The number of participants experiencing an adverse event (AE) was monitored. An AE is any unfavorable and unintended medical occurrence, symptom, or disease witnessed in a participant, regardless of whether or not a causal relationship with the study treatment can be demonstrated. Further, any worsening of a preexisting condition that is temporally associated with the use of the study treatment is also considered an AE.

The number of participants discontinuing study due to an AE was monitored.

Mean effective dose (ED) of \[18F\]MK-6240 was calculated from whole-body (WB) PET scans of healthy young participants included in Part 1 of study. ED, reported as microsieverts (µSv) / megabecquerel (MBq), is a measure of WB radiation exposure risk that accounts for differences in individual organ exposure and organ susceptibility to ionizing radiation. Following \[18F\]MK-6240 PET tracer administration, organ-specific time-activity curves (TACs) and radioactivity residence times were utilized to calculate exposure risk for individual organs. These values calculated for individual organs were then entered into a human biodistribution model to determine ED of \[18F\]MK-6240.

Mean organ ED of \[18F\]MK-6240 was calculated from WB PET scans of healthy young participants included in Part 1 of study. Organ ED, reported as micrograys (µGy) / MBq, is a measure of organ-specific radiation exposure risk. Following \[18F\]MK-6240 PET tracer administration, organ-specific TACs and radioactivity residence times were utilized to calculate organ ED for specific organs of the body.

As a surrogate of regional \[18F\[MK-6240 tracer distribution volume (VT), mean standardized uptake value ratios (SUVRs), were calculated for specific brain regions of interest (ROIs) in healthy elderly as well as AD/MCI elderly participants in Part 2 of the study. Calculated using calibrated PET scan images from each participant, SUVR is the relative ratio of pixel intensities at a specific brain ROI compared to a reference region (RR; cerebellar cortex, for this study). For an individual participant, the average SUVR for each brain ROI is calculated starting at 60 minutes and ending at 90 minutes following \[18F\]MK-6240 administration to quantify tracer retention; referred to as "SUVR (60-90min)." An SUVR (60-90 min) \< 1 indicates decreased tracer retention at brain ROI relative to RR. An SUVR (60-90 min) = 1 indicates no difference in tracer retention at brain ROI relative to RR. An SUVR (60-90 min) \> 1 indicates increased tracer retention at brain ROI relative to RR.

For each AD/MCI participant receiving 2 doses of MK-6240, the SUVR (60-90 min) during initial dose (SUVR\_1) was compared to the SUVR (60-90 min) during the second dose (SUVR\_2) to determine the percent test-retest (T-RT) variability of the SUVR (60-90 min) for each brain ROI. T-RT variability = (absolute value (SUVR\_1 - SUVR\_2) / average SUVR) \* 100. If T-RT variability = 0, indicates no variability between SUVR\_1 and SUVR\_2.