Phase 1/2 Trial of AEF0217 in Participants With Down Syndrome

Inclusion Criteria: * Male or female. * Age ≥18 to ≤35 years. * Body mass index (BMI) ≥18.5 to ≤32 kg/m2. * Clinical diagnosis of Down syndrome (full trisomy 21 and translocations) documented by chromosomal analysis (karyotyping). * Understands and accepts the trial procedures. * Independently mobile and have sufficient vision and hearing to participate in the trial evaluations. * Clinical Evaluation of Language Fundamentals Preschool-2 (CELF Preschool-2) test score ≥7. * IQ \>35-70 measured with KBIT. Individuals with IQ from \>35 to \<40 must have adequate adaptive functioning according to the judgment of the principal investigator. * Must have a parent or other reliable caregiver who agrees to accompany the participant to all clinic visits and be available for a telephone visit, provide information about the participant as required by the protocol, and ensure compliance with the medication schedule and protocol requirements. * Vital signs, electrocardiogram (ECG), and safety laboratory parameters must be within normal ranges or without clinically relevant abnormalities except for: 1. Stable type 1 or type 2 diabetes, i.e., HbA1c level \<7%, provided participants are monitored regularly prior to and during the trial to ensure adequate glucose control. 2. Hypothyroidism provided participants are euthyroid and stable on treatment for at least 6 weeks prior to screening. * Assent by the participant and consent by the legally authorized representative(s) on behalf of the participant or Consent by the participant in situations where consent rather than assent can be provided by the participant. * Informed consent by the participant's caregiver to take on the obligations of the caregiver in this trial. Exclusion Criteria: * Pregnant or nursing female. * Mosaic Down syndrome. * Active or clinically relevant conditions that could, in the investigator's judgment, affect absorption, distribution, or metabolism of the trial intervention (e.g., inflammatory bowel disease, gastric or duodenal ulcers). * Clinically relevant obstructive pulmonary disease or asthma that is untreated or not controlled by treatment within 6 weeks of screening or being treated with oral steroids. * Severe obstructive sleep apnea. * Recent (≤1 year) or ongoing hematologic or oncologic disorders (mild anemia is allowed). * Personal history of infantile spasms/convulsions/epilepsy, severe head trauma, or CNS infections (e.g., meningitis), except for isolated events of febrile seizures more than 8 years ago. * Clinically relevant unstable gastrointestinal, renal, hepatic, endocrine, or cardiovascular system disease. * Current Diagnostic and Statistical Manual of Mental Disorders (DSM-5) diagnosis including autism spectrum disorder or any primary psychiatric diagnosis. Diagnoses that are secondary, such as attention deficit hyperactivity disorder, depression, and conduct disorder are allowed if they are considered not to interfere with the trial conduct and are stable during the 3 months preceding randomization. Medical or behavioral treatments used for stabilization must be on stable regimen and dosing for the last 3 months and the type of treatment must be allowed according to the list of allowed and prohibited medication . * Treatment with medication known to induce CYP3A4/5 P450 isozymes. * Intake of vitamin supplements, catechins, or products containing epigallocatechin gallate (EGCG) (e.g., TEAVIGO, Mega Green Tea Capsules Life Extension, or Font-UP Grand Fontaine Laboratories) currently or during the 2 months prior to the baseline assessement. * Symptoms of early dementia as assessed by the National Task Group-Early Detection Screen for Dementia (NTG-EDSD). * Disclosure of drug or alcohol abuse during medical interview/anamnesis at screening and/or positive urine test for alcohol or drugs of abuse at screening or/and baseline. * Epileptiform abnormalities (excluding isolated sharp waves and beyond those expected for age) in the screening EEG performed over 10 minutes with concurrent video recording and evaluated by an expert. * Participants with a history of suicide attempt or deliberate self-harm due to suicidal ideation. Suicidal ideation (even in the absence of suicide attempt or deliberate self-harm) during the 12 months prior to screening. Assessed by 3 specific questions on suicidal ideation, suicidal behavior, and any self-injurious behavior. * Known hypersensitivity to any drug. * Participants with clinically significant illness from 2 weeks prior to screening until Day -1. * Covid-19 positive test and/or symptoms within the last 10 days prior to Day -1, or according to the requirements of the hospital. * History of or current life-threatening disease. * Any other clinically relevant concomitant disease or condition or finding at screening that in the investigator's judgment could jeopardize the participant's safety or interfere with, or the treatment thereof might interfere with, the conduct of the trial and related procedures and/or might bias interpretation of the trial results.