Alternatives for Reducing Chorea in Huntington Disease

The purpose of this study is to evaluate the safety, tolerability, and pharmacokinetics of SD-809 extended release (ER) in participants switching from tetrabenazine to SD-809 ER. In addition, the safety and tolerability of long-term treatment with SD-809 ER will be assessed in "Switch" participants as well as "Rollover" participants completing a randomized, double blind, placebo-controlled study of SD-809 ER.


Eligibility Criteria:
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:

  1. Has successfully completed participation in the First-HD Study (SD-809-C-15) or
  2. 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.