Inclusion Criteria:
1. Adult males and females, 50 to 80 years of age (inclusive) at the time of Screening.
2. Able to understand the consent form, and to provide voluntary written informed consent.
3. Willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study-related procedures to complete the study.
4. Confirmed diagnosis of idiopathic Parkinson's Disease according to 2015 Movement Disorder Society (MDS) clinical diagnostic criteria.
5. Off state Hoehn \& Yahr below Stage 3 at the time of Screening.
6. Participants must be on stable doses of L-dopa with or without other adjunctive PD therapy for at least 30 days prior to enrollment. Doses should be expected to remain stable for the duration of the study.
7. Participants must be in overall stable condition, as determined by pre-study medical history, physical examination, clinical laboratory tests, and 12 lead ECG measurements.
8. Participants must have normal or not clinically significant clinical laboratory test results, as determined by the study investigator, including coagulation panel, blood cell counts, comprehensive metabolic panel analytes, and creatinine clearance (60 cm3/min or greater). Clinical laboratory tests results that are consistent with known, stable comorbidities will be allowed as long as the comorbidities do not represent an exclusion criteria per se.
9. Participants must have a negative urinary drug screen (UDS) for illicit drugs and a negative alcohol breath test.
10. Abstention from use of other investigative or non-approved drugs for the duration of the trial
11. Male participants who are not surgically sterile (vasectomized) and their female sexual partners must agree to use contraception during the study period and for 2 months after receiving the last dose of study drug.
12. Male participants must not donate sperm during the study and for 12 months after receiving the last dose of study drug.
13. Female participants must be non-pregnant, non-lactating, and either postmenopausal for at least 1 year, or surgically sterile (bilateral tubal ligation, 'clipping or tying tubes,' or hysterectomy) for at least 3 months, or they must agree to use two forms of highly effective contraception method (less than 1 pregnancy per 100 people using the method for one year) from 28 days and/or their last confirmed menstrual period prior to study enrollment (whichever is longer) until 2 months after receiving the last dose of study drug. Postmenopausal status will be defined as follows: minimum 1 year; amenorrhea duration of 12 consecutive months and a serum FSH value \>40 IU/L; postmenopausal status must be confirmed by an FSH test at Screening). Highly effective contraception methods include: Intra-uterine device (IUD) containing either copper or levonorgestrel (e.g., Mirena®), and/or barrier methods of contraception, including condoms (external or internal) and diaphragm ('cap'). Hormonal methods of contraception (with the exception of hormonal IUD) are not permitted within this study. Female participants will refrain from using hormonal contraceptives for at least 28 days prior to study entry until the end of the study period. Participants/Participant's partner(s) must also use a barrier form of contraception, from the first dose of study drug through until 2 months after the last dose. For all females of childbearing potential, the pregnancy test result must be negative at Screening and Pre-Study Baseline (Day -1).
14. Participants must be able to speak, read, and understand English sufficiently to allow comprehension and completion of all study assessments.
Exclusion Criteria:
1. Atypical parkinsonian syndrome or secondary parkinsonism (e.g., due to drugs or toxins, metabolic neurogenetic disorders, encephalitis, cerebrovascular disease or non-PD degenerative disease).
2. Family history of early onset PD (age \<50 years) or known personal genetically causal etiology of PD.
3. Diagnosis of any other clinically significant neurologic disease that may confound the assessment of the study drug on PD symptoms
4. Not stabilized with current therapeutic regimen for PD or likely to require changes in L-dopa therapy over the duration of the trial.
5. Presence of PD psychosis or dementia, or other neuropsychiatric or psychiatric conditions impeding informed consent or compliance with study interventions.
6. Severe dyskinesia (defined as per MDS-UPDRS) during a "normal day" that would significantly interfere with the participant's ability to perform study assessments.
7. History of neurosurgery for PD or tremor.
8. Clinically significant medical, surgical, or laboratory abnormalities in the judgement of the Investigator.
9. Participants with any clinically unstable or significant cardiovascular (including acute coronary syndrome within the prior year to Screening), renal, hepatic, respiratory, gastrointestinal, hematological, endocrine, or infectious disease (including HIV infection).
10. Participants with clinically significant abnormalities on screening vital signs, laboratory tests, and/or ECG, per investigator's judgement. Participants with poor venous access will also be excluded.
11. Participants with a family history of significant cardiac disease (i.e., sudden death in first degree relative; myocardial infarction before the age of 50).
12. Participants with a history of disorders of the hypothalamic-pituitary-adrenal axis, including adrenal insufficiency and Cushing's, or with a history of disorders of the hypothalamic-pituitary-gonadal axis, including hypogonadism.
13. Participants with any of the following blood values at screening:
* Abnormal plasma renin and/or aldosterone value
* Morning cortisol level \<5 mcg/dL
* ACTH stimulated cortisol levels \<18 mcg/dL at 60 minutes after ACTH injection at screening, or
* Abnormal FSH, LH, testosterone (for males), or estradiol (for females, unless post-menopausal)
14. Participants who have used any topical, oral, or intravenous exogenous corticosteroids within 12 weeks and/or intra-articular exogenous corticosteroids within 6 months prior to the start of the trial, or who plan on using them during the study.
15. Participants who have used chemotherapy agents, or who have a personal history of cancer or cancer in first degree relatives suggestive of elevated cancer risk, other than nonmetastatic skin cancer that has been completely excised, within 5 years prior to Screening.
16. Participants who have used (within 14 days of randomization) or plan on using during the duration of the study any prescription or over-the-counter drugs that are moderate or strong CYP3A4 inducers or inhibitors.
17. Participants who have used (within 14 days of randomization) or plan on using during the duration of the study any dietary aids, supplements, or foods that are moderate or strong CYP3A4 inhibitors (e.g., grapefruit juice).
18. Participants with difficulty in swallowing oral medications
19. Participants who have used any other investigational drug within 1 month or 5 half-lives, whichever is longer, prior to enrollment.
20. Participants with a documented history of difficult lumbar puncture procedures, to the investigator's discretion.
