A Randomized, Double-Blind, Placebo-Controlled Study to Evaluate the Efficacy and Safety of TAK-861 for the Treatment of Narcolepsy With Cataplexy (Narcolepsy Type 1)
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.
• 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) \[or less than one-third of the mean values obtained in normal participants within the same standardized assay\].