A Trial of TAK-861 for the Treatment of Narcolepsy With Cataplexy

Inclusion Criteria: 1. The participant has a body mass index (BMI) within the range 18 to 40 kilograms per square meter (kg/m\^2). 2. The participant has an International Classification of Sleep Disorders, Third Edition (ICSD-3) or International Classification of Sleep Disorders, Third Edition, Text Revision (ICSD-3-TR) diagnosis of NT1. 3. The participant is positive for the human leukocyte antigen (HLA) genotype Major Histocompatibility Complex, Class II, DQ Beta 1 (HLA-DQB1\*06:02) or results from radioimmunoassay indicate the participant's CSF OX/hypocretin-1 concentration is ≤110 pg/mL (or less than one-third of the mean values obtained in normal participants within the same standardized assay). Exclusion Criteria: 1. The participant has a current medical disorder, other than narcolepsy with cataplexy, associated with EDS. 2. The participant a) has a history of myocardial infarction, b) has a history of clinically significant hepatic disease, thyroid disease, coronary artery disease, cardiac rhythm abnormality or heart failure, or c) has any medical condition (such as unstable cardiovascular, pulmonary, renal or gastrointestinal disease. 3. The participant has current or recent (within 6 months) gastrointestinal disease that is expected to influence the absorption of drugs. 4. The participant has a history of cancer in the past 5 years. 5. The participant has a clinically significant history of head injury or head trauma. 6. The participant has a history of epilepsy, seizure, or convulsion (except for a single febrile seizure in childhood). 7. The participant has a history of cerebral ischemia, transient ischemic attack (less than 5 years from screening), intracranial aneurysm, or arteriovenous malformation.