The primary purpose of the study is to evaluate the safety and tolerability of single intravenous (IV) infusions of E2025 in healthy adult participants.

Inclusion Criteria:

• Non-smoking, male or female age greater than or equal to (>=) 18 years and less than or equal to (<=) 55 years old at the time of informed consent. Females must be of nonchildbearing potential
• Body weight >=50 kilogram (kg) and a Body Mass Index (BMI) >=18 and less than (<) 30 kilogram per square meter (kg/m^2) at Screening

Exclusion Criteria:

• Females who are breastfeeding or pregnant at Screening or Baseline; Females of childbearing potential.
• Males who have not had a successful vasectomy (confirmed azoospermia) if their female partners are of childbearing potential and are not willing to use a highly effective contraceptive method throughout the study period or for 203 days after their partner's study drug administration.
• Clinically significant illness that requires medical treatment within 8 weeks or a clinically significant infection that requires medical treatment within 4 weeks of dosing
• Evidence of disease that may influence the outcome of the study within 4 weeks before dosing; example, psychiatric disorders and disorders of the gastrointestinal tract, liver, kidney, respiratory system, endocrine system, hematological system, neurological system, or cardiovascular system, or participants who have a congenital abnormality in metabolism
• Any clinically abnormal symptom or organ impairment found by medical history at Screening, and physical examinations, vital signs, ECG finding, or laboratory test results that require medical treatment at Screening
• A prolonged QT/QTc interval (QTcF >450 millisecond [ms]). A history of risk factors for torsade de pointes or the use of concomitant medications that prolonged the QT/QTc interval
• Persistent systolic blood pressure (BP) greater than 130 millimeter of mercury (mmHg) or diastolic BP greater than 85 mmHg at Screening or Baseline; Heart rate less than 50 or more than 100 beats per minute at Screening or Baseline
• Any lifetime history of suicidal ideation or any lifetime history of suicidal behavior as indicated by the Columbia-suicide Severity Rating Scale (C-SSRS) or equivalent scale or via interview with a psychiatrist
• Any lifetime history of psychiatric disease (including but not limited to depression or other mood disorders, bipolar disorder, psychotic disorders, including schizophrenia, panic attacks, anxiety disorders); any current psychiatric symptoms as indicated by a standard screening tool.
• Known history of clinically significant drug allergy; known history of food allergies or presently experiencing significant seasonal or perennial allergy at Screening
• Any history of hypersensitivity reaction to a foreign protein, with clinical features not limited to nasal or conjunctival symptoms such as in allergic rhinitis
• Known to be human immunodeficiency virus positive and/or active viral hepatitis (hepatitis B core antibody [HBcAb], hepatitis B viral protein [HBcAg], hepatitis B surface antigen [HBsAg], hepatitis C virus antibody [HCVAb]) as demonstrated by positive serology at Screening
• History of drug or alcohol dependency or abuse within the 2 years before Screening, or a positive urine drug test or breath alcohol test at Screening or Baseline
• Currently enrolled in another clinical study or used any investigational drug or device within 28 days (or 5*the half-life, whichever is longer) preceding informed consent
• Receipt of blood products within 4 weeks, or donation of blood within 8 weeks, or donation of plasma within 1 week of dosing
• Exposure to any biologic drug within 90 days or at least 5 half-lives (whichever is longer), or within 4 weeks for vaccines, before Screening, with the exception of flu (7 days before dosing) and COVID-19 vaccination (14 days before dosing until after the Follow-up visit).
• Any contraindication to continuous CSF sampling via indwelling lumbar catheter or via lumbar puncture (LP)
• Participants identified at risk for hemorrhage.
• Inadequate venous access that would interfere with study drug administration or obtaining blood samples
• Participants who contravene the restrictions on concomitant medications, food, beverages, physical activities, and others as defined in the protocol

Eisai's data sharing commitment and further information on how to request data can be found on our website http://eisaiclinicaltrials.com/.