Effect of Mild Hepatic Impairment on the Pharmacokinetics of Istradefylline

Inclusion Criteria: All subjects: * Non-smoking males and females 18-75 years of age, inclusive; * Men and women with procreative potential must practice medically reliable double barrier methods of birth control; * Body mass index (BMI): 18.0-35.0 kg/m2, inclusive: * Must abstain from drugs and nutrients known as moderate to potent inhibitors/inducers of CYP3A4 and CYP1A enzymes. These agents should be discontinued at least 4 weeks before the istradefylline dose (Day 1) until the Follow-up visit. * Negative results at Screening and Baseline for the following screening laboratory tests: urine drug screen (amphetamines, barbiturates, benzodiazepines, opiates, cannabinoids, and cocaine). Documented prescription use in subjects with mild HI for medications included in the urine drug of abuse test is permitted as long as the dose is stable for at least 2 weeks; Subjects with Normal Hepatic Function only * Medical history without clinically significant or ongoing pathology, which in the opinion of the Investigator will preclude the subject's participation in, or influence the outcome of the study; Subjects with Mild Hepatic Impairment only * Stable, mild liver disease (Child-Pugh A \[5 to 6 points\]); of cryptogenic, post-hepatic, hepatitis B/C virus, or alcoholic origin; * Stable hepatic impairment, defined as no clinically significant change in disease status within the last 30 days, as documented by the subject's recent medical history; Additional inclusion criteria apply Exclusion Criteria: * Female subjects who are taking oral contraceptives or long-term injectable or implantable hormonal contraceptives, pregnant, lactating, or breast-feeding; * Known history of treatment for drug or alcohol addiction within the previous 12 months or \> 14 untis of alcohol consumption per week, or alcohol consumption within 1 week prior to dosing; * Positive test results for human immunodeficiency virus (HIV), or Hepatitis B surface antigen; * Difficulty fasting or eating the standard meals that will be provided; * Use of tobacco or nicotine-containing products within 90 days of the study start to the Follow-up visit (to be confirmed by urine cotinine test); Subjects with Hepatic Impairment only * Severe ascites at Screening; * History of or current severe hepatic encephalopathy (Grade 3 or higher) * Any of the following laboratory parameters at screening: 1. Serum ALT \> 5 × the upper limit of normal range (ULN); 2. Serum albumin \< 2.4 g/dL; 3. Platelet count \< 80,000/mm3; 4. Hemoglobin \< 11 g/dL; 5. Absolute neutrophil count (ANC) \< 1.5 × 109/L (\< 1.5 × 103/μL); * Biliary liver cirrhosis or other causes of HI not related to parenchymal disorder and/or disease of the liver, including hepatocellular carcinoma. Additional exclusion criteria apply