This is a phase 2B multicenter, randomized, double-blind, placebo-controlled, parallel group dose finding study to evaluate the safety, tolerability and efficacy of PQ912, an inhibitor of the glutaminyl cyclase enzyme, in 250 subjects with mild cognitive impairment and mild dementia due to Alzheimer 's Disease.

In the parallel group dose finding part of the study the first 90 subjects will be randomized 1:1:1 between PQ912 300 mg BID, 600 mg BID, and placebo. When the 90th patient has completed the week 24 treatment visit, the DSMB will decide on the dose of PQ912 to be continued. The decision is based on safety findings only, no efficacy data will be considered. After the DSMB has reached a decision on the dose to be continued, all subjects randomized to receive PQ912 will be reallocated to this dose (1:1). The duration of Subjects participation in the study is either 48, 60, 72, 84 or 96 weeks of treatment (depending on time of randomization). Subjects recruited early into the study will be kept on treatment for 96 weeks or until the regular, scheduled study visit which is closest to the scheduled week 48 visit of the last subject recruited in the study, whichever comes first.

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