A Study of MK-2214 in Adults With Mild Cognitive Impairment or Mild-to-Moderate Alzheimer's Disease (MK-2214-002)

Prior treatment

• Has had a major surgery and/or donated or lost 1 unit of blood (approximately 500 mL) within 4 weeks prior to the prestudy visit
• Has a contraindication to lumbar dural puncture, such as coagulopathy, concomitant anticoagulation beyond low dose aspirin, thrombocytopenia, or other factors that could preclude safe lumbar puncture
• Currently receiving or has received aducanumab or another anti-amyloid therapy within the last 6 months
• Has a history of receiving biological therapy within 3 months or 5 half-lives (whichever is longer) or any human immunoglobulin preparation within the last year
• Has received any non-live vaccine starting from 14 days prior to first study intervention or is scheduled to receive any non-live vaccine through 14 days following the final dose of study intervention. Exception: coronavirus disease of 2019 (COVID-19) and influenza vaccines may be administered
• Is receiving systemic immunosuppression, including corticosteroids exceeding physiologic replacement doses

Other illness

• History of unstable or poorly controlled endocrine, gastrointestinal (GI), cardiovascular, hematological, hepatic, renal, respiratory, or genitourinary abnormalities or diseases
• History of clinically significant active neurological disease (except for AD or MCI for participants in Part 1)
• History of clinically significant active autoimmune disease requiring ongoing systemic immunosuppressant therapy
• History of cancer (malignancy)
• History of significant multiple and/or severe allergies (eg, food, drug, latex allergy), or has had an anaphylactic reaction or significant intolerability to prescription or nonprescription drugs or food
• Positive test(s) for Hepatitis B Surface Antigen (HBsAg), hepatitis C antibodies or human immunodeficiency virus (HIV)

Other

• The main exclusion criteria include but are not limited to the following:
• Based on clinical interview and Columbia-Suicide Severity Rating Scale (C-SSRS), has reported suicidal ideation with intent, with or without a plan or method

• The main inclusion criteria include but are not limited to the following:
• Participant is in overall good health based on medical history and laboratory safety tests
• Body mass index between 18.5 and 35 kg/m\^2
  • Part 1 (Mild Cognitive Impairment \[MCI\] and Mild-to-Moderate Alzheimer's Disease \[AD\]) Only:
• History of cognitive and functional decline with gradual onset and slow progression for at least one year before Screening
• Mini-Mental State Examination (MMSE) score \>12 at the prestudy visit
• Modified Hachinski Ischemic Score (MHIS) \<4 at the prestudy visit