This is a Phase 1b study to evaluate different doses of the drug and see whether a drug is safe and how it behaves in the body.

THN391 has already been assessed in healthy people without Alzheimer's disease. This is the first study of THN391 in patients with Early Alzheimer's disease. Later studies will evaluate THN391 to see if it is effective for the treatment of Alzheimer's disease.

In this study, THN391 will be compared with a placebo (a look-alike substance that contains no drug). The study duration depends on the number of dose administrations: for the 3 doses administration, the duration is approx. 8 months, in which the participants will visit the clinic approximately 13 times and have 2 telephone calls with the site. For the 6 doses administration group (starting in Jan 2026), the duration is approx. 11 months, with 19 clinic visits and 5 telephone calls with the site.

Patients who fulfill all criteria to participate in the study, will receive 3 or 6 times a monthly dose of THN391 or placebo in the clinic.

Assessments that will be done at several timepoints during the study will be blood collection, physical examinations and neurological examinations, 5-7x an MRI-scan of the head, 3x a spinal tap and some testing of the memory and thinking skills.

This is a Phase 1b, randomized, double-blind, multi-center, placebo-controlled, multiple ascending dose trial in male and female participants, aged 60 to 85 years with Early Alzheimer's disease and cSVD.

For the 3 dose administration group, the study duration is approximately 8 months: first screening to assess eligibility, then 2 months' treatment period (3 monthly doses), followed by a 6 month follow-up period. For the 6 dose administration group (starting in January 2026), the study duration is approximately 11 months: first screening to assess eligibility, then 5 months' treatment period (6 monthly doses), followed by a 6 month follow-up period.

The trial will investigate THN391 in at least 3 dose cohorts, Depending on preliminary, blinded results of the first two cohorts, the sample sizes of the following dose cohort may be increased and/or additional dose cohorts may be added.

Eligible participants will be randomized to receive either THN391 or placebo.

Three or six dose administrations will be provided monthly. Participants will undergo clinical and laboratory-based safety-related assessments, as well as Pharmacodynamics (PD), immunogenicity, and blood Pharmacokinetic (PK) collections at different time points.

Assessments will include 5-7 brain MRIs (Magnetic Resonance Imaging), 3 spinal taps, electrocardiograms (ECGs), vital signs, physical and neurological examinations, adverse event recordings, monitoring of mental health, and tests to determine the severity of Alzheimer's disease.

Inclusion Criteria:

• Be willing and able to understand the study procedures and the risks involved and provide written informed consent before the first study-related activity
• 60 to 85 years of age (inclusive at the time of informed consent).
• Diagnosis of Early Alzheimer's Disease (AD)
• Diagnosis of cerebral Small Vessel Disease (cSVD), and having at least one of the following vascular risk factors: hypertension, Type 2 diabetes mellitus, or hyperlipidemia

Exclusion criteria:

• Diagnosis of moderate or severe dementia
• Any other medical condition except for early AD (e.g. any clinically significant neurological, psychiatric or large vessel disease) that could affect interpretation of study assessments
• Use of anticoagulant, except for either clopidogrel or low dose aspirin, unless taken simultaneously