The purpose of this research study is to test the study drug, referred to as remternetug, to determine its effectiveness for the study treatment of asymptomatic (at risk) Alzheimer disease in individuals with AD-causing mutations. This study will also investigate the effects of remternetug on biomarkers (measures of the disease including brain scans, blood and spinal fluid tests), examine safety data to identify any potential benefits or risks, and examine how well participants can tolerate remternetug.

Stage 1 will determine if treatment with the study drug prevents or reverses amyloid beta (Aβ) accumulation compared with placebo in participants with dominantly inherited Alzheimer's disease (DIAD).

Stage 2 will evaluate the effect of early anti-amyloid treatment on downstream biomarkers of AD in treated participants compared to external control groups.

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by progressive decline in cognitive function and the ability to perform activities of daily living. The amyloid hypothesis of AD postulates that the accumulation of amyloid beta (Aβ) is an early and necessary event in the pathogenesis of AD. This hypothesis suggests that interventions that slow the accumulation of Aβ plaque in the brain or increase clearance of Aβ may be able to slow the progression of the AD clinical syndrome. AD occurs on a continuum from asymptomatic (preclinical) to mild cognitive impairment (MCI), and then to dementia in mild, moderate, and severe stages. Evidence from both genetic at-risk and age at-risk cohorts, such as in dominantly inherited AD (DIAD) suggests that the pathophysiological process of AD begins well more than a decade before the clinical stage now recognized as AD dementia, and that neurodegeneration is already apparent on MRI by the stage of mild cognitive impairment. Recent clinical trial data suggest that treating AD during the earlier stages could have the greatest potential benefit on the disease by slowing progression

The ability to identify individuals destined to develop Alzheimer's disease (AD) with a high degree of confidence provides a unique opportunity to assess the efficacy of therapies at asymptomatic and very early stages of dementia. Families with known disease-causing mutations are extremely rare and are geographically dispersed throughout the world.

Participants in this study will not yet have developed any clinical symptoms of AD; they will be "asymptomatic" carriers of mutations that cause DIAD and would be expected to perform normally on standard cognitive and functional testing. Further, most mutation carriers will have levels of AD-associated amyloid beta (Aβ) and non-Aβ biomarkers that are the same as non-carriers.

Amyloid beta is a protein that accumulates in the brain of people with AD. Although we do not understand exactly what causes AD, the abnormal accumulation of amyloid beta protein in the brain is thought to play an important role in the symptoms of AD. Recent research studies indicate that amyloid beta may start building up in the brain 15 years or more before the onset of memory loss.

Imaging and fluid biomarkers will be used to demonstrate that the treatment compounds have engaged their therapeutic targets. A set of cognitive measures designed to assess the very earliest and most subtle cognitive changes will be collected. The overall objectives of this study are to evaluate the biomarker effect, safety, and tolerability of investigational study drugs in participants who are known to have an AD-causing mutation.

The primary objective of Stage 1 is to determine if treatment with the study drug prevents or slows the rate of Aβ pathological disease accumulation demonstrated by Aβ PiB positron emission tomography (PET) imaging.

The primary objective of Stage 2 is to evaluate the effect of early anti-amyloid treatment on disease progression by assessing downstream non-Aβ biomarkers of AD (e.g., CSF total tau, NfL, MRI volume) compared to a control group from the DIAN Obs natural history study and the DIAN-TU-001 placebo-treated participants.

Remternetug is a monoclonal antibody. The mechanism of action of remternetug is to target and remove aggregated amyloid plaque, a key pathological hallmark of AD, via microglial-mediated clearance. Remternetug has demonstrated the ability to reduce brain amyloid plaque.

The remternetug arm is part of Master Protocol DIAN-TU-002 (NCT05552157)

Inclusion Criteria:

1. Provide written informed consent, signed, and dated by the participant and study partner, or by the participant's legally authorized representative if applicable, according to local regulations for the ICF and, if applicable, country specific ICFs.

2. Participant is at least 18 years old.

3. People of childbearing potential

   1. Must have a negative serum pregnancy test at screening (V1)
   2. Must agree not to try to become pregnant during the study until 5 half-lives after the last dose of any study drug.
   3. Must agree not to breastfeed from the time of signed ICF until 5 half-lives after the last dose of any study drug.
   4. If partner is not sterilized, must agree to use highly effective contraceptive measures from screening (V1) until 5 half lives after last dose of any study drug

4. Mutation Status:

   1. Participant is a carrier of a mutation in an APP, PSEN1, or PSEN2 gene that is associated with DIAD or does not know their mutation status and there is a mutation in their family pedigree that puts them at a direct risk of inheriting the known mutation;
   2. Participant is -25 to -11 years from predicted age of cognitive symptom onset based on their mutation type or family pedigree Note: If the at-risk parent is deemed a non-carrier through confirmed genetic testing at any time during the study, the participant will be withdrawn.

5. Cognitive status of participant is normal (CDR-SB 0).

6. Fluency in DIAN-TU trial approved language and evidence of adequate premorbid intellectual functioning. Participants must be fluent in languages for which cognitive and clinical measures have been translated and validated for use in the DIAN-TU. Fluency is generally defined as daily or frequent functional use of a language generally from birth or a young age. In cultures where multiple languages are spoken or for participants who are multilingual, determination as to whether a participant's level of fluency in languages for which clinical and cognitive measures are available meets qualification for the study should be made by the site PI.

7. Participant has adequate visual and auditory abilities to perform all aspects of the cognitive and clinical assessments.

8. Participant is receiving stable doses of medication(s) for the treatment of non-excluded medical condition(s) for at least 30 days prior to baseline visit (V2) except for medications taken for episodic conditions (e.g., migraine abortive therapy, antibiotics, and other medications for upper respiratory and gastrointestinal ailments).

9. Participant has a study partner who in the PI's judgment can provide accurate information as to the participant's cognitive and functional abilities, who agrees to provide information at the study visits that require study partner input for scale completion, and who signs the necessary ICF, if applicable.

10. The participant agrees not to donate blood or blood products for transfusion from the time of Screening (V1) for a study drug arm, for the duration of the study, and for 5 half lives after the final dose of study drug.

11. In the opinion of the PI, the participant will be compliant and have a high probability of completing the study.

12. The participant is able and willing to complete all study-related testing, evaluations, and procedures.

Exclusion Criteria:

1. Significant neurologic disease (other than AD) or psychiatric disease that may currently or during the study affect cognition or the participant's ability to complete the study. This would include disorders such as: recent or severe head trauma causing cognitive change, seizure disorder, neurodegenerative disease other than DIAD, hydrocephalus, cerebral/spinal hematoma, inflammatory disease, CNS infection (e.g., encephalitis or meningitis), neoplasm, toxic exposure, metabolic disorder (including hypoxic or hypoglycemic episodes) or endocrine disorder; psychiatric disorders such as schizophrenia, schizoaffective disorder, bipolar disorder or major depression, or any other psychiatric condition/disorder which could significantly interfere with the participant's cooperative participation (e.g., prominent anxiety, agitation or behavioral problems). Disorders that are controlled medically or remote history of these disorders (e.g., history of febrile seizures in childhood) that are not likely to interfere with cognitive function and compliance with study procedures are not exclusionary.

2. At high risk for suicide, e.g., significant suicidal ideation or attempt within last 12 months, current major depression (as defined in Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition [DSM-V]), or increased suicide risk based on screening Columbia Suicide Severity Rating Scale (C-SSRS). Current stable mild depression or current use of antidepressant medications are not exclusionary.

3. History of clinically evident stroke or history of clinically important carotid or vertebrobasilar stenosis, plaque, or other prominent risk factor for stroke or cerebral hemorrhage (including atrial fibrillation and anticoagulation, documented transient ischemic attack [TIA] in the last 12 months) that may be interfering with cognition or is likely to impact with the participant's ability to complete the study. Low dose aspirin (≤ 325 mg daily) is not exclusionary.

4. Alcohol or substance use sufficient to meet DSM-V criteria currently or within the past year.

5. History of or Baseline (V2) visit brain MRI scan indicative of any other significant abnormality, definite microhemorrhages, evidence of a cerebral contusion, encephalomalacia, or aneurysms. Minor or clinically insignificant imaging findings are not exclusionary.

6. Presence of certain implanted medical devices, such as some pacemakers, aneurysm clips, artificial heart valves, ear implants, or foreign metal objects in the eyes, skin, or body which would preclude MRI scan.

7. Cardiovascular complications such as uncontrolled hypertension, history of myocardial infarcts, heart failure, atrial fibrillation, long QT interval on ECG likely to interfere with participation in or analysis of the trial in the opinion of the investigator

8. Hepatic or renal abnormalities that in the opinion of the investigator would interfere with participation in or analysis of the trial.

9. History of Human Immunodeficiency Virus (HIV) infection, history of Hepatitis B infection within the past year, history of Hepatitis C infection which has not been adequately treated, history of spirochete infection (e.g., syphilis, Lyme) of the CNS or history of other infection with high risk for interfering with participation or interpretation of the study in the opinion of the investigator.

10. History of clinically significant multiple or severe drug allergies, significant atopy, or severe post-treatment hypersensitivity reactions (including but not limited to erythema multiforme major, linear IgA dermatosis, toxic epidermal necrolysis, and/or exfoliative dermatitis) or sensitivity to study-drug specific PET imaging agents with a high risk for interfering with participation or interpretation of the study in the opinion of the investigator.

11. Treatment with immunosuppressive medications (e.g., systemic corticosteroids) within 90 days prior to Baseline (V2) visit (topical and nasal corticosteroids and inhaled corticosteroids for asthma are permitted) or chemotherapeutic agents for malignancy within the last 3 years.

12. Current clinically significant abnormalities of thyroid function, or clinically significant deficiency in vitamin B12. Vitamin B12 less than the lower limits of normal with normal methylmalonic acid (MMA)/homocysteine is not deemed clinically significant, therefore not exclusionary.

13. Unstable or poorly controlled diabetes which the investigator believes may interfere with participation in or analysis of the study protocol. Participants may be rescreened after 3 months to allow optimization of diabetic control

14. Morbid obesity with significant comorbidities or that would preclude MRI imaging.

15. Current use of anticoagulants (e.g., warfarin, dabigatran, rivaroxaban, or apixaban). Daily use of low dose (< 325 mg) aspirin is not exclusionary.

16. Have been exposed to a monoclonal antibody targeting Aβ peptide within the past 6 months or 5 half-lives from screening, whichever is longer.

17. Received any other investigational pharmacological treatment within 3 months of Screening or 5 half-lives, whichever is longer.

    Note: Use of approved treatments for AD and other medications may be permitted in this study.

18. Lack of sufficient venous access.

19. Clinically relevant abnormalities in hematology, coagulation, or clinical chemistry.

20. History of cancer that the investigator believes has high risk of recurrence and impacting study participation or analysis.

21. Any other medical condition that could be expected to progress, recur, or change to such an extent that it could bias the assessment of the clinical or mental status of the participant to a significant degree or put the participant at special risk.

22. Currently, or within the last month prior to screening, participated in a clinical study, including a nonpharmacological study, without prior approval.

23. Participants with the "Dutch" APP E693Q mutation.

24. Unable to complete baseline visit (V2) procedures with appropriate cognitive and clinical scores for eligibility

25. A centrally read MRI demonstrating presence of ARIA-E, > 4 cerebral microhemorrhages, any superficial siderosis, any macrohemorrhage, or severe white matter disease at screening.

26. Exposure to lecanemab, donanemab, or other investigational amyloid lowering agents within the past 6 months or five half-lives from screening, whichever is longer.

    Note: Use of approved treatments for AD and other medications may be permitted.

27. Investigator site personnel directly affiliated with this trial and/or their immediate families, defined as a spouse, parent, child, or sibling, whether biological or legally adopted

28. Lilly employees or employees of a third-party organization (TPO) involved in this study that requires exclusion of their employees or have study partners who are Lilly employees or are employees of TPOs involved in this study that require exclusion of their employees

Access to DIAN-TU trial data will follow the DIAN-TU data access policy, which complies with the guidelines established by the Collaboration for Alzheimer's Prevention [CAP REF].