A Study Assessing the Safety of Oral ATH-399A in Healthy Adult Participants

Inclusion Criteria

• Healthy, as determined by the Investigator based on a medical evaluation including medical history, physical examination, neurological examination, laboratory tests, and cardiac monitoring.
• Population
• Part 1a and 1b: Men and women, age 18-55 years inclusive at the date of screening.
• Part 2: Men and women aged 18-55 years inclusive at the date of screening. Additional cohort: Participants of the additional cohort will be of approximately equal numbers of male and post-menopausal or surgically sterile females, with a minimum of 2 of each gender, aged >55-80 years, inclusive.
• Women of childbearing potential (WOCBP) must be non-pregnant and non-lactating.
• Postmenopausal women must have had ≥12 months of spontaneous amenorrhea (with follicle-stimulating hormone [FSH] ≥40 milli-international units per milliliter (mIU/mL)).
• Surgically sterile women are defined as those who have had a hysterectomy and/or bilateral oophorectomy. Women who are surgically sterile must provide verbal confirmation.
• Male participants who are sexually active with WOCBP must:
• Agree to use condoms to protect their partners from becoming pregnant during the study (including washout periods) and not to donate sperm for at least 90 days after the last dose of the study drug, and
• Agree to ensure that they and their partners are routinely using a medically approved contraceptive method. It is important that male participants not impregnate others while in the study.
• Body weight ≥50.0 kilograms (kg) for men and ≥45.0 kg for women and body mass index within the range of 18.0-30.0 kilogram/square meter (kg/m^2) (inclusive).
• Participants participating in Part 1b must be willing and able to consume the entire high-fat, high-calorie breakfast in the designated timeframe.
• Participants must understand the nature of the study, must be willing to participate in the study, and must provide signed and dated written informed consent in accordance with local regulations before the conduct of any study-related procedures.
• Participants must be, in the opinion of the Investigator, able to participate in all scheduled evaluations, likely to complete all required tests, and likely to be compliant.
• Participants must be fluent in English or French.
• Participants must agree not to post any personal medical data related to the study or information related to the study on any website or social media site.

Exclusion Criteria

• A positive urine cotinine, drug screen, or alcohol breath test at screening or Day -1.
• Any history of psychiatric disorders, including substance use disorders, according to the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) criteria that requires current treatment with psychiatric medications. Participants with mild anxiety or depression which is stable for >6 months are permitted.
• History of drug abuse within 1 year prior to screening or recreational use of soft drugs (such as marijuana) within 1 month or hard drugs (such as cocaine, phencyclidine [PCP], crack, opioid derivatives including heroin, and amphetamine derivatives) within 3 months prior to screening.
• A diagnosis of intellectual disability (intellectual developmental disorder) or mental retardation.
• A serious mental illness, dementia, or other neuropsychiatric disorder that would interfere with participation in the trial, or ability to provide informed consent in the opinion of the Investigator.
• Any active suicidal ideation as indicated by the C-SSRS (score of ≥4) or history of suicidal behavior within the 12 months prior to screening.
• A positive Hepatitis B surface antigen or positive Hepatitis C antibody result at screening.
• A positive test at screening for human immunodeficiency virus (HIV) antigen or antibody or a history of positive test.
• Alanine aminotransferase or aspartate aminotransferase levels greater than 1.2 times the ULN at screening or Day -1.
• Frequently use (>