This is a safety and tolerability study investigating the effect of HPP854 in subjects with mild cognitive impairment or a diagnosis of mild Alzheimer's disease. The study will assess the pharmacokinetic and pharmacodynamic relationships of HPP854 in plasma, pharmacodynamic relationship in cerebral spinal fluid and plasma concentration profiles for Amyloid-Beta.

Inclusion Criteria:

• Medical history for at least 6 months prior to screening of mild cognitive impairment with a Mini Mental State Exam (MMSE) score between 20 and 26 or diagnosis of mild Alzheimer's disease;
• Must be able to swallow dose of study medication;
• Body Mass Index (BMI) between 18.0 and 35.0; and
• Subject and Project Partner are willing to participate and agree to comply with all study requirements.

Exclusion Criteria:

• Blood pressure > 160 mmHg (systolic) and > 90 mmHg (diastolic);
• Received HPP854 in a previous trial;
• Participation in another clinical trial involving any marketed or investigational drug within 30 days of screening and until after the final study visit.
• Current evidence or history within the last 3 years of a neurological or psychiatric illness that could contribute to dementia including but not limited to: anxiety, epilepsy, focal brain lesion, Parkinson's disease, seizure disorder, or head injury with loss of consciousness and alcohol or substance abuse;
• Clinically significant cardiovascular, cerebrovascular disease, diabetic condition, hematologic, renal hepatic, pulmonary, endocrine, neurological, coagulation disorder;
• History or presence of cancer except for non melanoma skin cancer. Subjects with a history of prostate cancer stable for > 3 yrs with no active treatment for > 3 years prior to Screening may be considered for eligibility;
• Use of the following medications/therapy from 14 days before dosing until after the Final Visit: anti-cholinergic, tricyclic antidepressants, lithium, typical or atypical antipsychotic medications, anticonvulsant medications, immunosuppressive agents, oral corticosteroids, and radiotherapy;
• HbA1C > 6.5 % at the Screening Visit;
• Vitamin B12 level < 211 pg/mL at the Screening Visit;
• Any suicidal risk determined by C-SSRS administered by a study staff member appropriately certified for administration of the scale (Baseline/Screening, Phase 1 Study Version) at Screening Visit, Day -6 or Day -1;
• A score of 15 or more on the modified Geriatric Depression Scale (GDS); and
• A score of 5 or more on the Hashinski Ischemic Scale (Rosen modification.
• Contraindications of MRI including: Metallic fragments, clips or devices in the brain, eye, spinal canal, etc; Cardiac pacemakers, insulin pumps, neurostimulators, cochlear implants, etc.
• Contraindications for blood or CSF sampling, including: Bleeding disorder or taking anticoagulants/antiplatelet, chronic active infection.