The purpose of this double-blind, placebo-controlled, comparative study and open-label extension study is to confirm the efficacy and safety of E2020 in subjects with Down syndrome having regression symptoms and disabled activities of daily living.

This is a multicenter, randomized, double-blind, multiple-dose (two doses), placebo-controlled, parallel-group comparative study followed by an open-label extension study of E2020 in subjects with Down syndrome having regression and disabled ADL. A total of 60 subjects will be randomized to one of three dosing groups (at 1:1:1) to receive 3 mg of E2020, 5 mg of E2020, or placebo for 24 weeks based on their total scores of Body Functionality Checklist, sex, and study site as the allocation factors. This study consists of Pre-randomization Phase (4 weeks), Double-blind Phase (28 weeks), and Extension Phase (24 weeks). The Double-blind Phase includes a 24-week treatment period plus a 4-week transition period.

Inclusion Criteria

At enrollment in Pre-randomization Phase

1. With definitive diagnosis of Down syndrome
2. Have greater than or equal to 3 of the following 4 symptoms among 9 items according to the diagnostic criteria issued by the Intractable Diseases Treatment Research Program 2010 (Research paper on Intractable Diseases Treatment Research Program; Survey on Sudden Regression (21 trisomy) and Preparation of Diagnostic Criteria.) Motor retardation, mutism, social withdrawal (homeboundness), sleep disorder
3. Insufficiently improved with environmental adjustment and psychotherapies including counseling for greater than or equal to 8 weeks before enrollment
4. Have a suspected diagnosis with neuropsychiatric disorder without sufficient effect on a disease even after medical treatment for greater than or equal to 8 weeks before enrollment.
5. A total score of Body Functionality Checklist (51 items) is lesser than or equal to 210 at enrollment
6. Aged 15 to 39 years inclusive
7. Males and females
8. Must have a family member or a caregiver who will provide written informed consent and will be able to spend 3 days a week with the subject (at least 4 hours per day) and will be able to support the subject during the study by providing necessary study information to the subject, assisting treatment compliance, and accompanying the subject to all scheduled visits, supporting study-related tests for the efficacy and safety assessments throughout the study period
9. Males and females of childbearing potential must practice highly effective contraception
10. Able to comply with scheduled study visits according to the investigator's instruction
11. Able to visit for scheduled assessments (except for walking difficulty due to development of regression)
12. Submitted written informed consent for study entry (to obtain from subjects as much as possible; mandatory from their legal guardian)

Exclusion Criteria

At enrollment in Pre-randomization Phase

1. Suspected to have progressive neuropsychiatric disease (e.g., neurodegenerative disorder and progressive tumor) evidenced by MRI or CT within 1 year before the Pre-randomization Phase (if not tested within 1 year before the Pre-randomization Phase, reconfirm during the Pre-randomization Phase).

2. Have a history of significant neurological disorders such as stroke, brain tumor, encephalitis, meningitis, normal pressure hydrocephalus, brain trauma accompanying unconsciousness, and experience of brain surgery causing unsolved deficiency

3. Previously diagnosed with autism

4. With evidence of atlantoaxial subluxation, or underwent surgical operation for atlantoaxial subluxation within 2 years

5. Have seizure symptoms within 2 years or used antiepileptic drug within 1 year before enrollment of Pre-randomization Phase.

6. With severe hearing or visual impairment which may affect regression

7. Have a complication of cardiac disease (angina pectoris, congestive heart failure, bundle branch block, arrythmia) or peripheral vascular disease with unstable condition in 3 months before enrollment of Pre-randomization Phase

8. Have a complication of clinically significant active and unstable diseases in the gastrointestinal, hepatic, renal, respiratory, or cardiovascular system

9. Have a history of clinically significant gastrointestinal ulcer, bronchial asthma, or obstructive pulmonary disease

10. Have a complication of disease affecting absorption, distribution, and metabolism of study drug (e.g., inflammatory colon disease, gastric ulcer, duodenal ulcer, hepatic disorder, serious lactose intolerance)

11. With a present or past history of malignant tumor within 5 years before informed consent (except for basal cell carcinoma, squamous cell carcinoma)

12. With a complication or history of drug or alcohol dependency within recent 10 years

13. Have a known hypersensitivity to ingredient(s) of donepezil hydrochloride or peperidine derivatives

14. Not meet the criteria of prohibited and restricted concomitant medications, or anticipated to deviate from the above criteria of prohibited and restricted concomitant medications/therapies during the study

15. Pregnant or lactating women

16. Have participated in another clinical study and received the study drug within 12 weeks before the enrollment of this study

17. Have used donepezil hydrochloride or have participated in a clinical study of E2020 and received E2020 in the past

18. With a history of a treatment for Alzheimer's type dementia

19. With severe extrapyramidal disorder

    At enrollment in the Double-blind Phase

20. Suspected to have a complication of severe disease considering from the laboratory parameters at enrollment in the Pre-randomization Phase (visit 1) and the safety is not protected in the opinion of the principal investigator or subinvestigator