Evaluation of Safety and Tolerability of ENT-01 for the Treatment of Parkinson's Disease Related Constipation

Inclusion Criteria: 1. Parkinson's disease diagnosis confirmed by a neurologist specializing in movement disorders 2. Insufficient criteria for a diagnosis of Irritable Bowel Syndrome 3. Constipation for over 6 months, unresponsive to Milk of Magnesia, and requiring at least weekly treatment using an oral laxative, stool softener, bulking agent, and/or a suppository, and dissatisfaction with current treatment. 4. Body Mass Index is 18-40 kg/m2 5. At least 2 of the Rome IV functional constipation criteria are met 6. Loose stools are rarely present without the use of laxatives 7. Patient is willing and able to sign informed consent and comply with all study procedures 8. Patients must be able to read, understand, and accurately record data into the diary to guarantee full participation in the study Females only: 9. Must have negative serum or urine pregnancy tests and must not be lactating 10. If of child-bearing age: Must agree to using a hormonal (i.e., oral, implantable, or injectable) and either single- or double-barrier method of birth control throughout the study period. A vasectomized partner will be allowed as one in conjunction with another single-barrier method. 11. If unable to have children: Must have this documented in the case report form (i.e., ubal ligation, hysterectomy, or post-menopausal \[defined as a minimum of one year since last menstrual period\]). Post-menopausal status will be confirmed by follicle stimulating hormone in women less than 60 years of age. Exclusion Criteria: 1. Unable or unwilling to provide informed consent or to comply with study procedures 2. Diagnosis of secondary constipation beyond that of PD 3. Structural or metabolic diseases that affect the GI system 4. Functional GI disorder 5. Unable or unwilling to withdraw from taking the following medications 2 weeks prior to the dose-escalation period and throughout the study: Laxatives, opiates, sedatives, hypnotics, anti-histamines, protein pump inhibitors or any medications which may cause constipation 6. History of recent major surgery (within 60 days of screening) 7. Any clinically significant abnormalities on screening laboratories or physical examination as determined by the Investigator 8. Neurological disorder other than PD 9. On treatment with intra-jejunal dopamine 10. Treatment with COMT inhibitors for fewer than 4 weeks (entacapone, tolcapone, Stalevo) 11. Unable to maintain a stable diet regimen 12. Patients with a cognitive impairment that preclude them from understanding the informed consent 13. Patients placed under legal guardianship 14. Acute GI illness within 48 hours of the baseline period 15. History of major GI surgery (e.g. previous abdominal surgery, including cholecystectomy), except that patients with uncomplicated appendectomy are allowed 16. ALT or AST \> 1.5 X upper limit of normal (ULN) during screening 17. Females who are pregnant or breastfeeding 18. History of excessive alcohol use or substance abuse 19. Patient or caregiver unable to administer daily oral dosing 20. Participation in an investigational clinical study within the 6 months prior to dosing in the present study 21. Any other reason, which in the opinion of the Investigator would confound proper interpretation of the study