First Time Use of SD-809 in Huntington Disease

Inclusion Criteria: * Subject is at least 18 years of age or the age of majority (whichever is older) at Screening. * Subject has been diagnosed with manifest HD * Subject is able to swallow study medication whole. * Female subjects of childbearing potential agree to use an acceptable method of contraception from screening through study completion. * The subject has a reliable caregiver who interacts with the patient on a daily basis, oversees study drug administration, assures attendance at study visits and participates in evaluations, as required. * Subject is able to ambulate without assistance for at least 20 yards (Note: The use of assistive devices (i.e., walker, cane) is permitted during ambulation). Exclusion Criteria: * Subject has a serious untreated or under-treated psychiatric illness, such as depression, at Screening or Baseline. * Subject has active suicidal ideation at Screening or Baseline. * Subject has history of suicidal behavior at Screening or Baseline: * Subject has evidence for depression at Screening or Baseline. * Subject has an unstable or serious medical or psychiatric illness at Screening or Baseline. * Subject has been recently exposed to tetrabenazine. * Subject 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 * Subject has significantly impaired swallowing function at Screening. * Subject has significantly impaired speaking at Screening. * Subject requires treatment with drugs known to prolong the QT interval. * Subject has a prolonged QT interval on 12-lead ECG at Screening. * Subject has evidence of hepatic impairment at Screening. * Subject has evidence of significant renal impairment at Screening. * Subject has known allergy to any of the components of study medication. * Subject has participated in an investigational drug or device trial within 30 days (or 5 drug half-lives) of Screening, whichever is longer. * Subject is pregnant or breast-feeding at Screening or Baseline. * Subject acknowledges present use of illicit drugs at Screening. * Subject has a history of alcohol or substance abuse in the previous 12 months.