Background: The 3’-F analog of Pittsburgh Compound-B (3’-F-PiB; flutemetamol) is a new promising [F-18]-
labeled amyloid binding agent for in vivo imaging of β-amyloid (Ab) deposits in brains of subjects with Alzheimer’s
disease (AD). Preliminary imaging data demonstrated that compared to [C-11]PiB, [F-18]flutemetamol has similar
uptake and retention characteristics although somewhat higher, non-specific retention in white matter (Mathis et
al. J Nucl Med 2007; 48: (Supplement 2): 56P). Whether the two tracers have comparable ability to identify cortical
Aβ deposits is unknown.
Methods: Consecutive 12 μm paraffin sections of frontal and temporal cortical autopsy tissues from two AD
patients were processed for histofluorescence analyses using highly fluorescent derivatives of flutemetamol
(6-CN-flutemetamol) and PiB (6-CN-PiB), as well as for Aβ immunohistochemistry (clone 4G8 mab) and
Bielschowsky silver staining. The pattern of plaque labeling and the extent of overlap with Aβ-immunoreactive
deposits and Bielschowsky positive neuritic plaques were compared between 6-CN-flutemetamol and 6-CN-PiB.
In vitro binding characteristics of 6-CN-PiB and 6-CN-flutemetamol, compared to PiB and flutemetamol, were
determined in homogenates of frozen AD frontal cortex.
Results: 6-CN-flutemetamol and 6-CN-PiB labeled parenchymal amyloid plaques and cerebral vascular amyloid
deposits in cortical regions, while no labeling of neurofibrillary tangles was detectable. Aβ-immunoreactive
plaques were labeled with both compounds; a more intense fluorescence signal was detected in compact/cored
Aβ plaques, while diffuse Aβ plaques were less intensely labeled. Bielschowsky positive neuritic plaques were
prominently labeled with 6-CN-flutemetamol and 6-CN-PiB. The Ki values of the tested compounds were 8.6 nM
(6-CN-PiB) and 9.3 nM (6-CN-flutemetamol), versus 4.3 nM (PiB) and 5.9 nM (flutemetamol).
Conclusions: Our data demonstrate that 6-CN-flutemetamol and 6-CN-PiB have comparable patterns of binding
to Aβ plaque deposits in postmortem neocortical tissue sections and in tissue homogenates from AD brains. This
suggests that in vivo PET retention of flutemetamol in AD brains reflects neocortical Aβ plaque load in a manner
similar to PiB binding.