Safety, Tolerability, and Pharmacokinetics Study of ATH-1020

Inclusion Criteria: All Subjects 1. Body mass index (BMI) of ≥ 18.0 and ≤ 32.0 kg/m2 at Screening, with minimum weight of 60 kg. 2. Subjects in generally good health per the investigator's discretion. 3. Male subjects and their partners must be willing to comply with the contraceptive requirements of the study. 4. Subjects must have adequate venous access. Part A (SAD) 5. Male subjects aged 18 to 50 years at the time of signing the informed consent. Part B (MAD) 6. Male subjects aged 18 to 50 years (Cohorts B1, B2, B3, and B5); male and post-menopausal female subjects aged 65 to 85 years (Cohort B4) at the time of signing the informed consent. Exclusion Criteria: 1. History of significant drug allergies (including to any excipients) or of anaphylactic reaction. 2. Any condition per the investigator's discretion, which while not requiring chronic medication use, is likely to require intermittent/acute therapeutic intervention. 3. Any history of seizures or loss of consciousness for an unknown reason. 4. History of or positive results of serology screening for hepatitis B, hepatitis C or human immunodeficiency virus (HIV). 5. Abnormal liver tests 6. Impaired renal function. 7. History of having taken another investigational drug within 30 days prior to Admission (Day -1). 8. Major surgery within 90 days prior to Admission (Day -1) or anticipated surgery during the study. Part A (SAD) 9. Female subjects are not permitted. 10. Any medical condition that requires chronic medication use. Part B (MAD) 11. A history of intermittent benzodiazepine (short-acting only) or other treatments for insomnia and anxiety are allowed, provided that the subject is able to abstain from their use during the Screening period, and from Admission until discharge from the study. 12. Reported changes in cognition and reported history of declines in everyday life in the last year. Part B (MAD) CSF Sampling (Cohort B5) 13. Subject history of or current contraindication to lumbar puncture/spinal catheterization. 14. Clinically significant abnormalities in coagulation parameters.