Placebo matching to JNJ-63733657 will be administered by IV infusion.

ADCS-ADL MCI is a functional measure based on information provided by the study partner (informant) that describes the performance of participants in several ADLs. It assesses 18 instrumental activities of daily living (higher level daily functions) and one basic daily function (dressing). Total score ranges from 0 to 53 with higher scores indicating less impairment.

The RBANS includes 12 subtests that are divided into 5 RBANS indices: 1-Immediate memory (List learning and story memory); 2-Visuospatial/constructional (figure copy and line orientation); 3-Language (picture naming and semantic fluency); 4-Attention (digit span and coding); 5-Delayed memory (list recall, list recognition, story memory, and figure recall) will be reported. Index scores are expressed as an age-adjusted standard score with a normal mean of 100 and an SD of 15. The sum of Index Scores is the sum of the 5 index scores, and the Sum of Index Scores is converted to an RBANS Total Scale Index Score via a mapping table. RBANS Total Scale Index Score is a norm-based t-score, based on a distribution with a mean of 100 and standard deviation (SD) of 15. Higher scores on each sub measure and index indicate better performance.

Change from baseline in RBANS indices will be assessed. The RBANS includes 12 subtests that are divided into 5 RBANS indices: 1-Immediate memory (List learning and story memory); 2-Visuospatial/constructional (figure copy and line orientation); 3-Language (picture naming and semantic fluency); 4-Attention (digit span and coding); 5-Delayed memory (list recall, list recognition, story memory, and figure recall) will be reported.

CDR is a global clinical staging instrument that includes 3 cognitive and 3 functional ratings, including: memory, orientation, judgment and problem solving, involvement in community affairs, home and hobbies, and personal care based on the CDR interview. The CDR is scored 2 ways yielding a global CDR score (CDR GS, derived from an algorithm including categorical scoring of 0, 0.5, 1, 2, and 3), as well as CDR-Sum of Boxes (CDR-SB, the continuous sum of 6 domains, up to a total score of 18, with higher scores representing worse disease state). The Sum of boxes and global score is calculated from the overall CDR.

The NPI is a measure of psychobehavioral disturbances, assessing the frequency and severity of disturbances in 12 domains. Frequency for each domain is rated on a 4 point scale (from 1=rarely to 4=very often) and severity on a 3 point scale (from 1=mild to 3=severe), with the score for each domain being the product of the frequency and severity scores, such that each domain is scored from 1 to 12. The NPI total score is the sum of the 12 domain scores, ranging from 0 (best) to 144 (worst).

The CDR is a subjectively rated outcome measure that serves as a global clinical staging instrument that includes 3 cognitive and 3 functional ratings, including: 1) memory, 2) orientation, 3) judgment and problem solving, 4) involvement in community affairs, 5) home and hobbies, and 6) personal care based on the CDR interview. The CDR is scored 2 ways yielding a global CDR score (CDR GS, derived from an algorithm developed by the Knight Alzheimer Disease Research Center at Washington University School of Medicine in St. Louis, Missouri, US, and including categorical scoring of 0= cognitively unimpaired, 0.5= mild cognitive impairment, 1= mild dementia, 2= moderate dementia, and 3= severe dementia), as well as CDR-Sum of Boxes (CDR-SB, the continuous sum of 6 domains, up to a total score of 18, with higher scores representing worse disease state).

Change from baseline in brain tau burden, as measured by tau positron emission tomography (PET) will be assessed.

Change from baseline in CSF concentrations of total, free, and bound p217+tau (phosphorylated tau) fragments will be assessed.

CSF concentrations of JNJ-63733657 will be assessed.

Serum concentrations of JNJ-63733657 will be assessed.

Anti-drug antibody to JNJ-63733657 will be assessed.

An AE is any untoward medical occurrence in a clinical study participant administered a medicinal (investigational or non investigational) product. An AE does not necessarily have a causal relationship with the intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal finding), symptom, or disease temporally associated with the use of a medicinal product, whether or not related to that medicinal product.

Number of participants with a treatment-emergent AESI will be reported.

An SAE is any untoward medical occurrence that at any dose results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, is a suspected transmission of any infectious agent via a medicinal product.

Number of participants with ECG abnormalities will be reported.

Number of participants with clinical laboratory (hematology, clinical chemistry, and urinalysis) abnormalities will be assessed.

Number of participants with physical (body weight, height) and neurological (evaluation of mental status, cranial nerves, motor ability [including strength, tone, and involuntary movements], coordination [including finger-to-nose, gait, and postural reflexes], and sensation [including proprioception, cold, light touch, and deep tendon reflexes]) examination abnormalities will be reported.

Percentage of participants with vital sign abnormalities (temperature, pulse rate, systolic blood pressure [BP], diastolic BP) will be reported.

Changes from baseline in brain MRI safety findings (brain tumors, aneurysm or atrioventricular malformations, territorial stroke (excluding smaller watershed strokes), recent hemorrhage (parenchymal or subdural), or obstructive hydrocephalus) will be assessed.

C-SSRS is semi structured clinician-administered questionnaire designed to solicit the occurrence, severity, and frequency of suicide-related ideation and behaviors . Total score ranges from 1 to 10, a score of 0 will be assigned (0="no event that can be assessed on the basis of C-SSRS"). Higher scores indicate greater severity. The maximum score assigned for each participant will also be summarized into one of three broad categories: no suicidal ideation or behavior (0), suicidal ideation (1 to 5), suicidal behavior (6 to 10).