Astroglial EAAT2 Changes in Live TBI Brain Determined by PET Imaging

Rio Pharmaceuticals, Inc., San Francisco CA

Investigators

Abstract

PROJECT SUMMARY Traumatic Brain Injury (TBI) is one of the most significant Public Health trauma diseases of our time. Diagnoses of various TBI forms rely on patient self-reporting, established clinical rating scales, and multi- modality imaging to an extent. TBI elicits complex, heterogeneous pathological events, including mechanical injury, hemorrhage, and progressive secondary processes such as central and peripheral inflammation dynamics and excitotoxic driven necrotic and apoptotic events. A focal contusion TBI model that recapitulates various aspects of TBI observed in human is the controlled cortical contusion impact (CCI). The CCI rodent model can be used to determine longitudinal measures of TBI promoted neuropathological tissue changes. The major central nervous system (CNS) neurotransmitter L-glutamate (L-Glu) targets the excitatory amino acid transporter 2 (EAAT2) that clears 90 % of synaptic L-Glu. EAAT2 is expressed throughout the CNS and is primarily found on astroglial (astrocyte) cell membranes. Spatiotemporal changes to CNS EAAT2 protein densities are found in TBI postmortem brain tissues, including those from rodent TBI models where 20-40% reductions of EAAT2 is observed at acute times after injury. Positron emission tomography (PET) imaging is a technique that is sensitive and quantitative at the molecular level where it uniquely determines a positron-labeled tracer binding potential to its targeted CNS protein, thereby quantifying target protein density in live CNS tissues. A first-in-class fluorine-18 labeled (18F) tracer known as 18F-FAA has been discovered and possesses outstanding in vivo PET imaging qualities for quantitative determinations of CNS astrocyte EAAT2 target protein densities in live brain. Rio Pharmaceuticals, Inc., has licensed this EAAT2 PET imaging tracer technology and is further developing it for eventual clinical use. It is thought that the EAAT2 tracer is suitable to mark severity and localization of TBI in live brain. We hypothesize that CCI-promoted traumatic brain injury results in acute and latent regional cerebral tissue pathologies that can be marked in vivo by determining CNS EAAT2 protein density changes measured by quantitative dynamic PET imaging and correlated to in-vitro regional cerebral tissue EAAT2 and GFAP density alterations over time. Our long-term objective is to advance the development of a quantitative PET imaging tracer for CNS EAAT2 astrocyte protein target as an effective marker of TBI severity and localization. Ultimately, clinical EAAT2 CNS PET imaging will aid quantitative TBI diagnoses and afford a means to follow CNS tissue changes as a result of novel TBI therapies. The goal of this Phase 1 investigation is to establish an initial proof-of-concept (POC) demonstrating that dynamic PET imaging of the astrocyte EAAT2 target protein in live CCI TBI rat brain is a quantitative marker of TBI severity and localization. Establishing the Phase 1 POC will permit subsequent Phase 2 tracer development investigation in both males and female rodents, with experiments to further interrogate EAAT2 in model TBI brain (e.g., CCI and closed-head concussive injury) and studies to satisfy FDA criteria for evaluation of the PET imaging technology for clinical safety and TBI use efficacy. The industrial-academic collaborative Phase 1 POC project goal will be accomplished with the following three specific aims over one year. Specific Aim 1: Evaluate cerebral PET-CT-MR imaging 1, 3, 7, 30, 60 and 90 days after controlled CCI to afford quantitative cerebral signatures of 18F-FAA tracer binding potentials to EAAT2 target. Specific Aim 2: Measure regional cerebral EAAT2 and GFAP densities and the expression profiles of related markers in postmortem rat brain tissues 1, 3, 7, 30, 60 and 90 days after CCI TBI injury. Specific Aim 3: Establish the Phase 1 POC by determining correlations between regional in-vivo EAAT2 PET imaging tracer binding potential values and postmortem EAAT2 and GFAP measures.