Study to Assess the Clinical Benefit and Safety of Droxidopa in Parkinson's Disease

Inclusion Criteria: * Provide written informed consent to participate in the study * Diagnosed with probable levodopa-responsive idiopathic PD (meeting United Kingdom PD Brain Bank diagnostic criteria), and receiving levodopa therapy for this disorder. Other PD medications can also be used. * Must have AT LEAST ONE of below two criteria: 1. At least 3 months incidence of typical freezing of gait symptoms, occurring while levodopa is otherwise providing an "on" mobility state (including at least one of the following Freezing of Gait patterns: start hesitancy, freezing at making turns or when passing through a doorway, spontaneous freezing during continued walking, or Freezing of Gait related to a simultaneous mental or physical activity) OR 2. Have a screening score between 22 and 26 (inclusive) on the Montreal Cognitive Assessment Exclusion Criteria: * Taking direct acting vasoconstriction agent (i.e. ephedrine or midodrine). Subjects taking vasoconstrictor agents such as ephedrine or midodrine must stop taking these drugs at least 2 days or 5 half-lives (whichever is longer) prior to their baseline visit * Taking anti-hypertensive medication for the treatment of hypertension. Anti-hypertensive medication taken at night to prevent supine hypertension will be allowed * Changing dose or frequency of PD medication within 2 weeks of baseline * Use of cognitive-enhancing medications (donepezil, galantamine, rivastigmine, tacrine, or memantine), catechol-O-methyltransferase inhibitors (tolcapone or entacapone), anticholinergic drugs, or antipsychotic drugs (including quetiapine or clozapine). * Known or suspected alcohol or substance abuse within 1 year (e.g. DSM-IV definition of alcoholism) * Past or current history of chronic severe hypertension (with repeated findings of BP 150/90 mmHg in the supine or standing position) * Symptomatic coronary artery disease, severe congestive heart failure (NYHA Class 3 or 4) * Unable to remain off any effective Freezing of Gait medications for 12 hrs prior to Evaluation visits) * Women who are pregnant, lactating, or plan to become pregnant during the course of this study * Women of child bearing potential who are not using two methods of contraception (at least one barrier, i.e. condom) with their partner. * Male subjects who are sexually active with a woman of child bearing potential and not using two methods of contraception (at least one barrier, for example, condom) * A history of closed angle glaucoma; * Active (in the last 6 months) atrial fibrillation or, in the investigator's opinion, any other significant cardiac arrhythmia that should preclude the subject from this trial * History of myocardial infarction or unstable angina * Congestive heart failure (NYHA Class 3 or 4) * Diabetes insipidus, insulin-dependent diabetes mellitus, or diabetic neuropathy * In the investigator's opinion, any other significant systemic illness * Known or suspected malignancy (other than basal cell carcinoma) * Known gastrointestinal illness or other gastrointestinal disorder that may, in the investigator's opinion, affect the absorption of study drug * Any major surgical procedure within 30 days of the baseline visit * Currently receiving any investigational drug or have received an investigational drug within 30 days of the baseline visit * In the investigator's opinion, clinically significant abnormalities on clinical examination or laboratory testing that should preclude the subject from this trial. * Findings from suicidality screening that are compatible with risk for suicide