Inclusion Criteria:  Diagnosis of PD consistent with United Kingdom Brain Bank Criteria or the Movement Disorder Society Research Criteria for the Diagnosis of Parkinson's Disease, with bradykinesia and sequence effect being present, and prominent motor asymmetry (if no rest tremor). Current treatment with levodopa. Stable on levodopa and other study-approved medications for PD motor symptoms for at least four weeks prior to screening. Clinically significant wearing-off / fluctuating symptomatology warranting treatment with ISD Presence of apathy at baseline (LARS > -22) Living with adult informant  Exclusion Criteria:  Current or prior treatment with istradefylline. 2. History of Deep Brain Stimulation (DBS) surgery, ablative surgery (e.g., pallidotomy, thalamotomy, focused ultrasound, etc.), Duopa pump implantation, or other invasive intervention for Parkinson's disease symptoms. 3. Injury or concomitant health condition at screening that would preclude engagement in light physical activity. 10 4. Prior history of cerebrovascular accident (e.g., stroke or aneurysm) or seizure disorder (other than febrile seizures during childhood). 5. Dementia (MoCA<22) 6. Psychotic symptoms that would raise concern for safe use of ISD, as indicated by domains A (delusions) and B (hallucinations) of the Neuropsychiatric Inventory (NPI), and defined as a score of ≥4 on either the A (frequency × severity) or B (frequency × severity) scales of the NPI. 7. Depressive mood symptoms at baseline likely to interfere with response to treatment (BDI-II>19) 8. Active suicidal ideation within 1 year prior to Screening Visit as determined by a positive response to Question 4 or 5 on the C-SSRS. 9. Diagnosis or treatment for any central nervous system disorder other than Parkinson's disease that could be expected in the eyes of the investigator to impact ability to participate in study. 10. Contraindications / conditions that would preclude safe dosing of 40mg ISD 11. Change in medications for mood or anxiety within 6 weeks of enrollment or anticipation of change in medications for mood or anxiety during the 8-week study period. 12. Moderate or severe hepatic impairment. 13. Current treatment with strong CYP3A4 inhibitors 14. Current treatment with strong CYP3A4 inducers 15. Pregnant women are not able to participate due to unknown safety risks associated with ISD