A Study of TAK-861 in Participants With Narcolepsy Type 1

Inclusion Criteria:

The participant is aged 18 to 70 years, inclusive, at the time of signing the informed consent form (ICF).

Note: In Japan, participants aged 16 to 70 years, inclusive, may be included.

1. The participant has body mass index (BMI) within the range 18 to 40 kilogram per square meter [kg/m^2] (inclusive).
2. The participant has an International Classification of Sleep Disorders, 3rd Edition (ICSD-3) diagnosis of narcolepsy type 1 (NT1) by polysomnography (PSG)/Multiple Sleep Latency Test (MSLT), performed within the past 10 years.
3. The participant is positive for the human leukocyte antigen (HLA) genotype HLA-DQB1*06:02 or results from cerebrospinal fluid (CSF) testing indicate the participant's CSF orexin (OX)/hypocretin-1 concentration is <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:

1. The participant has a current medical disorder, other than narcolepsy with cataplexy, associated with EDS.
2. The participant has medically significant hepatic or thyroid disease.
3. The participant has a history of cancer in the past 5 years (does not apply to participants with carcinoma in situ that has been resolved without further treatment or basal cell cancer).
4. The participant has clinically significant coronary artery disease, a history of myocardial infarction, clinically significant angina, clinically significant cardiac rhythm abnormality, or heart failure.
5. The participant has a clinically significant history of head injury or head trauma.
6. The participant has history of epilepsy, seizure, or convulsion, or has a family history of inherited disorders associated with seizure (except for a single febrile seizure in childhood).

The participant has one or more of the following psychiatric disorders:

1. Any current unstable psychiatric disorder.
2. Current or history of manic or hypomanic episode, schizophrenia or any other psychotic disorder, including schizoaffective disorder, major depression with psychotic features, bipolar depression with psychotic features, obsessive compulsive disorder, intellectual disability, organic mental disorders, or mental disorders due to a general medical condition as defined in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5).
3. Current diagnosis or history of substance use disorder as defined in the DSM-5.
4. Current active major depressive episode (MDE) or who have had an active MDE in the past 6 months.
5. The participant has a history of cerebral ischemia, transient ischemic attack (<5 years ago), intracranial aneurysm, or arteriovenous malformation.
6. The participant has a positive test result for hepatitis B surface antigen, hepatitis C virus antibody, or human immunodeficiency virus (HIV) antibody/antigen.
7. The participant's renal creatinine clearance (Cockcroft-Gault Equation) is ≤50 mL/minute.
8. The participant has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) values >1.5 times the upper limit of normal (ULN).
9. The participant is considered by the investigator to be at imminent risk of suicide or injury to self, others, or property, or the participant has attempted suicide within the past year.