A Study of TAK-861 in Participants With Narcolepsy Type 2

The main aim is to evaluate the effect of TAK-861 on symptoms of narcolepsy, including excessive daytime sleepiness (EDS) as measured by sleep latency from the Maintenance of Wakefulness Test (MWT).

The study will enroll approximately 60 participants and they will be randomly assigned to 3 groups (20 per group) to take one of two different doses of TAK-861 or a placebo. All the participants will receive the treatment for 8 weeks. Participants will be asked to complete some questionnaires during the study. This trial will be conducted in North America, Europe, and Asia Pacific.

The drug being tested in this study is called TAK-861. TAK-861 is being tested to treat people who have narcolepsy without cataplexy \[Narcolepsy Type 2 (NT2)\]. This study will evaluate the efficacy, safety, and tolerability of 2 oral doses of TAK-861.

The study will enroll approximately 60 patients. Participants will be randomly assigned (by chance, like flipping a coin) to one of the three treatment groups-which will remain undisclosed to the patient and study doctor during the study (unless there is an urgent medical need):

* TAK-861 Dose 1
* TAK-861 Dose 2
* Placebo (dummy inactive pill) - this is a tablet that looks like the study drug but has no active ingredient

This is a multi-center trial to be conducted worldwide. The overall time to participate in this study is approximately 18 weeks.

Eligibility Criteria:
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.

- The participant has an International Classification of Sleep Disorders, 3rd edition (ICSD-3) diagnosis of NT2 by preceding polysomnography (PSG)/ multiple sleep latency test (MSLT), performed within the past 5 years.

Note: If there is a potential participant with NT2 for whom a diagnostic nocturnal polysomnography (nPSG)/MSLT was performed more than 5 years ago or is not available, the site may repeat the diagnostic PSG/MSLT.

Exclusion Criteria:

* The participant has a current medical disorder, other than narcolepsy without cataplexy, associated with EDS.
* 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, mental retardation, 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. Note: If the history of substance use disorder is more than 12 months before baseline, the participant may be allowed to enroll in the study after consultation with the sponsor or designee. (Participant must also have negative urine drug screen at the screening and Day -2 visit.)
  4. Current active major depressive episode (MDE) or who have had an active MDE in the past 6 months.
* The participant has a history of cerebral ischemia, transient ischemic attack (\<5 years ago), intracranial aneurysm, or arteriovenous malformation.
* The participant had major surgery or donated or lost 1 unit of blood (approximately 500 mL) within 4 weeks before the screening visit.