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.


Introduction
Serotonin-5-hydroxytryptamine (5-HT) or enteramine was discovered in the 1930s [1], and its presence in the brain was identified in the year 1953 [2]. Since then, advancements have been made in characterizing 5-HT and its receptors. 5-HT functions as an inhibitory monoamine neurotransmitter, a hormone, and a mitogen that plays an important role in the regulation of several basic functions of the central and peripheral nervous system [3]. The 5-HT transporter regulates the availability of 5-HT in the synapse. 5-HT mediates its functions through seven distinct families of receptors (5-HT 1-7 ), which comprise 14 distinct receptor subtypes. Most of the 5-HT receptor types are G-protein coupled receptors (GPCRs), except for the 5-HT 3 receptor, which is a ligand-gated cation channel. GPCRs can modulate diverse downstream signaling pathways, and this may partly explain the critical role of serotonin in controlling/modulating multiple physiological functions [4,5].
Only a fraction (~5%) of the total amount of 5-HT is produced in the brain [3,6], but 5-HT and 5-HT receptors play an important role in the regulation of many brain functions. In addition, the 5-HT system also interacts with other neurotransmitter systems in the brain [7]. Dysregulation of the 5-HT and/or 5-HT receptors has been implicated in the pathogenesis of several disorders including, but not limited to depression, anxiety, Alzheimer's disease (AD), schizophrenia, Parkinson's disease, and sleep disorders [8][9][10]. The majority of drugs (~40%) approved for use in humans for the treatment of various disorders target the 5-HT system [11]. The distribution of 5-HT receptors in the brain and diverse signaling pathways involved make the 5-HT system an important potential target for therapeutic interventions. The development and characterization of gene knock-out animals; expanding knowledge of receptor expression levels, specific localization, distribution and function; and progress in receptor subtype-specific agents comprise significant advancements in the area of 5-HT research. Considering the widespread distribution and functions of the serotonergic system, targeting the 5-HT receptors with specific characteristics and localization may define a drug development pathway. One of the 5-HT receptors-serotonin-6 (5-HT 6 ) receptor-has been an area of substantial research.
The 5-HT 6 receptor was first cloned and characterized from rat striatum by Monsma and colleagues in 1993 [12]. The human 5-HT 6 receptor was cloned and characterized in 1996 by Kohen and colleagues [13]. Detailed review of the discovery and characterization of the 5-HT 6 receptor has been previously reported [14][15][16]. Briefly, the 5-HT 6 receptor is a GPCR and is coupled to Gs/adenylyl cyclase pathway (adenylyl cyclase3 isoform) [16,17]. 5-HT 6 receptors are uniquely localized in the brain regions, including the olfactory tubercle, cerebral cortex, nucleus accumbens, striatum, hippocampus, thalamus, and cerebellum [12,18], providing an opportunity to identify new treatments modulating functions of specific brain regions. Although peripheral 5-HT 6 receptors expression has been reported in rodents, their expression is faint [16]. Thus, selective targeting of 5-HT 6 receptors may avoid unwanted side effects arising from other body systems (peripheral as well as other brain regions). In this report, we review the efforts and progress in the drug discovery research for agents targeting the 5-HT 6 receptors.

Literature Search
PubMed (United States National Library of Medicine, Bethesda, MD, USA) and clinical trial registries (ClinicalTrials.gov registry) were searched with terms including, but not limited to, "(serotonin-6 receptor) AND (Clinical trials)", "(5-HT 6 receptor) AND (Clinical trials)", (5-HT 6 receptor antagonist) AND (Clinical trials)", with publication date restricted to between 1 January 1996 to 30 November 2022. The choice of this search date was based on the cloning of human 5-HT 6 receptors in the year 1996. Additionally, searches were performed using internet-based search engines, such as Google and Google Scholar, to manually search for other relevant articles.
The 5-HT 6 receptor antagonists that were evaluated in patient populations (clinical trials) were considered and listed by code and compound name (if any). The literature on the non-clinical biology and clinical investigation of these 5-HT 6 receptor antagonists were collected from PubMed or ClinicalTrials.gov registry or Google or Google scholar by searching with the term "(compound name) and (code)", "(compound name) OR (code)".

Data Extraction
Information extracted from the clinical literature included indication, NCT number, endpoint and outcome. The information extracted from the non-clinical literature included the code(s), chemical structure, in-vitro profile, and in-vivo profile.

Results
The searches resulted in 306 publications. Of these, 87 publications (reports, press releases or documents) were included in this review ( Figure 1
In animal models, both 5-HT 6 receptor agonists and antagonists produced similar behavioral effects [37,38]. Most agents that were progressed to clinical evaluation were 5-HT 6 receptor antagonists, and we focus on the therapeutic indications of this class of agents. The chemical structures of these agents are included in Figure 2 and their non-clinical and clinical profiles are included in Tables 1-3, respectively. Biomolecules 2023, 13, x FOR PEER REVIEW 4 of 22 receptor agonists and antagonists with diverse physicochemical properties [20,[33][34][35] and specific interaction modes with the receptors have been identified [20,26,27,[33][34][35][36], such concepts are yet to be a basis for studies in patient populations. In animal models, both 5-HT6 receptor agonists and antagonists produced similar behavioral effects [37,38]. Most agents that were progressed to clinical evaluation were 5-HT6 receptor antagonists, and we focus on the therapeutic indications of this class of agents. The chemical structures of these agents are included in Figure 2 and their nonclinical and clinical profiles are included in Tables 1, 2     Attenuated the memory deficits induced by MK-801, and scopolamine in the object recognition task, passive avoidance task, and Morris water maze task [39] Cerlapirdine 24 [40] Not reported None [40] Attenuated the memory deficits induced by MK-801, scopolamine, combined scopolamine and MK-801 treatment in the object recognition task [40] Idalopirdine 60 # [52] CYP3A4 and CYP2D6 [53] Not reported Low [52] Improved cognition in phencyclidine-challenged rats; modulated dopamine, norepinephrine and glutamate neurotransmitters in brain; potentiated the effects of donepezil on neuronal oscillations, acetylcholine modulation and blood oxygen level-dependent functional signaling [54][55][56][57]  CYP2D6 [62] Not reported Enhanced memory in the social recognition task and object recognition task [43,63] Masupirdine 24.9 [49] CYP3A4 [64] Yes [64] No $ [64] Attenuated scopolamine, MK-801 and ageing associated memory deficits; potentiated the effects of donepezil on neuronal oscillations, and acetylcholine modulation; potentiated the effects of memantine on acetylcholine modulation; potentiated the effects of donepezil and memantine on cognition in the Morris water maze task, neuronal oscillations, and acetylcholine modulation; reduced aggression-like behaviors in resident intruder task and dominance levels in the dominant-submissive assay; Modulated cortical dopamine and norepinephrine [65,66]

SAM-760
Not reported CYP3A4/5 [67] Not reported No [67] Attenuated the memory deficits induced by MK-801, scopolamine, combined scopolamine and MK-801 treatment in the object recognition task [50,51] # Absolute bioavailability of the idalopirdine immediate release tablet in healthy male subjects; * Estimated oral bioavailability in extensive and poor CYP2D6 metabolizers; $ Based on in-vitro CYP profile.     The initial interest of targeting schizophrenia through 5-HT 6 receptors evolved from research indicating a potent affinity of several antipsychotics for the 5-HT 6 receptors [8]. In addition, 5-HT 6 receptor mRNA was found in the limbic system and striatum, brain regions implicated in the pathogenesis of schizophrenia [12]. A few 5-HT 6 receptors antagonists have been studied in schizophrenia patients.
Subsequently, the focus from schizophrenia shifted to AD. Progressive cognitive impairment is the major disease symptom of AD and neuropsychiatric abnormalities are not uncommon [91]. Cholinergic hypofunction is a contributor to memory dysfunction [92]. Considering the involvement of 5-HT 6 receptors in the modulation of cholinergic neurotransmission [93], 5-HT 6 receptor antagonists were explored as potential treatments for cognitive impairment. Several 5-HT 6 receptor antagonists were shown to enhance learning and memory in animal models (for reviews, see [16,42,[94][95][96]). Most 5-HT 6 receptor antagonists have been studied for potential cognitive benefits in AD patients based on the inferred cognitive benefits from animal models.
Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB) are some of the other neurodegenerative disorders with dementia as a characteristic feature. Deficits/imbalance in the cholinergic neurotransmission has been thought to play a role in the dementias of PDD and DLB [97][98][99]. Considering the localization of 5-HT 6 receptors in substantia nigra and cerebral cortex [12,18], and the modulatory role of 5-HT 6 receptors on cholinergic neurotransmission, 5-HT 6 receptor antagonists may have utility in the treatment of PDD or DLB. Few 5-HT 6 receptor antagonists have been studied in PDD or DLB.
Neuropsychiatric symptoms (NPS) are heterogeneous non-cognitive behavioral or mood manifestations of neurodegenerative disorders like AD, PDD and DLB [100,101]. These symptoms include psychosis, agitation/aggression, depression, anxiety, euphoria, apathy, disinhibition, irritability, motor disturbance, night-time behavior, and appetite and eating changes [102,103]. NPS are associated with increased levels of caregiver burden and distress [104,105]. There is no approved treatment for the management of NPS, except pimavanserin for PD psychosis and risperidone for short-term use for behavioral and psychological symptoms of AD (in Europe, Canada, Australia and New Zealand). Considering the affinity of antipsychotics and antidepressants for 5-HT 6 receptors and the wide distribution of 5-HT 6 receptors in the brain regions implicated for the control of mood and behavior, 5-HT 6 agents are candidates for use in the treatment of NPS of AD.
The following section describes the profile of the 5-HT 6 receptor antagonists in patient populations.

Avisetron
In addition to affinity for 5-HT 6 receptors, avisetron has affinity for 5-HT 2B receptors [39]. In an animal model, avisetron attenuated the effects of apomorphine on the startle prepulse inhibition, suggesting potential effects on the positive symptoms of schizophrenia. Avisetron also attenuated the memory deficits induced by MK-801 in task involving object recognition, passive avoidance, and route memory in the Morris water maze task [39,68] indicating potential effects on the cognitive symptoms of schizophrenia.
Initially the effect of avisetron (4 mg, every day (QD)) was studied in schizophrenia patients receiving stable antipsychotic medication. After treatment for 28 days, those treated with avisetron showed improvement in the PANSS total scores compared to the baseline, whereas no changes were observed with placebo treatment. In addition, significant decreases in PANSS positive subscores were observed with avisetron treatment. Significant changes were also noted in the continuous attention task. Overall, the results were suggestive of beneficial cognitive and anti-psychotic effects of avisetron [69,70].
Subsequently, avisetron was studied for its effects in schizophrenia patients with incomplete remission after receiving stable doses of antipsychotics. The starting dose of avisetron was 4 mg (QD) and was increased to 8 mg (QD) after 1 week. Avisetron treatment showed a nonsignificant trend towards improvement over placebo in PANSS total scores, PANSS positive subscores, and PANSS general psychopathology scale. Overall, avisetron was safe and well-tolerated [70]; however, no further evaluation of avisetron for schizophrenia is reported.

Cerlapirdine
This agent is a potent 5-HT 6 receptor antagonist with an affinity for 5-HT 7 and 5-HT 2B receptors. Cerlapirdine attenuated scopolamine-and MK-801-induced deficits in the object recognition and contextual fear conditioning tasks. It also attenuated the combined scopolamine and MK-801-induced deficit in the object recognition task. Cerlapirdine was shown to modulate acetylcholine and glutamate in the hippocampus [40].
Based on the pharmacological profile, cerlapirdine (0.5 mg, 1.5 mg, 3 mg and 5 mg, QD) was evaluated in an exploratory study as a monotherapy in mild to moderate AD patients for treatment duration of 4 weeks. Although the primary objective was to assess the safety and tolerability, the efficacy was explored using the MMSE, ADAS-Cog, and subtests of the Cambridge Neuropsychological Test Automated Battery (CANTAB). Trends towards improvement favoring the cerlapirdine treatment were observed on the ADAS-Cog scores and CANTAB [71].
Subsequently, cerlapirdine (1.5 mg, 3 mg and 5 mg, QD) was evaluated in a 52-week phase 2 study for its effects on cognition in mild to moderate AD patients. The primary outcome measure was change from baseline in ADAS-Cog scores at week 24. The study was terminated for reasons of futility (NCT00895895) [72]. Futility may be attributed to low 5-HT 6 receptor occupancy i.e., <30% after multiple 5 mg doses [106]. No safety concerns were noted with cerlapirdine in the clinical studies.

Idalopirdine
In addition to potently blocking the 5-HT 6 receptors, idalopirdine has potent affinity for adrenergic receptors (α 1A and α 1B ) and moderate affinity for 5-HT 2A and 5-HT 2C receptors [41]. In the conditioned avoidance response task, idalopirdine potentiated the effects of haloperidol and risperidone [54] suggesting its potential as an adjunct treatment for schizophrenia. In a rat model, idalopirdine as a standalone treatment attenuated the subchronic phencyclidine-induced cognitive impairment, suggesting its potential utility in the treatment of cognitive impairment associated with schizophrenia [41].
In clinical studies, idalopirdine was evaluated as monotherapy in a small population of 20 schizophrenic patients treated with a dose-escalating regimen of idalopirdine (60/180 mg or 120/240 mg) or placebo. The treatment duration was 14 days, and frequency was once a day. The Brief Assessment of Cognition in Schizophrenia (BACS) was used to assess the changes in cognition. A dose-dependent improvement was observed on BACS, with the effect reaching statistical significance at the highest tested dose of 240 mg. No changes were observed in the placebo treatment arm [73].
Further, idalopirdine was evaluated as an augmentation therapy to risperidone (NCT00810667). Idalopirdine (60 mg, BID) did not show improvement over placebo in schizophrenia symptoms as assessed by Positive and Negative Syndrome Scale (PANSS) total scores at the end of 12 weeks' treatment. In addition, there were no changes in the BACS scores or the PANSS cognitive subscale scores [74]. Overall, idalopirdine was safe and well-tolerated in schizophrenic patients. No further evaluation of idalopirdine in schizophrenia population has been reported.
Subsequently, idalopirdine was repositioned for the treatment of AD. In animal models, no procognitive-like effects of idalopirdine were observed as a standalone treatment. However, idalopirdine potentiated the effects of donepezil on neuronal oscillations, extracellular brain acetylcholine levels, and blood oxygen level-dependent functional signaling [55][56][57].
Guided by observations in animal models, idalopirdine (30 mg, three times a day (TID)) was studied for its effects on cognition in moderate AD patients as an add-on therapy to donepezil in a phase 2 study. The effect on cognition was assessed based on the change from baseline in ADAS-Cog 11 scores at week 24. A significant improvement in ADAS-Cog 11 scores was observed in the idalopirdine treatment arm compared to placebo. Parallel improvement was also observed in the MMSE scale. A trend towards improvement was observed in functional (ADCS-ADL) and global (Alzheimer's Disease Cooperative Study Clinical Global Impression of Change (ADCS-CGIC)) outcome measures [75]. Post hoc analysis of the phase 2 study suggested that treatment was associated with improvements in the anxiety and hallucinations domains of the 12-item Neuropsychiatric Inventory scale (NPI-12) [107].
Based on the effects on cognition noted in the phase 2 trial, idalopirdine was evaluated in phase 3 studies as an add-on therapy to donepezil or other cholinesterase inhibitors. The phase 3 studies differed from the phase 2 study, in dosage (10 mg or 30 mg or 60 mg for phase 3 vs. 30 mg for phase 2), and frequency of the idalopirdine treatment (QD for phase 3 vs. TID for phase 2), background therapy (all cholinesterase inhibitors for phase 3 vs. only donepezil for phase 2), cognitive impairment severity (MMSE12-22, mild to moderate for phase 3 vs. MMSE12-19, moderate for phase 2), and study geography (worldwide for phase 3 vs. Europe, Australia, Czech Republic and Canada for phase 2). No effects of idalopirdine were observed on ADAS-Cog 11 scores or other secondary endpoints compared to placebo [76]. In phase 3 studies, the effect of idalopirdine on anxiety was followed-up by selectively studying the anxiety domain of the NPI-12 scale as one of the secondary endpoints. No notable effect was observed (NCT02006641). Overall, idalopirdine was found to be safe and well-tolerated in the clinical studies.
In AD patients with mild to moderate cognitive deficits, intepirdine (5 mg, 15 mg and 35 mg, QD) was evaluated as a monotherapy for 24 weeks [77]. The effects on global function and cognition were assessed based on the Clinician's Interview-Based Impression of Change with caregiver input (CIBIC+) and change from baseline in Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-Cog) scores, respectively. Intepirdine treatment over 24 weeks showed improvement in CIBIC+, and the effect reached nominal statistical significance over placebo. No treatment-related effects were observed on the change from baseline in ADAS-Cog scores; however, slope of the dose response from linear trend analysis showed an improvement. Subsequent post hoc analysis suggested beneficial effects of intepirdine on the ADAS-Cog scores in a subgroup of the population with a Mini-Mental State Examination (MMSE) score of <18.
Two trials evaluated the effects of contemporary treatment with intepirdine and donepezil for 24 weeks in mild to moderate AD patients [78,79]. Intepirdine was evaluated at doses of 15 mg and 35 mg (QD) in one study, and another study titrated intepirdine from 15 mg (QD) to 35 mg (QD) at week 4. The co-primary endpoints were change from baseline in CIBIC+ and ADAS-Cog scores. No significant effects of either intepirdine or donepezil were observed on either of the endpoints. The effects of intepirdine were consistent between studies.
Intepirdine (15 mg and 35 mg, QD) was also studied as an add-on therapy to donepezil for 48 weeks in mild to moderate AD patients [79]. The primary endpoints were Clinical Dementia Rating-Sum of Boxes (CDR-SB) and change from baseline on the ADAS-Cog scores at week 24. Treatment with intepirdine was associated with significant improvement in ADAS-Cog scores at week 24 over placebo, and the effects persisted for up to 48 weeks. Parallel improvements were observed in the function as assessed by Alzheimer's Disease Cooperative Study-Activities of Daily Living (ADCS-ADL).
The observation of potential beneficial effects in earlier studies led to a large phase 3 study evaluating intepirdine (35 mg, QD) as an add-on treatment to donepezil for 24 weeks [80]. The change from baseline in ADAS-Cog scores and ADCS-ADL were co-primary endpoints. No notable effect of intepirdine treatment was observed on either of the endpoints. Intepirdine treatment was associated with a favorable safety profile across all the studies.
The effects of intepirdine (35 mg and 70 mg, QD) were assessed in patients with DLB. The primary efficacy measure was assessment of the effects of intepirdine on motor functions as change from baseline in the Unified Parkinson's Disease Rating Scale-Part III (UPDRS-III) scores at week 24. Secondary outcome measures assessed the effects on cognition as change from baseline in the ADAS-Cog scores. No significant effects of intepirdine were observed in the change from baseline in the UPDRS-III or ADAS-Cog 11 scores compared to placebo. Intepirdine was well-tolerated, with higher incidences of gastrointestinal adverse events [81].

Landipirdine
Landipirdine is a potent blocker of both 5-HT 6 and 5-HT 2A receptors. No reports on the non-clinical profile of landipirdine are publicly available. Landipirdine was evaluated for its effect on cognition in PDD patients as an add-on therapy to cholinesterase inhibitor for 16 weeks. The dose of landipirdine was up-titrated: 20 mg (QD) for week 1, 50 mg (QD) for week 2, and 100 mg (QD) from week 3 through week 16. The effect on cognition was assessed as change from baseline in Cognitive Drug Research Computerized Drug Research Cognition Battery Continuity of Attention (CDRCOA) captured in the ON state. No effects were observed on change from baseline in CDRCOA compared to placebo. Worsening of motor symptoms was observed in those receiving landipirdine [82]. In a post hoc analysis, examination of NPS based on the NPI-12 scale suggested potential improvements in apathy, anxiety, and irritability/lability domains with landipirdine treatment over placebo [109]. No further development of landipirdine has been reported.

Latrepirdine
This 5-HT 6 receptor antagonist has affinities for acetylcholinesterases, N-methyl-Daspartate receptors, and voltage-gated calcium channels. Many of the effects of latrepirdine were believed to be mediated through the blockade of 5-HT 6 receptors [43]. Based on the affinity for 5-HT 6 receptors, latrepirdine (20 mg) was studied as an add-on to risperidone therapy in schizophrenic patients transitioning from an acute psychotic episode to symptom remission [83]. No notable differences were observed in the PANSS total or subscale scores between the latrepirdine and placebo treated groups. However, latrepirdine showed statistically significant improvement over the placebo in the 16-item Negative Symptom Assessment (NSA-16) and numerical improvement in cognitive dimensions (working memory, attention, psycho-motor coordination and planning) in comparison with the placebo-treated group. No further studies for latrepirdine in schizophrenic patients have been reported.
In animal models, latrepirdine treatment was associated with memory improvement in a social recognition task [43] and an object recognition task [63]. The interest of developing latrepirdine for AD was based on the outcome from an 8-week open label pilot study. Latrepirdine was evaluated at a dose of 20 mg, TID. The study found improvements in cognition, function, and NPS [44]. An ensuing proof-of-concept study reported similar observations on cognition (assessed based on change from baseline in ADAS-Cog), function (assessed based on change from baseline in ADCS-ADL) and neuropsychiatric symptoms (assessed based on change from baseline in NPI-12). Latrepirdine was evaluated at a dose of 20 mg, TID after 26 weeks of treatment [84]. The effect of latrepirdine on the individual NPS was not reported, except for an absence of effects on dysphoria/depression [44,84]. In the subsequent larger phase 3 studies that were conducted to replicate the earlier findings, no effect was observed on cognition or functions as a standalone treatment for 26 weeks (NCT00675623) or as an add-on treatment to donepezil for 52 weeks (NCT00829374). In the above studies, latrepirdine was evaluated at doses of 5 mg and 20 mg, TID [85]. The effects on NPS were not reported in the results from the phase 3 studies. Latrepirdine was safe and well-tolerated in AD patients.

Masupirdine
This therapeutic candidate is a potent 5-HT 6 receptor antagonist and lacks affinity for other serotonergic receptors subtypes at clinically relevant doses. Masupirdine showed procognitive effects in diverse animal models as standalone and add-on treatment to donepezil. Masupirdine showed beneficial effects on cognition in animals receiving treatment with donepezil and memantine [49,65]. In animal models, masupirdine significantly reduced aggression-like behaviors in the resident-intruder task and decreased dominance levels in the dominant-submissive assay. In addition, masupirdine modulated the cortical dopamine and norepinephrine levels assessed using the brain microdialysis in rats [66].
Contingent on the procognitive effects of masupirdine in combination with donepezil and memantine, a phase 2 study evaluated masupirdine (50 mg and 100 mg, QD) as an addon treatment to donepezil and memantine in moderate AD patients. The primary efficacy endpoint was the change from baseline in ADAS-Cog scores after 26 weeks of treatment. No notable effects of masupirdine were observed on cognition when compared to the placebo treatment. Overall, masupirdine was safe and well-tolerated in AD patients [86]. A post hoc analysis revealed that masupirdine may have cognitive benefits in patients who are not concurrently treated with memantine high dose (28 mg) [87]. Post hoc observations from the phase 2 study suggested that treatment with masupirdine was associated with improvements in domains of agitation/aggression and psychosis [88].

SAM-760
It is a potent 5-HT 6 receptor antagonist. SAM-760 also acts as an antagonist at 5-HT 2A receptors; however, it did not show any significant occupancy of cortical 5-HT 2A receptors in humans [110]. SAM-760 showed procognitive effects in diverse animal models including cholinergic and glutamatergic deficit models. It enhanced the cholinergic and glutamatergic neurotransmission in the rat hippocampus and prefrontal cortex [50,51].
SAM-760 was advanced for clinical evaluation to assess its effect on cognition in moderate AD patients with existing NPS. Twelve weeks of treatment with SAM-760 (30 mg, QD) produced no significant effects in the change from baseline in ADAS-Cog scores. No beneficial effects were observed in the secondary outcome measures including NPS as assessed by the NPI-12 scale. Overall, SAM-760 was safe and well-tolerated [90]. 6 receptors have received considerable attention as potential treatments for cognitive deficits or NPS associated with AD and other neurocognitive disorders. Blocking the functions of 5-HT 6 receptors has been the biological effect of agents evaluated in clinical trials. Although the initial focus of 5-HT 6 receptors research was intended for the treatment of schizophrenia, most of the advanced clinical studies focus on memory deficits associated with AD or other dementias. However, no conclusive evidence has been observed in clinical trials to support the utility of targeting 5-HT 6 receptor antagonists for the treatment of schizophrenia or memory deficits in dementias. The majority of these agents had affinity for other (non-5-HT 6 ) serotonergic receptors, which might partially explain their failure(s). 5-HT 6 receptor agents have been shown to have anxiolytic and antidepressant-like properties in animal models [37]. 5-HT 6 receptor antagonists also facilitate release of neurotransmitters, such as dopamine and norepinephrine, implicated in mood and behavior [42,59,111]. Based on the affinity of many psychotropic drugs for 5-HT 6 receptors, the observations of modulatory role of the receptor on mood, and potential beneficial effects on NPS in patients with dementia, 5-HT 6 receptors antagonists may have potential as treatments of NPS in dementia. Many of the 5-HT 6 receptor antagonists also blocked the 5-HT 2A receptors. Considering the beneficial effects of 5-HT 2A receptors blockade in psychiatry, it is essential to understand the role of 5-HT 6 receptor on NPS. Characterizing the clinically evaluated agents for their interaction modes with the receptor may further help to delineate agents, and selective 5-HT 6 receptor antagonist may target specific downstream pathways and avoid unwanted effects. Among the 5-HT 6 receptor antagonists, masupirdine is being evaluated for its effects on agitation in patients with Alzheimer's-type dementia in a potentially pivotal clinical trial (NCT05397639). The outcome of 5-HT 6 receptor antagonists in the past clinical trials has been disappointing. The current trial of masupirdine for agitation will inform future approaches to the clinical utility of selective 5-HT 6 receptor antagonists.
Author Contributions: All authors were involved in preparation of manuscript. Authors critically reviewed the manuscript, commented on drafts, and approved the final manuscript. All authors have read and agreed to the published version of the manuscript.

Funding: This work was sponsored by Suven Life Sciences Ltd.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study is available in the Supplementary Materials.