The primary purpose of the study is to evaluate the safety and tolerability of single ascending doses of UCB7853 in healthy male study participants and to evaluate the safety and tolerability of multiple ascending doses of UCB7853 administered in study participants with Parkinson's Disease (PD)

Inclusion Criteria:

Part 1 (healthy participants):

• Participant must be male and 18 to 55 years of age inclusive
• Body weight within 50 kg to 110 kg and body mass index (BMI) within the range 18.0 to 30.0 kg/m^2 (inclusive)
• Participant must be healthy as determined by medical evaluation, including medical history, physical examination, laboratory tests, and cardiac monitoring
• Participant has clinical laboratory test results within the reference ranges of the laboratory

Part 2 (participants with Parkinson's Disease):

• Participant must be 40 to 80 years of age, inclusive, at the time of signing the informed consent form (ICF)
• Participant may be male or female
• Participant must have body weight of between 50 and 110 kg and a body mass index within the range of 18 to 32 kg/m^2 (inclusive)
• Participant must have a clinical diagnosis of Parkinson's disease (PD) according to the Movement Disorders Society criteria. The following diagnostic criteria must be met: Bradykinesia AND at least ONE of the following: muscular rigidity or resting tremor
• Participant must have a Hoehn and Yahr Stage of ≤3
• Participant must be either untreated, or treated with a stable regimen (at least 4 weeks prior to Baseline Visit) of antiparkinsonian drugs and is expected to remain on this regimen for the duration of the study
• Participant must be in good physical and mental health, in particular not affected by any neurological disorder other than Parkinson's disease (PD), in the opinion of the Investigator, determined on the basis of medical history and a general clinical examination at Screening
• Participant has clinical laboratory test results within the reference ranges of the laboratory

Exclusion Criteria:

Part 1 (healthy participants):

• Participant has a history or presence of cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrinological, hematological, pancreatic, musculoskeletal, genitourinary, immunological, dermatological, or neurological disorders capable of significantly altering the absorption, metabolism, or elimination of drugs; constituting a risk when taking the study intervention; or interfering with the interpretation of data
• Participant has a known hypersensitivity to any components of the study medication or comparative drugs
• Participant has any clinically relevant abnormal findings in physical examination, laboratory tests, or vital signs, which, in the opinion of the Investigator, may place the participant at risk because of participation in the study
• Participant has any clinically relevant brain magnetic resonance imaging (MRI) abnormality at Screening

Part 2 (participants with Parkinson's Disease):

• Participant has a diagnosis of a significant Central nervous system (CNS) disease other than PD or history of epilepsy or seizure disorder other than febrile seizures as a child
• Study participant has a history of levodopa-induced motor fluctuations or dyskinesia expected to interfere with his/her ability to participate in the study
• Participant has a known hypersensitivity to any components of the study medication or comparative drugs
• Study participant has had prior surgical treatment for PD involving intracranial surgery or implantation of a device (including deep brain stimulation) or duodopa
• Participant has any clinically relevant abnormal findings in physical examination (other than symptoms of PD), laboratory tests, or vital signs, which, in the opinion of the Investigator, may place the participant at risk because of participation in the study
• Participant has any clinically relevant brain MRI abnormality at Screening

Due to the small sample size in this trial, IPD cannot be adequately anonymized i.e., there is a reasonable likelihood that individual participants could be re-identified. For this reason, data from this trial cannot be shared.