Abstract
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Objectives: Synaptic loss is one of the hallmarks of Alzheimer’s disease and region-specific changes in synaptic density have been implicated in other neuropsychiatric diseases such as epilepsy, depression and schizophrenia. Thus, in vivo imaging of synaptic density could be an extremely useful tool for the diagnosis of various diseases and monitoring of therapeutic outcome. We have previously demonstrated that PET imaging of the synaptic vesicle glycoprotein 2A (SV2A) with 11C-UCB-J can serve as a surrogate biomarker for synaptic density [1]. To overcome the limitations of 11C-UCB-J for clinical diagnostic application as imposed by the short C-11 half-life, we synthesized and evaluated a series of readily accessible 18F-labeled SV2A ligands.
Methods: Using a divergent synthetic strategy, novel SV2A ligands derived from the pyrrolidinone pharmacophore were designed and synthesized [2]. Binding affinities were assessed through radioligand competition assays using rat brain homogenates. The corresponding 18F-labeled radiotracers were synthesized via iodonium salts or ylides for aromatic radiofluorination. PET scans in monkeys were performed on a FOCUS 220 scanner. Arterial blood was drawn during each scan for metabolite analysis and construction of the plasma input functions. Regional brain time-activity curves (TACs) were analyzed by one-tissue (1T), two-tissue (2T), and multilinear analysis-1 (MA1) models to obtain regional volumes of distribution (VT) and binding potential (BPND). Displacement experiments with levetiracetam (30 mg/kg, i.v.) were carried out to determine the binding specificity of the radiotracers. Results obtained with the novel SV2A radiotracers were compared with those of 11C-UCB-J and 18F-UCB-H.
Results: A number of compounds were synthesized from commercially available starting materials. They were found to display moderate to high binding affinities to rat brain SV2A. Two ligands (termed SDM-1 and SDM-2) with the highest binding affinities (Ki of 44.7 nM and 15.2 nM, on par with that of UCB-J, with Ki of 25 nM for rat brain SV2A) [3] were chosen for radiolabeling and PET imaging evaluation in monkeys. The 18F-labeled radiotracers were successfully prepared in good radiochemical yield (14-30%) and high specific activity. All radiotracers showed similar rates and patterns of metabolism, with radio-metabolites more polar than the parent tracers. Plasma free fractions were 61% and 43% for 18F-SDM-1 and 18F-SDM-2, respectively, compared with 44% and 52% for 11C-UCB-J and 18F-UCB-H. 18F-SDM-1 and 18F-SDM-2 displayed high brain uptake (peak SUV of 6-7) and distribution pattern similar to that of 11C-UCB-J and 18F-UCB-H. Kinetics was fast, with radioactivity peaking within 10 min for 18F-SDM-1 and 18F-SDM-2, compared with 10-50 min for 11C-UCB-J [3]. Both 1T and MA1 models produced reliable VT values with good fits. Regional BPND values were estimated using the white matter as reference region, with rank order of 11C-UCB-J > 18F-SDM-2 > 18F-SDM-1 >> 18F-UCB-H. For SDM-2, BPND values ranged from 0.77 in amygdala to 2.14 in cingulate cortex. Displacement with levetiracetam reduced the binding of 18F-SDM-1 and 18F-SDM-2 to background levels, confirming their binding specificity to SV2A.
Conclusion: We have successfully synthesized a number of novel SV2A ligands and evaluated two of the radiotracers in nonhuman primates. Both 18F-SDM-1 and 18F-SDM-2 were found to have attractive imaging properties: high brain uptake, fast tissue kinetics, uptake pattern consistent with SV2A distribution in the brain, and good specific binding signals. 18F-SDM-2 displays the highest specific binding signals among the new radiotracers and is judged to be a promising PET imaging agent for SV2A and synaptic density. Comprehensive characterization in non-human primates is underway and advancement to human evaluation is expected. Research Support: