Study To Assess Adverse Events and Change in Disease Activity Of 24-hour Continuous Subcutaneous Infusion Of ABBV-951 In Adult Participants With Advanced Parkinson's Disease

Parkinson's disease (PD) is a neurological condition, which affects the brain. PD gets worse over time, but how quickly it progresses varies a lot from person to person. Some symptoms of PD are tremors, stiffness, and slowness of movement. This study will assess how safe and effective ABBV-951 is in adult participants with PD. Adverse events and change in disease activity is evaluated. ABBV-951 is an investigational (unapproved) drug containing Levodopa Phosphate/Carbidopa Phosphate (LDP/CDP) given as an infusion under the skin for the treatment of Parkinson's Disease. Adult participants with advanced PD and who have completed M15-736 or M20-339 study will be enrolled. Approximately 130 participants will be enrolled in the study in approximately 60 sites in the United States and Australia. Participants will receive continuous subcutaneous infusion (CSCI) (under the skin) of ABBV-951 for 96 weeks during the Primary Treatment Period and during the optional Extended Treatment Period. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the course of the study at a hospital or clinic. The effect of the treatment will be checked by medical and remote telephone assessments, blood tests, checking for side effects, and completing questionnaires.

Inclusion Criteria: \- Completion of the parent study, Study M15-736 or Study M20-339. Exclusion Criteria: \- Participant considered by the investigator to be an unsuitable candidate to receive ABBV-951 for any reason.