First Time Use of SD-809 in Huntington Disease

This is a randomized, double blind, placebo controlled, parallel group study designed to
evaluate the efficacy, safety and tolerability of SD-809 ER for the treatment of chorea
associated with Huntington's Disease. Approximately 90 subjects will be randomized (1:1)
into the study, with approximately 45 subjects receiving SD-809 ER and 45 subjects receiving
placebo. The study will be conducted at approximately 30 centers in the U.S. and Canada.

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 characteristic motor
exam features and has a documented expanded CAG repeat (≥ 37) at or before Screening.

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, 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.

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.

9. 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:

1. Subject has a serious untreated or under-treated psychiatric illness, such as
depression, at Screening or Baseline.

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

3. Subject has history of suicidal behavior at Screening or Baseline:

4. Subject has evidence for depression at Screening or Baseline.

5. Subject has an unstable or serious medical or psychiatric illness at Screening or
Baseline.

6. Subject has been recently exposed to tetrabenazine.

7. 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

8. Subject has significantly impaired swallowing function at Screening.

9. Subject has significantly impaired speaking at Screening.

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

11. Subject has a prolonged QT interval on 12-lead ECG at Screening.

12. Subject has evidence of hepatic impairment at Screening.

13. Subject has evidence of significant renal impairment at Screening.

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.