The number of participants experiencing an adverse event (AE) was assessed. An AE is defined as any unfavorable and unintended medical occurrence, sign, symptom, or disease temporally associated with the use of a pharmaceutical product or protocol-specified procedure, whether or not considered related to the pharmaceutical product or protocol-specified procedure. Any worsening of a preexisting condition, temporally associated with the use of the Sponsor's product, is also an AE.

The number of participants discontinuing the study due to an AE was assessed. An AE is defined as any unfavorable and unintended medical occurrence, sign, symptom, or disease temporally associated with the use of a pharmaceutical product or protocol-specified procedure, whether or not considered related to the pharmaceutical product or protocol-specified procedure. Any worsening of a preexisting condition, temporally associated with the use of the Sponsor's product, is also an AE.

Mean effective dose (ED) of \[11C\]MK-6884 was calculated as a measure of risk associated with exposure of the whole body (WB) to low levels of ionizing radiation. Following \[11C\]MK-6884 injection, WB positron emission tomography (PET) scans were collected to visually identify organs absorbing \[11C\]MK-6884 in significant amounts. Around identified organs, three-dimensional (3D) volumes were drawn to estimate the percentage of injected activity absorbed. These data were converted into time-activity curves (TACs) and retention of radioactivity in these regions was entered into a human biodistribution model to determine ED of \[11C\]MK-6884. ED is expressed in units millisieverts (mSv) / MBq.

Mean organ ED of \[11C\]MK-6884 was calculated as a measure of risk associated with exposure of individual organs to low levels of ionizing radiation. Following \[11C\]MK-6884 injection, WB PET scans were collected to visually identify organs absorbing \[11C\]MK-6884 in significant amounts. Around identified organs, 3D volumes were drawn to estimate the percentage of injected activity absorbed. These data were converted into TACs and retention of radioactivity in these regions was used to calculate organ-specific ED of \[11C\]MK-6884. For sex organs (testes/ovaries), organ EDs were derived for participants of the respective sex. However, organ ED for the uterus was estimable in all participants as the male radiologic phantom was sufficiently hermaphroditic.

Mean BPND of \[11C\]MK-6884 in each brain ROI was assessed. BPND is the ratio at equilibrium of specifically bound \[11C\]MK-6884 to that of non-displaceable \[11C\]MK-6884 in tissue. At time "0", a single IV bolus of \[11\]MK-6884 is administered and PET scanning initiated, yielding brain regional TACs. These TACs are then used to determine peak standard uptake value (SUV) and area under the curve (AUC) in order to quantify brain regional \[11C\]MK-6884 uptake. The target region BPND is estimated using the cerebellum as the reference region with the transient equilibrium tissue ratio (TE-TR) method. Higher values indicate increased specific \[11C\]MK-6884 binding in the brain ROI.

Intra-subject T-RT variability in BPND of \[11C\]MK-6884 in each brain ROI was assessed. For each healthy elderly participant receiving 2 doses of \[11C\]MK-6884 in study Part 2, the BPND calculated during the first dose (BPND-1) was compared to the BPND calculated during the second dose (BPND-2) to determine the percent T-RT variability of the BPND of \[11C\]MK-6884 for each brain ROI. Percent T-RT variability = \[absolute value (BPND-1 - BPND-2) / (average BPND)\] \* 100. A percent T-RT variability = 0, indicates no variability between BPND-1 and BPND-2.

Mean BPND of \[11C\]MK-6884 in each brain ROI was assessed. BPND is the ratio at equilibrium of specifically bound \[11C\]MK-6884 to that of non-displaceable \[11C\]MK-6884 in tissue. At time "0", a single IV bolus of \[11\]MK-6884 is administered and PET scanning initiated, yielding brain regional TACs. These TACs are then used to determine SUV and AUC in order to quantify brain regional \[11C\]MK-6884 uptake. The target region BPND is estimated using the cerebellum as the reference region with the TE-TR method. Higher values indicate increased specific \[11C\]MK-6884 binding in the brain ROI.