[18F]MK-6240 Positron Emission Tomography (PET) Tracer First-in-Human Validation Study (MK-6240-001)

Inclusion Criteria: Part 1 and Part 2: * Male, or non-pregnant and non-breast feeding female; in addition: * Male participant who is sexually active with females of childbearing potential must be willing to use a condom from the first dose of study drug until 3 months post the last dose of study drug * Female participant with reproductive potential must have serum β-human chorionic gonadotropin (β-hCG) test result consistent with non-pregnant state at screening and agree to use two acceptable methods of birth control beginning at screening visit, during study and until 2 weeks after the last dose of study drug * Post-menopausal female participant has been without menses for at least 1 year and has a documented follicle stimulating hormone (FSH) level in the postmenopausal range at screening * Surgically sterile female participant may enroll in study if procedure (hysterectomy, oophorectomy, or tubal ligation) is documented/confirmed by medical records or protocol-defined examination/tests * Nonsmoker and/or has not used nicotine or nicotine-containing products for at least approximately 3 months Part 1 only: * 18 to 55 years of age * Body Mass Index (BMI) between 18-32 kg/m\^2 * In good health based on medical history, physical examination, vital sign measurements, electrocardiogram (ECG) and laboratory safety tests Part 2 only: * 56 to 85 years of age * Body weight \<136 kg * In stable medical condition based on medical history, physical examination, vital sign measurements and ECG * In good health based on laboratory safety tests * For some participants, willing to allow placement of an arterial catheter in the radial artery * For AD participants: * Mini-Mental State Examination (MMSE) score ≤28 * Meets National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria for probable AD * Meets Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR) criteria for AD * Modified Hachinski score ≤4 * Screening magnetic resonance imaging (MRI) scan consistent with a diagnosis of AD * Able to read at a 6th grade level or equivalent and has a history of academic achievement and/or employment sufficient to exclude mental retardation * Has a reliable informant/caregiver who is able to accompany the participant to all clinic visits and provide information to study investigator/staff via telephone contact * If taking symptomatic treatment for AD, be on a stable dose for at least 4 weeks prior to study * For amnestic MCI participants: * MMSE score ≥26 * A history of subjective memory decline before screening * Objective impairment in verbal memory based on investigator's clinical assessment * General cognitive function and activities of daily living sufficiently intact, so as not to meet criteria for mild AD dementia * Modified Hachinski score ≤4 * MRI scan obtained at the screening visit is either normal or consistent with a diagnosis of AD * Able to read at a 6th grade level or equivalent and has a history of academic achievement and/or employment sufficient to exclude mental retardation * For non-AD/non-MCI healthy elderly participants: * MMSE score ≥27 * No history of subjective memory or other cognitive complaints * No objective evidence of memory or cognitive impairment Exclusion Criteria: Part 1 and Part 2: * Subject has participated in another investigational trial within 4 weeks of screening * Subject has participated in a PET research study or other study involving administration of a radioactive substance or ionizing radiation within 12 months prior to screening or has undergone an extensive radiological examination within this period * History within 2 years prior to screening, or current evidence of a psychotic disorder or a major depressive disorder * History of alcoholism or drug dependency/abuse within the last 2 years before screening * History of cancer * History of significant multiple and/or severe allergies or has had an anaphylactic reaction or significant intolerability to prescription or non-prescription drugs or food * Has positive test result for hepatitis B surface antigen, hepatitis C antibodies or human immunodeficiency virus (HIV) * Participant has had major surgery or donated or lost 1 unit of blood (approximately 500 mL) within 4 weeks prior to screening * QTc interval ≥470 msec (for males) or ≥480 msec (for females) * Participant consumes \>3 servings of alcohol a day * Participant consumes \>6 caffeine servings a day * Participant is currently a regular or recreational user of cannabis, any illicit drugs or has a history of drug (including alcohol) abuse within approximately 3 months * Suffers from claustrophobia or an inability to tolerate confinement in small places and would be unable to undergo PET or (for Part 2 only) MRI scanning Part 1 Only: * Evidence of a clinically relevant neurological disorder at screening * History of clinically significant endocrine, gastrointestinal, cardiovascular, hematological, hepatic, immunological, renal, respiratory, genitourinary or major neurological abnormality or disease * Participant is unable to refrain from or anticipates the use of any medication, including prescription and non-prescription drugs or herbal remedies, beginning approximately 2 weeks prior to administration of the initial dose of study drug and throughout the study Part 2 Only: * Evidence of a clinically relevant neurological disorder other than AD at screening * History or current evidence of clinically significant endocrine, gastrointestinal, cardiovascular, hematological, hepatic, immunological, renal, respiratory, genitourinary or major neurological abnormality or disease, which is not adequately controlled through a stable medication regimen * Participant has or is suspected to have implanted or embedded metal objects, or fragments in the head or body that would present a risk during the MRI scanning procedure * For participants undergoing arterial catheter placement only: * Allergy to lidocaine which may be locally injected as an anesthetic * Currently uses aspirin or aspirin-containing medications at doses exceeding 100 mg daily, or nonsteroidal anti-inflammatory drugs (NSAIDs), which cannot be discontinued 2 weeks prior to dosing and throughout the course of the study