Neuroinflammation Radiotracers

Inflammation and alterations in the immune response are associated with various age-related neurodegenerative diseases. Stress factors, such as alpha-synuclein (aSyn) aggregates, can disrupt the balance between protective and harmful glial responses. This disruption may lead to neurotoxicity induced by microglia and astroglia through reactive oxygen species (ROS) and pro-inflammatory cytokines, resulting in chronic neuroinflammation and glutamatergic excitotoxicity. Chronic or maladaptive neuroinflammation can have detrimental effects in many neurologic disorders.

The principal target for imaging neuroinflammation is the overexpression of the translocator protein (TSPO) in activated microglia. TSPO, located in the outer mitochondrial membrane, is absent in healthy brain parenchyma but is commonly found in microglial cells involved in neuroinflammatory processes and dementia neuropathology. Historically, the TSPO PET radiopharmaceutical tracer [11C]-(R)-PK11195 has been used. In the past decade, fluorinated tracers have been developed, including phenoxyarylacetamide derivatives ([18F]-FEDAA1106, [18F]-FEPPA, [18F]-PBR06), imidazopyridine derivatives ([18F]-PBR111), and pyrazolopyrimidine derivatives ([18F]-DPA-714).

However, a polymorphism in the TSPO gene (rs6971) affects TSPO binding, significantly impacting visualization and quantification. To address this issue, new generations of rs6971-insensitive TSPO radioligands have been developed, such as flutriciclamide ([18F]-GE180). These latest tracers are currently under evaluation ^,

Dopaminergic System Tracers

Dopaminergic pathways are extensively studied in PD. Various components of the dopamine synapse can be examined, such as dopamine transporter density (DAT). DAT ligands, which enable the study of presynaptic denervation, are available for SPECT. Recently, phase III clinical trials have evaluated a promising PET ligand, [18F]LBT-999. PET imaging with [18F]LBT-999 could offer an alternative for assessing dopaminergic presynaptic injury in clinical settings with a single 10 minute acquisition. LBT 999 is highly promising due to its exceptional selectivity and specificity combined with the high sensitivity of PET technology. Additionally, the kinetics of this tracer allows imaging to be conducted just 30 minutes after injection, significantly improving patient comfort. Its automated radiosynthesis on various devices also enhance its suitability for clinical use.

Vesicular Monoamine Transporter Tracer

Vesicular monoamine transporter (VMAT) density can also be investigated. VMAT is a membrane protein responsible for transporting monoamines such as dopamine. As VMAT2 imaging by PET provides reliable information on the degeneration of nigrostriatal dopaminergic neurons, fluorinated tracers like [18F]AV133 and [18F]FP-DTBZ have been tested in several neurodegenerative disorders, including PD, LBD, and MSA, and are currently in phase III trials. Previous studies have shown that [18F]-FP-DTBZ uptake in the striatum is significantly associated with the severity of PD. In PD, various image processing techniques and brain segmentation methods have been studied to aid clinical practice. In PSP, VAMT2 binding in the striatum and hippocampus reflects the severity of fall/postural stability and cognition, respectively.

The postsynaptic side of the dopaminergic synapse can be studied using D2 receptor tracers such as [18F]fallypride. This tracer has completed phase II trials and has been notably tested in PD and Huntington’s disease.

Glutamatergic System Tracers

Glutamate receptors, the primary excitatory neurotransmitters in the central nervous system, are crucial targets for study. The N-methyl- d -aspartate (NMDA) receptor, a type of glutamate receptor, is part of the neurotransmitter channel receptor family. This receptor is conditionally open, with its activation dependent on the binding of its endogenous ligand, glutamate. Under normal physiologic conditions, NMDA receptors are activated briefly during synaptic transmission. However, in pathologic conditions, excessive activation can lead to excessive Ca ²⁺ influx into nerve cells, potentially resulting in cell death. This abnormal activation mediates excitotoxic neuronal injury following acute brain damage and is thought to contribute to disorders of neuronal hyperexcitability (such as essential tremor) and chronic neurodegenerative and psychotic disorders (including Huntington’s disease and Tourette’s syndrome). Several tracers have been developed to better understand the pathophysiology of these diseases.

Currently, a clinical trial is underway using an NMDA receptor tracer, [18F]FNM, to study Tourette’s syndrome. This phase I trial aims to correlate NMDA receptor hyperactivity with the severity of tics.

Another target is metabotropic glutamate receptors (mGluRs). These G protein-coupled receptors activate intracellular second messenger systems to modulate glutamatergic neurotransmission. The mGluR5 subtype, in particular, plays a significant role in regulating the glutamatergic system due to its involvement in neurogenesis and synaptic maintenance. An advanced phase I clinical trial is utilizing the [18F]FPEB tracer, which targets mGluR5, a metabotropic glutamatergic receptor. In a clinical trial on PD, Kang and colleagues demonstrated that mGluR5 is upregulated in key dopaminergic brain regions adversely affected by PD. They suggested that mGluR5 tracers may warrant further investigation as potential biomarkers for response in clinical trials.

Serotonergic System Tracers

Serotonergic system tracers have been established for many years and are currently undergoing phase III to IV clinical studies. Well-known examples include [18F]MPPF and [18F]WAY, which target the 5HT1A receptor. These tracers have been tested in AD, epilepsy, and panic disorder and could potentially be relevant for PD. Recent studies suggest that the noradrenergic and/or serotonergic systems might play a role in parkinsonian pain. ^, A clinical trial is underway to investigate the involvement of the serotonergic system in the pathophysiology of pain in patients with PD suffering from central pain.

Synaptic Density Tracers

The SV2 (synaptic vesicule) family consists of glycoproteins found in the vesicles of neuronal and endocrine cells. SV2A modulates calcium-dependent neurotransmitter release and is also involved in the maturation of neurotransmitter vesicles, making them responsive to calcium-induced release. Antiepileptic drugs such as levetiracetam and brivaracetam target SV2A, improve the regulation of synaptic activity. Preclinical studies have shown that SV2A tracers are sensitive to disease-related differences and intervention-induced changes. Reduced synaptic density in PD-related regions has been observed in several studies, potentially reflecting the breakdown of structural and functional connections associated with the disease. Synaptic density is a recent target of interest, particularly with [18F]SynVest1, which may correlate with results from DAT and Dopa imaging.

Mitochondrial Complex Tracers

Mitochondria, and their regulation by mitophagy, appears to be altered in certain pathologies such as PD. A tracer like [18F]BCPP, which targets mitochondrial complex I, could provide insights into this relatively new aspect of the pathology. [18F]BCPP is currently in phase I trials for use in PD and AD.

Cholinergic System Tracers

The vesicular acetylcholine transporter (VAChT) PET tracer [18F]fluoroethoxybenzovesamicol has recently demonstrated its high value to detect alterations of the cholinergic system in AD, PD, and DLB. A recent study has demonstrated that visual hallucination in PD is associated with a marked cholinergic deficiency in the left ventral visual stream and the left superior temporal lobe, in addition to an extensive global cholinergic denervation in the general PD population. Another study using this tracer demonstrated that cholinergic function differs between PD patients with the GBA1 genetic mutation and those without a genetic risk factor. There is now evidence of a relationship between pathology in the basal forebrain, acetylcholine denervation, and cognitive decline in PD, suggesting that the VAChT tracer could be helpful in detecting these changes.

The molecular mechanisms underlying PD and other neurodegenerative disorders are not yet fully understood. To gain a better understanding of these pathologies, it is essential to develop new diagnostic tools that enable the longitudinal study of disease progression. Two promising yet unexplored pathways are emerging: the adenosine A2A receptor pathway and the oxidative stress pathway. Significant efforts have been made to develop new PET radiotracers to study these mechanisms. This research is currently in the preclinical stage, with plans for clinical trials if the preclinical results are favorable.

Reactive Oxygen Species Tracer

The term ROS refers to a group of highly reactive oxygen-containing molecules, including superoxide radical anion, hydrogen peroxide, hydroxyl radical, and hypochlorous acid. The primary source of ROS is oxidative phosphorylation in the mitochondria, particularly due to mitochondrial dysfunction involving reduced Complex I activity. This dysfunction is considered one of the main contributors to elevated ROS levels in PD. ^, Oxidative stresses have also been reported in postmortem samples of AD brain and in transgenic mouse models of AD. Synucleinopathies are a diverse group of neurodegenerative disorders characterized by the accumulation of aggregated aSyn in vulnerable populations of brain cells. Oxidative stress is an early and persistent process in the development of synucleinopathies, suggesting the potential utility of PET imaging with a tracer targeting ROS. A research team has developed an 18F radioligand ([18F]-ROStrace) based on a known compound, the fluorescent probe dihydroethidium, which is used to measure superoxide levels in cells and tissues through microscopy and optical imaging techniques. [18F]-ROStrace holds promise as a noninvasive method for detecting aSyn-associated oxidative stress. This process is also involved in AD, and [18F]-ROStrace has been shown to identify increased oxidative stress and neuroinflammation in APP/PS1 female mice, a model of AD, concurrent with an increased amyloid burden in midlife. ^{, ,}