A Trial of ONO-2808 in Participants With the Parkinsonian Subtype of Multiple System Atrophy (MSA-P)

Key Inclusion Criteria: * Participants with a diagnosis of clinically established or clinically probable MSA-P according to the 2022 Movement Disorder Society (MDS) criteria for MSA diagnosis. * Participants in the early stages of the disease, defined as a maximum of 5 years since the onset of one of the following symptoms associated with MSA-P: * Parkinsonism * Orthostatic hypotension * Urinary dysautonomia * Participants with an anticipated survival of at least 3 years in the opinion of the Investigator. * Participants who are able to ambulate without the assistance of another person, defined as the ability to take at least 10 steps and then to turn around and walk at least another 10 steps. Use of assistive devices (eg, walker or cane) is allowed. * Ability to swallow oral medication and willingness to adhere to the study drug regimen. * Normal range of laboratory values at screening and baseline, especially liver function tests. * Participants receiving treatment (including chronic medications, and herbal or dietary supplements) for symptoms associated with MSA must be on a stable dosage for at least 60 days prior to randomization based on the judgment of the Investigator. Participants must not initiate new treatment within 60 days prior to randomization. Key Exclusion Criteria: * Participants with the cerebellar subtype of MSA according to the 2022 MDS criteria for MSA diagnosis. * Female participants who are pregnant, planning to become pregnant during the study, or breastfeeding. * Participants with a clinically significant or unstable medical or surgical condition other than MSA-P that, in the opinion of the Investigator, might preclude safe completion of the study or might affect the results of the study (eg, pulmonary, cardiovascular \[including bradyarrhythmia\], macular edema, and significant renal or hepatic dysfunction). * Neurological diseases/disorders other than MSA-P, such as Parkinson's disease, dementia with Lewy bodies, essential tremor, progressive supranuclear palsy, spinocerebellar ataxia, spastic paraparesis, corticobasal degeneration, or vascular, normal pressure hydrocephalus, pharmacological, or postencephalitic parkinsonism. * Any abnormalities, other than MSA, found on the centrally read brain MRI that, in the opinion of the Investigator, may constitute a confounder for the study or preclude safe participation. * Participants with documented liver diseases, cirrhosis, prior drug-induced liver injury (DILI), or ascites or symptoms and signs of encephalopathy due to hepatic dysfunction. Other protocol-defined inclusion and exclusion criteria apply.