Alternatives for Reducing Chorea in Huntington Disease

Inclusion Criteria

• Participant is at least 18 years of age or the age of majority (whichever is older) at Screening.
• Participant has been diagnosed with manifest HD, as indicated by characteristic motor exam features, and has a documented expanded cytosine adenine guanine (CAG) repeat (greater than or equal to >= [37]) at or before Screening.
• Participant meets either of the following:
• Has successfully completed participation in the First-HD Study (SD-809-C-15) or
• Has been receiving an Food and Drug Administration (FDA)-approved dose of tetrabenazine that has been stable for >=8 weeks before Screening and is providing a therapeutic benefit for control of chorea.
• Participant has a Total Functional Capacity (TFC) score >=5 at Screening.
• Participant is able to swallow study medication whole.
• Participant has provided written, informed consent or, a legally authorized representative (LAR) has provided written informed consent and the subject has provided assent.
• Participant has provided a Research Advance Directive.
• Female participants of childbearing potential agree to use an acceptable method of contraception from screening through study completion.
• The participant has a reliable caregiver who interacts with the participant on a daily basis, oversees study drug administration, assures attendance at study visits and participates in evaluations, as required.
• Participant is able to ambulate without assistance for at least 20 yards (Note: The use of assistive devices (such as; walker, cane) are permitted during ambulation).
• Has sufficient reading skills to comprehend the participant completed rating scales.

Exclusion Criteria

• Participant has a serious untreated or under-treated psychiatric illness, such as depression, at Screening or Baseline.
• Participant has active suicidal ideation at Screening or Baseline.
• Participant has history of suicidal behavior at Screening or Baseline.
• Participant has evidence for depression at Baseline.
• Participant has an unstable or serious medical illness at Screening or Baseline.
• Participant has received tetrabenazine within 7 days of Baseline (Rollover participants only).
• Participant has received any of the following concomitant medications within 30 days of Screening or Baseline: Antipsychotics, Metoclopramide, Monoamine oxidase inhibitors (MAOI), Levodopa or dopamine agonists, Reserpine, Amantadine, Memantine (Rollover participants only)
• Switch participants may receive Memantine if on a stable, approved dose for at least 30 days
• Participant has significantly impaired swallowing function at Screening or Baseline.
• Participant has significantly impaired speaking at Screening or Baseline.
• Participant requires treatment with drugs known to prolong the QT interval.
• Participant has prolonged QT interval on 12-lead electrocardiogram (ECG) at Screening.
• Participant has evidence of hepatic impairment at Screening.
• Participant has evidence of significant renal impairment at Screening.
• Participant has known allergy to any of the components of study medication.
• Participant has participated in an investigational drug or device trial other than SD-809-C-15 within 30 days (or 5 drug half-lives) of Screening, whichever is longer.
• Participant is pregnant or breast-feeding at Screening or Baseline.
• Participant acknowledges present use of illicit drugs at Screening or Baseline.
• Participant has a history of alcohol or substance abuse in the previous 12 months.