling pathways may be an effective therapeutic strategy for AD. Studies have found few drugs targeting autophagy dysregulation in AD. These drugs include metformin that has been found to modulate the expression of TRIB3 for autophagy regulation. Another drug, resveratrol has been reported to augment the activity of Nmnat thus, increases autophagy flux. BACE1 and mTOR inhibitors like arctigenin, nilvadipine and dapagliflozin were also found to restore autophagy. This study elaborates recent advances in signaling and molecular pathways and discusses current and emerging therapeutic interventions targeting autophagy dysfunction in AD.

Targeting 5-HT Is a Potential Therapeutic Strategy for Neurodegenerative Diseases

There is increasing interest in the potential therapeutic role of 5-HT (serotonin) in the treatment of neurodegenerative diseases, which are characterized by the progressive degeneration and death of nerve cells. 5-HT is a vital neurotransmitter that plays a central role in regulating mood, cognition, and various physiological processes in the body. Disruptions in the 5-HT system have been linked to several neurological and psychiatric disorders, making it an attractive target for therapeutic intervention. Although the exact causes of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) are not fully understood, researchers believe that regulating the 5-HT system could help alleviate symptoms and potentially slow the progression of these diseases. Here, we delve into the potential of harnessing 5-HT as a therapeutic target for the treatment of neurodegenerative diseases. It is important to note that the current clinical drugs targeting 5-HT are still limited in the treatment of these complex diseases. Therefore, further research and clinical trials are needed to evaluate the feasibility and effectiveness of its clinical application.

Future Therapeutic Strategies for Alzheimer's Disease: Focus on Behavioral and Psychological Symptoms

Alzheimer's disease (AD) is a progressive, degenerative brain disorder that impairs memory and thinking skills, leading to significant economic and humanistic burdens. It is associated with various neuropsychiatric symptoms (NPS) such as anxiety, agitation, depression, aggression, apathy, and psychosis. NPSs are common in patients with AD, affecting up to 97% of individuals diagnosed with AD. The severity of NPS is linked to disease progression and cognitive decline. NPS in Alzheimer's disease leads to increased morbidity, mortality, caregiver burden, earlier nursing home placement, and higher healthcare costs. Despite their significant impact, clinical research on NPS in AD is limited. In clinical settings, accurately distinguishing and diagnosing NPS related to AD remains a challenge. Additionally, conventional treatments for NPS in AD are often ineffective, highlighting the need for new therapies that target these specific symptoms. Understanding these comorbidities can aid in early diagnosis and better management of AD. In this review, we provide a summary of the various neurological and psychiatric symptoms (NPS) associated with AD and new candidates under development for the treatment of NPS based on their therapeutic targets and mechanisms. On top of the conventional NPS studied so far, this review adds recent advancements in the understanding of social functional impairment in AD. This review also provides information that can contribute to the advancement of studies and translational research in this field by emphasizing therapeutic targets and mechanisms of action focused on AD-related NPS rather than conventional mechanisms targeted in AD drug development. Above all, considering the relative lack of research in this new field despite the importance of clinical, medical, and translational research, it may increase interest in NPS in AD, its pathophysiological mechanisms, and potential therapeutic candidates such as molecules with antioxidant potential.

5-HT6 receptor neutral antagonists protect astrocytes: A lesson from 2-phenylpyrrole derivatives

The serotonin type 6 receptor (5-HT6R) displays a strong constitutive activity, suggesting it participates largely in the physiological and pathological processes controlled by the receptor. The active states of 5-HT6R engage particular signal transduction pathways that lead to different biological responses. In this study, we present the development of 5-HT6R neutral antagonists at Gs signaling built upon the 2-phenylpyrrole scaffold. Using molecular dynamics simulations, we outline the relationship between the exposure of the basic center of the molecules and their ability to target the agonist-activated state of the receptor. Our study identifies compound 30 as a potent and selective neutral antagonist at 5-HT6R-operated Gs signaling. Furthermore, we demonstrate the cytoprotective effects of 30 and structurally diverse 5-HT6R neutral antagonists at Gs signaling in C8-D1A cells and human astrocytes exposed to rotenone. This effect is not observed for 5-HT6R agonists or inverse agonists. In light of these findings, we propose compound 30 as a valuable molecular probe to study the biological effects associated with the agonist-activated state of 5-HT6R and provide insight into the glioprotective properties of 5-HT6R neutral antagonists at Gs signaling.

Indole-Based Compounds in the Development of Anti-Neurodegenerative Agents

Neurodegeneration is a gradual decay process leading to the depletion of neurons in both the central and peripheral nervous systems, ultimately resulting in cognitive dysfunctions and the deterioration of brain functions, alongside a decline in motor skills and behavioral capabilities. Neurodegenerative disorders (NDs) impose a substantial socio-economic strain on society, aggravated by the advancing age of the world population and the absence of effective remedies, predicting a negative future. In this context, the urgency of discovering viable therapies is critical and, despite significant efforts by medicinal chemists in developing potential drug candidates and exploring various small molecules as therapeutics, regrettably, a truly effective treatment is yet to be found. Nitrogen heterocyclic compounds, and particularly those containing the indole nucleus, which has emerged as privileged scaffold, have attracted particular attention for a variety of pharmacological applications. This review analyzes the rational design strategy adopted by different research groups for the development of anti-neurodegenerative indole-based compounds which have the potential to modulate various molecular targets involved in NDs, with reference to the most recent advances between 2018 and 2023.

Converged avenues: depression and Alzheimer's disease- shared pathophysiology and novel therapeutics

Depression, a highly prevalent disorder affecting over 280 million people worldwide, is comorbid with many neurological disorders, particularly Alzheimer's disease (AD). Depression and AD share overlapping pathophysiology, and the search for accountable biological substrates made it an essential and intriguing field of research. The paper outlines the neurobiological pathways coinciding with depression and AD, including neurotrophin signalling, the hypothalamic-pituitary-adrenal axis (HPA), cellular apoptosis, neuroinflammation, and other aetiological factors. Understanding overlapping pathways is crucial in identifying common pathophysiological substrates that can be targeted for effective management of disease state. Antidepressants, particularly monoaminergic drugs (first-line therapy), are shown to have modest or no clinical benefits. Regardless of the ineffectiveness of conventional antidepressants, these drugs remain the mainstay for treating depressive symptoms in AD. To overcome the ineffectiveness of traditional pharmacological agents in treating comorbid conditions, a novel therapeutic class has been discussed in the paper. This includes neurotransmitter modulators, glutamatergic system modulators, mitochondrial modulators, antioxidant agents, HPA axis targeted therapy, inflammatory system targeted therapy, neurogenesis targeted therapy, repurposed anti-diabetic agents, and others. The primary clinical challenge is the development of therapeutic agents and the effective diagnosis of the comorbid condition for which no specific diagnosable scale is present. Hence, introducing Artificial Intelligence (AI) into the healthcare system is revolutionary. AI implemented with interdisciplinary strategies (neuroimaging, EEG, molecular biomarkers) bound to have accurate clinical interpretation of symptoms. Moreover, AI has the potential to forecast neurodegenerative and psychiatric illness much in advance before visible/observable clinical symptoms get precipitated.

Structural insights into constitutive activity of 5-HT6 receptor

While most therapeutic research on G-protein-coupled receptors (GPCRs) focuses on receptor activation by (endogenous) agonists, significant therapeutic potential exists through agonist-independent intrinsic constitutive activity that can occur in various physiological and pathophysiological settings. For example, inhibiting the constitutive activity of 5-HT6R-a receptor that is found almost exclusively in the brain and mediates excitatory neurotransmission-has demonstrated a therapeutic effect on cognitive/memory impairment associated with several neuropsychiatric disorders. However, the structural basis of such constitutive activity remains unclear. Here, we present a cryo-EM structure of serotonin-bound human 5-HT6R-Gs heterotrimer at 3.0-Å resolution. Detailed analyses of the structure complemented by comprehensive interrogation of signaling illuminate key structural determinants essential for constitutive 5-HT6R activity. Additional structure-guided mutagenesis leads to a nanobody mimic Gαs for 5-HT6R that can reduce its constitutive activity. Given the importance of 5-HT6R for a large number of neuropsychiatric disorders, insights derived from these studies will accelerate the design of more effective medications, and shed light on the molecular basis of constitutive activity.

Novel thienocycloalkylpyridazinones as useful scaffolds for acetylcholinesterase inhibition and serotonin 5-HT6 receptor interaction

A library of eighteen thienocycloalkylpyridazinones was synthesized for human acetylcholinesterase (hAChE) and butyrylcholinesterase (hBChE) inhibition and serotonin 5-HT₆ receptor subtype interaction by following a multitarget-directed ligand approach (MTDL), as a suitable strategy for treatment of Alzheimer's disease (AD). The novel compounds featured a tricyclic scaffold, namely thieno[3,2-h]cinnolinone, thienocyclopentapyridazinone and thienocycloheptapyridazinone, connected through alkyl chains of variable length to proper amine moieties, most often represented by N-benzylpiperazine or 1-(phenylsulfonyl)-4-(piperazin-1-ylmethyl)-1H-indole as structural elements addressing AChE and 5-HT₆ interaction, respectively. Our study highlighted the versatility of thienocycloalkylpyridazinones as useful architectures for AChE interaction, with several N-benzylpiperazine-based analogues emerging as potent and selective hAChE inhibitors with IC₅₀ in the 0.17-1.23 μM range, exhibiting low to poor activity for hBChE (IC₅₀ = 4.13-9.70 μM). The introduction of 5-HT₆ structural moiety phenylsulfonylindole in place of N-benzylpiperazine, in tandem with a pentamethylene linker, gave potent 5-HT₆ thieno[3,2-h]cinnolinone and thienocyclopentapyridazinone-based ligands both displaying hAChE inhibition in the low micromolar range and unappreciable activity towards hBChE. While docking studies provided a rational structural explanation for AChE/BChE enzyme and 5-HT₆ receptor interaction, in silico prediction of ADME properties of tested compounds suggested further optimization for development of such compounds in the field of MTDL for AD.

Potential Anti-Amnesic Activity of a Novel Multimodal Derivative of Salicylamide, JJGW08, in Mice

Memory impairments constitute a significant problem worldwide, and the COVID-19 pandemic dramatically increased the prevalence of cognitive deficits. Patients with cognitive deficits, specifically memory disturbances, have underlying comorbid conditions such as schizophrenia, anxiety, or depression. Moreover, the available treatment options have unsatisfactory effectiveness. Therefore, there is a need to search for novel procognitive and anti-amnesic drugs with additional pharmacological activity. One of the important therapeutic targets involved in the modulation of learning and memory processes are serotonin receptors, including 5-HT1A, 5-HT6, and 5-HT7, which also play a role in the pathophysiology of depression. Therefore, this study aimed to assess the anti-amnesic and antidepressant-like potential of JJGW08, a novel arylpiperazine alkyl derivative of salicylamide with strong antagonistic properties at 5-HT1A and D2 receptors and weak at 5-HT2A and 5-HT7 receptors in rodents. First, we investigated the compound’s affinity for 5-HT6 receptors using the radioligand assays. Next, we assessed the influence of the compound on long-term emotional and recognition memory. Further, we evaluated whether the compound could protect against MK-801-induced cognitive impairments. Finally, we determined the potential antidepressant-like activity of the tested compound. We found that JJGW08 possessed no affinity for 5-HT6 receptors. Furthermore, JJGW08 protected mice against MK-801-induced recognition and emotional memory deficits but showed no antidepressant-like effects in rodents. Therefore, our preliminary study may suggest that blocking serotonin receptors, especially 5-HT1A and 5-HT7, might be beneficial in treating cognitive impairments, but it requires further investigation.

Investigating CNS distribution of PF-05212377, a P-glycoprotein substrate, by translation of 5-HT6 receptor occupancy from non-human primates to humans

PF-05212377 (SAM760) is a potent and selective 5-HT₆ antagonist, previously under development for the treatment of Alzheimer's disease. In vitro, PF-05212377 was determined to be a P-gp/non-BCRP human transporter substrate. Species differences were observed in the in vivo brain penetration of PF-05212377 with a ratio of the unbound concentration in brain/unbound concentration in plasma (C_bu /C_pu ) of 0.05 in rat and 0.64 in non-human primates (NHP). Based on pre-clinical evidence, brain penetration and target engagement of PF-05212377 was confirmed in NHP using positron emission tomography (PET) measured 5-HT₆ receptor occupancy (%RO). The NHP C_pu EC₅₀ of PF-05212377 was 0.31 nM (consistent with the in vitro human 5HT6 K_i : 0.32 nM). P-gp has been reported to be expressed in higher abundance at the rat BBB and in similar abundance at the BBB of non-human primates and human; brain penetration of PF-05212377 in humans was postulated to be similar to that in non-human primates. In humans, PF-05212377 demonstrated dose and concentration dependent increases in 5-HT₆ RO; maximal 5-HT6 RO of ∼80% was measured in humans at doses of ≥15 mg with an estimated unbound plasma EC₅₀ of 0.37 nM (which was similar to the in vitro human 5HT6 binding K_i 0.32 nM). In conclusion, cumulative evidence from NHP and human PET RO assessments confirmed that NHP is more appropriate than the rat for the prediction of human brain penetration of PF-05212377, a P-gp/non-BCRP substrate. Clinical trial number: NCT01258751.

New Triazine Derivatives as Serotonin 5-HT6 Receptor Ligands

Since the number of people with Alzheimer's disease (AD) continues to rise, new and effective drugs are urgently needed to not only slow down the progression of the disease, but to stop or even prevent its development. Serotonin 5-HT₆ receptor (5-HT₆R) ligands are still a promising therapeutic target for the treatment of AD. 1,3,5-Triazine derivatives, as novel structures lacking an indole or a sulfone moiety, have proven to be potent ligands for this receptor. In present work, new derivatives of the compound MST4 (4-((2-isopropyl-5-methylphenoxy)methyl)-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine), the potent 5-HT₆R antagonist (K_i = 11 nM) with promising ADMET and in vivo properties, were designed. The synthesized compounds were tested for their affinity towards 5-HT₆R and other receptor (off)targets (serotonin 5-HT_2A, 5-HT₇ and dopamine D₂). Based on the new results, 4-(2-tert-butylphenoxy)-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine (3) was selected for extended in vitro studies as a potent and selective 5-HT₆R ligand (K_i = 13 nM). Its ability to permeate the blood-brain barrier (BBB) and its hepatotoxicity were evaluated. In addition, X-ray crystallography and solubility studies were also performed. The results obtained confirm that 6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine derivatives, especially compound 3, are promising structures for further pharmacological studies as 5-HT₆R ligands.

Impact of the Substitution Pattern at the Basic Center and Geometry of the Amine Fragment on 5-HT6 and D3R Affinity in the 1H-Pyrrolo[3,2-c]quinoline Series

Salt bridge (SB, double-charge-assisted hydrogen bonds) formation is one of the strongest molecular non-covalent interactions in biological systems, including ligand-receptor complexes. In the case of G-protein-coupled receptors, such an interaction is formed by the conserved aspartic acid (D3.32) residue and the basic moiety of the aminergic ligand. This study aims to determine the influence of the substitution pattern at the basic nitrogen atom and the geometry of the amine moiety at position 4 of 1H-pyrrolo[3,2-c]quinoline on the quality of the salt bridge formed in the 5-HT₆ receptor and D₃ receptor. To reach this goal, we synthetized and biologically evaluated a new series of 1H-pyrrolo[3,2-c]quinoline derivatives modified with various amines. The selected compounds displayed a significantly higher 5-HT₆R affinity and more potent 5-HT₆R antagonist properties when compared with the previously identified compound PZ-1643, a dual-acting 5-HT₆R/D₃R antagonist; nevertheless, the proposed modifications did not improve the activity at D₃R. As demonstrated by the in silico experiments, including molecular dynamics simulations, the applied structural modifications were highly beneficial for the formation and quality of the SB formation at the 5-HT₆R binding site; however, they are unfavorable for such interactions at D₃R.

Serotonin Receptor Gene Polymorphisms Are Associated with Cerebrospinal Fluid, Genetic, and Neuropsychological Biomarkers of Alzheimer's Disease

A decrease in serotonergic transmission throughout the brain is among the earliest pathological changes in Alzheimer's disease (AD). Serotonergic receptors are also affected in AD. Polymorphisms in genes of serotonin (5HT) receptors have been mostly associated with behavioral and psychological symptoms of dementia (BPSD). In this study, we examined if AD patients carrying different genotypes in 5HTR1B rs13212041, 5HTR2A rs6313 (T102C), 5HTR2C rs3813929 (-759C/T), and 5HTR6 rs1805054 (C267T) polymorphisms have a higher risk of faster disease progression (assessed by neuropsychological testing), are more prone to develop AD-related pathology (reflected by levels of cerebrospinal fluid [CSF] AD biomarkers), or have an association with an apolipoprotein E (APOE) haplotype. This study included 115 patients with AD, 53 patients with mild cognitive impairment (MCI), and 2701 healthy controls. AD biomarkers were determined in the CSF of AD and MCI patients using enzyme-linked immunosorbent assays (ELISA), while polymorphisms were determined using either TaqMan SNP Genotyping Assays or Illumina genotyping platforms. We detected a significant decrease in the CSF amyloid β1-42 (Aβ1-42) and an increase in p-tau181/Aβ1-42 ratio in carriers of the T allele in the 5HTR2C rs3813929 (-759C/T) polymorphism. A significantly higher number of APOE ε4 allele carriers was observed among individuals carrying a TT genotype within the 5HTR2A T102C polymorphism, a C allele within the 5HTR1B rs13212041 polymorphism, and a T allele within the 5HTR6 rs1805054 (C267T) polymorphism. Additionally, individuals carrying the C allele within the 5HTR1B rs13212041 polymorphism were significantly more represented among AD patients and had poorer performances on the Rey-Osterrieth test. Carriers of the T allele within the 5HTR6 rs1805054 had poorer performances on the MMSE and ADAS-Cog. As all four analyzed polymorphisms of serotonin receptor genes showed an association with either genetic, CSF, or neuropsychological biomarkers of AD, they deserve further investigation as potential early genetic biomarkers of AD.

Effect of Concurrent Use of Memantine on the Efficacy of Masupirdine (SUVN-502): A Post Hoc Analysis of a Phase 2 Randomized Placebo-Controlled Study

INTRODUCTION

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive deterioration in cognition, memory and activities of daily living. Selective blockade of serotonin-6 (5-HT6) receptors, which are exclusively localized to the central nervous system, is reported to play an important role in learning and memory. Masupirdine is a potent and selective 5-HT6 receptor antagonist with pro-cognitive properties in animal models of cognition.

METHODS

The efficacy and safety of masupirdine were evaluated in patients with moderate AD concurrently treated with donepezil and memantine. A total of 564 patients were randomized in a 1:1:1 ratio. The study consisted of a 26-week double-blind treatment period. The primary efficacy outcome was the 11-item cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog 11). Changes from baseline were analyzed using a mixed effects model for repeated measures (MMRM). In exploratory post hoc analyses, patients were subdivided based on the use of memantine dosage forms and memantine plasma concentrations, to evaluate the impact of memantine on the efficacy of masupirdine.

RESULTS

In an exploratory post hoc analysis, less worsening in cognition (ADAS-Cog 11 scores) was observed with masupirdine treatment as compared with placebo in patients whose trough memantine plasma concentrations were ≤ 100 ng/mL.

CONCLUSIONS

Although prespecified study endpoints of the phase 2 study were not met, these exploratory post hoc subgroup observations are hypothesis-generating and suggest that the efficacy of masupirdine was adversely affected by concurrent therapy with memantine. Further assessment of masupirdine to determine its potential role as a treatment option for cognitive deficits associated with AD is warranted.

TRIAL REGISTRATION

The study was registered at ClinicalTrials.gov (NCT02580305).

Potential beneficial effects of masupirdine (SUVN-502) on agitation/aggression and psychosis in patients with moderate Alzheimer's disease: Exploratory post hoc analyses

OBJECTIVES

The effects of masupirdine on the neuropsychiatric symptoms were explored.

METHODS

Masupirdine (SUVN-502) was evaluated for its effects on cognition in patients with moderate AD. The prespecified primary outcome showed no drug-placebo difference. Post hoc analyses of domains of the 12-item neuropsychiatric inventory scale were carried out.

RESULTS

In a subgroup of patients (placebo, n = 57; masupirdine 50 mg, n = 53; masupirdine 100 mg, n = 48) with baseline agitation/aggression symptoms ≥1, a statistically significant reduction in agitation/aggression scores was observed in masupirdine 50 mg (95% confidence interval (CI), -1.9 to -0.5, p < 0.001) and masupirdine 100 mg (95% CI, -1.7 to -0.3, p = 0.007) treated arms at Week 13 in comparison to placebo and the effect was sustained for trial duration of 26 weeks in the masupirdine 50 mg treatment arm (95% CI, -2.3 to -0.8, p < 0.001). Similar observations were noted in the subgroup of patients (placebo, n = 29; masupirdine 50 mg, n = 30; masupirdine 100 mg, n = 21) with baseline agitation/aggression symptoms ≥3. In the subgroup of patients (placebo, n = 28; masupirdine 50 mg, n = 28; masupirdine 100 mg, n = 28) who had baseline psychosis symptoms and/or symptom emergence, a significant reduction in psychosis scores was observed in the masupirdine 50 mg (Week 4: 95% CI, -2.8 to -1.4, p < 0.001; Week 13: 95% CI, -3.3 to -1.3, p < 0.001) and masupirdine 100 mg (Week 4: 95% CI, -1.4 to 0, p = 0.046; Week 13: 95% CI, -1.9 to 0.1, p = 0.073) treatment arms in comparison to placebo.

CONCLUSION

Further research is warranted to explore the potential beneficial effects of masupirdine on NPS.

Effect of masupirdine (SUVN-502) on cognition in patients with moderate Alzheimer's disease: A randomized, double-blind, phase 2, proof-of-concept study

Introduction

This study explored the efficacy and safety of a serotonin-6 receptor antagonist, masupirdine, as adjunct treatment in patients with moderate Alzheimer's disease (AD) concomitantly treated with donepezil and memantine.

Methods

The effects of masupirdine were evaluated in patients with moderate AD dementia on background treatment with donepezil and memantine. Five hundred thirty-seven patients were expected to be randomized in a 1:1:1 ratio, using permuted blocked randomization. After a 2- to 4-week screening period, the study consisted of a 26-week double-blind treatment period, and a 4-week washout period. The primary efficacy measure was the 11-item cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog 11). Secondary efficacy measures were Clinical Dementia Rating Scale-Sum of Boxes, Mini-Mental State Examination, 23-item Alzheimer's Disease Co-operative Study Activities of Daily Living, and 12-item Neuropsychiatric Inventory. Changes from baseline were analyzed using a mixed effects model for repeated measures (MMRM). A total of 564 patients were randomized to receive either daily masupirdine 50 mg (190 patients), masupirdine 100 mg (185 patients), or placebo (189 patients). The study is registered at ClinicalTrials.gov (NCT02580305).

Results

The MMRM results showed statistically non-significant treatment differences in change from baseline in ADAS-Cog 11 scores at week 26, comparing each masupirdine dose arm to the placebo arm. No significant treatment effects were observed in the secondary evaluations.

Discussion

Masupirdine was generally safe and well tolerated. Possible reasons for the observed trial results are discussed.

Highlights

Masupirdine was evaluated in moderate Alzheimer's disease patients.First trial in class with background treatment of donepezil and memantine.Masupirdine was generally safe and well tolerated.Possible reasons for the observed trial results are discussed.

In Silico Screening of Natural Compounds for Candidates 5HT6 Receptor Antagonists against Alzheimer's Disease

Alzheimer's disease (AD), a devastating neurodegenerative disease, is the focus of pharmacological research. One of the targets that attract the most attention for the potential therapy of AD is the serotonin 5HT6 receptor, which is the receptor situated exclusively in CNS on glutamatergic and GABAergic neurons. The neurochemical impact of this receptor supports the hypothesis about its role in cognitive, learning, and memory systems, which are of critical importance for AD. Natural products are a promising source of novel bioactive compounds with potential therapeutic potential as a 5HT6 receptor antagonist in the treatment of AD dementia. The ZINC-natural product database was in silico screened in order to find the candidate antagonists of 5-HT6 receptor against AD. A virtual screening protocol that includes both short-and long-range interactions between interacting molecules was employed. First, the EIIP/AQVN filter was applied for in silico screening of the ZINC database followed by 3D QSAR and molecular docking. Ten best candidate compounds were selected from the ZINC Natural Product database as potential 5HT6 Receptor antagonists and were proposed for further evaluation. The best candidate was evaluated by molecular dynamics simulations and free energy calculations.

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

Evidence-based complementary and alternative medicine bioinformatics approach through network pharmacology and molecular docking to determine the molecular mechanisms of Erjing pill in Alzheimer's disease

Erjing pill, a Traditional Chinese Medicine (TCM) formulation composed of Polygonatum sibiricum and Lycium chinense, has an important role in the treatment of Alzheimer's disease (AD). However, the underlying mechanisms of the action of Erjing pill in AD have remained elusive. In the present study, the key ingredients of Erjing pill were investigated and the active components and their mechanisms of action on AD were analyzed based on networks pharmacology. By using the TCM and TCM Systems Pharmacology and databases, the components of Erjing pill were screened and the data were captured using Discovery Studio. The SwissTarget webserver database was used to predict the potential protein targets of Erjing pill components for pathologies related to AD. The data were further analyzed with the disease targets of AD based on analysis of the Online Mendelian Inheritance in Man, DiGSeE and Therapeutic Target Database. Subsequent analysis of mechanistic pathways of the screened components and protein targets allowed us to construct a network by using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes, which revealed potential molecular mechanisms of Erjing pill against AD. Finally, the protective effect of three active components on neurons was verified using an in vitro injury model of PC12 cells induced by Aβ25-35. The results indicated that 65 bioactive components of Erjing pill, including lauric acid and zederone, and 6 targets, including acetylcholinesterase, butylcholinesterase and amyloid protein precursor, were closely associated with the prevention and treatment of AD. The molecular components of Erjing pill were indicated to be involved in various biological signaling processes, mainly in synaptic signal transmission, transsynaptic signal transmission and chemical synaptic transmission. Furthermore, related pathways targeted by Erjing pill in AD included the regulation of neuroactive ligand-receptor interactions, the PI3K-Akt signaling pathway, serotoninergic synapses, calcium signaling pathways and dopaminergic synapses. A cell viability assay indicated that the compounds (polygonatine A, polygonatine C and 4',5-dihydroxyflavone) assessed were able to significantly improve the survival rate and increase the Ca2+ level in a PC12 cell model of AD induced by amyloid-β25-35. The present study revealed that the mechanisms of action of Erjing pill to prevent and treat AD included a multicompound, multitarget and multipathway regulatory network.

Toward Predicting Impact of Common Genetic Variants on Schizophrenia Clinical Responses With Antipsychotics: A Quantitative System Pharmacology Study

CNS disorders are lagging behind other indications in implementing genotype-dependent treatment algorithms for personalized medicine. This report uses a biophysically realistic computer model of an associative and dorsal motor cortico-striatal-thalamo-cortical loop and a working memory cortical model to investigate the pharmacodynamic effects of COMTVal158Met rs4680, 5-HTTLPR rs 25531 s/L and D2DRTaq1A1 genotypes on the clinical response of 7 antipsychotics. The effect of the genotypes on dopamine and serotonin dynamics and the level of target exposure for the drugs was calibrated from PET displacement studies. The simulations suggest strong gene-gene pharmacodynamic interactions unique to each antipsychotic. For PANSS Total, the D2DRTaq1 allele has the biggest impact, followed by the 5-HTTLPR rs25531. The A2A2 genotype improved efficacy for all drugs, with a more complex outcome for the 5-HTTLPR rs25531 genotype. Maximal range in PANSS Total for all 27 individual combinations is 3 (aripiprazole) to 5 points (clozapine). The 5-HTTLPR L/L with aripiprazole and risperidone and the D2DRTaq1A2A2 allele with haloperidol, clozapine and quetiapine reduce the motor side-effects with opposite effects for the s/s genotype. The COMT genotype has a limited effect on antipsychotic effect and EPS. For cognition, the COMT MM 5-HTTLPR L/L genotype combination has the best performance for all antipsychotics, except clozapine. Maximal difference is 25% of the total dynamic range in a 2-back working memory task. Aripiprazole is the medication that is best suited for the largest number of genotype combinations (10) followed by Clozapine and risperidone (6), haloperidol and olanzapine (3) and quetiapine and paliperidone for one genotype. In principle, the platform could identify the best antipsychotic treatment balancing efficacy and side-effects for a specific individual genotype. Once the predictions of this platform are validated in a clinical setting the platform has potential to support rational personalized treatment guidance in clinical practice.

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.

Neuropathic pain-alleviating activity of novel 5-HT6 receptor inverse agonists derived from 2-aryl-1H-pyrrole-3-carboxamide

The diverse signaling pathways engaged by serotonin type 6 receptor (5-HT₆R) together with its high constitutive activity suggests different types of pharmacological interventions for the treatment of CNS disorders. Non-physiological activation of mTOR kinase by constitutively active 5-HT₆R under neuropathic pain conditions focused our attention on the possible repurposing of 5-HT₆R inverse agonists as a strategy to treat painful symptoms associated with neuropathies of different etiologies. Herein, we report the identification of compound 33 derived from the library of 2-aryl-1H-pyrrole-3-carboxamides as a potential analgesic agent. Compound 33 behaves as a potent 5-HT₆R inverse agonist at Gs, Cdk5, and mTOR signaling. Preliminary ADME/Tox studies revealed preferential distribution of 33 to the CNS and placed it in the low-risk safety space. Finally, compound 33 dose-dependently reduced tactile allodynia in spinal nerve ligation (SNL)-induced neuropathic rats.

Therapy for Alzheimer's disease: Missing targets and functional markers?

The development of the next generation therapy for Alzheimer's disease (AD) presents a huge challenge given the number of promising treatment candidates that failed in trials, despite recent advancements in understanding of genetic, pathophysiologic and clinical characteristics of the disease. This review reflects some of the most current concepts and controversies in developing disease-modifying and new symptomatic treatments. It elaborates on recent changes in the AD research strategy for broadening drug targets, and potentials of emerging non-pharmacological treatment interventions. Established and novel biomarkers are discussed, including emerging cerebrospinal fluid and plasma biomarkers reflecting tau pathology, neuroinflammation and neurodegeneration. These fluid biomarkers together with neuroimaging findings can provide innovative objective assessments of subtle changes in brain reflecting disease progression. A particular emphasis is given to neurophysiological biomarkers which are well-suited for evaluating the brain overall neural network integrity and function. Combination of multiple biomarkers, including target engagement and outcome biomarkers will empower translational studies and facilitate successful development of effective therapies.

Serotonin 6 receptor modulation reduces locomotor activity in C57BL/6J mice

The serotonin 6 receptor (5-HT6) is a more recently identified therapeutic target for several neuropsychiatric disorders. While the 5-HT6 receptor has gained interest as a target for novel therapeutics, determining the basic sex differences is lacking in the literature. To address this, the present study examined the effects of 5-HT6 receptor modulation on locomotor activity and open field measures of anxiety in C57BL/6J mice. Female and male mice were tested after acute treatment with either 5-HT6 receptor antagonist SB 271046 or 5-HT6 receptor agonist EMD 386088. Acute 5-HT6 receptor blockade with SB 271046 attenuated locomotor activity in C57BL6/J mice, irrespective of sex. When locomotor activity was analyzed for six 10 min time blocks, 0.1, 5, or 15 mg/kg of SB 271046 reduced locomotor activity for the initial 40 min of testing, but only 5 and 15 mg/kg SB 271046 exhibited a reduction in locomotor activity for at least 60 min. EMD 386088 only attenuated locomotor activity when mice were treated with the high dose of 15 mg/kg EMD 386088. This was true for all time blocks except for the 40-50 min time block. In addition, EMD 386088 at the 15 mg/kg dose reduced locomotor activity in female mice more than males during the 20-30 and 30-40 minute time blocks. Analysis of the anxiolytic properties of 5-HT6 receptor modulation via the open field, showed that SB 271046 did not demonstrate anxiogenic properties in either sex at the doses tested. Instead, 15 mg/kg EMD 386088 produced an anxiogenic effect in both female and male mice. Together these findings highlight the differing impact of specific 5-HT6 receptor modulation on locomotor activity in C57BL/6J mice.

2-Phenyl-1H-pyrrole-3-carboxamide as a New Scaffold for Developing 5-HT6 Receptor Inverse Agonists with Cognition-Enhancing Activity

Serotonin type 6 receptor (5-HT₆R) has gained particular interest as a promising target for treating cognitive deficits, given the positive effects of its antagonists in a wide range of memory impairment paradigms. Herein, we report on degradation of the 1H-pyrrolo[3,2-c]quinoline scaffold to provide the 2-phenyl-1H-pyrrole-3-carboxamide, which is devoid of canonical indole-like skeleton and retains recognition of 5-HT₆R. This modification has changed the compound’s activity at 5-HT₆R-operated signaling pathways from neutral antagonism to inverse agonism. The study identified compound 27 that behaves as an inverse agonist of the 5-HT₆R at the Gs and Cdk5 signaling pathways. Compound 27 showed high selectivity and metabolic stability and was brain penetrant. Finally, 27 reversed scopolamine-induced memory decline in the novel object recognition test and exhibited procognitive properties in the attentional set-shifting task in rats. In light of these findings, 27 might be considered for further evaluation as a new cognition-enhancing agent, while 2-phenyl-1H-pyrrole-3-carboxamide might be used as a template for designing 5-HT₆R inverse agonists.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Anti-PTSD Effects of Hypidone Hydrochloride (YL-0919): A Novel Combined Selective 5-HT Reuptake Inhibitor/5-HT1A Receptor Partial Agonist/5-HT6 Receptor Full Agonist

Posttraumatic stress disorder (PTSD) is a debilitating trauma and stressor-related disorder that has become a major neuropsychiatric problem, leading to substantial disruptions in individual health and societal costs. Our previous studies have demonstrated that hypidone hydrochloride (YL-0919), a novel combined selective 5-HT reuptake inhibitor/5-HT_1A receptor partial agonist/5-HT₆ receptor full agonist, exerts notable antidepressant- and anxiolytic-like as well as procognitive effects. However, whether YL-0919 exerts anti-PTSD effects and its underlying mechanisms are still unclear. In the present study, we showed that repeated treatment with YL-0919 caused significant suppression of contextual fear, enhanced anxiety and cognitive dysfunction induced by the time-dependent sensitization (TDS) procedure in rats and by inescapable electric foot-shock in a mouse model of PTSD. Furthermore, we found that repeated treatment with YL-0919 significantly reversed the accompanying decreased expression of the brain-derived neurotrophic factor (BDNF) and the synaptic proteins (synapsin1 and GluA1), and ameliorated the neuroplasticity disruption in the prefrontal cortex (PFC), including the dendritic complexity and spine density of pyramidal neurons. Taken together, the current study indicated that YL-0919 exerts clear anti-PTSD effects, which might be partially mediated by ameliorating the structural neuroplasticity by increasing the expression of BDNF and the formation of synaptic proteins in the PFC.

Imidazopyridine-Based 5-HT6 Receptor Neutral Antagonists: Impact of N1-Benzyl and N1-Phenylsulfonyl Fragments on Different Receptor Conformational States

G-protein coupled receptors (GPCRs) exist in an equilibrium of multiple conformational states, including different active states, which depend on the nature of the bound ligand. In consequence, different conformational states can initiate specific signal transduction pathways. The study identified compound 7e, which acts as a potent 5-hydroxytryptamine type 6 receptor (5-HT₆R) neutral antagonist at Gs and does not impact neurite growth (process controlled by Cdk5). MD simulations highlighted receptor conformational changes for 7e and inverse agonist PZ-1444. In cell-based assays, neutral antagonists of the 5-HT₆R (7e and CPPQ), but not inverse agonists (SB-258585, intepirdine, PZ-1444), displayed glioprotective properties against 6-hydroxydopamine-induced and doxorubicin-induced cytotoxicity. These suggest that targeting the activated conformational state of the 5-HT₆R with neutral antagonists implicates the protecting properties of astrocytes. Additionally, 7e prevented scopolamine-induced learning deficits in the novel object recognition test in rats. We propose 7e as a probe for further understanding of the functional outcomes of different states of the 5-HT₆R.

Procognitive profiling of a serotonin 5-HT6 receptor antagonist in a complex model system in rats: A novel translational approach for clinical prediction

INTRODUCTION

The serial clinical failures of novel cognitive enhancer candidates point out the lack of predictive power in the preceding animal experimentation. For a more predictive profiling of putative procognitive drugs in rodents, we recently elaborated a methodical approach which consists of three fundamental steps: 1. teaching various learning tasks representing different cognitive domains to the same cohort of animals with the aim to create a population with 'widespread knowledge'. 2. Applying a cognitive deficit-inducing intervention to transform this cohort of animals to a 'patient population'. 3. Testing putative procognitive drugs with a 'clinical trial-like' design on the wide spectrum of cognitive (dys)functions in the actual 'patient population'. The present study has been the first trial to test the feasibility and utility of the proposed system.

METHODS

The population with 'widespread knowledge' consisted of 2 year old male Long-Evans rats with a learning history in five-choice serial reaction time task (5-CSRTT, attentional paradigm), Morris water maze (MWM, spatial learning), a cooperative task carried out in pairs (social learning), and a skill-learning task, "pot-jumping". For inducing cognitive deficit, thus creating a 'patient population' we increased the difficulty of the tasks. For the cognitive enhancer mechanism to test in the system we chose a serotonin 5-HT₆ receptor antagonist compound, RO4368554. Animals were randomly assigned to vehicle- and drug treated groups based on their baseline learning performance and their response in a pilot test of increase in task difficulty. During the 13-day long treatment with 3 mg/kg ip. RO4368554 all the learning paradigms were repeatedly run with increased difficulty supplemented with a novel object recognition test (NOR, episodic memory).

RESULTS

In the 5-CSRTT, reducing the stimulus duration from 1 s to 0.25 s caused a significant decrease in the percentage of correct responses (from 52 % to 31 % in the control group) which was not affected by the 5-HT₆ receptor antagonist treatment (correct responses decreased from 58 % to 31 %). In the MWM, replacing the escape platform to a new location did not mean a hard challenge for the rats. Members of both groups could find it within a relatively short time: mean escape latencies were 83 s and 65 s at the first replacement trial and 58 s and 74 s at the second one in the control and drug-treated groups, respectively. In the cooperation paradigm, where the rats had to perform simultaneous nose-pokes to get a reward, task difficulty was increased by requiring two consecutive simultaneous nose-poking from the animals. This caused a fall in the percentage of successful trials in both groups (from 48 % to 12 % and from 50 % to 20 % in the saline - and drug-treated group, respectively), however, by the end of the treatment RO4368554-treated animals showed significantly higher performance (29 %) than saline treated rats (2%). The NOR test, carried out with a 5 -h delay, revealed poor recognition memory in both groups (discrimination index (DI) values were 0.13 and 0.06 for saline and RO4368554, respectively). Performance in the pot jumping test was also not improved by the drug-treatment.

CONCLUSIONS

The applied study design allowed parallel measurements of the action of the test compound on several cognitive functions and to follow its time course. RO4368554 did not show notable effects on impaired attention and visual recognition; nor did it affect spatial and procedural learning, but it exerted beneficial effect on cooperative behaviour. The revealed activity pattern highlight the cognitive domain most sensitive to the particular drug effect and may give hints for further target validating and clinical studies.

The 5-HT6 Receptors in the Ventrolateral Orbital Cortex Attenuate Allodynia in a Rodent Model of Neuropathic Pain

Mechanical allodynia, characterized by a painful sensation induced by innocuous stimuli, is thought to be caused by disruption in pain-related regions. Identification and reversal of this pathologic neuroadaptation are therefore beneficial for clinical treatment. Previous evidence suggests that 5-HT₆ receptors in the ventrolateral orbital cortex (VLO) are involved in neuropathic pain, but their function is poorly understood. The aim of the present study is to unveil the role of 5-HT₆ receptors in the VLO and the underlying mechanisms in pain modulation. Here, by using the spared nerve injury (SNI) pain model, first, we report that 5-HT₆ receptor protein decreased in the contralateral VLO compared with the ipsilateral VLO in rats with allodynia. Second, microinjection of the selective 5-HT₆ receptor agonists EMD-386088 and WAY-208466 into the contralateral VLO consistently and significantly depressed allodynia. Third, microinjection of the selective antagonist SB-258585 blocked the agonist-induced anti-allodynic effect, while the antagonist applied alone to the VLO had no effect. Furthermore, the anti-nociceptive effect of EMD-386088 on neuropathic pain was prevented by the adenylate cyclase (AC) inhibitor SQ-22536, and protein kinase A (PKA) inhibitor H89, suggesting that AC/PKA signaling might underlie the antinociception of agonists. Finally, the 5-HT₆ receptors were found to be colocalized with a glutamate transporter (EAAC1) by immunofluorescent staining, and the glutamate receptor antagonist kynurenic acid was found to completely block antinociception. These findings indicated that the antinociceptive effect of 5-HT₆ receptor agonists might occur via interaction with the glutamatergic system. Altogether, the agonists activated 5-HT₆ receptors present in the glutamatergic neurons in the VLO to facilitate the AC/PKA cascade, which subsequently might evoke glutamate release, thus depressing allodynia. These findings suggest a potential therapeutic role of 5-HT₆ receptor agonists in treating neuropathic pain.

Opportunities for multiscale computational modelling of serotonergic drug effects in Alzheimer's disease

Alzheimer's disease (AD) is an age-specific neurodegenerative disease that compromises cognitive functioning and impacts the quality of life of an individual. Pathologically, AD is characterised by abnormal accumulation of beta-amyloid (Aβ) and hyperphosphorylated tau protein. Despite research advances over the last few decades, there is currently still no cure for AD. Although, medications are available to control some behavioural symptoms and slow the disease's progression, most prescribed medications are based on cholinesterase inhibitors. Over the last decade, there has been increased attention towards novel drugs, targeting alternative neurotransmitter pathways, particularly those targeting serotonergic (5-HT) system. In this review, we focused on 5-HT receptor (5-HTR) mediated signalling and drugs that target these receptors. These pathways regulate key proteins and kinases such as GSK-3 that are associated with abnormal levels of Aβ and tau in AD. We then review computational studies related to 5-HT signalling pathways with the potential for providing deeper understanding of AD pathologies. In particular, we suggest that multiscale and multilevel modelling approaches could potentially provide new insights into AD mechanisms, and towards discovering novel 5-HTR based therapeutic targets.

A dual-acting 5-HT6 receptor inverse agonist/MAO-B inhibitor displays glioprotective and pro-cognitive properties

The complex etiology of Alzheimer's disease has initiated a quest for multi-target ligands to address the multifactorial causes of this neurodegenerative disorder. In this context, we designed dual-acting 5-HT₆ receptor (5-HT₆R) antagonists/MAO-B inhibitors using pharmacophore hybridization strategy. Our approach involved linking priviliged scaffolds of 5-HT₆R with aryloxy fragments derived from reversible and irreversible MAO-B inhibitors. The study identified compound 48 that acts as an inverse agonist of 5-HT₆R at G_s signaling and an irreversible MAO-B inhibitor. Compound 48 showed moderate metabolic stability in rat microsomal assay, artificial membrane permeability, no hepatotoxicity, and it was well distributed to the brain. Additionally, 48 showed glioprotective properties in a model of cultured astrocytes using 6-OHDA as the cytotoxic agent. Finally, compound 48 (MED = 1 mg/kg, p.o.) fully reversed memory deficits in the NOR task induced by scopolamine in rats. A better understanding of effects exerted by dual-acting 5-HT₆R/MAO-B modulators may impact the future development of neurodegenerative-directed treatment strategies.

Stress-induced aggression in heterozygous TPH2 mutant mice is associated with alterations in serotonin turnover and expression of 5-HT6 and AMPA subunit 2A receptors

BACKGROUND

The contribution of gene-environment interactions that lead to excessive aggression is poorly understood. Environmental stressors and mutations of the gene encoding tryptophan hydroxylase-2 (TPH2) are known to influence aggression. For example, TPH2 null mutant mice (Tph2-/-) are naturally highly aggressive, while heterozygous mice (Tph2+/-) lack a behavioral phenotype and are considered endophenotypically normal. Here we sought to discover whether an environmental stressor would affect the phenotype of the genetically 'susceptible' heterozygous mice (Tph2+/-).

METHODS

Tph2+/- male mice or Tph2+/+ controls were subjected to a five-day long rat exposure stress paradigm. Brain serotonin metabolism and the expression of selected genes encoding serotonin receptors, AMPA receptors, and stress markers were studied.

RESULTS

Stressed Tph2+/- mice displayed increased levels of aggression and social dominance, whereas Tph2+/+ animals became less aggressive and less dominant. Brain tissue concentrations of serotonin, its precursor hydroxytryptophan and its metabolite 5-hydroxyindoleacetic acid were significantly altered in all groups in the prefrontal cortex, striatum, amygdala, hippocampus and dorsal raphe after stress. Compared to non-stressed animals, the concentration of 5-hydroxytryptophan was elevated in the amygdala though decreased in the other brain structures. The overexpression of the AMPA receptor subunit, GluA2, and downregulation of 5-HT6 receptor, as well as overexpression of c-fos and glycogen-synthase-kinase-3β (GSK3-β), were found in most structures of the stressed Tph2+/- mice.

LIMITATIONS

Rescue experiments would help to verify causal relationships of reported changes.

CONCLUSIONS

The interaction of a partial TPH2 gene deficit with stress results in pathological aggression and molecular changes, and suggests that the presence of genetic susceptibility can augment aggression in seemingly resistant phenotypes.

Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Change in Expression of 5-HT6 Receptor at Different Stages of Alzheimer's Disease: A Postmortem Study with the PET Radiopharmaceutical [18F]2FNQ1P

BACKGROUND

The 5-HT6 receptor is one of the most recently identified serotonin receptors in the central nervous system. Because of its role in memory and cognitive process, this receptor might be implicated in Alzheimer's disease (AD) and associated disorders.

OBJECTIVE

The aim of this study was to investigate the binding of [18F]2FNQ1P, a new specific radiotracer of 5-HT6 receptors, and to quantify 5-HT6 receptor density in caudate nucleus in a population of patients with different AD stages.

METHODS

Patients were classified according to the "ABC" NIA-AA classification. In vitro binding assays were performed in postmortem brain tissue from the healthy control (HC; n = 8) and severe AD ("High"; n = 8) groups. In vitro quantitative autoradiography was performed in human brain tissue (caudate nucleus) from patients with different stages of AD: HC (n = 15), "Low" (n = 18), "Int" (n = 20), and "High" (n = 15).

RESULTS

In vitro binding assays did not show significant differences for the KD and Bmax parameters between "High" and HC groups. In vitro quantitative autoradiography showed a significant difference between the "High" and HC groups (p = 0.0025). We also showed a progressive diminution in [18F]2FNQ1P specific binding, which parallels 5-HT6 receptors expression, according to increasing AD stage. Significant differences were observed between the HC group and all AD stages combined ("Low", "Intermediate", and "High") (p = 0.011).

CONCLUSION

This study confirms the interest of investigating the role of 5-HT6 receptors in AD and related disorders. [18F]2FNQ1P demonstrated specific binding to 5-HT6 receptors.

Learning from amyloid trials in Alzheimer's disease. A virtual patient analysis using a quantitative systems pharmacology approach

BACKGROUND

Many trials of amyloid-modulating agents fail to improve cognitive outcome in Alzheimer's disease despite substantial reduction of amyloid β levels.

METHODS

We applied a mechanism-based Quantitative Systems Pharmacology model exploring the pharmacodynamic interactions of apolipoprotein E (APOE), Catechol -O -methyl Transferase (COMTVal158Met), and 5-HT transporter (5-HTTLPR) rs25531 genotypes and aducanumab.

RESULTS

The model predicts large clinical variability. Anticipated placebo differences on Alzheimer's Disease Assessment Scale (ADAS)-COG in the aducanumab ENGAGE and EMERGE ranged from 0.77 worsening to 1.56 points improvement, depending on the genotype-comedication combination. 5-HTTLPR L/L subjects are found to be the most resilient. Virtual patient simulations suggest improvements over placebo between 4% and 20% at the 10 mg/kg dose, depending on the imbalance of the 5-HTTLPR genotype and exposure. In the Phase II PRIME trial, maximal anticipated placebo difference at 10 mg/kg ranges from 0.3 worsening to 5.3 points improvement.

DISCUSSION

These virtual patient simulations, once validated against clinical data, could lead to better informed future clinical trial designs.

Multifunctional 6-fluoro-3-[3-(pyrrolidin-1-yl)propyl]-1,2-benzoxazoles targeting behavioral and psychological symptoms of dementia (BPSD)

Patients suffering from dementia experience cognitive deficits and 90% of them show non-cognitive behavioral and psychological symptoms of dementia (BPSD). The spectrum of BPSD includes agitation, depression, anxiety and psychosis. Antipsychotics, e.g. quetiapine, have been commonly used off-label to control the burdensome symptoms, though they cause serious side effects and further cognitive impairment. Therefore, the development of targeted therapy for BPSD, suitable for elderly patients, remains relevant. A multitarget-directed ligand, acting on serotonin 5-HT_2A and dopamine D₂ receptors (R) and thus exerting anti-aggressive and antipsychotic activity, as well as on 5-HT₆Rs and 5-HT₇Rs (potential pro-cognitive, antidepressant and anxiolytic activity), poses a promising strategy for the treatment of BPSD. Antitargeting muscarinic M₃R and hERG channel is expected to reduce the risk of side effects. We obtained a series of stereoisomeric compounds by combining 6-fluoro-1,2-benzoxazole moiety and arylsulfonamide fragment through pyrrolidin-1-yl-propyl linker. N-[(3R)-1-[3-(6-fluoro-1,2-benzoxazol-3-yl)propyl]pyrrolidin-3-yl]-1-benzothiophene-2-sulfonamide showed a substantial affinity for the targets of interest (pK_i = 8.32-9.35) and no significant interaction with the antitargets. Functional studies revealed its antagonist efficacy (pK_B = 7.41-9.03). The lead compound showed a promising profile of antipsychotic-like activity in amphetamine- and MK-801-induced hyperlocomotion (MED = 2.5 mg/kg), antidepressant-like, as well as anxiolytic-like activity in mice (MED = 0.312 and 1.25 mg/kg in the forced swim and four-plate tests, respectively). Notably, the novel compound didn't affect spontaneous locomotor activity, nor induced catalepsy or memory deficits (step-through passive avoidance test) in therapeutically relevant doses, which proved its benign safety profile. The overall pharmacological characteristics of the lead compound outperformed the reference drug quetiapine, making it a promising option for evaluation in the treatment of BPSD.

Serotonin in Animal Cognition and Behavior

Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator of nervous systems in both invertebrates and vertebrates. It has been proposed for several decades that it impacts animal cognition and behavior. In spite of a completely distinct organization of the 5-HT systems across the animal kingdom, several lines of evidence suggest that the influences of 5-HT on behavior and cognition are evolutionary conserved. In this review, we have selected some behaviors classically evoked when addressing the roles of 5-HT on nervous system functions. In particular, we focus on the motor activity, arousal, sleep and circadian rhythm, feeding, social interactions and aggressiveness, anxiety, mood, learning and memory, or impulsive/compulsive dimension and behavioral flexibility. The roles of 5-HT, illustrated in both invertebrates and vertebrates, show that it is more able to potentiate or mitigate the neuronal responses necessary for the fine-tuning of most behaviors, rather than to trigger or halt a specific behavior. 5-HT is, therefore, the prototypical neuromodulator fundamentally involved in the adaptation of all organisms across the animal kingdom.

Are the Hydantoin-1,3,5-triazine 5-HT6R Ligands a Hope to a Find New Procognitive and Anti-Obesity Drug? Considerations Based on Primary In Vivo Assays and ADME-Tox Profile In Vitro

Though the 5-HT₆ serotonin receptor is an important target giving both agonists and antagonists similar therapeutic potency in the treatment of topic CNS-diseases, no 5-HT₆R ligand has reached the pharmaceutical market yet due to the too narrow chemical space of the known 5-HT₆R agents and insufficient “drugability.” Recently, a new group of non-indole and non-sulfone hydantoin-triazine 5-HT₆R ligands was found, where 3-((4-amino-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-yl)methyl)-5-methyl-5-(naphthalen-2-yl)imidazolidine-2,4-dione (KMP-10) was the most active member. This study is focused on wider pharmacological and “druglikeness” characteristics for KMP-10. A computer-aided insight into molecular interactions with 5-HT₆R has been performed. “Druglikeness” was examined using an eight-test panel in vitro, i.e., a parallel artificial membrane permeability assay (PAMPA), and Caco-2 permeability-, P-glycoprotein (Pgp) affinity-, plasma protein binding-, metabolic stability- and drug–drug interaction-assays, as well as mutagenicity- and HepG2-hepatotoxicity risk tests. Behavioral studies in vivo, i.e., elevated plus-maze (EPM) and novel object recognition (NOR) tests, were performed. Extended studies on the influence of KMP-10 on rats’ metabolism, including biochemical tests, were conducted in vivo. Results indicated significant anxiolytic and precognitive properties, as well as some anti-obesity properties in vivo, and it was found to satisfy the “druglikeness” profile in vitro for KMP-10. The compound seems to be a good lead-structure and candidate for wider pharmacological studies in search for new CNS-drugs acting via 5-HT₆R.

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.

The 5-HT6 receptor interactome: New insight in receptor signaling and its impact on brain physiology and pathologies

The serotonin (5-HT)₆ receptor is a Gs-coupled receptor exclusively expressed in the central nervous system. Highest receptor densities are found in brain regions implicated in mnemonic functions where the receptor is primarily but not exclusively located in the primary cilium of neurons. The 5-HT₆ receptor continues to raise particular interest for neuropharmacologists, given the pro-cognitive effects of antagonists in a wide range of cognitive impairment paradigms in rodents and human. The 5-HT₆ receptor also finely controls key neuro-developmental processes including neuron migration and differentiation. However, its influence upon neurodevelopment and cognition is not solely mediated by its coupling to the Gs-adenylyl cyclase pathway, suggesting alternative signal transduction mechanisms. This prompted studies aimed at characterizing the receptor interactome that identified 125 candidate receptor partners, making the 5-HT₆ receptor one of the G protein-coupled receptors with the most extensively characterized interactome. These studies showed that the receptor localization at the plasma membrane and, consequently, its signal transduction, are finely modulated by several receptor partners. They demonstrated that prefrontal 5-HT₆ receptors engage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT₆-mTOR pathway in temporal epilepsy. Finally, they revealed that the receptor activates Cdk5 signaling in an agonist-independent manner through a mechanism involving receptor phosphorylation by the associated Cdk5 and highlighted its key role in the migration of neurons and neurite growth. These new receptor-operated signaling mechanisms should be considered in the future development of drugs acting on 5-HT₆ receptors. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

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.