A study of chorea after tetrabenazine withdrawal in patients with Huntington disease

Sex differences in Huntington's disease from a neuroinflammation perspective

Huntington's disease (HD) is a debilitating neurodegenerative condition characterized by motor, cognitive and psychiatric abnormalities. Immune dysregulation, prominently featuring increased immune activity, plays a significant role in HD pathogenesis. In addition to the central nervous system (CNS), systemic innate immune activation and inflammation are observed in HD patients, exacerbating the effects of the Huntingtin (HTT) gene mutation. Recent attention to sex differences in HD symptom severity underscores the need to consider gender as a biological variable in neurodegenerative disease research. Understanding sex-specific immune responses holds promise for elucidating HD pathophysiology and informing targeted treatment strategies to mitigate cognitive and functional decline. This perspective will highlight the importance of investigating gender influence in HD, particularly focusing on sex-specific immune responses predisposing individuals to disease.

Mutant-Huntingtin Molecular Pathways Elucidate New Targets for Drug Repurposing

The spectrum of neurodegenerative diseases known today is quite extensive. The complexities of their research and treatment lie not only in their diversity. Even many years of struggle and narrowly focused research on common pathologies such as Alzheimer's, Parkinson's, and other brain diseases have not brought cures for these illnesses. What can be said about orphan diseases? In particular, Huntington's disease (HD), despite affecting a smaller part of the human population, still attracts many researchers. This disorder is known to result from a mutation in the HTT gene, but having this information still does not simplify the task of drug development and studying the mechanisms of disease progression. Nonetheless, the data accumulated over the years and their analysis provide a good basis for further research. Here, we review studies devoted to understanding the mechanisms of HD. We analyze genes and molecular pathways involved in HD pathogenesis to describe the action of repurposed drugs and try to find new therapeutic targets.

Current Drug Repurposing Strategies for Rare Neurodegenerative Disorders

Rare diseases are life-threatening or chronically debilitating low-prevalent disorders caused by pathogenic mutations or particular environmental insults. Due to their high complexity and low frequency, important gaps still exist in their prevention, diagnosis, and treatment. Since new drug discovery is a very costly and time-consuming process, leading pharmaceutical companies show relatively low interest in orphan drug research and development due to the high cost of investments compared to the low market return of the product. Drug repurposing–based approaches appear then as cost- and time-saving strategies for the development of therapeutic opportunities for rare diseases. In this article, we discuss the scientific, regulatory, and economic aspects of the development of repurposed drugs for the treatment of rare neurodegenerative disorders with a particular focus on Huntington’s disease, Friedreich’s ataxia, Wolfram syndrome, and amyotrophic lateral sclerosis. The role of academia, pharmaceutical companies, patient associations, and foundations in the identification of candidate compounds and their preclinical and clinical evaluation will also be discussed.

Zebrafish an experimental model of Huntington's disease: molecular aspects, therapeutic targets and current challenges

Huntington disease (HD) is a lethal autosomal dominant neurodegenerative disease whose exact causative mechanism is still unknown. It can transform from one generation to another generation. The CAG triplet expansion on polyglutamine (PolyQ) tract on Huntingtin protein primarily contributes in HD pathogenesis. Apart from this some another molecular mechanisms are also involved in HD pathology such as loss of Brain derived neurotrophic factor in medium spiny neurons, mitochondrial dysfunction, and alterations in synaptic plasticity are briefly discussed in this review. However, several chemicals (3-nitropropionic acid, and Quinolinic acid) and genetic (mHTT-ΔN17-97Q over expression) experimental models are used to explore the exact pathogenic mechanism and finding of new drug targets for the development of novel therapeutic approaches. The zebrafish (Danio rerio) is widely used in in-vivo screening of several central nervous system (CNS) diseases such as HD, Alzheimer's disease (AD), Parkinson's disease (PD), and in memory deficits. Thus, this makes zebrafish as an excellent animal model for the development of new therapeutic strategies against various CNS disorders. We had reviewed several publications utilizing zebrafish and rodents to explore the disease pathology. Studies suggested that zebrafish genes and their human homologues have conserved functions. Zebrafish advantages and their characteristics over the other experimental animals make it an excellent tool for the disease study. This review explains the possible pathogenic mechanism of HD and also discusses about possible treatment therapies, apart from this we also discussed about possible potential therapeutic targets which will helps in designing of novel therapeutic approaches to overcome the disease progression.  Diagrammatic depiction shows prevention of HD pathogenesis through attenuation of various biochemical alterations.

New Avenues for the Treatment of Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by a CAG expansion in the HD gene. The disease is characterized by neurodegeneration, particularly in the striatum and cortex. The first symptoms usually appear in mid-life and include cognitive deficits and motor disturbances that progress over time. Despite being a genetic disorder with a known cause, several mechanisms are thought to contribute to neurodegeneration in HD, and numerous pre-clinical and clinical studies have been conducted and are currently underway to test the efficacy of therapeutic approaches targeting some of these mechanisms with varying degrees of success. Although current clinical trials may lead to the identification or refinement of treatments that are likely to improve the quality of life of those living with HD, major efforts continue to be invested at the pre-clinical level, with numerous studies testing novel approaches that show promise as disease-modifying strategies. This review offers a detailed overview of the currently approved treatment options for HD and the clinical trials for this neurodegenerative disorder that are underway and concludes by discussing potential disease-modifying treatments that have shown promise in pre-clinical studies, including increasing neurotropic support, modulating autophagy, epigenetic and genetic manipulations, and the use of nanocarriers and stem cells.

Modulation of dopamine D1 receptors via histamine H3 receptors is a novel therapeutic target for Huntington's disease

Early Huntington's disease (HD) include over-activation of dopamine D₁ receptors (D₁R), producing an imbalance in dopaminergic neurotransmission and cell death. To reduce D₁R over-activation, we present a strategy based on targeting complexes of D₁R and histamine H₃ receptors (H₃R). Using an HD mouse striatal cell model and HD mouse organotypic brain slices we found that D₁R-induced cell death signaling and neuronal degeneration, are mitigated by an H₃R antagonist. We demonstrate that the D₁R-H₃R heteromer is expressed in HD mice at early but not late stages of HD, correlating with HD progression. In accordance, we found this target expressed in human control subjects and low-grade HD patients. Finally, treatment of HD mice with an H₃R antagonist prevented cognitive and motor learning deficits and the loss of heteromer expression. Taken together, our results indicate that D₁R - H₃R heteromers play a pivotal role in dopamine signaling and represent novel targets for treating HD.

Sex differences in movement disorders

In a range of neurological conditions, including movement disorders, sex-related differences are emerging not only in brain anatomy and function, but also in pathogenesis, clinical features and response to treatment. In Parkinson disease (PD), for example, oestrogens can influence the severity of motor symptoms, whereas elevation of androgens can exacerbate tic disorders. Nevertheless, the real impact of sex differences in movement disorders remains under-recognized. In this article, we provide an up-to-date review of sex-related differences in PD and the most common hyperkinetic movement disorders, namely, essential tremor, dystonia, Huntington disease and other chorea syndromes, and Tourette syndrome and other chronic tic disorders. We highlight the most relevant clinical aspects of movement disorders that differ between men and women. Increased recognition of these differences and their impact on patient care could aid the development of tailored approaches to the management of movement disorders and enable the optimization of preclinical research and clinical studies.

Perceptions of the impact of chorea on health-related quality of life in Huntington disease (HD): A qualitative analysis of individuals across the HD spectrum, family members, and clinicians

Chorea, a hallmark symptom of Huntington's disease (HD), is characterized by jerky involuntary movements affecting the whole body that can interfere with daily functioning and impact health-related quality of life (HRQOL). To characterize chorea's impact on everyday functioning and HRQOL and identify patterns of perception and experiences of chorea among patients, caregivers, and providers. Data from focus groups of individuals with manifest HD (n = 8 early-stage HD; n = 16 late-stage HD), individuals at-risk or prodromal HD (n = 16), family HD caregivers (n = 17), and HD clinicians (n = 25). Focus group recordings were transcribed verbatim and analysed via constant comparison to identify meaningful and salient themes of living with chorea. Global themes of chorea's impact identified included: watching for chorea, experiences of stigma, and constraints on independence and relationships. Themes distinct to specific respondent groups included: Vigilance (at risk, prodromal); adaptation to chorea (early-stage); loss of autonomy and social life (late-stage); monitoring engagement (family caregivers) and safety (clinical providers). Living with chorea significantly constrains daily functioning, interactions, and HRQOL across the HD disease spectrum. Addressing these impacts via appropriate management of chorea can potentially enhance functioning, HRQOL, and overall satisfaction for persons with HD and their families.

Treatment options for chorea

INTRODUCTION

Chorea is defined as jerk-like movements that move randomly from one body part to another. It is due to a variety of disorders and although current symptomatic therapy is quite effective there are few etiology- or pathogenesis-targeted therapies. The aim of this review is to summarize our own experience and published evidence in the treatment of chorea. Areas covered: After evaluating current guidelines and clinical practices for chorea of all etiologies, PubMed was searched for the most recent clinical trials and reviews using the term 'chorea' cross referenced with specific drug names. Expert commentary: Inhibitors of presynaptic vesicular monoamine transporter type 2 (VMAT2) that cause striatal dopamine depletion, such as tetrabenazine, deutetrabenazine, and valbenazine, are considered the treatment of choice in patients with chorea. Some clinicians also use dopamine receptor blockers (e.g. antipsychotics) and other drugs, including anti-epileptics and anti-glutamatargics. 'Dopamine stabilizers' such as pridopidine and other experimental drugs are currently being investigated in the treatment of chorea. Deep brain stimulation is usually reserved for patients with disabling chorea despite optimal medical therapy.

Dysregulation of Corticostriatal Connectivity in Huntington's Disease: A Role for Dopamine Modulation

Aberrant communication between striatum, the main information processing unit of the basal ganglia, and cerebral cortex plays a critical role in the emergence of Huntington’s disease (HD), a fatal monogenetic condition that typically strikes in the prime of life. Although both striatum and cortex undergo substantial cell loss over the course of HD, corticostriatal circuits become dysfunctional long before neurons die. Understanding the dysfunction is key to developing effective strategies for treating a progressively worsening triad of motor, cognitive, and psychiatric symptoms. Cortical output neurons drive striatal activity through the release of glutamate, an excitatory amino acid. Striatal outputs, in turn, release γ-amino butyric acid (GABA) and exert inhibitory control over downstream basal ganglia targets. Ample evidence from transgenic rodent models points to dysregulation of corticostriatal glutamate transmission along with corresponding changes in striatal GABA release as underlying factors in the HD behavioral phenotype. Another contributor is dysregulation of dopamine (DA), a modulator of both glutamate and GABA transmission. In fact, pharmacological manipulation of DA is the only currently available treatment for HD symptoms. Here, we review data from animal models and human patients to evaluate the role of DA in HD, including DA interactions with glutamate and GABA within the context of dysfunctional corticostriatal circuitry.

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.

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.

Tetrabenazine: Spotlight on Drug Review

BACKGROUND

Tetrabenazine (TBZ) is the only US Food and Drug Administration-approved drug for the treatment of chorea related to Huntington's disease and other hyperkinetic disorders. TBZ was first synthesized in 1950, and was then used for the treatment of psychosis. But later its potential in treating hyperkinetic disorders was proved by its ability to block vesicular monoamine transporters 2 and deplete monoamine stores. There is still lack of awareness about the therapeutic potential of this drug.

SUMMARY

TBZ had been approved only for the treatment of chorea, but several clinical studies have been conducted by different research groups and it was concluded that TBZ is effective in various other conditions such as tardive dyskinesia, dystonia, tics, and Tourette's syndrome, thus, highlighting the need for further clinical trials in these conditions.

KEY MESSAGE

The intention of this review is to sum up the information regarding chemistry, mechanism of action, pharmacokinetics, interactions, contraindications, adverse effects, and clinical efficacy of TBZ in diseases other than Huntington's chorea.

Advancing pharmacotherapy for treating Huntington's disease: a review of the existing literature

INTRODUCTION

Huntington's disease (HD) is an incurable chronic neurodegenerative disorder that typically presents in mid-life with a range of motor, cognitive and affective problems. Patients are currently managed using a combination of drug treatments and non-pharmacological therapies but at present there is no "gold standard" treatment for any aspect of the disease.

AREAS COVERED

In this review the empirical evidence supporting the use of drugs commonly used to treat HD was discussed. In particular, we focus on therapeutics that have either reached phase 3 clinical trials or are already in clinical use.

EXPERT OPINION

The results confirmed that there is a striking lack of evidence to support the efficacy of the drugs currently used in the management of HD. In fact, many drugs are prescribed on the basis of case reports, open label studies, small double blind placebo control trials of limited duration, or personal clinical experience. However of late, the establishment of large international databases, capturing all patients and their clinical details regardless of stage or geographical location has led to an increase in the number of clinical trials conducted on new therapies. Unfortunately, the same is not true for the existing therapies which look set to remain untested for the foreseeable future.

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.

Preclinical and clinical investigations of mood stabilizers for Huntington's disease: what have we learned?

Huntington's disease (HD) is a lethal, autosomal dominant neurodegenerative disorder caused by CAG repeat expansions at exon 1 of the huntingtin (Htt) gene, which encodes for a mutant huntingtin protein (mHtt). Prominent symptoms of HD include motor dysfunction, characterized by chorea; psychiatric disturbances such as mood and personality changes; and cognitive decline that may lead to dementia. Pathologically multiple complex processes and pathways are involved in the development of HD, including selective loss of neurons in the striatum and cortex, dysregulation of cellular autophagy, mitochondrial dysfunction, decreased neurotrophic and growth factor levels, and aberrant regulation of gene expression and epigenetic patterns. No cure for HD presently exists, nor are there drugs that can halt the progression of this devastating disease. Therefore, the need to discover neuroprotective modalities to combat HD is critical. In basic and preclinical studies using cellular and animal HD models, the mood stabilizers lithium and valproic acid (VPA) have shown multiple beneficial effects, including behavioral and motor improvement, enhanced neuroprotection, and lifespan extension. Recent studies in transgenic HD mice support the notion that combined lithium/VPA treatment is more effective than treatment with either drug alone. In humans, several clinical studies of HD patients found that lithium treatment improved mood, and that VPA treatment both stabilized mood and moderately reduced chorea. In contrast, other studies observed that the hallmark features of HD were unaffected by treatment with either lithium or VPA. The current review discusses preclinical and clinical investigations of the beneficial effects of lithium and VPA on HD pathophysiology.

Safety and Efficacy of Tetrabenazine and Use of Concomitant Medications During Long-Term, Open-Label Treatment of Chorea Associated with Huntington's and Other Diseases

Background

Although tetrabenazine, a drug that depletes presynaptic dopamine by inhibiting vesicular monoamine transporter 2 (VMAT2), was approved by the U.S. Food and Drug Administration in 2008 for the treatment of chorea associated with Huntington’s disease (HD), there is a paucity of data on its long-term efficacy and safety.

Methods

Approximately 2,000 patients with a variety of hyperkinetic movement disorders had been treated with open-label tetrabenazine at the Movement Disorders Clinic, Baylor College of Medicine, since 1979. Tetrabenazine was usually started at 12.5 mg/day, and the dosage was gradually increased (up to 300 mg/day). Responses were rated by the investigator 1–5, with 1  =  marked chorea reduction, excellent improvement in function; 2  =  moderate chorea reduction, very good improvement in function; 3  =  fair chorea improvement, only mild improvement in function; 4  =  poor or no response for chorea and function; and 5  =  worsening chorea, some functional deterioration. Efficacy and safety were analyzed retrospectively.

Results

By 2004, 98 HD chorea patients had received tetrabenazine for a mean of 3.1 years (range ≤1–11.4 years). Of those with valid ratings, 75% had either marked or very good responses (rating 1 or 2) at their optimal dosages. The most common adverse events occurring in ≥5% of the patients were somnolence (39%), insomnia (33%), depression (31%), accidental injury (26%), and dysphagia (19%). Efficacy and safety were comparable to results for non-HD chorea patients.

Discussion

Tetrabenazine treatment was associated with long-term improvement in chorea. Adverse event rates were comparable to those reported from controlled trials.

Antisense therapy in neurology

Antisense therapy is an approach to fighting diseases using short DNA-like molecules called antisense oligonucleotides. Recently, antisense therapy has emerged as an exciting and promising strategy for the treatment of various neurodegenerative and neuromuscular disorders. Previous and ongoing pre-clinical and clinical trials have provided encouraging early results. Spinal muscular atrophy (SMA), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS), Duchenne muscular dystrophy (DMD), Fukuyama congenital muscular dystrophy (FCMD), dysferlinopathy (including limb-girdle muscular dystrophy 2B; LGMD2B, Miyoshi myopathy; MM, and distal myopathy with anterior tibial onset; DMAT), and myotonic dystrophy (DM) are all reported to be promising targets for antisense therapy. This paper focuses on the current progress of antisense therapies in neurology.

Recent advances in the management of choreas

The management of patients with chorea, in particular Huntington's disease, is a complex task requiring skills in a number of areas. This paper reviews new knowledge on this topic and places it in the context of established procedures. It is focused on Huntington's disease, since this is the disorder, for which most publications on management have been published in the past few years. Management starts with appropriate diagnosis and differential diagnosis, with the aim of finding disorders with chorea amenable to causative treatment. The place of genetic testing and the importance of genetic counselling is stressed, as well as the importance of precise observation in the course of the disorder to tailor appropriate therapies. Pharmacological treatment is based on poor evidence but to a large extent on expertise from centres devoted to the care of patients with Huntington's disease. It is focused mainly on motor and psychiatric aspects of the phenotype. Nonpharmacological treatment is important and is best offered in a multidisciplinary care setting.

Translational research in Huntington's disease: opening up for disease modifying treatment

Research on the molecular mechanisms involved in Huntington's disease, a monogenic disorder with a complex phenotype including motor, behaviour, and cognitive impairments, is advancing at a rapid path. Knowledge on several of the multimodal pathways has now lead to the establishment of rational strategies to prepare trials of several compounds in affected people. Furthermore, improved understanding of the phenotype and on ways of assessing it, as well as the process of developing biomarkers, allows setting the frame for such studies. In this brief review, the present status of some of these aspects is examined.

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.

Effect of tetrabenazine on motor function in patients with huntington disease

Introduction

Tetrabenazine (TBZ) reduces chorea related to Huntington disease (HD); however, it is uncertain whether this effect improves functionally relevant motor skills such as hand coordination and balance. The objective of this study was to provide pilot data regarding three motor function tests, which might be useful in monitoring symptom progression and therapeutic response, pending formal validation.

Methods

The authors assessed 11 ambulatory patients with HD-related chorea on two occasions: (1) while off TBZ (either prior to starting therapy or following a >24 h washout) and (2) when on a stable dose of TBZ, titrated to optimal effect. Study evaluations included the Jebsen-Taylor Hand Function Test (JTHFT) and Berg Balance Scale, a timed 25-foot walk, the Montreal Cognitive Assessment (MoCA) and the complete United Huntington Disease Rating Scale (UHDRS).

Results

Maximal chorea scores (UHDRS item 12) improved from 11.1 ± 2.9 to 8.5 ± 3.9 while on TBZ (P = 0.03), but we could not detect an improvement in functional measures while on TBZ in this small cohort. Scores of the JTHFT were globally slower than published normative data and correlated with MoCA summary scores, but not UHDRS chorea scores.

Conclusions

This pilot study did not detect significant functional gains with chorea suppression. The fact that performance on tests of hand function correlates with MoCA but not UHDRS chorea scores highlights the need for additional treatments targeted toward the cognitive aspects of HD.

Clinical assessment of the effect of tetrabenazine on functional scales in huntington disease: a pilot open label study

Background

Tetrabenazine is a monoamine depleter with a well-documented effect against chorea associated with Huntington disease (HD). There is a paucity of data about how reduction in chorea relates to better performance on motor, gait, cognitive, and psychiatric assessments.

Methods

We designed an open label tetrabenazine withdrawal study to test performance using validated scales. The following instruments were used to assess cognitive, behavioral, and motor function in 10 patients with documented HD: The Montreal Cognitive Assessment, Beck Depression Inventory II, Dynamic Gait Index (DGI), Jebsen Hand Test, Timed 25-foot walk, Berg Balance Test (BBT), QuickDASH, and the Unified Huntington Disease Rating Scale (UHDRS) Motor, Stroop Color Word, Behavioral Assessment, Functional Checklist, and Independence Scale.

Results

Subjects performed significantly better while on tetrabenazine as measured by the DGI (p = 0.041), BBT (p = 0.007), and the UHDRS Total Motor (p = 0.009), Maximum Chorea (p = 0.005), and Stroop Color-Word tests (p = 0.028).

Discussion

This pilot study demonstrates tetrabenazine's potential effects beyond improvement in chorea.

Tetrabenazine for the treatment of chorea and other hyperkinetic movement disorders

Tetrabenazine (TBZ; Xenazine) is a potent, selective, reversible depletor of monoamines from nerve terminals. TBZ inhibits the vesicular monoamine transporter type 2 which, in humans, is expressed nearly exclusively in the brain. TBZ is rapidly metabolized in the liver by carbonyl reductase to stereoisomers of hydrotetrabenazine, some of which are potent inhibitors of vesicular monoamine transporter type 2. Initially developed in the 1950s for schizophrenia, since the 1970s several publications have reported on the efficacy of TBZ in the treatment of various hyperkinetic movement disorders. Although quite effective in controlling the involuntary movements, there were considerable inter-individual differences in the optimal dose, defined as the dose judged by the investigator to provide the greatest efficacy with minimal or tolerable adverse events. This variability is in part owing to differences in severity and mechanism of the target symptoms and to variable activity of the enzyme carbonyl reductase that metabolizes TBZ to its active metabolites. Dose-limiting adverse events, consisting mainly of sedation, parkinsonism, akathisia and depression, are usually rapidly reversible upon dosage reduction. In addition to its established antichorea efficacy in Huntington's disease, the drug has been reported to also be effective in a variety of other hyperkinetic movement disorders, including tardive dyskinesia and tics associated with Tourette's syndrome.

Evidence-based guideline: pharmacologic treatment of chorea in Huntington disease: report of the guideline development subcommittee of the American Academy of Neurology

OBJECTIVE

To develop an evidence-based guideline assessing pharmacologic options for treating Huntington disease (HD) chorea.

METHODS

We evaluated available evidence from a structured literature review performed through February 2011.

RESULTS AND RECOMMENDATIONS

If HD chorea requires treatment, clinicians should prescribe tetrabenazine (up to 100 mg/day), amantadine (300-400 mg/day), or riluzole (200 mg/day) (Level B) for varying degrees of expected benefit. Occurrence of adverse events should be discussed and monitored, particularly depression/suicidality and parkinsonism with tetrabenazine and elevated liver enzymes with riluzole. Clinicians may also prescribe nabilone for modest decreases (1- to <2-point changes on the Unified Huntington's Disease Rating Scale [UHDRS] chorea score) in HD chorea (Level C), but information is insufficient to recommend long-term use, particularly given abuse potential concerns (Level U). Clinicians should not prescribe riluzole 100 mg/day for moderate (2- to < 3-point UHDRS chorea change) short-term benefits (Level B) or for any long-term (3-year) HD antichoreic goals (Level B). Clinicians may choose not to prescribe ethyl-EPA (Level B), minocycline (Level B), or creatine (Level C) for very important improvements (>3-point UHDRS chorea change) in HD chorea. Clinicians may choose not to prescribe coenzyme Q10 (Level B) for moderate improvements in HD chorea. Data are insufficient to make recommendations regarding the use of neuroleptics or donepezil for HD chorea treatment (Level U).

Tetrabenazine: for chorea associated with Huntington's disease

Oral tetrabenazine is currently the only drug approved by the US FDA for the treatment of chorea associated with Huntington's disease (HD). Although the precise antichorea mechanism of action is unknown, it most likely involves reversible depletion of monoamines, particularly dopamine, from presynaptic terminals via inhibition of human vesicular monoamine transporter type 2. In a 12-week, double-blind, placebo-controlled trial conducted in the US in patients with HD, oral tetrabenazine (≤100 mg/day; n = 54) was significantly (p = 0.0001) more efficacious than placebo (n = 30) at improving adjusted mean Unified HD Rating Scale (UHDRS) total maximum chorea scores (reduced from baseline by 5 vs 1.5) [primary endpoint]. After 12 weeks, improvements in UHDRS total maximum chorea scores of >3 were achieved by significantly (p < 0.0001) more patients in the tetrabenazine group than in the placebo group. The antichorea efficacy of tetrabenazine was maintained in an 80-week extension study (n = 75), with the adjusted mean UHDRS total maximum chorea score significantly (p < 0.001) reduced from baseline (score of 14.9) by 4.6 points (primary outcome). In the 12-week trial and 80-week extension study, treatment-emergent adverse events in the tetrabenazine group mainly occurred during the dosage-titration phase, a period during which the dosage was individually optimized. Most of these events were mild to moderate and were manageable with dosage adjustments or discontinuation of study drug.

Targeting the neurovascular unit for treatment of neurological disorders

Drug discovery for CNS disorders has been restricted by the inability for therapeutic agents to cross the blood-brain barrier (BBB). Moreover, current drugs aim to correct neuron cell signaling, thereby neglecting pathophysiological changes affecting other cell types of the neurovascular unit (NVU). Components of the NVU (pericytes, microglia, astrocytes, and neurons, and basal lamina) act as an intricate network to maintain the neuronal homeostatic microenvironment. Consequently, disruptions to this intricate cell network lead to neuron malfunction and symptoms characteristic of CNS diseases. A lack of understanding in NVU signaling cascades may explain why current treatments for CNS diseases are not curative. Current therapies treat symptoms by maintaining neuron function. Refocusing drug discovery to sustain NVU function may provide a better method of treatment by promoting neuron survival. In this review, we will examine current therapeutics for common CNS diseases, describe the importance of the NVU in cerebral homeostasis and discuss new possible drug targets and technologies that aim to improve treatment and drug delivery to the diseased brain.

Tetrabenazine: the first approved drug for the treatment of chorea in US patients with Huntington disease

Huntington disease (HD) is a dominantly inherited progressive neurological disease characterized by chorea, an involuntary brief movement that tends to flow between body regions. HD is typically diagnosed based on clinical findings in the setting of a family history and may be confirmed with genetic testing. Predictive testing is available to those at risk, but only experienced clinicians should perform the counseling and testing. Multiple areas of the brain degenerate mainly involving the neurotransmitters dopamine, glutamate, and γ-aminobutyric acid. Although pharmacotherapies theoretically target these neurotransmitters, few well-conducted trials for symptomatic or neuroprotective interventions yielded positive results. Tetrabenazine (TBZ) is a dopamine-depleting agent that may be one of the more effective agents for reducing chorea, although it has a risk of potentially serious adverse effects. Some newer antipsychotic agents, such as olanzapine and aripiprazole, may have adequate efficacy with a more favorable adverse-effect profile than older antipsychotic agents for treating chorea and psychosis. This review will address the epidemiology and diagnosis of HD as background for understanding potential pharmacological treatment options. Because TBZ is the only US Food and Drug Administration-approved medication in the United States for HD, the focus of this review will be on its pharmacology, efficacy, safety, and practical uses. There are no current treatments to change the course of HD, but education and symptomatic therapies can be effective tools for clinicians to use with patients and families affected by HD.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding for huntingtin protein. A lot has been learned about this disease since its first description in 1872 and the identification of its causative gene and mutation in 1993. We now know that the disease is characterized by several molecular and cellular abnormalities whose precise timing and relative roles in pathogenesis have yet to be understood. HD is triggered by the mutant protein, and both gain-of-function (of the mutant protein) and loss-of-function (of the normal protein) mechanisms are involved. Here we review the data that describe the emergence of the ancient huntingtin gene and of the polyglutamine trait during the last 800 million years of evolution. We focus on the known functions of wild-type huntingtin that are fundamental for the survival and functioning of the brain neurons that predominantly degenerate in HD. We summarize data indicating how the loss of these beneficial activities reduces the ability of these neurons to survive. We also review the different mechanisms by which the mutation in huntingtin causes toxicity. This may arise both from cell-autonomous processes and dysfunction of neuronal circuitries. We then focus on novel therapeutical targets and pathways and on the attractive option to counteract HD at its primary source, i.e., by blocking the production of the mutant protein. Strategies and technologies used to screen for candidate HD biomarkers and their potential application are presented. Furthermore, we discuss the opportunities offered by intracerebral cell transplantation and the likely need for these multiple routes into therapies to converge at some point as, ideally, one would wish to stop the disease process and, at the same time, possibly replace the damaged neurons.

Tetrabenazine

BACKGROUND

Tetrabenazine (TBZ) depletes presynaptic dopamine in the CNS. It has been found to be beneficial in hyperkinetic movement disorders without carrying the extrapyramidal side effects that are characteristic of neuroleptics.

OBJECTIVE

To summarize current knowledge on the use of TBZ and draw conclusions about its efficacy and safety.

METHODS

PubMed literature searches using the term 'tetrabenazine' were carried out for the period prior to May 2009. Additional relevant studies referenced by these publications were included.

CONCLUSIONS

Both short- and long-term studies have consistently yielded favorable results for the use of TBZ in the treatment of hyperkinetic movement in terms of efficacy and safety. TBZ is most effective in reducing chorea (including Huntington's disease associated chorea), tic associated with Tourette's syndrome and tardive dyskinesias. Furthermore, TBZ might also have potential for use in other hyperkinetic disorders (e.g., myoclonus and dystonia), for which future clinical trials are needed.

Tetrabenazine as anti-chorea therapy in Huntington disease: an open-label continuation study. Huntington Study Group/TETRA-HD Investigators

Background

Tetrabenazine (TBZ) selectively depletes central monoamines by reversibly binding to the type-2 vesicular monoamine transporter. A previous double blind study in Huntington disease (HD) demonstrated that TBZ effectively suppressed chorea, with a favorable short-term safety profile (Neurology 2006;66:366-372). The objective of this study was to assess the long-term safety and effectiveness of TBZ for chorea in HD.

Methods

Subjects who completed the 13-week, double blind protocol were invited to participate in this open label extension study for up to 80 weeks. Subjects were titrated to the best individual dose or a maximum of 200 mg/day. Chorea was assessed using the Total Maximal Chorea (TMC) score from the Unified Huntington Disease Rating Scale.

Results

Of the 75 participants, 45 subjects completed 80 weeks. Three participants terminated due to adverse events (AEs) including depression, delusions with associated previous suicidal behavior, and vocal tics. One subject died due to breast cancer. The other 26 subjects chose not to continue on with each ensuing extension for various reasons. When mild and unrelated AEs were excluded, the most commonly reported AEs (number of subjects) were sedation/somnolence (18), depressed mood (17), anxiety (13), insomnia (10), and akathisia (9). Parkinsonism and dysphagia scores were significantly increased at week 80 compared to baseline. At week 80, chorea had significantly improved from baseline with a mean reduction in the TMC score of 4.6 (SD 5.5) units. The mean dosage at week 80 was 63.4 mg (range 12.5-175 mg).

Conclusions

TBZ effectively suppresses HD-related chorea for up to 80 weeks. Patients treated chronically with TBZ should be monitored for parkinsonism, dysphagia and other side effects including sleep disturbance, depression, anxiety, and akathisia.

Trial Registration

Clinicaltrials.gov registration number (initial study): NCT00219804

Emerging drug therapies in Huntington's disease

Huntington's disease (HD) is a relentless neurodegenerative disease that results in profound disability through a triad of motor, cognitive and neuropsychiatric symptoms. At present, there are very few therapeutic interventions available with the exception of a limited number of drugs that offer mild symptomatic relief. Although the genetic basis of the disease has been identified, the mechanisms behind the cellular pathogenesis are still not clear and as a result no candidate drugs with the potential for disease modification have been found clinically until now. One of the major limitations in assessing the usefulness of drug treatments in HD is the lack of well-designed, double-blind, placebo-controlled clinical trials. Most studies have been open-label, using a small number of patients and tend to concentrate on the motor features of the disease, primarily the chorea. This review discusses the treatments now used for HD before evaluating the newer drugs at present being explored in both the clinic and in the laboratory in mouse models of the disease.

Symptomatic treatment of Huntington disease

Huntington disease (HD) is a progressive heredoneurodegenerative disease manifested by chorea and other hyperkinetic (dystonia, myoclonus, tics) and hypokinetic (parkinsonism) movement disorders. In addition, a variety of psychiatric and behavioral symptoms, along with cognitive decline, contribute significantly to the patient's disability. Because there are no effective neuroprotective therapies that delay the progression of the disease, symptomatic treatment remains the cornerstone of medical management. Several classes of medications have been used to ameliorate the various symptoms of HD, including typical and atypical neuroleptics, dopamine depleters, antidepressants, antiglutamatergic drugs, GABA agonists, antiepileptic medications, acetylcholinesterase inhibitors, and botulinum toxin. Recently, surgical approaches including pallidotomy, deep brain stimulation, and fetal cell transplants have been used for the symptomatic treatment of HD. The selected therapy must be customized to the needs of each patient, minimizing the potential adverse effects. The primary aim of this article is to review the role of the different therapies, both available and investigational, for the treatment of the motor, psychiatric, behavioral, and cognitive symptoms of HD, and to examine their impact on the patient's functionality and quality of life.