A Study of TAK-994 in Adults With Type 1 and Type 2 Narcolepsy

The main aims of the study are:

* To check for side effects from TAK-994 and check what dose of TAK-994 participants can tolerate.
* To check what dose range provides adequate relief of narcolepsy symptoms.
* To check how much TAK-994 stays in the blood of participants, over time.

The study will have 4 parts. Participants can only join 1 of the parts.

A. Participants with type 1 narcolepsy will take either TAK-994 or placebo tablets for 28 days. A placebo looks just like TAK-994 but will not have any medicine in it.

B. Participants with type 1 narcolepsy will take 1 of 3 doses of TAK-994 or placebo tablets for 56 days.

C. Participants with type 1 narcolepsy in China only will take TAK-994 or placebo tablets for 56 days.

D. Participants with type 2 narcolepsy will take either TAK-994 or placebo tablets for 28 days.

The drug being tested in this study is called TAK-994. TAK-994 is being tested in participants with NT1 and NT2.

The study will enroll up to approximately 202 participants. The study has 4 Parts: Parts A, B, C (China only) and D. Part A - Part A has 2 cohorts \[Cohorts (A1a and A1b) A2\] In both of these Cohorts, participants will be randomly assigned (by chance, like flipping a coin) in a 2:1 ratio to receive TAK-994 or placebo up to 28 days:

* Part B: In Part B, participants will be randomized in 1:1:1:1 ratio in four parallel arms to receive TAK-994 Dose 1, 2 or 3 or placebo for 56 days. Depending upon their eligibility participants completing Part B of the study treatment will be enrolled to participate in an Extension study.
* Part C: In Part C, participants only from China will be enrolled and randomized in a 2:1 ratio to receive TAK-994 and placebo for 56 days.
* Part D: Participants will be included in two cohorts \[Cohorts (D1a and D1b) and D2\] and will be randomized in 2:1 ratio to receive TAK-994 or placebo for 28 days. The dose will be selected based on the safety and tolerability in Part A.

This multi-center trial will be conducted in the United States, Japan, China, Italy, France, and European Union. The overall duration of the study is 63 days. Participants will be followed up for 7 days after the last dose of study drug.

Eligibility Criteria:
Inclusion Criteria:

1. Has a diagnosis of narcolepsy type 1 (NT1) (Parts A-C) or NT2 (Part D) by polysomnography (PSG)/ multiple sleep latency test (MSLT) performed within the past 10 years meeting the minimal acceptable criteria for the proper performance of the PSG/MSLT as outlined by the International Classification of Sleep Disorders, 3rd edition criteria.
2. The participant's Epworth Sleepiness Scale (ESS) score must be greater than or equal to (\>=) 10 at Day -1.
3. Must be willing to discontinue all medications used for the treatment of NT1/NT2.
4. The human leukocyte antigen (HLA) genotype: Part A: should test positive for human leukocyte antigen (HLADQB1)\* 06:02 (PARTs A-C)- (positive results for either homozygous or heterozygous alleles will be considered "positive" and acceptable). However, if the HLA test is negative (i.e. negative for the heterozygous allele) and the PI feels strongly that the participant has narcolepsy with cataplexy (NT1) then a discussion should be initiated between the PI and the sponsor or designee about the advisability of doing a spinal tap to determine the participant's cerebrospinal fluid (CSF) orexin-1 (OX-1) level. If the CSF result shows the orexin 1 (OX-1) concentration is either less than or equal to\<110 pg/mL, or less than one-third of mean values obtained in normal participants with the same standardized assay, then the diagnosis of NT1 is established allowing the participant to be enrolled and randomized, If the CSF OX-1 concentration is \>110 pg/mL then the participant will not be allowed to continue in the study .
5. For Parts A, B, and C, during the screening period, participant, must have \>=4 partial or complete episodes of cataplexy/week (WCR), and \>=4 partial or complete episodes of cataplexy/week during the screening period when off of anticataplexy medications, averaged over 2 weeks (14 consecutive days) minimum. WCR recording taken during following period will be considered for study eligibility: after the participant has stopped taking anticataplexy medications for at least 7 days (minimum 7-day washout) and study Day -2.

Exclusion Criteria:

1. Has a risk of suicide according to endorsement of Item 4 or 5 of the screening/baseline visit Columbia suicide severity rating scale (C-SSRS) or has made a suicide attempt in the previous 12 months.
2. Is an excessive (\>600 mg/day) caffeine user 1 week before to the study screening.
3. Has a history of cancer (except carcinoma in situ that has been resolved without further treatment or basal cell skin cancer); past or current epilepsy, seizure; a lifetime history of major psychiatric disorder other than depression or anxiety; a clinically significant history of head injury or head trauma; a history of cerebral ischemia, transient ischemic attack, intracranial aneurysm, or arteriovenous malformation; known coronary artery disease, a history of myocardial infarction, angina, cardiac rhythm abnormality, or heart failure; or current or recent (within 6 months) gastrointestinal disease expected to influence the absorption of drugs. Any history of Roux-en-Y gastric bypass is considered exclusionary and any other surgical intervention that may influence the absorption of drugs should be discussed and approved by the sponsor or designee before enrolling the participants.
4. Has a medical disorder, other than narcolepsy, associated with EDS. This includes clinically significant moderate to severe obstructive sleep apnea and/or with or without treatment with mandibular advanced device hypoglossal nerve stimulation and/or positive airway pressure (PAP) therapy) and/or restless legs syndrome (RLS)/periodic limb movement disorder that has a significant impact on daytime sleepiness. This is evidenced by a clinical history of sleep apnea syndrome (loud snoring with observed respiratory pauses in the absence of nPSG) and/or RLS causing historical sleep onset/maintenance insomnia with resultant insufficient sleep. Or any as evaluated during the clinical interview at screening. pPast PSG data demonstrating any of the following sleep disturbances: apnea Hypopnea Index ≥15 or apnea index ≥10, an oxygen saturation of \<80 for \>10 seconds, periodic leg movement arousal index of ≥15/h) or as evaluated on interview at the time of screening. Asshould be considered exclusionary unless, based on a clinical evaluation by the investigator, a meaningful change in clinical status has occurred that would impact the results. Because nPSG data is obtained on Day -2, subjects may fail screening if criteria are not meet on the Day -2 nPSG.
5. Has a usual bedtime later than 2400 (12:00 AM, midnight) or an occupation requiring nighttime shift work or variable shift work within the past 6 months or travel within more than 3 time zones, within 14 days before Study Day -2.
6. Has a nicotine dependence that is likely to have an effect on sleep (e.g., a participant who routinely awakens at night to smoke) and/or an unwillingness to discontinue all smoking and nicotine use during the confinement portions of the study. Participants undergoing optional CSF collection.
7. Has a local infection at the puncture site.
8. Has developed signs of lumbar radiculopathy, including lower extremity pain and paresthesia.
9. Has any known focal neurological deficit that might suggest an increase in intracranial pressure.