Assessing Synaptic Density in Alzheimer Disease With Synaptic Vesicle Glycoprotein 2A Positron Emission Tomographic Imaging

<h3>Importance</h3> Synaptic loss is well established as the major structural correlate of cognitive impairment in Alzheimer disease (AD). The ability to measure synaptic density in vivo could accelerate the development of disease-modifying treatments for AD. Synaptic vesicle glycoprotein 2A is an essential vesicle membrane protein expressed in virtually all synapses and could serve as a suitable target for synaptic density. <h3>Objective</h3> To compare hippocampal synaptic vesicle glycoprotein 2A (SV2A) binding in participants with AD and cognitively normal participants using positron emission tomographic (PET) imaging. <h3>Design, Setting, and Participants</h3> This cross-sectional study recruited 10 participants with AD and 11 participants who were cognitively normal between November 2015 and June 2017. We hypothesized a reduction in hippocampal SV2A binding in AD, based on the early degeneration of entorhinal cortical cell projections to the hippocampus (via the perforant path) and hippocampal SV2A reductions that had been observed in postmortem studies. Participants underwent high-resolution PET scanning with (<i>(R)</i>-1-((3-(11C-methyl-11C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one), a compound more commonly known as¹¹C-UCB-J, for SV2A. They also underwent high-resolution PET scanning with carbon 11–labeled Pittsburgh Compound B (¹¹C-PiB) for β-amyloid, magnetic resonance imaging, and cognitive and neurologic evaluation. <h3>Main Outcomes and Measures</h3> Outcomes were¹¹C-UCB-J–specific binding (binding potential [<i>BP</i>_ND]) via PET imaging in brain regions of interest in participants with AD and participants who were cognitively normal. <h3>Results</h3> Ten participants with AD (5 male and 5 female; mean [SD] age, 72.7 [6.3] years; 10 [100%] β-amyloid positive) were compared with 11 participants who were cognitively normal (5 male and 6 female; mean [SD] age, 72.9 [8.7] years; 11 [100%] β-amyloid negative). Participants with AD spanned the disease stages from amnestic mild cognitive impairment (n = 5) to mild dementia (n = 5). Participants with AD had significant reduction in hippocampal SV2A specific binding (41%) compared with cognitively normal participants, as assessed by¹¹C-UCB-J–PET<i>BP</i>_ND(cognitively normal participants: mean [SD]<i>BP</i>_ND, 1.47 [0.37]; participants with AD: 0.87 [0.50];<i>P</i> = .005). These reductions remained significant after correction for atrophy (ie, partial volume correction; participants who were cognitively normal: mean [SD], 2.71 [0.46]; participants with AD: 2.15 [0.55];<i>P</i> = .02). Hippocampal SV2A-specific binding<i>BP</i>_NDwas correlated with a composite episodic memory score in the overall sample (<i>R</i> = 0.56;<i>P</i> = .01). <h3>Conclusions and Relevance</h3> To our knowledge, this is the first study to investigate synaptic density in vivo in AD using¹¹C-UCB-J–PET imaging. This approach may provide a direct measure of synaptic density, and it therefore holds promise as an in vivo biomarker for AD and as an outcome measure for trials of disease-modifying therapies, particularly those targeted at the preservation and restoration of synapses.