Efficacy and Safety of SOM3355 in Huntington's Disease Chorea

Inclusion Criteria: * Subject is at least 18 years of age at time of consent. * Diagnosis of HD definite by a movement disorders expert and confirmed by a number of HTT gene CAG repeats equal or greater than 36. * Female of child bearing potential (FCBP) and non-vasectomized male agree to practice appropriate methods of birth control. * Ability to walk independently or with minimal assistance. * UHDRS TMC score equal or greater than 8. * UHDRS TFC equal or greater than 4. * Subject has provided written informed consent or through his/her legally authorized representative. Exclusion Criteria: * Onset of HD symptoms prior to age 18 (Juvenile forms of HD). * Non-ambulatory patients. * A past medical history of clinically significant ECG abnormalities or a family history (grandparents, parents and siblings) of a prolonged QT-interval syndrome. * Pregnant or breastfeeding female patients, including those planning to conceive during the period of the trial. * Patients with psychiatric symptoms, or other impairments, that would interfere with their full compliance with the Investigator instructions and testing, unless there is an identified caregiver to support the patient. * Any surgical or medical condition which might significantly alter the absorption, distribution, metabolism, or excretion of drugs, or which may jeopardize the subject in case of participation in the study. The Investigator should make this determination in consideration of the subject's medical history and/or clinical laboratory test results at screening and baseline. * Known allergy/sensitivity/intolerance to the study drugs or their excipients. * Any significant laboratory results which, in the Investigator's opinion, would not be compatible with study participation or represent a risk for the subject while in the study. * Prescribed anti-hypertensive medication, tetrabenazine, deutetrabenazine or valbenazine within 15 days prior starting the investigational treatment. * Excluded concomitant medications: any anti-hypertensive medication, tetrabenazine, deutetrabenazine or valbenazine, all typical neuroleptics and all MAO inhibitors * Subject has a history of alcohol or substance abuse in the previous 12 months. * Patients with diabetic ketoacidosis or metabolic acidosis. * Patients with cardiogenic shock, congestive heart failure, pulmonary hypertension due to right-sided heart failure, severe sinus bradycardia, atrioventricular block (grades II and III) or sinoatrial block. * Subject has participated in an investigational drug or device trial within 30 days prior starting the investigational treatment.