An AE is defined as any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and which does not necessarily have a causal relationship with this treatment. An SAE is an event that results in death, is life-threatening, requires or prolongs hospitalization, results in a congenital anomaly, persistent or significant disability/incapacity or is an important medical event that, based on medical judgment, may jeopardize the subject and may require medical or surgical intervention to prevent any of the outcomes listed above.

AESIs are defined as AEs from "special situations," such as accidental or intentional overdose, medication error, occupational or accidental exposure, off-label use, drug abuse, drug misuse, or drug withdrawal, all which must be reported whether associated with an AE or not.

The Infusion Site Evaluation Scale will be used to assess infusion sites. Infusion Site Evaluation Scale is an eight-point numeric scale used to assess irritation at the infusion site area (0 being "no evidence of irritation" and 7 being "strong reaction spreading beyond the test site").

The Infusion Site Evaluation Scale will be used to assess infusion sites. Infusion Site Evaluation Scale is an A to G letter grade scale, used to assess irritation at the infusion site area (A being "no finding" to G being "Small petechial erosions and/or scabs").

C-SSRS is a systematically administered instrument designed to assess suicidal behavior and ideation, track and assess all suicidal events, and assess the lethality of attempts. Any participant who has suicidal behavior or suicidal ideation with plan since the last C-SSRS completed, will be evaluated immediately by the investigator.

The QUIP-RS is a brief, self-completed or rater-administered rating scale to assess the severity of symptoms of impulse control disorders (ICDs) and related behaviors reported to occur in PD. The QUIP-RS uses a 5-point Likert scale that requires individuals to rate the severity of each symptom based on its frequency.

Cognitive impairment is assessed by the Mini-Mental State Examination (MMSE). MMSE is a brief 30-point questionnaire, administered by a trained rater, that provides a quantitative measure of cognitive status in adults and is used widely to screen for cognitive impairment and to estimate the severity of cognitive impairment at a given point in time, to follow the course of changes in a patient over time, and to document response to treatment.

Number of participants with abnormal change in clinical laboratory test results like hematology will be assessed..

Number of participants with abnormal change from baseline in vital sign measurements like systolic and diastolic blood pressure will be assessed.

12-lead resting ECGs will be recorded. Parameters include RR interval, PR interval, QT interval, and QRS duration.

"On" time is defined as periods of good motor symptom control, and was assessed by the Parkinson's Disease (PD) diary. The normalized "On" time without troublesome dyskinesia is the sum of the normalized "On" time without dyskinesia and the normalized "On" time with non-troublesome dyskinesia. "On" time without dyskinesia plus "On" time with non-troublesome dyskinesia are based on the PD Diary (normalized to a 16-hour waking day averaged over 3 consecutive days). Baseline value is defined as the average of normalized "On" time without troublesome dyskinesia collected over the 3 PD Diary days before randomization.

An AE is defined as any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and which does not necessarily have a causal relationship with the treatment. The investigator assesses the relationship of each event to the use of the study drug or device as either reasonable possibility or no reasonable possibility. A serious adverse event (SAE) is an event that results in death, is life-threatening, requires or prolongs hospitalization, results in a congenital anomaly, persistent or significant disability/incapacity or is an important medical event that, based on medical judgment, may jeopardize the participant and may require medical or surgical intervention to prevent any of the outcomes listed above. Adverse Events of Special Interest (AESI) - polyneuropathy, weight loss, hallucinations/psychosis, and somnolence will be monitored throughout the study.

Number of participants with clinically significant change from baseline in laboratory parameters (hematology, biochemistry, coagulation, and urinalysis) will be reported throughout the study.

Number of participants with clinically significant change from baseline in vital signs will be reported throughout the study.

An adverse event (AE) is defined as any untoward medical occurrence in a patient or clinical investigation participant administered a pharmaceutical product which does not necessarily have a causal relationship with this treatment. The investigator assesses the relationship of each event to the use of study drug. A serious adverse event (SAE) is an event that results in death, is life-threatening, requires or prolongs hospitalization, results in a congenital anomaly, persistent or significant disability/incapacity or is an important medical event that, based on medical judgment, may jeopardize the participant and may require medical or surgical intervention to prevent any of the outcomes listed above. Treatment-emergent adverse events/treatment-emergent serious adverse events (TEAEs/TESAEs) are defined as any event that began or worsened in severity on or after the first dose of study drug.

Treatment emergent adverse events of special interest are defined as any adverse event of infusion site infections, infusion site reactions, hallucinations/psychosis, falls and associated injuries, polyneuropathy (peripheral neuropathy), weight loss, or somnolence from the first dose of study drug until 30 days following last dose of study drug.

Skin tolerability was assessed using the Infusion Site Evaluation Scale, a 2-part numeric (0-7) and letter (A-G) grade scale, where a notable skin reaction is defined as a reaction with a numeric grade of 6 or 7 or a letter grade of D, E, F, or G. Any observation of infusion site reaction with irritation criteria \> 2 or \> C was recorded as an adverse event (AE).

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Samples for clinical laboratory tests were collected at study visits, and a certified central laboratory was used to process the samples and provide results.

Orthostatic blood pressure was measured after the participant had been supine (lying down with their face up) for at least 5 minutes and then, after the participant had been standing for 2 minutes. When vital sign measurements were scheduled at the same time as a blood collection, vital sign measurements were obtained prior to blood collection.

Orthostatic blood pressure was measured after the participant had been supine (lying down with their face up) for at least 5 minutes and then, after the participant had been standing for 2 minutes. When vital sign measurements were scheduled at the same time as a blood collection, vital sign measurements were obtained prior to blood collection.

Orthostatic pulse rate was measured after the participant had been supine (lying down with their face up) for at least 5 minutes and then, after the participant had been standing for 2 minutes. When vital sign measurements were scheduled at the same time as a blood collection, vital sign measurements were obtained prior to blood collection.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

12-lead electrocardiograms (ECGs) were recorded at study visits after the participant had been supine for at least 5 minutes. Participants were instructed to remain stationary (no talking, laughing, deep breathing, sleeping, or swallowing) for approximately 10 seconds during the ECG recording. When an ECG was recorded at a time near that of a blood collection, the ECG was obtained prior to the blood collection. ECGs on Day 1 were recorded after initiation of the continuous subcutaneous infusion (CSCI) of ABBV-951 prior to the end of the study visit.

Local tolerability (infusion site evaluation) was assessed using the Infusion Site Assessment 2-part rating scale.

Vital signs will be collected both supine and standing.

The C-SSRS is an instrument designed to assess suicidal behavior and ideation.

Blood samples for serum chemistry tests will be collected following a minimum 8-hour fast.

A 12-lead ECG will be performed after the participant has been supine for at least 5 minutes.

Maximum observed plasma concentration of levodopa following a single infusion of ABBV-951.

Time to maximum observed plasma concentration of levodopa following a single infusion of ABBV-951.

Area under the plasma concentration-time curve following a single infusion of ABBV-951.

Number of participants reporting adverse events

Apparent terminal phase elimination rate constant (β or Beta)

Terminal phase elimination half-life (t1/2) will be assessed.