The purpose of the study is to assess the safety, tolerability, and pharmacokinetics of single, ascending doses of ABBV-951 administered as a subcutaneous bolus infusion followed by a continuous subcutaneous infusion in subjects with Parkinson's disease.

Inclusion Criteria:

• Subject has a diagnosis of definite idiopathic PD, which is LD responsive, according to the United Kingdom Parkinson's Disease Society Brain Bank Criteria.
• Subject must be taking an optimized and stable regimen of oral medications for PD, which has remained stable and unchanged for at least 30 days before dosing in this study. This regimen must include oral carbidopa/levodopa (CD/LD) tablets (e.g., Sinemet).
• Females must have negative results for pregnancy tests at screening and prior to confinement.
• If male, subject must be surgically sterile or practicing an adequate method of birth control from initial study drug administration until 30 days after last dose of study drug.
• Body Mass Index (BMI) is 18.0 to 38.0, inclusive.
• A condition of general good health, based upon the results of a medical history, physical examination, vital signs, laboratory profile, and a 12-lead electrocardiogram (ECG).

Exclusion Criteria:

• Receipt of any drug by injection within 30 days or within a period defined by 5 half-lives, whichever is longer, prior to study drug administration.
• History of significant skin conditions or disorders (e.g., psoriasis, atopic dermatitis, etc.) or evidence of recent sunburn, acne, scar tissue, tattoo, open wound, branding, or colorations that in the Investigator's opinion would interfere with the infusion of the study drug or could interfere with study assessments.
• Use of any medication from the prohibited concomitant therapies.
• Subjects with glomerular filtration rate (GFR) less than 45 ml/min/1.73 m2 as determined by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation.
• Donation or loss of 550 mL or more blood volume (including plasmapheresis) or receipt of a transfusion of any blood product within 8 weeks prior to initial study drug administration.
• Consideration by the investigator for any reason that the subject is an unsuitable candidate to receive ABBV-951.