Researchers from the University of South Florida found in a retrospective analysis of clinical trial data that long-term use of monoamine oxidase type B, or MAO-B, inhibitors slows the clinical decline of patients with Parkinson's disease.

Parkinson's decline slowed with extended MAO-B inhibitors - UPI.com

Unfortunately, the paper on which this report is based [1] is behind a paywall. But, Medicalxpress are reporting [2]:

"A one year increase in duration of MAO-B inhibitor exposure was associated with a benefit equivalent in magnitude to approximately 20% of the annual decline in Activities of Daily Living, Ambulatory Capacity, and Global Disability observed in participants not receiving an MAO-B inhibitor."

References:

[1] "Longer Duration of MAO-B Inhibitor Exposure is Associated with Less Clinical Decline in Parkinson’s Disease: An Analysis of NET-PD LS1"
Hauser Robert, et al.
Journal of Parkinson's Disease, 2017
Longer Duration of MAO-B Inhibitor Exposure is Associated with Less Clinical Decline in Parkinson’s Disease: An Analysis of NET-PD LS1 - IOS Press

[2] Extended use of MAO-B inhibitors slows decline in Parkinson's disease patients

John