The main aim of this study is to learn how effective TAK-861 is in improving excessive sleepiness during the day (called excessive daytime sleepiness or EDS) after 3 months of treatment. Other aims are to learn how effective TAK-861 is in lowering the number of sudden, unexpected attacks of muscle weakness while staying conscious (cataplexy) in a week; to learn the effect TAK-861 has on participants' ability to maintain attention, participant's overall quality of life, the spectrum of narcolepsy symptoms and daily life functions; and to learn about the safety of TAK-861.
Who can participate
Age range16 Years – 70 Years
SexALL
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Inclusion criteria
✓. The participant has a body mass index (BMI) within the range 18 to 40 kilograms per meter square (kg/m\^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.
✓. The participant has greater than or equal to (≥)4 partial or complete episodes of cataplexy/week (WCR).
✓. The participant is positive for the human leukocyte antigen (HLA) genotype HLA-DQB1\*06:02 or results from radioimmunoassay indicate the participant's cerebrospinal fluid (CSF) orexin (OX)/hypocretin-1 concentration is less than or equal to (≤)110 picograms per milliliter (pg/mL) \[or less than one-third of the mean values obtained in normal participants within the same standardized assay\].
Exclusion criteria
✕. The participant has a current medical disorder, other than narcolepsy with cataplexy, associated with EDS.
✕. 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).
✕. The participant has current or recent (within 6 months) gastrointestinal disease that is expected to influence the absorption of drugs.
✕. The participant has a history of cancer in the past 5 years.
✕. The participant has a clinically significant history of head injury or head trauma.
What they're measuring
1
Change From Baseline to Week 12 in Mean Sleep Latency From the 4 Maintenance of Wakefulness Test (MWT) Wake Trials
✕. The participant has a history of epilepsy, seizure, or convulsion.
✕. The participant has any current unstable psychiatric disorder or current active major depressive episode (MDE) or an active MDE in the past 6 months.