NEUROLOGY Imaging synaptic density in the living human brain Sjoerd J. Finnema, 1 * Nabeel B. Nabulsi, 1 Tore Eid, 2 Kamil Detyniecki, 3 Shu-fei Lin, 1 Ming-Kai Chen, 1 Roni Dhaher, 2 David Matuskey, 1 Evan Baum, 1 Daniel Holden, 1 Dennis D. Spencer, 4 Joël Mercier, 5 Jonas Hannestad, 5† Yiyun Huang, 1 Richard E. Carson 1,6 Chemical synapses are the predominant neuron-to-neuron contact in the central nervous system. Presynaptic boutons of neurons contain hundreds of vesicles filled with neurotransmitters, the diffusible signaling chemicals. Changes in the number of synapses are associated with numerous brain disorders, including Alzheimer’s disease and epilepsy. However, all current approaches for measuring synaptic density in humans require brain tissue from autopsy or surgical resection. We report the use of the synaptic vesicle glycoprotein 2A (SV2A) radioligand [ 11 C]UCB-J combined with positron emission tomography (PET) to quantify synaptic density in the living human brain. Validation studies in a baboon confirmed that SV2A is an alternative synaptic density marker to synaptophysin. First-in-human PET studies demonstrated that [ 11 C]UCB-J had excellent imaging properties. Finally, we confirmed that PET imaging of SV2A was sensitive to synaptic loss in patients with temporal lobe epilepsy. Thus, [ 11 C]UCB-J PET imaging is a promising approach for in vivo quantification of synaptic density with several potential applications in diagnosis and therapeutic monitoring of neurological and psychiatric disorders. INTRODUCTION The human brain is estimated to contain 100 trillion synapses (1). Structural disruption or loss of synapses can result in network dys- function with aberrant neuronal signaling. Synaptic pathology has been associated with many brain disorders. Reduced synaptic density in the seizure onset zone in patients with epilepsy has been shown in neuro- pathological studies (2–5), and synapse loss in the hippocampus and cerebral cortex has been closely associated with cognitive impairment in Alzheimer’s disease (6–9). Moreover, regional synaptic changes have been demonstrated in patients with autism (10), depression (11), or schizophrenia (12, 13). However, all approaches for synapse quantifica- tion in humans depend on examination of brain tissue from autopsy or surgical resection, thereby greatly limiting the utility of this approach for early diagnosis and therapeutic monitoring. A minimally invasive method for quantification of synaptic density in the living human brain is therefore desired. Quantification of synaptic density in brain tissue is typically per- formed using immunohistochemistry or electron microscopy. Antibodies targeting key proteins located in the pre- or postsynaptic neurons, such as synaptophysin (SYN), which is present in the membrane of synaptic vesicles (14), are commonly used for immunohistochemical detection of synaptic elements. In vivo quantification of proteins in the living brain is possible using positron emission tomography (PET), and the use of PET imaging is well established for a wide range of brain receptors, transporters, and enzymes, as well as other proteins (15). In vivo quantification of synapses throughout the entire brain may be possible using PET when combined with a synapse-specific radioligand, but to date, no radio- ligands have been reported that bind to a protein ubiquitously present on synaptic vessels. To this end, we developed [ 11 C]UCB-J [(R)-1- ((3-( 11 C-methyl- 11 C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophe- nyl)pyrrolidin-2-one] as a best-in-class PET radioligand for imaging of the synaptic vesicle glycoprotein 2A (SV2A) and demonstrated its potential in nonhuman primates (16). SV2 is present in all vertebrates and is an integral membrane protein located similarly to SYN in the presynaptic vesicle membranes. SV2 consists of three isoforms, with SV2A as the only isoform ubiquitously and homogeneously located in synapses across the brain (17). Thus, PET imaging and quantification of SV2A signal may be an excellent in vivo proxy of synaptic density. Other SV2A radioligands have been reported recently, including [ 11 C]UCB-A and [ 18 F]UCB-H (18). [ 11 C] UCB-A binding could be quantified in rats and pigs (19). [ 18 F]UCB-H displayed good kinetics in rats (20) and nonhuman primates (21), and acceptable dosimetry in humans (22), but a human brain imaging study has not yet been reported. Here, we demonstrate that [ 11 C]UCB-J binds to SV2A and serves as a synaptic density marker in nonhuman primates as well as in people, in a first-in-human imaging study in healthy controls and in patients with epilepsy. In healthy humans, we characterized the pharmacokinetic and imaging properties of [ 11 C]UCB-J and confirmed SV2A-specific binding. We also demonstrated proof of concept in epilepsy patients that changes in synaptic density can be monitored noninvasively. [ 11 C]UCB-J PET is a promising approach for research, clinical diagnosis, and therapeutic monitoring in neurological and psychiatric disorders. RESULTS SV2A is a marker of synaptic density—A baboon case study To validate [ 11 C]UCB-J as a marker of synaptic density, we performed a PET measurement in an olive baboon (Papio anubis), after which the animal was sacrificed and the brain was dissected for postmortem tissue studies. After injection of [ 11 C]UCB-J, brain uptake was rapid, with highest radioactivity concentrations in the cerebral cortex and lowest in the white matter region centrum semiovale (Fig. 1, A and B). Regional time-activity curves (TACs) displayed rapid kinetics, with peak uptake of 1 Yale Positron Emission Tomography Center, Department of Radiology and Bio- medical Imaging, Yale University, New Haven, CT 06520, USA. 2 Department of Labo- ratory Medicine, Yale University, New Haven, CT 06520, USA. 3 Department of Neurology, Yale University, New Haven, CT 06520, USA. 4 Department of Neurosurgery, Yale University, New Haven, CT 06520, USA. 5 UCB Pharma, B-1420 Braine-l’Alleud, Bel- gium. 6 Department of Biomedical Engineering, Yale University, New Haven, CT 06520, USA. *Corresponding author. Email: sjoerd.finnema@yale.edu †Present address: Denali Therapeutics, South San Francisco, CA 94080, USA. RESEARCH ARTICLE www.ScienceTranslationalMedicine.org 20 July 2016 Vol 8 Issue 348 348ra96 1 by guest on June 13, 2020 http://stm.sciencemag.org/ Downloaded from