MK-0947 PN-002: An Open-label Study to Evaluate Brain α-synuclein Deposition Using Positron Emission Tomography (PET) and [¹⁸F]MK-0947 in Participants With Parkinson's Disease (PD)

Inclusion Criteria: * All Participants (PD, HE) 1. Must voluntarily sign and date each informed consent, approved by the Research Ethics Committee (REC) prior to the initiation of screening or study-specific procedures. 2. For women of childbearing potential, agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception, and agreement to refrain from donating eggs for 90 days following the last PET scan in each study period, as defined below: 3. A woman is considered to be of childbearing potential if she is postmenarchal, has not reached a postmenopausal state (12 continuous months of amenorrhea with no identified cause other than menopause), and is not permanently infertile due to surgery (i.e., removal of ovaries, fallopian tubes, and/or uterus) or another cause as determined by the Investigator (e.g., Müllerian agenesis). The definition of childbearing potential may be adapted for alignment with local guidelines or regulations. Women of childbearing potential must remain abstinent or use 2 methods of contraception, one of which is a barrier method (i.e., male or female condom), for the study duration and 90 days after the last investigational agent injection. 4. Male participants with partners of childbearing potential must commit to the use of 2 methods of contraception, one of which is a barrier method (i.e., male condom with or without spermicidal jelly), for the study duration and 90 days after the last investigational agent injection. 5. Male participants must not donate sperm for the duration of the study and 90 days after the last investigational agent injection. 6. In the opinion of the Investigator, deemed to be physically able to participate in all visits throughout the duration of the trial. * PD and HE Participants 1. Adult males and/or females between the ages of 40 and 80 years old (inclusive) at the time of providing informed consent. 2. The participant is in good health based on medical history and physical examination at screening. 3. Score of ≥25 out of 30 on the MMSE, or score of ≥25 out of 30 on the MoCA. * Participants with PD 1. Diagnosed with clinically established Parkinson's disease according to UK-Brain bank clinical criteria. At least 2 cardinal motor features (including bradykinesia with resting tremor, rigidity, or postural instability) PD diagnosis established for at least 3 years, at the Screening Visit. 2. Hoehn and Yahr score between 2-3. 3. UPSIT score of ≤26 4. Currently on stable PD medication in the judgment of the Investigator 5. Participants are judged to be L-DOPA responsive and able to ambulate independently and without significant history of falls within 6 months. Exclusion Criteria: * All Participants (PD, HE) 1. Unwilling or unable to provide written informed consent and/or follow the procedures outlined in the protocol, in the opinion of the investigator. 2. A general medical condition that in the opinion of the investigator may impair participant safety or the scientific integrity of the study, including, but not limited to: 1. Significant hepatic impairment 2. Significant renal insufficiency 3. Significant cardiovascular, pulmonary, endocrine, or systemic chronic inflammatory disorder 4. Clinically significant infection within 30 days at study entry 5. History of severe allergic reactions or hypersensitivity to PET tracers or related compounds 3. History of cancer that in the opinion of the investigator may impair participant safety or the scientific integrity of the study. 4. Any disability that would, in the opinion of the investigator, prevent completion of study procedures or assessments (including but not limited to blindness or significant visual impairment, deafness or significant hearing impairment, speech impairment, or sensory or motor dysfunction). 5. Participant is unable or unwilling to undergo CT, PET procedures or for whom MRI (Part 1 only) is contraindicated (e.g. have aneurysm clip, metal fragments, internal electrical devices such as a cochlear implant, spinal cord stimulator or pacemaker), have claustrophobia that cannot be medically managed or are unable to lie still for the imaging procedures. 6. Prior participation in other radiologic research protocols or clinical care in the last year in addition to the radiation exposure expected from participation in this clinical study, such that radiation exposure exceeds the effective dose of 50 mSv, which would be above the acceptable annual limit established by the US Federal Guidelines. 7. Pregnant or breastfeeding female. 8. Refusal to use highly effective contraception if of childbearing potential (if applicable). 9. Approximately 30 days prior enrollment in an experimental study evaluating pharmacological therapies. 10. Current or prior participation in a trial of an investigational therapeutic product targeting alpha-synuclein, unless documented to have only received placebo. 11. Any finding (including history and medication use) that would in the opinion of the Investigator compromise participant safety or the scientific integrity of the study. 12. Unwillingness or inability to follow the procedures outlined in the protocol. 13. Has received an investigational drug within 30 days or 5 plasma half-lives (whichever is longer) before inclusion into this study, unless in the opinion of the Investigator, such medication will not constitute a risk for participant safety or the scientific integrity of the study. 14. Participant is mentally or legally incapacitated. 15. Has any other clinically significant condition or situation that has the potential to interfere with study conduct, in the judgment of the investigator * HE participants 1. Current or active neurological brain disorder. 2. First degree relative with idiopathic PD (parent, sibling, child). 3. Major surgery or donation or loss of 400 mL or more of blood within 4 weeks prior to dosing, or longer if required by local regulation * Participants with PD 1. Have evidence of an alternative form of parkinsonism (e.g., drug-induced parkinsonism due to neuroleptic use) or other parkinsonian disorders (e.g., MSA, PSP), as determined by UK Brain Bank Clinical criteria or investigator judgement. 2. Participant is a known mutation carrier of LRRK2, PINK1, PARK2, or DJ-1 3. Have known history of negative SAA test (via skin punch or CSF sample) and/or DaTscan without evidence of striatal dopaminergic deficit. 4. Have a modified Hoehn and Yahr score of 4 or greater 5. Be living in a nursing home or assisted living facility 6. Have a history of dementia (due to AD, PD, or other cause) 7. Have a history of significant disabling CNS disease other than PD 8. Have had a prior history of brain surgery 9. Major surgery or donation or loss of 400 mL or more of blood within 4 weeks prior to dosing, or longer if required by local regulation