Characterization of neuroinflammatory positron emission tomography biomarkers in chronic traumatic encephalopathy

Identification of novel tau positron emission tomography tracers for chronic traumatic encephalopathy by comprehensive in silico screening and molecular dynamics simulation

Chronic traumatic encephalopathy (CTE), a neurodegenerative disease associated with repetitive mild traumatic brain injury, is characterized neuropathologically by abnormal hyperphosphorylated tau accumulation. Early detection of tau deposition in the brain is crucial for the prevention and evaluation of CTE. Positron emission tomography (PET) tracers can image specific proteins, while the optimal PET tracer for CTE tau fibrils remains unidentified. In this study, structure-based virtual screening and CNS PET MPO algorithms were utilized to identify candidates for novel tau PET tracers from 23 000 compounds in the ChemDiv CNS BBB library. A total of 8 μs molecular dynamics simulations were then employed to evaluate their binding affinity and atomic-level interaction with CTE tau protofibrils. The results indicate that V017-7820 (CNS-4), S776-0061 (CNS-12), S567-0465 (CNS-18), and T828-0465 (CNS-25) exhibit higher docking scores and binding free energies with CTE tau protofibrils while also satisfying the fundamental physicochemical properties of PET tracers. Further simulation analyses reveal that CNS-4 has the strongest binding affinity to tau protofibrils among the four compounds. Hydrophobic, π-π stacking, and hydrogen bonding interactions are the primary driving forces for the binding of these compounds to CTE tau protofibrils. In particular, CNS-12 and CNS-25 exhibit more intense hydrophobic and π-π stacking interactions, whereas CNS-4 and CNS-25 exhibit stronger hydrogen bonding interactions. This study identifies promising lead compounds for tau PET tracers and highlights their mechanism of binding to CTE tau protofibrils, which provides new insights for further screening and development of novel PET tracers for CTE diagnosis.

Fluid biomarkers of chronic traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of long-term disability across the world. Evidence for the usefulness of imaging and fluid biomarkers to predict outcomes and screen for the need to monitor complications in the acute stage is steadily increasing. Still, many people experience symptoms such as fatigue and cognitive and motor dysfunction in the chronic phase of TBI, where objective assessments for brain injury are lacking. Consensus criteria for traumatic encephalopathy syndrome, a clinical syndrome possibly associated with the neurodegenerative disease chronic traumatic encephalopathy, which is commonly associated with sports concussion, have been defined only recently. However, these criteria do not fit all individuals living with chronic consequences of TBI. The pathophysiology of chronic TBI shares many similarities with other neurodegenerative and neuroinflammatory conditions, such as Alzheimer disease. As with Alzheimer disease, advancements in fluid biomarkers represent one of the most promising paths for unravelling the chain of pathophysiological events to enable discrimination between these conditions and, with time, provide prediction modelling and therapeutic end points. This Review summarizes fluid biomarker findings in the chronic phase of TBI (≥6 months after injury) that demonstrate the involvement of inflammation, glial biology and neurodegeneration in the long-term complications of TBI. We explore how the biomarkers associate with outcome and imaging findings and aim to establish mechanistic differences in biomarker patterns between types of chronic TBI and other neurodegenerative conditions. Finally, current limitations and areas of priority for future fluid biomarker research are highlighted.

Emerging frontiers in Chronic Traumatic Encephalopathy: early diagnosis and implications for neurotherapeutic interventions

INTRODUCTION

Chronic Traumatic Encephalopathy (CTE) is a neurodegenerative disorder associated with repetitive head trauma. Historically, the diagnosis has been primarily clinical, which has hindered definitive early diagnosis and proactive intervention.

AREAS COVERED

The authors analyze the recent advancements in early diagnosis of CTE by examining biomarkers, imaging, and clinical decision tools. They discuss the identification of neuropathologies - such as tau aggregates - through novel techniques ranging from blood sampling and to brain density scanning. The reader will walk away with a better understanding of current advancements in early detection and be better equipped to deal with encephalopathies secondary to trauma in clinical practice.

EXPERT OPINION

Tremendous progress has been made in understanding the pathophysiology of CTE. Despite these advancements, CTE treatment is still primarily symptomatic rather than underlying disease. Future research should focus on integrating current understanding of CTE pathophysiology with treatment modalities.

From spreading depolarization to blood-brain barrier dysfunction: navigating traumatic brain injury for novel diagnosis and therapy

Considerable strides in medical interventions during the acute phase of traumatic brain injury (TBI) have brought improved overall survival rates. However, following TBI, people often face ongoing, persistent and debilitating long-term complications. Here, we review the recent literature to propose possible mechanisms that lead from TBI to long-term complications, focusing particularly on the involvement of a compromised blood-brain barrier (BBB). We discuss evidence for the role of spreading depolarization as a key pathological mechanism associated with microvascular dysfunction and the transformation of astrocytes to an inflammatory phenotype. Finally, we summarize new predictive and diagnostic biomarkers and explore potential therapeutic targets for treating long-term complications of TBI.

Flortaucipir tau PET findings from former professional and college American football players in the DIAGNOSE CTE research project

INTRODUCTION

Tau is a key pathology in chronic traumatic encephalopathy (CTE). Here, we report our findings in tau positron emission tomography (PET) measurements from the DIAGNOSE CTE Research Project.

METHOD

We compare flortaucipir PET measures from 104 former professional players (PRO), 58 former college football players (COL), and 56 same-age men without exposure to repetitive head impacts (RHI) or traumatic brain injury (unexposed [UE]); characterize their associations with RHI exposure; and compare players who did or did not meet diagnostic criteria for traumatic encephalopathy syndrome (TES).

RESULTS

Significantly elevated flortaucipir uptake was observed in former football players (PRO+COL) in prespecified regions (p < 0.05). Association between regional flortaucipir uptake and estimated cumulative head impact exposure was only observed in the superior frontal region in former players over 60 years old. Flortaucipir PET was not able to differentiate TES groups.

DISCUSSION

Additional studies are needed to further understand tau pathology in CTE and other individuals with a history of RHI.

Synthesis and Preclinical Positron Emission Tomography Imaging of the p38 MAPK Inhibitor [11C]Talmapimod: Effects of Drug Efflux and Sex Differences

Stress-activated kinases are targets of interest in neurodegenerative disease due to their involvement in inflammatory signaling and synaptic dysfunction. The p38α kinase has shown clinical and preclinical promise as a druggable target in several neurodegenerative conditions. We report the radiosynthesis and evaluation of the first positron emission tomography (PET) radiotracer for imaging MAPK p38α/β through radiolabeling of the inhibitor talmapimod (SCIO-469) with carbon-11. [¹¹C]Talmapimod was reliably synthesized by carbon-11 methylation with non-decay corrected radiochemical yields of 3.1 ± 0.7%, molar activities of 38.9 ± 13 GBq/μmol, and >95% radiochemical purity (n = 20). Preclinical PET imaging in rodents revealed a low baseline brain uptake and retention with standardized uptake values (SUV) of ∼0.2 over 90 min; however, pretreatment with the P-glycoprotein (P-gp) drug efflux transporter inhibitor elacridar enabled [¹¹C]talmapimod to pass the blood-brain barrier (>1.0 SUV) with distinct sex differences in washout kinetics. Blocking studies with a structurally dissimilar p38α/β inhibitor, neflamapimod (VX-745), and displacement imaging studies with talmapimod were attempted in elacridar-pretreated rodents, but neither compound displaced radiotracer uptake in the brain of either sex. Ex vivo radiometabolite analysis revealed substantial differences in the composition of radioactive species present in blood plasma but not in brain homogenates at 40 min post radiotracer injection. Digital autoradiography in fresh-frozen rodent brain tissue confirmed that the radiotracer signal was largely non-displaceable in vitro, where self-blocking and blocking with neflamapimod marginally decreased the total signal by 12.9 ± 8.8% and 2.66 ± 2.1% in C57bl/6 healthy controls and 29.3 ± 2.7% and 26.7 ± 12% in Tg2576 rodent brains, respectively. An MDCK-MDR1 assay suggests that talmapimod is likely to suffer from drug efflux in humans as well as rodents. Future efforts should focus on radiolabeling p38 inhibitors from other structural classes to avoid P-gp efflux and non-displaceable binding.

Evaluation of Tau Radiotracers in Chronic Traumatic Encephalopathy

Chronic traumatic encephalopathy (CTE) is a neurologic disorder associated with head injuries, diagnosed by the perivascular accumulation of hyperphosphorylated tau protein (phospho-tau) identified at autopsy. Tau PET radiopharmaceuticals developed for imaging Alzheimer disease are under evaluation for brain injuries. The goal of this study was to conduct a head-to-head in vitro evaluation of 5 tau PET radiotracers in subjects pathologically diagnosed with CTE. Methods: Autoradiography was used to assess the specific binding and distribution of 3H-flortaucipir (also known as Tauvid, AV-1451, and T807), 3H-MK-6240 (also known as florquinitau), 3H-PI-2620, 3H-APN-1607 (also known as PM-PBB3 and florzolotau), and 3H-CBD-2115 (also known as 3H-OXD-2115) in fresh-frozen human postmortem CTE brain tissue (stages I-IV). Immunohistochemistry was performed for phospho-tau with AT8, 3R tau with RD3, 4R tau with RD4 and amyloid-β with 6F/3D antibodies. Tau target density (maximum specific binding) was quantified by saturation analysis with 3H-flortaucipir in tissue sections. Results: 3H-flortaucipir demonstrated a positive signal in all CTE cases examined, with varying degrees of specific binding (68.7% ± 10.5%; n = 12) defined by homologous blockade and to a lesser extent by heterologous blockade with MK-6240 (27.3% ± 13.6%; n = 12). The 3H-flortaucipir signal was also displaced by the monoamine oxidase (MAO)-A inhibitor clorgyline (43.9% ± 4.6%; n = 3), indicating off-target binding to MAO-A. 3H-APN-1607 was moderately displaced in homologous blocking studies and was not displaced by 3H-flortaucipir; however, substantial displacement was observed when blocking with the β-amyloid-targeting compound NAV-4694. 3H-MK-6240 and 3H-PI-2620 had negligible binding in all but 2 CTE IV cases, and binding may be attributed to pathology severity or mixed Alzheimer disease/CTE pathology. 3H-CBD-2115 showed moderate binding, displaced under homologous blockade, and aligned with 4R-tau immunostaining. Conclusion: In human CTE tissues, 3H-flortaucipir and 3H-APN-1607 revealed off-target binding to MAO-A and amyloid-β, respectively, and should be considered if these radiotracers are used in PET imaging studies of patients with brain injuries. 3H-MK-6240 and 3H-PI-2620 bind to CTE tau in severe- or mixed-pathology cases, and their respective 18F PET radiotracers warrant further evaluation in patients with severe suspected CTE.