A Study to Evaluate the Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of TAK-925 in Healthy Volunteers and Participants With Narcolepsy

The purpose of this study is to investigate the safety, tolerability, pharmacokinetics, and pharmacodynamics of TAK-925 when administered to healthy participants and narcolepsy participants.

The drug being tested in this study is called TAK-925. TAK-925 is being tested in healthy participants and participants with narcolepsy. This study will look at the safety, tolerability, pharmacokinetics, and efficacy of TAK-925.

This study will consist of three parts. Part A will be a randomized, double-blind, placebo-controlled, multiple rising dose (MRD) study in healthy participants. Part B is a randomized, double-blind, placebo-controlled MRD study in participants with narcolepsy. Part C is a randomized, double-blind, placebo-controlled, parallel group, multiple repeat dose study in participants with narcolepsy. Part A' is a single dose study in healthy participants.

The study will enroll approximately 96 participants planned as total. All participants except Part A' will be randomly assigned (by chance, like flipping a coin) to one of the two treatment groups-which will remain undisclosed to the participants and study doctor during the study (unless there is an urgent medical need):

Part A:

* TAK-925 (Dose Levels A1-A6)
* Placebo

Part B:

* TAK-925 (Dose Levels B1-B4)
* Placebo

Part C:

* TAK-925 (Dose Levels C1-C2)
* Placebo

Part A':

• TAK-925 (Dose Levels A'1-A'2)

All participants will be asked to take TAK-925 or Placebo at the same time each day from Day 1 to Day 7 in Parts A, B and C, and take TAK-925 on Day 1 in Part A'.

This multi-center trial will be conducted in Japan. The overall study period is approximately 15 days in Parts A, B and C, and approximately 7 days for Part A'. Participants will be partly admitted to a hospital during the study.

Eligibility Criteria:
Inclusion Criteria:

Healthy adult participants and Healthy elderly participants:

• Participant weighs at least 50 kg (Healthy adults participants) / 40 kilogram (kg) (Healthy elderly participants) and has a body mass index (BMI) from 18.5 to 30 kilogram per square meter (kg/m\^2), inclusive at Screening.

Narcolepsy participants:

* Participants weighs at least 40 kg inclusive at Screening (\>=50 kg is required for Cohort B4).
* A diagnosis of narcolepsy, as defined by the International Classification of Sleep Disorders, Third Edition (ICSD-3).
* At Day -1, Epworth sleepiness scale (ESS) score \>=10

Exclusion Criteria:

All Participants:

* Participants consume excessive amounts, defined as greater than 6 servings (1 serving is approximately equivalent to 120 mg of caffeine) of coffee, tea, cola, energy drinks, or other caffeinated beverages per day.
* Participants have a moderate to severe substance use disorder.
* Participants have a risk of suicide according to endorsement of item 4 or 5 with Screening/Baseline visit C-SSRS (Columbia Suicide Severity Rating Scale) or has made a suicide attempt in the previous 6 months.
* Participants have a lifetime history of major psychiatric disorder, such as bipolar disorder or schizophrenia.
* Participants experienced sleep wake cycle disturbance with external factors such as irregular work hours.