Study Comparing Continuous Subcutaneous Infusion Of ABBV-951 With Oral Carbidopa/Levodopa Tablets For Treatment Of Motor Fluctuations 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 measures the efficacy, safety, and tolerability of ABBV-951 versus oral Levodopa (LD)/Carbidopa (CD) \[LD/CD\] in advanced PD participants to achieve reduction in motor fluctuations. ABBV-951 is an investigational (unapproved) drug containing Levodopa Phosphate/Carbidopa Phosphate (LDP/CDP) given subcutaneously (under the skin) for the treatment of Parkinson's Disease. Adult participants with advanced PD will be enrolled. Approximately 130 participants will be enrolled in the study in approximately 80 sites across the world. In one arm, participants will receive ABBV-951 solution as a continuous infusion under the skin plus oral placebo capsules for LD/CD. In the second arm, participants will receive placebo solution for ABBV-951 as a continuous infusion under the skin plus oral capsules containing LD/CD tablets. The treatment duration is 12 weeks. 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 assessments, blood tests, checking for side effects, and completing questionnaires.