Alternatives for Reducing Chorea in 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 characteristic motor exam
features, and has a documented expanded CAG repeat (≥ 37) at or before Screening.

3. Subject meets either of the following:

- Has successfully completed participation in the First-HD Study (SD-809-C-15) OR

- Has been receiving an FDA-approved dose of tetrabenazine that has been stable for
≥ 8 weeks before Screening and is providing a therapeutic benefit for control of
chorea.

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. Subject has provided a Research Advance Directive.

8. Female subjects of childbearing potential agree to use an acceptable method of
contraception from screening through study completion.

9. 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.

10. 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).

11. Has sufficient reading skills to comprehend the subject completed rating scales.

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 Baseline.

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

6. Subject has received tetrabenazine within 7 days of Baseline (Rollover subjects only).

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 (Rollover subjects only)

- Switch subjects may receive Memantine if on a stable, approved dose for at
least 30 days

8. Subject has significantly impaired swallowing function at Screening or Baseline.

9. Subject has significantly impaired speaking at Screening or Baseline.

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

11. Subject has 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 other than
SD-809-C-15 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 or Baseline.

18. Subject has a history of alcohol or substance abuse in the previous 12 months.