First Time Use of SD-809 in Huntington Disease (First-HD)

Inclusion Criteria:

1. Subject is at least 18 years of age or the age of majority (whichever is older) at Screening.
2. Subject has been diagnosed with manifest HD, as indicated by a positive family history or characteristic motor exam features and an expanded CAG repeat (≥37).
3. Subject has a Total Maximal Chorea Score (TMC) ≥ 8 at Screening and Baseline.
4. Subject has a Total Functional Capacity (TFC) score ≥ 5 at Screening.
5. Subject is able to swallow study medication whole.
6. Subject has provided written, informed consent or, if subject lacks the capacity to provide informed consent (as determined by an independent assessment by a qualified healthcare provider not directly involved in other study activities), a legally authorized representative (LAR) has provided written informed consent and the subject has provided assent.

7. Female subjects of childbearing potential agree to use an acceptable method of contraception from screening through study completion. Female subjects of childbearing potential must be using one of the following acceptable birth control methods if sexually active:

   • IUD or intrauterine system in place for at least 3 months prior to screening;
   • Subject or partner using barrier method (e.g., condom, diaphragm, or cervical cap) with spermicide from screening through study completion;
   • Partner has a documented vasectomy > 6 months prior to enrollment.
   • Stable hormonal contraception (with approved oral, transdermal, or depot regimen) for at least 3 months prior to screening.
8. 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. (Note: Subjects with a TFC score of 5-7 at Screening must have a live-in caregiver). Subjects with live-in caregivers and who enrolled with the consent of an LAR must have caregivers present at study visits.
9. Subject is able to ambulate without assistance for at least 20 yards (Note: The use of assistive devices (i.e., walker, cane) are permitted during ambulation).

Exclusion Criteria:

1. Subject has a serious psychiatric illness, such as untreated or undertreated depression, schizophrenia, or bipolar disorder at Screening or Baseline.

   • Note: Subjects receiving antidepressant therapy may be enrolled if on a stable dose for at least 8 weeks before Screening.

2. Subject has active suicidal ideation at Screening or Baseline.

3. Subject has history of any of the following suicidal thoughts or behavior at Screening or Baseline:

   • Previous intent to act on suicidal ideation with a specific plan (positive answer to question 5 on C-SSRS), irrespective of level of ambivalence at the time of suicidal thought
   • Previous preparatory acts or behavior
   • A previous actual, interrupted or aborted suicide attempt
4. Subject has a score ≥11 on the depression subscale of the Hospital Anxiety and Depression Scale (HADS) at Screening or Baseline.
5. Subject has an unstable or serious medical illness at Screening or Baseline.
6. Subject has been previously exposed to tetrabenazine.

7. Subject has received any of the following concomitant medications within 30 days of Screening or Baseline:

   • D2 blockers, including metoclopromide.
   • Monoamine oxidase inhibitors (MAOI)
   • Levodopa or dopamine agonists
   • Reserpine
   • Amantadine
   • Memantine
8. Subject has a score of ≥11 on the Swallowing Disturbance Questionnaire (SDQ) at Screening.
9. Subject has a Unified Parkinson's Disease Rating Scale (UPDRS) dysarthria score of ≥3 at Screening.

10. Subject requires treatment with drugs known to prolong the QT interval.

    • Note: Celexa, Levitra, and Lexapro in doses >10 mg/day are not allowed.
    • See Appendix 16 for a coplete list of prohibited drugs.

11. Subject has a QTcF value >450 ms (males) or >460 ms (females), or >480 ms (with right bundle branch block) on 12-lead ECG at Screening.

    • Note: Subjects with left bundle branch block are not eligible).

12. Subject has evidence of hepatic impairment at Screening, as indicated by:

    • AST or ALT >2.5 times the upper limit of normal.

    • Alkaline phosphatase (ALP) or total bilirubin (TBil) >2 times the upper limit of normal (ULN)

      • Note: Subjects with Gilbert's Syndrome are eligible to participate if approved by the medical monitor.
      • Note: Subjects with abnormalities in two or more of these analytes (AST, ALT, ALP, TBil) must be approved by the medical monitor in order to be enrolled.
    • Prothrombin time > 4 sec prolonged.
    • Positive Hepatitis B surface antigen (HBsAg).
13. Subject has evidence of significant renal impairment at Screening, indicated by a creatinine clearance <50 mL/min, as estimated by the Cockroft-Gault formula.
14. Subject has known allergy to any of the components of study medication.
15. Subject has participated in an investigational drug or device trial within 30 days (or 5 drug half-lives) of Screening, whichever is longer.
16. Subject is pregnant or breast-feeding at Screening or Baseline.
17. Subject acknowledges present use of illicit drugs at Screening.
18. Subject has a history of alcohol or substance abuse in the previous 12 months, as defined in the DSM-IV, or subject is unable to refrain from substance abuse throughout the study.