Extension Study To Evaluate Safety And Tolerability Of 24-Hour Daily Exposure Of Continuous Subcutaneous Infusion of ABBV-951 In Adult Participants With 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. The purpose of this study is to continue testing whether ABBV-951 is safe, effective, and tolerable in participants with Parkinson's disease after completion of the parent study M15-741. ABBV-951 is an investigational (unapproved) drug containing levodopa phosphate/carbidopa phosphate (LDP/CDP) given as infusion under the skin for the treatment of Parkinson's Disease. Participants who have successfully completed M15-741 study will immediately enter this study's treatment period to continue receiving ABBV-951. Adult participants with advanced PD will be enrolled. Approximately 130 adult participants will be enrolled in the study at approximately 65 sites worldwide. Participants will receive continuous subcutaneous infusion (CSCI) of ABBV-951 for 24 hours daily 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 clinic visits and have remote assessments completed via phone calls during the course of the study. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects, and completing questionnaires.

Inclusion Criteria: * Participants who have Parkinson's Disease and who have successfully completed the parent study M15-741. * Participants willing and able to comply with procedures required in the protocol. Exclusion Criteria: \- Participants, if judged by the investigator to be unsuitable candidates to continue to receive ABBV-951 for any reason.