Preclinical In Vitro and In Vivo Characterization of Synaptic Vesicle 2A–Targeting Compounds Amenable to F-18 Labeling as Potential PET Radioligands for Imaging of Synapse Integrity

• DOI: 10.1007/s11307-019-01428-0
• Publication Date: 2019-11-14T18:02:37Z
• Authors: Shil Patel; Ashley Knight; Stephen Krause; Tyler Teceno; Cedric Tresse; Songye Li; Zhengxin Cai; Alexandra Gouasmat; Vincent M. Carroll; Olivier Barret; Vijay Gottmukkala; Wenjie Zhang; Xianhong Xiang; Thomas Morley; Yiyun Huang; Jan Passchier
• Keywords: Cerebral Cortex; Mammals; 0301 basic medicine; Fluorine Radioisotopes; Membrane Glycoproteins; Time Factors; Staining and Labeling; Nerve Tissue Proteins; Ligands; Binding, Competitive; Rats, Sprague-Dawley; 03 medical and health sciences; Positron-Emission Tomography; Synapses; Animals; Autoradiography; Humans; Tissue Distribution; Radiopharmaceuticals

Current synaptic vesicle 2A (SV2A) positron emission tomography (PET) imaging agents include the nanomolar affinity probes [11C]UCB-J and [18F]UCB-H derived from the anti-epileptic drug levitaracetam (Keppra®). An industry-utilized "de-risking" approach was used to carry out initial pharmacological characterization and to assess potential next-generation candidates amenable to F-18 radiolabeling for preliminary evaluation.Radioligand binding methods were employed in mammalian brain homogenates to determine the SV2A affinity (Kd) and maximal binding capacity (Bmax) of [3H]UCB-J. Novel leads were then screened to identify compounds minimally with comparable binding affinities with UCB-J in order to select a F-18-labeled candidate for subsequent in vivo assessment in rat. In parallel, mammalian brain tissue section autoradiography was performed to assess specific SV2A distribution.[3H]UCB-J bound with high affinity to a single population of sites in the rat brain (Kd = 2.6 ± 0.25 nM; Bmax = 810 ± 25 fmol/mg protein) and control human cortex (Kd = 2.9 ± 0.54 nM; Bmax = 10,000 ± 640 fmol/mg protein). Distribution of specific SV2A binding was shown to be homogeneous throughout the rodent brain and primarily in gray matter regions of rodent and human brain sections. Analog screening identified MNI-1038, MNI-1126/SDM-8, and SDM-2 as having comparable binding affinities with the currently available PET ligands. Subsequent [18F]MNI-1126/[18F]SDM-8 dynamic micro-PET imaging in rats revealed in vivo uptake and accumulation in the brain with favorable kinetics. Chase studies using 30 mg/kg levetiracetam confirmed that in vivo brain uptake of [18F]MNI-1126/[18F]SDM-8 was reversible.Taken together, these data suggest [18F]MNI-1126/[18F]SDM-8 (since renamed as [18F]SynVesT-1) characterized via an in vitro screening cascade provided a measurable in vivo SV2A specific signal in the rodent brain. This tracer as well as the close analog [18F]SDM-2 (since renamed as [18F]SynVesT-2) is currently undergoing further evaluation in preclinical and clinical studies.