Efficacy and Safety of the Dopaminergic Stabilizer Pridopidine (ACR16) in Patients With Huntington's Disease

Treatment of Depression in Huntington's Disease: A Systematic Review

Depression is a common psychiatric disorder among individuals with Huntington's disease (HD). Depression in HD and major depressive disorder appear to have different pathophysiological mechanisms. Despite the unique pathophysiology, the treatment of depression in HD is based on data from the treatment of major depressive disorder in the general population. The objective of this systematic review was to conduct a comprehensive evaluation of the available evidence. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. Studies on the treatment of depression in HD were identified by searching MEDLINE, Embase, and PsycInfo. The initial search yielded 2,771 records, 41 of which were ultimately included. There were 19 case reports, seven case series, three cross-sectional studies, one qualitative study, nine nonrandomized studies, and two randomized trials among the included studies. The most common assessment tools were the Hospital Anxiety and Depression Scale (N=8), the Beck Depression Inventory (N=6), and the Hamilton Depression Rating Scale (N=6). Only 59% of the included studies assessed depressive symptoms with a scoring system. The pharmacological options for the treatment of depression included antidepressants and antipsychotics. Nonpharmacological approaches were multidisciplinary rehabilitation, psychotherapy, and neurostimulation. Limited evidence on the treatment of depression in HD was available, and this literature consisted mainly of case reports and case series. This systematic review highlights the knowledge gap and the pressing need for HD-specific research to determine the efficacy of treatment approaches for depression in HD.

Gut microbiota dysbiosis and Huntington's disease: Exploring the gut-brain axis and novel microbiota-based interventions

Huntington's disease (HD) is a complex progressive neurodegenerative disorder affected by genetic, environmental, and metabolic factors contributing to its pathogenesis. Gut dysbiosis is termed as the alterations of intestinal microbial profile. Emerging research has highlighted the pivotal role of gut dysbiosis in HD, focusing on the gut-brain axis as a novel research parameter in science. This review article provides a comprehensive overview of gut microbiota dysbiosis and its relationship with HD and its pathogenesis along with the future challenges and opportunities. The focuses on the essential mechanisms which link gut dysbiosis to HD pathophysiology including neuroinflammation, immune system dysregulation, altered metabolites composition, and neurotransmitter imbalances. We also explored the impacts of gut dysbiosis on HD onset, severity, and symptoms such as cognitive decline, motor dysfunction, and psychiatric symptoms. Furthermore, we highlight recent advances in therapeutics including microbiota-based therapeutic approaches, including dietary interventions, prebiotics, probiotics, fecal microbiota transplantation, and combination therapies with conventional HD treatments and their applications in managing HD. The future challenges are also highlighted as the heterogeneity of gut microbiota, interindividual variability, establishing causality between gut dysbiosis and HD, identifying optimal therapeutic targets and strategies, and ensuring the long-term safety and efficacy of microbiota-based interventions. This review provides a better understanding of the potential role of gut microbiota in HD pathogenesis and guides the development of novel therapeutic approaches.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

Fountain of youth—Targeting autophagy in aging

As our society ages inexorably, geroscience and research focusing on healthy aging is becoming increasingly urgent. Macroautophagy (referred to as autophagy), a highly conserved process of cellular clearance and rejuvenation has attracted much attention due to its universal role in organismal life and death. Growing evidence points to autophagy process as being one of the key players in the determination of lifespan and health. Autophagy inducing interventions show significant improvement in organismal lifespan demonstrated in several experimental models. In line with this, preclinical models of age-related neurodegenerative diseases demonstrate pathology modulating effect of autophagy induction, implicating its potential to treat such disorders. In humans this specific process seems to be more complex. Recent clinical trials of drugs targeting autophagy point out some beneficial effects for clinical use, although with limited effectiveness, while others fail to show any significant improvement. We propose that using more human-relevant preclinical models for testing drug efficacy would significantly improve clinical trial outcomes. Lastly, the review discusses the available cellular reprogramming techniques used to model neuronal autophagy and neurodegeneration while exploring the existing evidence of autophagy’s role in aging and pathogenesis in human-derived in vitro models such as embryonic stem cells (ESCs), induced pluripotent stem cell derived neurons (iPSC-neurons) or induced neurons (iNs).

Current and Possible Future Therapeutic Options for Huntington’s Disease

Huntington’s disease (HD) is an autosomal neurodegenerative disease that is characterized by an excessive number of CAG trinucleotide repeats within the huntingtin gene ( HTT). HD patients can present with a variety of symptoms including chorea, behavioural and psychiatric abnormalities and cognitive decline. Each patient has a unique combination of symptoms, and although these can be managed using a range of medications and non-drug treatments there is currently no cure for the disease. Current therapies prescribed for HD can be categorized by the symptom they treat. These categories include chorea medication, antipsychotic medication, antidepressants, mood stabilizing medication as well as non-drug therapies. Fortunately, there are also many new HD therapeutics currently undergoing clinical trials that target the disease at its origin; lowering the levels of mutant huntingtin protein (mHTT). Currently, much attention is being directed to antisense oligonucleotide (ASO) therapies, which bind to pre-RNA or mRNA and can alter protein expression via RNA degradation, blocking translation or splice modulation. Other potential therapies in clinical development include RNA interference (RNAi) therapies, RNA targeting small molecule therapies, stem cell therapies, antibody therapies, non-RNA targeting small molecule therapies and neuroinflammation targeted therapies. Potential therapies in pre-clinical development include Zinc-Finger Protein (ZFP) therapies, transcription activator-like effector nuclease (TALEN) therapies and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system (Cas) therapies. This comprehensive review aims to discuss the efficacy of current HD treatments and explore the clinical trial progress of emerging potential HD therapeutics.

The Efficacy and Safety of Pridopidine on Treatment of Patients with Huntington's Disease: A Systematic Review and Meta-Analysis

BACKGROUND

Pridopidine is a novel drug that helps stabilize psychomotor function in patients with Huntington's disease (HD) by activating the cortical glutamate pathway. It promises to achieve the unmet needs of current therapies of HD without worsening other symptoms.

OBJECTIVE

To review the literature discussing the efficacy of pridopidine in alleviating motor symptoms and its safety in patients with HD.

METHODS

We searched Scopus, Web of Science, the Cochrane Library, Wiley, and PubMed for randomized controlled trials (RCTs) of pridopidine on HD. Data from eligible studies were extracted and pooled as mean differences for efficacy and risk ratios (RRs) for safety using RevMan software version 5.3.

RESULTS

A total of 4 relevant RCTs with 1130 patients were selected (816 in the pridopidine group and 314 in the placebo group). The pooled effect size favored pridopidine over placebo insignificantly in the Unified Huntington's Disease Rating Scale Total Motor Score (mean difference [MD], -0.93; 95% confidence interval [CI], -2.01 to 0.14; P = 0.09), whereas the effect size of 3 studies significantly favored pridopidine over placebo in the Unified Huntington's Disease Rating Scale Modified Motor Score (MD, -0.81; 95% CI, -1.48 to -0.13; P = 0.02). Pridopidine generally was well tolerated. None of the adverse effects were considerably higher in the case of pridopidine compared with placebo in overall adverse events (RR, 1.03; 95% CI, 0.94-1.13; P = 0.49) and serious adverse events (RR, 1.62; 95% CI, 0.88-2.99; P = 0.12).

CONCLUSION

The effects of pridopidine on motor functions (especially voluntary movements) in patients with HD are encouraging and provide a good safety profile that motivates further clinical trials on patients to confirm its effectiveness and safety.

A MDS Evidence-Based Review on Treatments for Huntington's Disease

BACKGROUND

Huntington's disease (HD) is a rare neurodegenerative disorder with protean clinical manifestations. Its management is challenging, consisting mainly of off-label treatments.

OBJECTIVES

The International Parkinson and Movement Disorder Society commissioned a task force to review and evaluate the evidence of available therapies for HD gene expansion carriers.

METHODS

We followed the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. Eligible randomized controlled trials were identified via an electronic search of the CENTRAL, MEDLINE, and EMBASE databases. All eligible trials that evaluated one or more of 33 predetermined clinical questions were included. Risk of bias was evaluated using the Cochrane Risk of Bias tool. A framework was adapted to allow for efficacy and safety conclusions to be drawn from the balance between the GRADE level of evidence and the importance of the benefit/harm of the intervention.

RESULTS

Twenty-two eligible studies involving 17 interventions were included, providing data to address 8 clinical questions. These data supported a likely effect of deutetrabenazine on motor impairment, chorea, and dystonia and of tetrabenazine on chorea. The data did not support a disease-modifying effect for premanifest and manifest HD. There was no eligible evidence to support the use of specific treatments for depression, psychosis, irritability, apathy, or suicidality. Similarly, no evidence was eligible to support the use of physiotherapy, occupational therapy, exercise, dietary, or surgical treatments.

CONCLUSIONS

Data for therapeutic interventions in HD are limited and support only the use of VMAT2 inhibitors for specific motor symptoms. © 2021 International Parkinson and Movement Disorder Society.

Sigma-1 Receptor: A Potential Therapeutic Target for Traumatic Brain Injury

The sigma-1 receptor (Sig-1R) is a chaperone receptor that primarily resides at the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) and acts as a dynamic pluripotent modulator regulating cellular pathophysiological processes. Multiple pharmacological studies have confirmed the beneficial effects of Sig-1R activation on cellular calcium homeostasis, excitotoxicity modulation, reactive oxygen species (ROS) clearance, and the structural and functional stability of the ER, mitochondria, and MAM. The Sig-1R is expressed broadly in cells of the central nervous system (CNS) and has been reported to be involved in various neurological disorders. Traumatic brain injury (TBI)-induced secondary injury involves complex and interrelated pathophysiological processes such as cellular apoptosis, glutamate excitotoxicity, inflammatory responses, endoplasmic reticulum stress, oxidative stress, and mitochondrial dysfunction. Thus, given the pluripotent modulation of the Sig-1R in diverse neurological disorders, we hypothesized that the Sig-1R may affect a series of pathophysiology after TBI. This review summarizes the current knowledge of the Sig-1R, its mechanistic role in various pathophysiological processes of multiple CNS diseases, and its potential therapeutic role in TBI.

Sigma receptors and neurological disorders

Sigma receptors were identified relatively recently, and their presence has been confirmed in the central nervous system and peripheral organs. Changes in sigma receptor function or expression may be involved in neurological diseases, and thus sigma receptors represent a potential target for treating central nervous system disorders. Many substances that are ligands for sigma receptors are widely used in therapies for neurological disorders. In the present review, we discuss the roles of sigma receptors, especially in the central nervous system disorders, and related therapies.

Pridopidine for the Improvement of Motor Function in Patients With Huntington's Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Background: Huntington's disease (HD) is a progressive neurodegenerative disorder. Generally, it is characterized by deficits in cognition, behavior, and movement. Recent studies have shown that pridopidine is a potential and effective drug candidate for the treatment of HD. In the present study, we performed a meta-analysis to evaluate the efficacy and safety of pridopidine in HD.

Methods: The MEDLINE, EMBASE, CENTRAL, and Clinicaltrials.gov databases were searched for randomized controlled trials (RCTs) which had that evaluated pridopidine therapy in HD patients.

Results: We pooled data from 1,119 patients across four RCTs. Patients in the pridopidine group had a significantly lower Unified Huntington's Disease Rating Scale (UHDRS)-modified Motor Score (mMS) (MD −0.79, 95% CI = −1.46 to −0.11, p = 0.02) than those in the placebo group. Additionally, no differences were observed in the UHDRS-Total Motor Score (TMS) (MD −0.91. 95% CI = −2.03 to 0.21, p = 0.11) or adverse events (RR 1.06, 95% CI = 0.96 to 1.16, p = 0.24) in the pridopidine and placebo groups. In the subgroup analysis, the short-term (≤12 weeks) and long-term (>12 weeks) subgroups exhibited similar efficacy and safety with no statistical significance in TMS, mMS, or adverse events. However, TMS (MD −1.50, 95% CI = −2.87 to −0.12, p = 0.03) and mMS (MD −1.03, 95% CI = −1.87 to −0.19, p = 0.02) were observed to be improved significantly when the dosage of pridopidine ≥90 mg/day. Additionally, pridopidine (≥90 mg/day) increased total adverse events (RR 1.11, 95% CI = 1.00 to 1.22, p = 0.04) compared with placebo. On this basis, we analyzed the incidence of various adverse events when the dosage was ≥90 mg/day. Nonetheless, these results were within the acceptable threshold, although patients developed symptoms, such as nasopharyngitis and insomnia.

Conclusion: Pridopidine improved mMS and had no statistical significance in association with TMS or adverse events. Pridopidine (≥90 mg/day) improved TMS and mMS but increased adverse events, such as nasopharyngitis and insomnia. More RCTs were expected to assess pridopidine in HD.

Mitochondrial and Redox-Based Therapeutic Strategies in Huntington's Disease

Significance: The molecular processes that determine Huntington's disease (HD) pathogenesis are not yet fully understood, and until now no effective neuroprotective therapeutic strategies have been developed. Mitochondria are one of most important organelles required for neuronal homeostasis, by providing metabolic pathways relevant for energy production, regulating calcium homeostasis, or controlling free radical generation and cell death. Because augmented reactive oxygen species (ROS) accompanied by mitochondrial dysfunction are relevant early HD mechanisms, targeting these cellular mechanisms may constitute relevant therapeutic approaches. Recent Advances: Previous findings point toward a close relationship between mitochondrial dysfunction and redox changes in HD. Mutant huntingtin (mHTT) can directly interact with mitochondrial proteins, as translocase of the inner membrane 23 (TIM23), disrupting mitochondrial proteostasis and favoring ROS production and HD progression. Furthermore, abnormal brain and muscle redox signaling contributes to altered proteostasis and motor impairment in HD, which can be improved with the mitochondria-targeted antioxidant mitoquinone or resveratrol, an SIRT1 activator that ameliorates mitochondrial biogenesis and function. Critical Issues: Various antioxidants and metabolic enhancers have been studied in HD; however, the real outcome of these molecules is still debatable. New compounds have proven to ameliorate mitochondrial and redox-based signaling pathways in early stages of HD, potentially precluding selective neurodegeneration. Future Directions: Unraveling the molecular etiology of deregulated mitochondrial function and dynamics, and oxidative stress opens new prospects for HD therapeutics. In this review, we explore the role of redox unbalance and mitochondrial dysfunction in HD progression, and further describe advances on clinical trials in HD based on mitochondrial and redox-based therapeutic strategies.

Dysregulation of Neuronal Calcium Signaling via Store-Operated Channels in Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder that is characterized by motor, cognitive, and psychiatric problems. It is caused by a polyglutamine expansion in the huntingtin protein that leads to striatal degeneration via the transcriptional dysregulation of several genes, including genes that are involved in the calcium (Ca²⁺) signalosome. Recent research has shown that one of the major Ca²⁺ signaling pathways, store-operated Ca²⁺ entry (SOCE), is significantly elevated in HD. SOCE refers to Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. The dysregulation of Ca²⁺ homeostasis is postulated to be a cause of HD progression because the SOCE pathway is indirectly and abnormally activated by mutant huntingtin (HTT) in γ-aminobutyric acid (GABA)ergic medium spiny neurons (MSNs) from the striatum in HD models before the first symptoms of the disease appear. The present review summarizes recent studies that revealed a relationship between HD pathology and elevations of SOCE in different models of HD, including YAC128 mice (a transgenic model of HD), cellular HD models, and induced pluripotent stem cell (iPSC)-based GABAergic medium spiny neurons (MSNs) that are obtained from adult HD patient fibroblasts. SOCE in MSNs was shown to be mediated by currents through at least two different channel groups, Ca²⁺ release-activated Ca²⁺ current (I_CRAC) and store-operated Ca²⁺ current (I_SOC), which are composed of stromal interaction molecule (STIM) proteins and Orai or transient receptor potential channel (TRPC) channels. Their role under physiological and pathological conditions in HD are discussed. The role of Huntingtin-associated protein 1 isoform A in elevations of SOCE in HD MSNs and potential compounds that may stabilize elevations of SOCE in HD are also summarized. Evidence is presented that shows that the dysregulation of molecular components of SOCE or pathways upstream of SOCE in HD MSN neurons is a hallmark of HD, and these changes could lead to HD pathology, making them potential therapeutic targets.

Huntington's Disease Clinical Trials Corner: January 2019

In this edition of the Huntington's Disease Clinical Trials Corner we expand on the GENERATION-HD1 and PACE-HD trials, and we list all currently registered and ongoing clinical trials in Huntington's disease.

Pridopidine: Overview of Pharmacology and Rationale for its Use in Huntington's Disease

Despite advances in understanding the pathophysiology of Huntington’s disease (HD), there are currently no effective pharmacological agents available to treat core symptoms or to stop or prevent the progression of this hereditary neurodegenerative disorder. Pridopidine, a novel small molecule compound, has demonstrated potential for both symptomatic treatment and disease modifying effects in HD. While pridopidine failed to achieve its primary efficacy outcomes (Modified motor score) in two trials (MermaiHD and HART) there were consistent effects on secondary outcomes (TMS). In the most recent study (PrideHD) pridiopidine did not differ from placebo on TMS, possibly due to a large enduring placebo effect.

This review describes the process, based on in vivo systems response profiling, by which pridopidine was discovered and discusses its pharmacological profile, aiming to provide a model for the system-level effects, and a rationale for the use of pridopidine in patients affected by HD. Considering the effects on brain neurochemistry, gene expression and behaviour in vivo, pridopidine displays a unique effect profile. A hallmark feature in the behavioural pharmacology of pridopidine is its state-dependent inhibition or activation of dopamine-dependent psychomotor functions. Such effects are paralleled by strengthening of synaptic connectivity in cortico-striatal pathways suggesting pridopidine has potential to modify phenotypic expression as well as progression of HD. The preclinical pharmacological profile is discussed with respect to the clinical results for pridopidine, and proposals are made for further investigation, including preclinical and clinical studies addressing disease progression and effects at different stages of HD.

Neuronal Sigma-1 Receptors: Signaling Functions and Protective Roles in Neurodegenerative Diseases

Sigma-1 receptor (S1R) is a multi-functional, ligand-operated protein situated in endoplasmic reticulum (ER) membranes and changes in its function and/or expression have been associated with various neurological disorders including amyotrophic lateral sclerosis/frontotemporal dementia, Alzheimer's (AD) and Huntington's diseases (HD). S1R agonists are broadly neuroprotective and this is achieved through a diversity of S1R-mediated signaling functions that are generally pro-survival and anti-apoptotic; yet, relatively little is known regarding the exact mechanisms of receptor functioning at the molecular level. This review summarizes therapeutically relevant mechanisms by which S1R modulates neurophysiology and implements neuroprotective functions in neurodegenerative diseases. These mechanisms are diverse due to the fact that S1R can bind to and modulate a large range of client proteins, including many ion channels in both ER and plasma membranes. We summarize the effect of S1R on its interaction partners and consider some of the cell type- and disease-specific aspects of these actions. Besides direct protein interactions in the endoplasmic reticulum, S1R is likely to function at the cellular/interorganellar level by altering the activity of several plasmalemmal ion channels through control of trafficking, which may help to reduce excitotoxicity. Moreover, S1R is situated in lipid rafts where it binds cholesterol and regulates lipid and protein trafficking and calcium flux at the mitochondrial-associated membrane (MAM) domain. This may have important implications for MAM stability and function in neurodegenerative diseases as well as cellular bioenergetics. We also summarize the structural and biochemical features of S1R proposed to underlie its activity. In conclusion, S1R is incredibly versatile in its ability to foster neuronal homeostasis in the context of several neurodegenerative disorders.

Pridopidine Induces Functional Neurorestoration Via the Sigma-1 Receptor in a Mouse Model of Parkinson's Disease

Pridopidine is a small molecule in clinical development for the treatment of Huntington's disease. It was recently found to have high binding affinity to the sigma-1 receptor, a chaperone protein involved in cellular defense mechanisms and neuroplasticity. Here, we have evaluated the neuroprotective and neurorestorative effects of pridopidine in a unilateral 6-hydroxydopamine (6-OHDA) lesion model of parkinsonism in mice. By 5 weeks of daily administration, a low dose of pridopidine (0.3 mg/kg) had significantly improved deficits in forelimb use (cylinder test, stepping test) and abolished the ipsilateral rotational bias typical of hemiparkinsonian animals. A higher dose of pridopidine (1 mg/kg) significantly improved only the rotational bias, with a trend towards improvement in forelimb use. The behavioral recovery induced by pridopidine 0.3 mg/kg was accompanied by a significant protection of nigral dopamine cell bodies, an increased dopaminergic fiber density in the striatum, and striatal upregulation of GDNF, BDNF, and phosphorylated ERK1/2. The beneficial effects of pridopidine 0.3 mg/kg were absent in 6-OHDA-lesioned mice lacking the sigma-1 receptor. Pharmacokinetic data confirmed that the effective dose of pridopidine reached brain concentrations sufficient to bind S1R. Our results are the first to show that pridopidine promotes functional neurorestoration in the damaged nigrostriatal system acting via the sigma-1 receptor.

Pridopidine stabilizes mushroom spines in mouse models of Alzheimer's disease by acting on the sigma-1 receptor

There is evidence that cognitive decline in Alzheimer's disease (AD) results from deficiencies in synaptic communication (e.g., loss of mushroom-shaped 'memory spines') and neurodegenerative processes. This might be treated with sigma-1 receptor (S1R) agonists, which are broadly neuroprotective and modulate synaptic plasticity. For example, we previously found that the mixed muscarinic/S1R agonist AF710B prevents mushroom spine loss in hippocampal cultures from APP knock-in (APP-KI) and presenilin-1-M146 V knock-in (PS1-KI) mice. We also found that the "dopaminergic stabilizer" pridopidine (structurally similar to the S1R agonist R(+)-3-PPP), is a high-affinity S1R agonist and is synaptoprotective in a mouse model of Huntington disease. Here we tested whether pridopidine and R(+)-3-PPP are synaptoprotective in models of AD and whether this requires S1R. We also examined the effects of pridopidine on long-term potentiation (LTP), endoplasmic reticulum calcium and neuronal store-operated calcium entry (nSOC) in spines, all of which are dysregulated in AD, contributing to synaptic pathology. We report here that pridopidine and 3-PPP protect mushroom spines from Aβ42 oligomer toxicity in primary WT hippocampal cultures from mice. Pridopidine also reversed LTP defects in hippocampal slices resulting from application of Aβ42 oligomers. Pridopidine and 3-PPP rescued mushroom spines in hippocampal cultures from APP-KI and PS1-KI mice. S1R knockdown from lenti-viral shRNA expression destabilized WT mushroom spines and prevented the synaptoprotective effects of pridopidine in PS1-KI cultures. Knockout of PS1/2 destabilized mushroom spines and pridopidine was unable to prevent this. Pridopidine lowered endoplasmic reticulum calcium levels in WT, PS1-KO, PS1-KI and PS2 KO neurons, but not in PS1/2 KO neurons. S1R was required for pridopidine to enhance spine nSOC in PS1-KI neurons. Pridopidine was unable to rescue PS1-KI mushroom spines during pharmacological or genetic inhibition of nSOC. Oral pridopidine treatment rescued mushroom spines in vivo in aged PS1-KI-GFP mice. Pridopidine stabilizes mushroom spines in mouse models of AD and this requires S1R, endoplasmic reticulum calcium leakage through PS1/2 and nSOC. Thus, pridopidine may be useful to explore for the treatment of AD.

Safety and efficacy of pridopidine in patients with Huntington's disease (PRIDE-HD): a phase 2, randomised, placebo-controlled, multicentre, dose-ranging study

BACKGROUND

Previous trials have shown that pridopidine might reduce motor impairment in patients with Huntington's disease. The aim of this study was to ascertain whether higher doses of pridopidine than previously tested reduce motor symptoms in a dose-dependent manner while maintaining acceptable safety and tolerability.

METHODS

PRIDE-HD was a randomised, placebo-controlled, phase 2, dose-ranging study in adults (aged ≥21 years) with Huntington's disease at outpatient clinics in 53 sites across 12 countries (Australia, Austria, Canada, Denmark, France, Germany, Italy, Poland, Russia, the Netherlands, the UK, and the USA). Eligible patients had clinical onset after age 18 years, 36 or more cytosine-adenine-guanine repeats in the huntingtin gene, motor symptoms (Unified Huntington's Disease Rating Scale total motor score [UHDRS-TMS] ≥25 points), and reduced independence (UHDRS independence score ≤90%). Patients were randomly assigned (1:1:1:1:1) with centralised interactive-response technology to receive one of four doses of pridopidine (45, 67·5, 90, or 112·5 mg) or placebo orally twice a day for 52 weeks. Randomisation was stratified within centres by neuroleptic drug use. The primary efficacy endpoint was change in the UHDRS-TMS from baseline to 26 weeks, which was assessed in all randomised patients who received at least one dose of study drug and had at least one post-baseline efficacy assessment (full analysis set). Participants and investigators were masked to treatment assignment. This trial is registered with EudraCT (2013-001888-23) and ClinicalTrials.gov (NCT02006472).

FINDINGS

Between Feb 13, 2014, and July 5, 2016, 408 patients were enrolled and randomly assigned to receive placebo (n=82) or pridopidine 45 mg (n=81), 67·5 mg (n=82), 90 mg (n=81), or 112·5 mg (n=82) twice daily for 26 weeks. The full analysis set included 397 patients (81 in the placebo group, 75 in the 45 mg group, 79 in the 67·5 mg group, 81 in the 90 mg group, and 81 in the 112·5 mg group). Pridopidine did not significantly change the UHDRS-TMS at 26 weeks compared with placebo at any dose. The most frequent adverse events across all groups were diarrhoea, vomiting, nasopharyngitis, falls, headache, insomnia, and anxiety. The most common treatment-related adverse events were insomnia, diarrhoea, nausea, and dizziness. Serious adverse events occurred in the pridopidine groups only and were most frequently falls (n=5), suicide attempt (n=4), suicidal ideation (n=3), head injury (n=3), and aspiration pneumonia (n=3). No new safety or tolerability concerns emerged in this study. One death in the pridopidine 112·5 mg group due to aspiration pneumonia was considered to be possibly related to the study drug.

INTERPRETATION

Pridopidine did not improve the UHDRS-TMS at week 26 compared with placebo and, thus, the results of secondary or tertiary analyses in previous trials were not replicated. A potentially strong placebo effect needs to be ruled out in future studies.

FUNDING

Teva Pharmaceutical Industries.

Repurposing drugs to treat l-DOPA-induced dyskinesia in Parkinson's disease

In this review, we discuss the opportunity for repurposing drugs for use in l-DOPA-induced dyskinesia (LID) in Parkinson's disease. LID is a particularly suitable indication for drug repurposing given its pharmacological diversity, translatability of animal-models, availability of Phase II proof-of-concept (PoC) methodologies and the indication-specific regulatory environment. A compound fit for repurposing is defined as one with appropriate human safety-data as well as animal safety, toxicology and pharmacokinetic data as found in an Investigational New Drug (IND) package for another indication. We first focus on how such repurposing candidates can be identified and then discuss development strategies that might progress such a candidate towards a Phase II clinical PoC. We discuss traditional means for identifying repurposing candidates and contrast these with newer approaches, especially focussing on the use of computational and artificial intelligence (AI) platforms. We discuss strategies that can be categorised broadly as: in vivo phenotypic screening in a hypothesis-free manner; in vivo phenotypic screening based on analogy to a related disorder; hypothesis-driven evaluation of candidates in vivo and in silico screening with a hypothesis-agnostic component to the selection. To highlight the power of AI approaches, we describe a case study using IBM Watson where a training set of compounds, with demonstrated ability to reduce LID, were employed to identify novel repurposing candidates. Using the approaches discussed, many diverse candidates for repurposing in LID, originally envisaged for other indications, will be described that have already been evaluated for efficacy in non-human primate models of LID and/or clinically. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Cause or compensation?-Altered neuronal Ca2+ handling in Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder of typically middle-aged onset for which there is no disease-modifying treatment. Caudate and putamen medium-sized spiny projection neurons (SPNs) most severely degenerate in HD. However, it is unclear why mutant huntingtin protein (mHTT) is preferentially toxic to these neurons or why symptoms manifest only relatively late in life. mHTT interacts with numerous neuronal proteins. Likewise, multiple SPN cellular processes have been described as altered in various HD models. Among these, altered neuronal Ca²⁺ influx and intracellular Ca²⁺ handling feature prominently and are addressed here. Specifically, we focus on extrasynaptic NMDA-type glutamate receptors, endoplasmic reticulum IP3 receptors, and mitochondria. As mHTT is expressed throughout development, compensatory processes will likely be mounted to mitigate any deleterious effects. Although some compensations can lessen mHTT's disruptive effects, others-such as upregulation of the ER-refilling store-operated Ca²⁺ channel response-contribute to pathogenesis. A causation-based approach is therefore necessary to decipher the complex sequence of events linking mHTT to neurodegeneration, and to design rational therapeutic interventions. With this in mind, we highlight evidence, or lack thereof, that the above alterations in Ca²⁺ handling occur early in the disease process, clearly interact with mHTT, and show disease-modifying potential when reversed in animals.

Rating scales for behavioral symptoms in Huntington's disease: Critique and recommendations

Behavioral symptoms are an important feature of Huntington's disease and contribute to impairment in quality of life. The Movement Disorder Society commissioned the assessment of the clinimetric properties of rating scales in Huntington's disease to make recommendations regarding their use, following previously used standardized criteria. A systematic literature search was conducted to identify the scales used to assess behavioral symptoms in Huntington's disease. For the purpose of this review, 7 behavioral domains were deemed significant in Huntington's disease: irritability, anxiety, depression, apathy, obsessive-compulsive behaviors, psychosis, and suicidal ideation. We included a total of 27 behavioral rating scales, 19 of which were of a single behavioral domain and the remaining 8 scales included multiple behavioral domains. Three rating scales were classified as "recommended" exclusively for screening purposes: the Irritability Scale for irritability, the Beck Depression Inventory-II, and the Hospital Anxiety and Depression Scale for depression. There were no "recommended" scales for other purposes such as diagnosis, severity, or change in time or to treatment. The main challenges identified for assessment of behavioral symptoms in Huntington's disease are the co-occurrence of multiple behavioral symptoms, the particular features of a behavioral symptom in Huntington's disease, and the need to address stage- and disease-specific features, including cognitive impairment and lack of insight. The committee concluded that there is a need to further validate currently available behavioral rating scales in Huntington's disease to address gaps in scale validation for specific behavioral domains and purpose of use. © 2016 International Parkinson and Movement Disorder Society.

Huntington's Disease-Update on Treatments

Huntington's disease (HD) is an autosomal dominantly inherited neurodegenerative disease characterized by progressive motor, behavioral, and cognitive decline, ending in death. Despite the discovery of the underlying genetic mutation more than 20 years ago, treatment remains focused on symptomatic management. Chorea, the most recognizable symptom, responds to medication that reduces dopaminergic neurotransmission. Psychiatric symptoms such as depression and anxiety may also respond well to symptomatic therapies. Unfortunately, many other symptoms do not respond to current treatments. Furthermore, high-quality evidence for treatment of HD in general remains limited. To date, there has been minimal success with identifying a disease-modifying therapy based upon molecular models. However, one of the emerging gene silencing techniques may provide a breakthrough in treating this devastating disease.

Biological Insights of the Dopaminergic Stabilizer ACR16 at the Binding Pocket of Dopamine D2 Receptor

The dopamine D2 receptor (D2R) plays an important part in the human central nervous system and it is considered to be a focal target of antipsychotic agents. It is structurally modeled in active and inactive states, in which homodimerization reaction of the D2R monomers is also applied. The ASP2314 (also known as ACR16) ligand, a D2R stabilizer, is used in tests to evaluate how dimerization and conformational changes may alter the ligand binding space and to provide information on alterations in inhibitory mechanisms upon activation. The administration of the D2R agonist ligand ACR16 [³H](+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ((+)PHNO) revealed K_i values of 32 nM for the D2^highR and 52 μM for the D2^lowR. The calculated binding affinities of ACR16 with post processing molecular dynamics (MD) simulations analyses using MM/PBSA for the monomeric and homodimeric forms of the D2^highR were -9.46 and -8.39 kcal/mol, respectively. The data suggests that the dimerization of the D2R leads negative cooperativity for ACR16 binding. The dimerization reaction of the D2^highR is energetically favorable by -22.95 kcal/mol. The dimerization reaction structurally and thermodynamically stabilizes the D2^highR conformation, which may be due to the intermolecular forces formed between the TM4 of each monomer, and the result strongly demonstrates dimerization essential for activation of the D2R.

Current Pharmacological Approaches to Reduce Chorea in Huntington's Disease

There are currently no effective pharmacological agents available to stop or prevent the progression of Huntington's disease (HD), a rare hereditary neurodegenerative disorder. In addition to psychiatric symptoms and cognitive impairments, HD causes progressive motor disturbances, in particular choreiform movements, which are characterized by unwanted contractions of the facial muscles, trunk and extremities. Management of choreiform movements is usually advised if chorea interferes with daily functioning, causes social isolation, gait instability, falls, or physical injury. Although drugs to reduce chorea are available, only few randomized controlled studies have assessed the efficacy of these drugs, resulting in a high variety of prescribed drugs in clinical practice. The current pharmacological treatment options to reduce chorea in HD are outlined in this review, including the latest results on deutetrabenazine, a newly developed pharmacological agent similar to tetrabenazine, but with suggested less peak dose side effects. A review of the existing literature was conducted using the PubMed, Cochrane and Medline databases. In conclusion, mainly tetrabenazine, tiapride (in European countries), olanzapine, and risperidone are the preferred first choice drugs to reduce chorea among HD experts. In the existing literature, these drugs also show a beneficial effect on motor symptom severity and improvement of psychiatric symptoms. Generally, it is recommended to start with a low dose and increase the dose with close monitoring of any adverse effects. New interesting agents, such as deutetrabenazine and pridopidine, are currently under development and more randomized controlled trials are warranted to assess the efficacy on chorea severity in HD.

Role of Sigma-1 Receptor in Cocaine Abuse and Neurodegenerative Disease

Sigma-1 receptors (Sig-1R) are recognized as a unique class of non-G protein-coupled intracellular protein. Sig-1R binds to its ligand such as cocaine , resulting in dissociation of Sig-1R from mitochondrion-associated ER membrane (MAM) to the endoplasmic reticulum (ER), plasma membrane, and nuclear membrane, regulating function of various proteins. Sig-1R has diverse roles in both physiological as well as in pathogenic processes. The disruption of Sig-1R pathways has been implicated as causative mechanism(s) in the development of both neurodegenerative disorders such as Alzheimer disease (AD ), Parkinson disease (PD ), amyotrophic lateral sclerosis (ALS ) and Huntington Disease (HD ) . Additionally, the interaction of cocaine and Sig-1R has more recently been implicated in potentiating the pathogenesis of HIV-associated neurocognitive disorders (HAND) through impairment of blood-brain barrier (BBB), microglial activation and astrogliosis. On the other hand, restoration of Sig-1R homeostasis has been shown to exert neuroprotective effects. In this review, we provide an overview of how Sig-1R plays a role in the pathogenesis of neurodegenerative disorders and cocaine and implications for future development of therapeutic strategies.

The sigma-1 receptor mediates the beneficial effects of pridopidine in a mouse model of Huntington disease

The tri-nucleotide repeat expansion underlying Huntington disease (HD) results in corticostriatal synaptic dysfunction and subsequent neurodegeneration of striatal medium spiny neurons (MSNs). HD is a devastating autosomal dominant disease with no disease-modifying treatments. Pridopidine, a postulated "dopamine stabilizer", has been shown to improve motor symptoms in clinical trials of HD. However, the target(s) and mechanism of action of pridopidine remain to be fully elucidated. As binding studies identified sigma-1 receptor (S1R) as a high-affinity receptor for pridopidine, we evaluated the relevance of S1R as a therapeutic target of pridopidine in HD. S1R is an endoplasmic reticulum - (ER) resident transmembrane protein and is regulated by ER calcium homeostasis, which is perturbed in HD. Consistent with ER calcium dysregulation, we observed striatal upregulation of S1R in aged YAC128 transgenic HD mice and HD patients. We previously demonstrated that dendritic MSN spines are lost in aged corticostriatal co-cultures from YAC128 mice. We report here that pridopidine and the chemically similar S1R agonist 3-PPP prevent MSN spine loss in aging YAC128 co-cultures. Spine protection was blocked by neuronal deletion of S1R. Pridopidine treatment suppressed supranormal ER Ca2+ release, restored ER calcium levels and reduced excessive store-operated calcium (SOC) entry in spines, which may account for its synaptoprotective effects. Normalization of ER Ca2+ levels by pridopidine was prevented by S1R deletion. To evaluate long-term effects of pridopidine, we analyzed expression profiles of calcium signaling genes. Pridopidine elevated striatal expression of calbindin and homer1a, whereas their striatal expression was reduced in aged Q175KI and YAC128 HD mouse models compared to WT. Pridopidine and 3-PPP are proposed to prevent calcium dysregulation and synaptic loss in a YAC128 corticostriatal co-culture model of HD. The actions of pridopidine were mediated by S1R and led to normalization of ER Ca2+ release, ER Ca2+ levels and spine SOC entry in YAC128 MSNs. This is a new potential mechanism of action for pridopidine, highlighting S1R as a potential target for HD therapy. Upregulation of striatal proteins that regulate calcium, including calbindin and homer1a, upon chronic therapy with pridopidine, may further contribute to long-term beneficial effects of pridopidine in HD.

Investigational agents for the management of Huntington's disease

INTRODUCTION

An inherited, chronic progressive, neurodegenerative disorder is Huntington's disease, characterized by motor, cognitive, and psychiatric symptoms. Predictive genetic testing allows earlier diagnosis and identification of gene carriers for Huntington's disease. These individuals are ideal candidates for testing of therapeutic interventions for disease modification. Areas covered: According to queries in Pubmed, Embase and clinical register databases, research and clinical studies emerge on symptomatic and neuroprotective therapies in Huntington's disease. This review discusses novel agents for symptomatic therapy and disease modification. They are currently in phase I and II of drug development Expert opinion: There are promising, safe and well tolerated compounds for amelioration of motor and neuropsychiatric symptoms, but their efficacy still needs to be proven in clinical trials. Deterioration of mutant huntingtin expression, antiapoptotic or cell death inhibition as disease modifying concepts was efficacious in models of Huntington's disease. However, the risk for clinical trial failures is high not only due to ineffectiveness of the tested agent. Negative study outcomes may also result from design misconceptions, underestimation of the heterogeneity of Huntington's disease, too short study durations and too small study cohorts.

Discovery of Therapeutic Approaches for Polyglutamine Diseases: A Summary of Recent Efforts

Polyglutamine (PolyQ) diseases are a group of neurodegenerative disorders caused by the expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats in the coding region of specific genes. This leads to the production of pathogenic proteins containing critically expanded tracts of glutamines. Although polyQ diseases are individually rare, the fact that these nine diseases are irreversibly progressive over 10 to 30 years, severely impairing and ultimately fatal, usually implicating the full-time patient support by a caregiver for long time periods, makes their economic and social impact quite significant. This has led several researchers worldwide to investigate the pathogenic mechanism(s) and therapeutic strategies for polyQ diseases. Although research in the field has grown notably in the last decades, we are still far from having an effective treatment to offer patients, and the decision of which compounds should be translated to the clinics may be very challenging. In this review, we provide a comprehensive and critical overview of the most recent drug discovery efforts in the field of polyQ diseases, including the most relevant findings emerging from two different types of approaches-hypothesis-based candidate molecule testing and hypothesis-free unbiased drug screenings. We hereby summarize and reflect on the preclinical studies as well as all the clinical trials performed to date, aiming to provide a useful framework for increasingly successful future drug discovery and development efforts.

Antipsychotic drugs in Huntington's disease

INTRODUCTION

The aim of this review is to overview the pharmacological features of neuroleptics experienced in the treatment of Huntington's disease (HD) symptoms. Despite a large number of case reports, randomized controlled trials (RCT) and drug comparison studies are lacking. Areas covered: After evaluating current guidelines and clinical unmet needs we searched PubMed for the term 'Huntington's disease' cross referenced with the terms 'Antipsychotic drugs' 'Neuroleptic drugs' and single drug specific names. Expert commentary: In clinical practice antipsychotics represent the first choice in the management of chorea in the presence of psychiatric symptoms, when poor compliance is suspected or when there is an increased risk of adverse events due to tetrabenazine. Antipsychotics are considered valid strategies, with the second generation preferred to reduce extrapyramidal adverse events, however they may cause more metabolic side effects. In the future 'dopamine stabilizers', such as pridopidine, could replace antipsychotics modulating dopamine transmission.

Pridopidine for the treatment of Huntington's disease

INTRODUCTION

Huntington's disease is a rare dominantly-inherited neurodegenerative disease with motor, cognitive and behavioral manifestations. It results from an expanded unstable trinucleotide repeat in the coding region of the huntingtin gene. Treatment is symptomatic, but a poor evidence baseguides selection of therapeutic agents. Non-choreic derangements in voluntary movement contribute to overall motor disability and are poorly addressed by current therapies. Pridopidine is a novel agent in the dopidine class believed to have 'state dependent' effects at dopamine receptors, thus show promise in the treatment of these disorders of voluntary movement.

AREAS COVERED

This review discusses the pharmacokinetics and pharmacodynamics of pridopidine and reviews clinical trials supporting development of the drug for HD. This information was culled from literature searches for dopidines, pridopidine, and HD experimental therapeutics in PubMed and at http://www.clinicaltrials.org .

EXPERT OPINION

There is a compelling need to discover new treatments for motor disability in HD, particularly for non-choreic motor symptoms. While pridopidine failed to achieve its primary efficacy outcomes in 2 large trials, reproducible effects on secondary motor outcomes have fueled an ongoing trial studying higher doses and more focused clinical endpoints. This and phase III trials will define define the utility of pridopidine for HD.

Profile of pridopidine and its potential in the treatment of Huntington disease: the evidence to date

Huntington disease (HD) is a chronic, genetic, neurodegenerative disease for which there is no cure. The main symptoms of HD are abnormal involuntary movements (chorea and dystonia), impaired voluntary movements (ie, incoordination and gait balance), progressive cognitive decline, and psychiatric disturbances. HD is caused by a CAG-repeat expanded mutation in the HTT gene, which encodes the huntingtin protein. The inherited mutation results in the production of an elongated polyQ mutant huntingtin protein (mHtt). The cellular functions of the Htt protein are not yet fully understood, but the functions of its mutant variant are thought to include alteration of gene transcription and energy production, and dysregulation of neurotransmitter metabolism, receptors, and growth factors. The phenylpiperidines pridopidine (4-[3-methanesulfonyl-phenyl]-1-propyl-piperidine; formerly known as ACR16) and OSU6162 ([S]-[-]-3-[3-methane [sulfonyl-phenyl]-1-propyl-piperidine) are members of a new class of pharmacologic agents known as “dopamine stabilizers”. Recent clinical trials have highlighted the potential of pridopidine for symptomatic treatment of patients with HD. More recently, the analysis of HD models (ie, in vitro and in mice) highlighted previously unknown effects of pridopidine (increase in brain-derived neurotrophic factor, reduction in mHtt levels, and σ-1 receptor binding and modulation). These additional functions of pridopidine suggest it might be a neuroprotective and disease-modifying drug. Data from ongoing clinical trials of pridopidine will help define its place in the treatment of HD. This commentary examines the available preclinical and clinical evidence regarding the use of pridopidine in HD.

Pridopidine, a dopamine stabilizer, improves motor performance and shows neuroprotective effects in Huntington disease R6/2 mouse model

Huntington disease (HD) is a neurodegenerative disorder for which new treatments are urgently needed. Pridopidine is a new dopaminergic stabilizer, recently developed for the treatment of motor symptoms associated with HD. The therapeutic effect of pridopidine in patients with HD has been determined in two double-blind randomized clinical trials, however, whether pridopidine exerts neuroprotection remains to be addressed. The main goal of this study was to define the potential neuroprotective effect of pridopidine, in HD in vivo and in vitro models, thus providing evidence that might support a potential disease-modifying action of the drug and possibly clarifying other aspects of pridopidine mode-of-action. Our data corroborated the hypothesis of neuroprotective action of pridopidine in HD experimental models. Administration of pridopidine protected cells from apoptosis, and resulted in highly improved motor performance in R6/2 mice. The anti-apoptotic effect observed in the in vitro system highlighted neuroprotective properties of the drug, and advanced the idea of sigma-1-receptor as an additional molecular target implicated in the mechanism of action of pridopidine. Coherent with protective effects, pridopidine-mediated beneficial effects in R6/2 mice were associated with an increased expression of pro-survival and neurostimulatory molecules, such as brain derived neurotrophic factor and DARPP32, and with a reduction in the size of mHtt aggregates in striatal tissues. Taken together, these findings support the theory of pridopidine as molecule with disease-modifying properties in HD and advance the idea of a valuable therapeutic strategy for effectively treating the disease.

Clinical diagnosis and management in early Huntington's disease: a review

This review focuses on clinical diagnosis and both pharmacological and nonpharmacological therapeutic options in early stages of the autosomal dominant inherited neurodegenerative Huntington's disease (HD). The available literature has been reviewed for motor, cognitive, and psychiatric alterations, which are the three major symptom domains of this devastating progressive disease. From a clinical point of view, one has to be aware that the HD phenotype can vary highly across individuals and during the course of the disease. Also, symptoms in juvenile HD can differ substantially from those with adult-onset of HD. Although there is no cure of HD and management is limited, motor and psychiatric symptoms often respond to pharmacotherapy, and nonpharmacological approaches as well as supportive care are essential. International treatment recommendations based on study results, critical statements, and expert opinions have been included. This review is restricted to symptomatic and supportive approaches since all attempts to establish a cure for the disease or modifying therapies have failed so far.

Dopamine and Huntington's disease

Huntington's disease (HD) is an incurable, inherited, progressive neurodegenerative disorder that is defined by a combination of motor, cognitive and psychiatric features. Pre-clinical and clinical studies have demonstrated an important role for the dopamine (DA) system in HD with dopaminergic dysfunction at the level of both DA release and DA receptors. It is, therefore, not surprising that the drug treatments most commonly used in HD are anti-dopaminergic agents. Their use is based primarily on the belief that the characteristic motor impairments are a result of overactivation of the central dopaminergic pathways. While this is a useful starting place, it is clear that the behavior of the central dopaminergic pathways is not fully understood in this condition and may change as a function of disease stage. In addition, how abnormalities in dopaminergic systems may underlie some of the non-motor features of HD has also been poorly investigated and this is especially important given the greater burden these place on the patients' and families' quality of life. In this review, we discuss what is known about central dopaminergic pathways in HD and how this informs us about the mechanisms of action of the dopaminergic therapies used to treat it. By doing so, we will highlight some of the paradoxes that exist and how solving them may reveal new insights for improved treatment of this currently incurable condition, including the possibility that such drugs may even have effects on disease progression and pathogenesis.

Update on Huntington's disease: advances in care and emerging therapeutic options

INTRODUCTION

Huntington's disease (HD) is the most common hereditary neurodegenerative disorder. Despite the fact that both the gene and the mutation causing this monogenetic disorder were identified more than 20 years ago, disease-modifying therapies for HD have not yet been established.

REVIEW

While intense preclinical research and large cohort studies in HD have laid foundations for tangible improvements in understanding HD and caring for HD patients, identifying targets for therapeutic interventions and developing novel therapeutic modalities (new chemical entities and advanced therapies using DNA and RNA molecules as therapeutic agents) continues to be an ongoing process. The authors review recent achievements in HD research and focus on approaches towards disease-modifying therapies, ranging from huntingtin-lowering strategies to improving huntingtin clearance that may be promoted by posttranslational HTT modifications.

CONCLUSION

The nature and number of upcoming clinical studies/trials in HD is a reason for hope for HD patients and their families.

Investigational drugs for the management of Huntington's disease: are we there yet?

INTRODUCTION

Huntington's disease is a hereditary neurodegenerative disease. It is designated as a rare disease in the US, which means there are < 200,000 patients in the country who suffer from it. The drugs that are currently used to treat this disease were not designed specifically for it but developed for other diseases. Presently, two classes of drugs are being developed; those that provide symptomatic relief and those that may modify course of the disease.

AREAS COVERED

This review is focused on seven selected drugs currently in clinical testing and describes their progress. Five of the seven drugs that are reviewed here, can be categorized as 'symptomatic' drugs, and, selisistat and PBT-2 are amongst the ones that would qualify as 'disease modifying' drugs.

EXPERT OPINION

The authors believe that the future treatment paradigm for this disease is best met by using a disease-modifying drug that can be administered together with symptomatic drugs. Towards that end, it is important for the industry to focus on disease-modifying drugs by targeting unique pathways and targets. Furthermore, they propose that neuroprotective drugs, that is, drugs that directly work by preserving neuronal health and function is an opportunity for such disease-modifying drugs.

Therapeutic Strategies for Huntington's Disease

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease, caused by expansion of the CAG repeat in the huntingtin (HTT) gene and characterized pathologically by the loss of pyramidal neurons in several cortical areas, of striatal medium spiny neurons, and of hypothalamic neurons. Clinically, a distinguishing feature of the disease is uncontrolled involuntary movements (chorea, dyskensias) accompanied by progressive cognitive, motor, and psychiatric impairment. This review focuses on the current state of therapeutic development for the treatment of HD, including the preclinical and clinical development of small molecules and molecular therapies.

Therapeutic approaches to preventing cell death in Huntington disease

Neurodegenerative diseases affect the lives of millions of patients and their families. Due to the complexity of these diseases and our limited understanding of their pathogenesis, the design of therapeutic agents that can effectively treat these diseases has been challenging. Huntington disease (HD) is one of several neurological disorders with few therapeutic options. HD, like numerous other neurodegenerative diseases, involves extensive neuronal cell loss. One potential strategy to combat HD and other neurodegenerative disorders is to intervene in the execution of neuronal cell death. Inhibiting neuronal cell death pathways may slow the development of neurodegeneration. However, discovering small molecule inhibitors of neuronal cell death remains a significant challenge. Here, we review candidate therapeutic targets controlling cell death mechanisms that have been the focus of research in HD, as well as an emerging strategy that has been applied to developing small molecule inhibitors-fragment-based drug discovery (FBDD). FBDD has been successfully used in both industry and academia to identify selective and potent small molecule inhibitors, with a focus on challenging proteins that are not amenable to traditional high-throughput screening approaches. FBDD has been used to generate potent leads, pre-clinical candidates, and has led to the development of an FDA approved drug. This approach can be valuable for identifying modulators of cell-death-regulating proteins; such compounds may prove to be the key to halting the progression of HD and other neurodegenerative disorders.

The monoamine stabilizer (-)-OSU6162 attenuates voluntary ethanol intake and ethanol-induced dopamine output in nucleus accumbens

BACKGROUND

New medications for alcohol use disorder (AUD) are needed. Long-term alcohol consumption leads to a dysregulated dopamine system. A novel approach to normalize these dysregulations might be treatment with "monoamine stabilizers," a novel class of compounds characterized by the ability to either suppress, stimulate, or not influence dopamine activity depending on the prevailing dopaminergic tone.

METHODS

The effects of the monoamine stabilizer (-)-OSU6162 (OSU6162) on voluntary ethanol intake and ethanol withdrawal symptoms were evaluated in rats voluntarily consuming ethanol for at least 3 months before testing. Furthermore, effects of OSU6162 on ethanol seeking behavior were evaluated with the progressive ratio and cue-induced reinstatement paradigms. Finally, the interaction of OSU6162 with ethanol on dopamine output and metabolism was studied with microdialysis.

RESULTS

The OSU6162 attenuated several ethanol-mediated behaviors, including voluntary ethanol consumption, ethanol withdrawal symptoms, operant ethanol self-administration under progressive ratio schedule, and cue-induced reinstatement of ethanol seeking in rats that had voluntarily consumed ethanol for at least 3 months before treatment. In addition, OSU6162 blunted ethanol-induced dopamine output in nucleus accumbens of ethanol-naïve rats.

CONCLUSIONS

These results highlight the ability of OSU6162 to stabilize dopamine activity depending on the prevailing dopaminergic tone and indicate that OSU6162 might decrease ethanol intake by attenuating the acute rewarding properties of ethanol. In addition, OSU6162 might have potential to prevent relapse triggered by alcohol craving, alcohol related cues, and or an urge to relieve abstinence symptoms. The present study is to our knowledge the first indicating that OSU6162 might serve as a novel medication for AUD.

Striatal atrophy and dendritic alterations in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disease characterized by progressive atrophy of the striatum, cerebral cortex, and white matter tracks. Major pathological hallmarks of HD include neuronal loss, primarily in the striatum, and dendritic anomalies in surviving striatal neurons. Although many mouse models of HD have been generated, their success at reproducing all pathological features of the disease is not fully known. Previously, we demonstrated extensive striatal neuronal loss and striatal atrophy at 20-26 months of age in a knock-in (KI) mouse model of HD. To further investigate this model, which carries a human exon 1 with ∼119 CAG repeats inserted into the mouse gene (initially 140 repeats), we have examined whether these mice exhibit the atrophy and neuronal anomalies characteristic of HD. Stereological analyses revealed no changes in the striatal volume of male and female homozygote mice at 4 months, however striatal atrophy was already present at 12 months in both sexes. Analysis of cortical and corpus callosum volume in male homozygotes revealed a loss in corpus callosum volume by 20-26 months. At this later age, the surviving striatal neurons displayed extensive loss of spines in distal branch orders that affected both immature and mature spines. Mirroring late stage HD striatal neuronal morphology, the striatal neurons at this late age also showed reduced dendritic complexity, as revealed by Sholl analysis. Tyrosine hydroxylase immunoreactivity was also decreased in the striatum of 20-26 month old KI mice, suggesting an alteration in striatal inputs. These data further indicate that CAG140 homozygote KI mice exhibit HD-like pathological features and are a useful model to test the effects of early and/or sustained administration of novel neuroprotective treatments.