A Study to Evaluate Satety, Tolerability, Biodistribution, Radiation Dosimetry, and Pharmacokinetics of SST001 in Healthy Volunteers, Patients With PD and Patients With MSA

Inclusion Criteria: * Sign the informed consent form approved by IEC. * Male or female participants aged ≥40 years old. * Adequate organ functions. * Proper contraception methods. * Willingness to follow the study procedures. * Additional inclusion criteria for healthy volunteers: Good health status; no history of motor disorders or cognitive disorders. * Additional inclusion criteria for MSA: Diagnosed with clinically established or clinically probable MSA according to the MDS MSA criteria (2022). If previously treated, the treatment regimen for MSA must have been stable for at least 4 weeks with no planned adjustments in the near term. * Additional inclusion criteria for PD: Diagnosed with clinically established or clinically probable PD according to the MDS PD criteria (2015). If previously treated, the treatment regimen for PD must have been stable for at least 4 weeks with no planned adjustments in the near term. Exclusion Criteria: * Being pregnant or lactating. * History of other severe neurological disorders. * History of serious or uncontrolled medical condition. * Active HBV/HCV/HIV infection, etc. * History of abuse of drugs or alcohol within 1 year. * Allergy to the study drug. * Intolerance to PET/CT or MRI (e.g. claustrophobia). * Any interventional clinical studies within 30 days. * Radiation exposure dose exceeding 50 mSv/year. * Prior therapy targeting α-Syn. * Other ineligible conditions for this study.