Primary objective is to evaluate the neuroprotective effect of T-817MA on Tau protein phosphorylated at threonine 181 (p-tau 181) in cerebrospinal fluid (CSF) compared with placebo in patients with a diagnosis of MCI due to AD or mild AD.

Secondary objectives are:

1. To evaluate in patients on T-817MA and placebo:

   • cognitive function measured by the Clinical Dementia Rating Scale Sum of Boxes (CDR-sb) and working memory and attention domain as measured by the Cognitive Functional Composite (CFC).
   • AD-related biomarkers in CSF and plasma
   • imaging analysis using volumetric magnetic resonance imaging (vMRI)
   • alpha/theta ratio of the electroencephalogram (EEG)

2. To evaluate the safety of T-817MA by clinical laboratory tests and adverse events (AEs).

3. To evaluate the pharmacokinetics of T-817MA

Key Inclusion Criteria:

• Female of non-childbearing potential or male, ages 50 to 80 years (inclusive)
• MCI due to AD or mild AD per NIA-AA diagnostic criteria (Jack et al., 2018), with MMSE 24 to 30 (inclusive)
• CSF results at Screening consistent with the presence of Aß and p-tau181 abnormality (≤1000 pg/ml for Aß, ≥19 pg/ml for p-tau181).
• Taking stable dose of AChE Inhibitor (donepezil, galantamine or rivastigmine) at least for 3 months prior to randomization, or not taking any AChE Inhibitors.

Key Exclusion Criteria:

• MRI of the brain within the previous 2 years that showed pathology that would be inconsistent with a diagnosis of AD
• Taking memantine
• Any contraindications to lumbar puncture
• Any contraindications to MRI

If male, patients must:

1. agree he will not donate sperm during the study and until 104 days after the last dose, AND
2. be required to use highly effective methods of contraception during the study and until 104 days after the last dose

If female, patients must:

1. be post-menopausal (i.e. no menses for 12 months without an alternative medical cause) OR
2. be permanently sterilized with methods including hysterectomy, bilateral salpingectomy and bilateral oophorectomy

Details for sharing data have not been decided yet.