Open-label Extension Study of Pridopidine (ACR16) in the Symptomatic Treatment of Huntington Disease (OPEN-HART)

Inclusion Criteria:

• Subject is able to, and has provided written Informed Consent prior to any study related procedure.
• Subject has completed the HART (ACR16C009) trial, including the follow-up period, or the PRIDE-HD (TV7820-CNS-20002) trial , and has remained on drug during the on treatment part of the trial (including de-escalated subjects).
• Willing and able to take oral medication and able to comply with the study specific procedures.

Exclusion Criteria:

• Ongoing treatment with tetrabenazine seizure threshold lowering medications, , certain antipsychotics and antidepressants.
• Newly instigated or changed treatment with neuroleptics/antipsychotics (< 6 weeks before Baseline Visit).
• Use of tricyclic antidepressants or class I antiarrhythmics within 6 weeks of Baseline Visit, or at any time during the study period.
• Any clinically significant, abnormal, laboratory result at any point during the randomized phase, including clinically significant hepatic or renal impairment, or any ongoing adverse events from the randomized phase, which in the opinion of the Investigator affects the subject's suitability for the study or puts the subject at risk if he/she enters the study.
• A prolonged QTc interval at Baseline Visit (defined as a QTc interval of >450 msec for both males and females using Fredericia's formula , or other clinically significant heart conditions as judged by the investigator.
• Severe intercurrent illness, which, in the opinion of the Investigator, may put the subject at risk when participating in the trial.
• Alcohol and/or drug abuse as defined by DSM IV-TR criteria for substance abuse - this includes the illicit use of cannabis.
• Subjects with suicidal ideation as defined as a positive score on criteria for major depressive episode, item A9 on the DSM -IV-TR criteria for a Major Depressive Episode.
• Subjects with a known history of epilepsy or a history of febrile seizure(s) or seizure(s) of unknown cause.
• Females who are pregnant or lactating.
• Females who are of child bearing potential and not taking adequate contraceptive precautions (either oral, barrier or chemical contraceptives) 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.