Alzheimer Disease Proof of Concept Study With BI 409306 Versus Placebo

Inclusion criteria: * Male and female patients with an age of at least 55 years * Body weight not lower than 50 kgs * Patients with a confirmed diagnosis of prodromal Alzheimer's Dementia (AD) on neuropsychological testing defined as: Mini-Mental State Examination (MMSE) score: greater or equal 24 and a global Clinical Dementia Rating (CDR)-score of 0 or 0.5 and Free and Cued Selective Recall Reminding Test (FCSRT) score: free recall test: lower or equal 20 (out of 48) and total recall test: lower or equal 42 (out of 48) Patients who do not reach the required score in FCSRT will additionally perform the Wechsler Memory Visual Paired Associates test. If the Wechsler Memory Visual Paired Associates test shows a cognitive deficit worse than 1 standard deviation to the mean (compared to the reference values of age and educational norms for inclusion), then the patients can be considered to be eligible for the study. * Confirmation of abnormal markers of AD pathology either via a), or alternatively b) mentioned below: 1. Presence in cerebrospinal fluid of (samples taken within past 4 months may be eligible,: low Aß1-42 concentrations (\< 640 pg/mL) and increased total tau concentrations (\> 375 pg/ml), or / and low Aß1-42 concentrations (\< 640 pg/mL) and increased phospho-tau concentrations (\> 52 pg/mL in cerebrospinal fluid), or 2. Abnormal amyloid deposition in a cerebral Positron Emission Tomography (PET) scan. Scans performed in the past according to the recommendation in the protocol are acceptable * Patients who have not received prescribed drugs for treatment of AD (including acetyl cholinesterase inhibitors (donepezil, galantamine, rivastigmine, tacrine, phenserine) and Memantine within three months prior to screening * Patients must have at least 6 years of formal education and fluency in the test language as verbally confirmed by the patient and documented by the study investigator. * Patients must have given written informed consent in accordance with Good Clinical Practice (GCP) and local legislation prior to any study procedures. All patients must be able to give informed consent personally and have capacity for such consent. An informed consent given by a legal representative will not be accepted. * Patients must have a reliable study partner (per investigator judgement, for instance a family member, partner, guardian etc.) Exclusion criteria: * Mild cognitive impairment with any etiology other than prodromal AD (for example: neurosyphilis, craniocerebral trauma, small vessel disease) based on clinical data and/or current laboratory findings and/or a pre-existing MRI or CT of the brain (CCT). If previous cranial imaging is not available or older than 12 months prior to screening then a CCT or MRI needs to be performed at screening * Substantial concomitant cerebrovascular disease (defined by a history of a stroke / intracranial haemorrhagia) temporally related to the onset of worsening of cognitive impairment per investigator judgement * Medical history of cancer (except for basal cell carcinoma) and/or treatment for cancer within the last 5 years * Medical history or diagnosis of any of symptomatic and unstable/uncontrolled conditions per investigator judgement * Severe renal impairment defined with a glomerular filtration rate (GFR) \< 30ml/min/1.73m2 in the screening central lab report * Any other psychiatric disorders such as schizophrenia, or mental retardation * Any suicidal actions in the past 2 years (per investigator judgement i.e. actual attempt, interrupted attempt, aborted attempt, or preparatory acts or behaviour) * Any suicidal ideation of type 4 or 5 in the Columbia Suicide Severity Rating Scale (C-SSRS) in the past 3 months (i.e. active suicidal thought with intent but without specific plan, or active suicidal thought with plan and intent) * Previous participation in investigational drug studies of mild cognitive impairment within three months prior to screening. Having received active treatment in any other study targeting disease modification like Aß immunization and tau therapies. Previous participation in studies with non-prescription medications, vitamins or other nutritional formulations is allowed. * Significant history of drug dependence or abuse (including alcohol, as defined in Diagnostic and Statistical Manual of Mental Disorders \[DSM-V\] or in the opinion of the investigator) within the last two years, or a positive urine drug screen for cocaine, heroin, or marijuana. * Known history of HIV infection * Any planned surgeries requiring general anaesthesia, or hospitalisation for more than 1 day during the study period * Pre-menopausal women (last menstruation \<= 1 year prior to informed consent) who are nursing or pregnant or are of child-bearing potential and are not practicing an acceptable method of birth control * For male patients: Men who are able to father a child, unwilling to be abstinent or to use an adequate form of effective contraception for the duration of study participation and for at least 28 days after treatment has ended. * Use of any investigational drug or procedure for other indications within 3 months or 6 half-lives (whichever is longer) prior to randomization. * Intake of the following medications within 3 months prior to randomization and intended to be initiated during the duration of the trial: 1. tricyclic antidepressants, 2. antidepressants that are monoamine oxidase inhibitors, 3. neuroleptics with moderate or greater anticholinergic potency (e.g. chlorpromazine, fluphenazine, loxapine, perphenazine, thioridazine), 4. anticholinergic medications The following drugs may be given as needed if the total daily dose was stable 8 weeks prior to randomisation and is expected to be for the duration of the trial: 1. neuroleptics listed in the protocol 2. benzodiazepines and sedatives listed in the protocol * Clinically significant uncompensated hearing loss in the judgment of the investigator. Use of hearing aids is allowed. * Known hypersensitivity to the drug product excipients