Study to Assess Droxidopa in the Treatment of Freezing Of Gait Symptoms in Patients With Parkinson's Disease

Inclusion Criteria: * Male or female patients, aged at least 30 years; * Diagnosed with probable levodopa-responsive idiopathic Parkinson's Disease , and receiving levodopa. Other Parkinson's Disease medications can also be used. * At least 3 months incidence of typical freezing of gait (FOG) symptoms, occurring while levodopa is otherwise providing an "on" mobility state (including at least one of the following FOG 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). * Provide written informed consent to participate in the study and understand that they may withdraw their consent at any time without prejudice to their future medical care * On a stable dose of carbidopa, alone or with other Parkinson's medication, providing a range of carbidopa between 100mg and 400mg daily Exclusion Criteria: * Taking direct acting vasoconstricting agent (i.e. ephedrine or midodrine). Patients 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 more than one anti-hypertensive medication for the treatment of high blood pressure. Short acting anti-hypertensive medication taken at night to prevent supine hypertension will be allowed. * Have changed dose or frequency of Parkinson's medication within 2 weeks of baseline * Known or suspected alcohol or substance abuse within 1 year * Sustained hypertension (BP greater than 140/90 mmHg in the sitting position) * Symptomatic coronary artery disease, severe congestive heart failure * Women who are pregnant, lactating, or plan to become pregnant during the course of this study; * Women of child bearing potential (WOCP) who are not using two methods of contraception (at least one barrier: i.e. condom) with their partner. * Male patients who are sexually active with a woman of child bearing potential (WOCP) and not using two methods of contraception (at least one barrier: i.e. condom) * Untreated closed angle glaucoma, or treated closed angle glaucoma that in the opinion of an ophthalmologist would cause increased risk to the patient; * Active (last 6 months) atrial fibrillation or, in the investigator's opinion, any other significant cardiac arrhythmia that should preclude the patient from this trial; * History of myocardial infarction or unstable angina * 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 patient from this trial; * Patient has only lower body Parkinson's Disease * In the investigators opinion, freezing of gait is attributable to previous stroke