A Study of Treatment With Pridopidine (ACR16) in Participants With Huntington's Disease

The purpose of this study is to determine if ACR16 is effective and safe in the symptomatic treatment of Huntington's disease.

The primary objective in the present study is to confirm whether ACR16 is efficacious in improving voluntary motor function in Huntington's disease based on the Unified Huntington"s Disease Rating Scale (UHDRS) subscale. These symptoms seem to be most important for the functional disability associated with the disorder. To achieve this, participants are randomized to ACR16 45 mg once daily, ACR16 45 mg twice daily, or placebo treatment in equal proportions in a parallel design for a treatment duration of 26 weeks.

Eligibility Criteria:
Inclusion Criteria:

* Able to provide written Informed Consent prior to any study related procedure.
* Huntington's disease diagnosed with the aid of clinical features and a positive family history and/or the presence of ≥ 36 CAG repeats in the Huntington gene.
* Male or female age ≥ 30 years.
* Willing and able to take oral medication and able to comply with the study specific procedures.
* Ambulatory, being able to travel to the assessment centre, and judged by the Investigator as likely to be able to continue to travel for the duration of the study.
* Availability of a caregiver or family member to accompany the participant.
* A sum of ≥ 10 points on the mMS at the screening visit.
* For participants taking allowed antipsychotic medication, the dosing of medication must have been kept constant for at least 6 weeks before randomization. The allowed antipsychotic medication is Amisulpride, Haloperidol, Olanzapine, Risperidone, Sulpiride, or Tiapride.
* For participants taking allowed antidepressant or other psychotropic medication, the dosing of medication must have been kept constant for at least 6 weeks before randomization.
* Willing to provide a blood sample for CAG analysis (where CAG result is not already available).
* In France only, the participant must be affiliated to a social security system or be a beneficiary of such a system.

Exclusion Criteria:

* Unable to give written informed consent.
* Treatment with any non-allowed antipsychotic medication within 12 weeks of randomization. The non-allowed antipsychotic medication is any medication other than Amisulpride, Haloperidol, Olanzapine, Risperidone, Sulpiride, or Tiapride.
* Treatment with the antidepressants Fluoxetine or Paroxetine within 6 weeks of randomization.
* Use of Tetrabenazine within 12 weeks of randomization, or at any time during the study period.
* Treatment with any investigational product within 4 weeks of randomization.
* Use of tricyclic antidepressants, class I antiarrhythmics, and strong CYP2D6 inhibitors such as Ajmalicine, Chinidin/Quinidine and Ritonavir, within 6 weeks of randomization.
* Participants previously included into this study.
* A prolonged QTc interval at screen (defined as a QTc interval of \> 450 milliseconds \[msec\] for males or \> 470 msec for females), or other clinically significant heart conditions.
* Creatinine clearance \<40 milliliters (mL)/minute (min) as measured at the screening visit.
* Any clinically significant, abnormal, baseline laboratory result which in the opinion of the Investigator, affects the participants' suitability for the study or puts the participant at risk if he/she enters the study.
* Clinically significant hepatic or renal impairment.
* Participants with a history of epilepsy or a history of seizure(s) of unknown cause.
* Severe intercurrent illness, which, in the opinion of the Investigator, may put the participant at risk when participating in the trial or may influence the results of the trial or affect the participants' ability to take part in the trial.
* Alcohol and/or drug abuse as defined by Diagnostic and Statistical Manual - Fourth Edition - Text Revision (DSM IV-TR) criteria for substance abuse - this includes the illicit use of cannabis within the last 12 months.
* Participants with suicidal ideation, defined as a positive score on criteria for major depressive episode, item A9 on the DSM-IV-TR criteria for a Major Depressive Episode.
* Females who are pregnant or lactating.
* Females who are of child bearing potential and not taking adequate contraceptive precautions are excluded from the trial. Females of child bearing potential taking acceptable contraceptive precautions can be included.
* Known allergy to any ingredients of the trial medication or placebo.
* Any previous participation in a clinical study with ACR16.
* Participants currently receiving deep brain stimulation.
* Participants with a history of surgical procedures aiming to improve the symptoms of Huntington's disease, such as neural transplantations, lesions of the central nervous system, infusions of neurotrophic agents or previous attempts of deep brain stimulation.