This is a Phase 2, double-blind, parallel-group, placebo-controlled study to assess safety, tolerability, pharmacokinetics, and efficacy of ONO-2020 in participants with mild to moderate Alzheimer's disease (AD). This study aims to determine whether administering ONO-2020, an epigenetic regulator, may improve cognitive functions like memory and cognition in individuals with Alzheimer's disease dementia.

In the study, participants will undergo a screening period of up to 6 weeks (42 days). Eligible participants will be assigned to receive one of 2 dose levels of ONO-2020 or placebo control arm. ONO-2020 or placebo will be administered orally QD for 26 weeks. All participants who received study intervention will be followed up for 4 weeks after treatment discontinuation. The target sample size is 240 participants , out of which up to 45 participants will undergo additional special CSF biomarker evaluation. After enrollment, participants will be randomized in a 1:1:1 ratio to one of 3 treatment arms.

Inclusion Criteria:

1. Have a diagnosis of Alzheimer's disease according to the recommendations from the revised criteria for diagnosis and staging of Alzheimer's disease: Alzheimer's Association Workgroup , along with any positive AD-specific biomarker results (abnormal Core 1 or Core 2 biomarkers) from a previous diagnosis or at screening.
2. Have a previous MRI or CT scan of the brain, which was performed within 1 year prior to enrollment in the study, to confirm that more recent neurological events (e.g., stroke) would not potentially constitute a confounder in the assessment of the etiology of the participant's cognitive status.
3. MMSE score of 15 to 24, inclusive, and MMSE score cannot deviate more than 3 points in either direction between the screening and baseline visits.
4. AD numeric clinical stage 4 or stage 5 based on NIA-AA criteria 2024, at screening and baseline visits
5. Participants receiving concurrent AD treatment (acetylcholinesterase inhibitors and /or memantine) must be on a stable dose for at least 90 days prior to randomization, and the participant must be willing to remain on the same dose for the duration of the study.
6. Have the ability to comply with procedures for cognitive and other tests in the opinion of the investigator
7. If female, postmenopausal for at least 1 year
8. Non-vasectomized male participants with female partners of childbearing potential must agree to use an effective method of contraception from dosing on Day 1 until 3 months after the last administration of study intervention and agree not to donate sperm until 3 months after the last administration of study intervention.
9. Participant must have a Caregiver who has frequent contact with the participant (defined as at least 8 hours per week spread across 3~4 visits per week) to provide support to the participant to ensure compliance with study requirements. The Caregiver must be willing to consent to participate in this study, to provide a rating of the extent and severity of change of the participant's memory, problem-solving abilities, or activities of daily living from prior abilities.
10. General health status acceptable for participation in the study, and the participant must be able to ingest pills.
11. Participant and his/her Caregiver have provided full written informed consent prior to the performance of any protocol-specified procedure; or if a participant is unable to provide informed consent due to cognitive status, he/she has provided assent, and a legally acceptable representative (LAR) has provided full written informed consent on behalf of the participant.

Exclusion Criteria:

1. Participants with dementia or other memory impairment not due to Alzheimer's disease, including, but not limited to, dementia with Lewy bodies, vascular dementia, Parkinson's disease, Huntington disease, corticobasal degeneration, Creutzfeldt-Jakob disease, progressive supranuclear palsy, frontotemporal degeneration, normal pressure hydrocephalus, hypoxia, severe sleep apnea or other chronic sleep disturbance, or baseline intellectual disability.
2. Participants with a history of stroke, well-documented transient ischemic attack, or pulmonary or cerebral embolism.
3. History of significant psychiatric illness such as schizophrenia or bipolar affective disorder, or history or current major depressive disorder in the past year and any other significant psychiatric illness that in the opinion of the investigator could interfere with participation in the study.
4. Participants with delirium or history of delirium within the 30 days prior to the screening visit.
5. Have suicide ideation according to the investigator's clinical judgment as per the Columbia-Suicide Severity Rating Scale (C-SSRS) at screening or have made a suicide attempt in the 6 months prior to screening.
6. Clinically significant ECG abnormality as judged by the investigator.
7. Confirmed absolute QTcF >450 msec for males or >470 msec for females.
8. Positive results at screening for active viral infections that include human immunodeficiency virus (HIV), hepatitis B surface antigen (HBsAg), and hepatitis C virus (HCV) RNA PCR test.
9. Participants with total bilirubin, alanine transaminase (ALT) or aspartate transaminase (AST) greater than 1.5×upper limit of normal (ULN), or international normalized ratio (INR) greater than 1.7 at screening.
10. Participants with estimated creatinine clearance (CrCL, Cockcroft-Gault equation) ≤30 mL/min at screening.
11. Participants with a history of treatment, and/or current treatment, with anti-Aβ antibodies
12. Changes in any medications that, in the opinion of the investigator, may potentially impair participants' ability to perform cognitive testing or study procedures during the study period (from Screening to EOT), and their dosing should be stable for at least 1 month before Screening (such as benzodiazepines and sedatives/hypnotics). All concomitant medications must be kept as stable as medically possible during the study.
13. Participants who have taken any investigational products, or used investigational medical devices, within 3 months or five half-lives of the therapy (whichever is longer) with respect to first dosing and throughout the study