A Double-blind, Placebo-Controlled, Randomized Withdrawal Trial 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 assess how effective TAK-861 is for treating narcolepsy type 1 and if this effect is maintained over time. Participants will take TAK-861 for a few months and if they meet certain criteria, they will be randomly assigned (by chance, like flipping a coin) to continue taking TAK-861 or take placebo (fake medicine) for up to 4 weeks to see if their narcolepsy symptoms return.
• The participant has a body mass index (BMI) within the range 18 to 40 kilograms per square meter (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 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).