Istradefylline for Parkinson Disease With Cognitive Impairment

Istradefylline has been approved by the U. S. Food and Drug Administration (FDA) to reduce "off" episodes in Parkinson disease. The period when levodopa has a positive effect on Parkinson's symptoms is called on-time. Once the medication stops working, a so called "off" episode starts, where symptoms recur. Usual care for treatment of Parkinson disease with cognitive impairment is use of cognition enhancing medications also called cholinesterase inhibitors. In this study, participants will receive usual care, and in addition, they will be asked to take istradefylline daily for 26 weeks.