A Study to Evaluate Safety and Tolerability of Different Doses and Efficacy of PQ912 in Subjects With MCI and Mild AD

Main Inclusion Criteria: * Positive CSF AD biomarker signature according to the AA-NIA criteria * Clinical syndrome of MCI or mild dementia according to the AA-NIA Research Framework * A cognitive impairment in the WAIS IV Coding Test of at least 0.5 standard deviation below the normative data * Adequate visual and auditory abilities to perform the cognitive and functional assessments in the opinion of the investigator * Meeting the completion and performance criteria for the CogState NTB * Outpatient with study partner capable of accompanying the subject on all applicable clinic visits Main Exclusion Criteria: * Significant neurological or psychiatric disorders, other than AD, that may affect cognition. * Atypical clinical presentations of MCI due to AD or mild dementia due to AD, such as the visual variant of AD (including posterior cortical atrophy), frontal variant or the language variant (including logopenic aphasia). * Moderate and severe dementia with a Mini-Mental State Examination score (MMSE) below 20. * Current presence of a clinically important major psychiatric disorder (e.g. major depressive disorder) as defined by DSM-5 criteria, or symptom(s) (e.g. hallucinations) that could affect the subject's ability to complete the study. * History of clinically evident stroke. * History of seizures within the last two years prior to the screening visit. * Myocardial infarction within the last six months prior to screening. * History of uncontrolled hypertension (in the opinion of the investigator) within six months prior to screening. * Contraindication to lumbar puncture and MRI