18F-THK5351 PET Can Identify Core Lesions in Different Amyotrophic Lateral Sclerosis Phenotypes

18F-THK5351 Uptake May Not Estimate Neurofibrillary Tangles in In Vivo Images: A Comparison With 18F-MK-6240 Uptake

ABSTRACT

Currently, monoamine oxidase B is recognized as the primary target of 18F-THK5351, although 18F-THK5351 was initially developed to target neurofibrillary tangles (NFTs) in Alzheimer disease. When clinically applying 18F-THK5351 PET to visualize ongoing astrogliosis via estimating monoamine oxidase B levels, a crucial concern is how much degree 18F-THK5351 uptake reflects NFTs in in vivo images. To unravel this concern, a head-to-head comparison between 18F-THK5351 and 18F-MK-6240 (estimating NFT) images in the NFT lesion ideally without accompanying astrogliosis is essential. Here, we present such a case suggesting that 18F-THK5351 uptake may not estimate NFTs in in vivo images.

Potential of neuroimaging as a biomarker in amyotrophic lateral sclerosis: from structure to metabolism

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron degeneration. The development of ALS involves metabolite alterations leading to tissue lesions in the nervous system. Recent advances in neuroimaging have significantly improved our understanding of the underlying pathophysiology of ALS, with findings supporting the corticoefferent axonal disease progression theory. Current studies on neuroimaging in ALS have demonstrated inconsistencies, which may be due to small sample sizes, insufficient statistical power, overinterpretation of findings, and the inherent heterogeneity of ALS. Deriving meaningful conclusions solely from individual imaging metrics in ALS studies remains challenging, and integrating multimodal imaging techniques shows promise for detecting valuable ALS biomarkers. In addition to giving an overview of the principles and techniques of different neuroimaging modalities, this review describes the potential of neuroimaging biomarkers in the diagnosis and prognostication of ALS. We provide an insight into the underlying pathology, highlighting the need for standardized protocols and multicenter collaborations to advance ALS research.

Clinical application of MAO-B PET using 18F-THK5351 in neurological disorders

Monoamine oxidase B (MAO-B) is an enzyme localized to the outer mitochondrial membrane and highly concentrated in astrocytes. Temporal changes in regional MAO-B levels can be used as an index of astrocytic proliferation, known as activated astrocytes or astrogliosis. MAO-B is a marker to evaluate the degree of astrogliosis. Therefore, MAO-B positron emission tomography (PET) is a powerful imaging technique for visualizing and quantifying ongoing astrogliosis through the estimate of regional MAO-B levels. Each neurodegenerative disorder generally has a characteristic distribution pattern of astrogliosis secondary to neuronal loss and pathological protein aggregation. Therefore, by imaging astrogliosis, MAO-B PET can be used as a neurodegeneration marker for identifying degenerative lesions. Any inflammation in the brain usually accompanies astrogliosis starting from an acute phase to a chronic phase. Therefore, by imaging astrogliosis, MAO-B PET can be used as a neuroinflammation marker for identifying inflammatory lesions. MAO-B levels are high in gliomas originating from astrocytes but low in lymphoid tumors. Therefore, MAO-B PET can be used as a brain tumor marker for identifying astrocytic gliomas by imaging MAO-B levels and distinguishing between astrocytic and lymphoid tumors. This review summarizes the clinical application of MAO-B PET using 18F-THK5351 as markers for neurodegeneration, neuroinflammation, and brain tumors in neurological disorders. Because we assume that MAO-B PET is clinically applied to an individual patient, we focus on visual inspection of MAO-B images at the individual patient level. Geriatr Gerontol Int 2024; 24: 31-43.

Detailed Assessment of 18F-THK5351 Distribution Pattern in the Midbrain: Comparison With Progressive Supranuclear Palsy and Corticobasal Syndrome

BACKGROUND

18F-THK5351 PET is used to image ongoing astrogliosis by estimating monoamine oxidase B levels. 18F-THK5351 preferentially accumulates around the substantia nigra (SN) and periaqueductal gray (PG) in the midbrain under healthy conditions and exhibits a "trimodal pattern." In progressive supranuclear palsy (PSP) and corticobasal syndrome (CBS), the midbrain 18F-THK5351 uptake can be increased by astrogliosis, collapsing the "trimodal pattern." We aimed to elucidate cases in which the "trimodal pattern" collapses in PSP and CBS.

PATIENTS AND METHODS

Participants in the PSP (n = 11), CBS (n = 17), Alzheimer disease (n = 11), and healthy control (n = 8) groups underwent 18F-THK5351 PET. Volumes of interest (VOIs) were placed on the SN, PG, and their midpoints. The midbrain uptake ratio (MUR) was calculated to assess the trimodal pattern as follows: MUR = (VOI value on the midpoint)/(VOI value on the SN and PG). Approximately, the trimodal pattern can be identified at MUR <1 but not at MUR >1.

RESULTS

Compared with the healthy control group, MUR significantly increased in the PSP (P < 0.01) and CBS (P < 0.01) groups, but was unchanged in the Alzheimer disease group (P = 0.10). In the PSP group, all patients, including 2 with mild symptoms and a short disease duration, showed MUR >1. In the CBS group, MUR varied widely.

CONCLUSIONS

In PSP, the trimodal pattern can collapse even in the early phase when symptoms are mild. In CBS, the trimodal pattern may or may not collapse depending on the underlying pathology.

Clinical Application of 18F-THK5351 PET to Identify Inflammatory Lesions Through Imaging Astrogliosis in a Case of Cytomegalovirus Ventriculoencephalitis

ABSTRACT

18F-THK5351 PET is used to estimate the degree of astrogliosis. Because inflammatory lesions usually accompany astrogliosis, 18F-THK5351 PET is potentially worthy of clinical application in inflammatory disorders. Here, we report a case of cytomegalovirus ventriculoencephalitis in an immunocompromised 75-year-old woman who underwent 18F-THK5351 PET and conventional neuroimaging modalities, including 11C-methionine, 18F-FDG, and MRI. 18F-THK5351 PET was clearly superior to the other modalities in identifying inflammatory lesions and can therefore be a useful marker for identifying inflammatory lesions through imaging astrogliosis. This feature of 18F-THK5351 may contribute to the early diagnosis of cytomegalovirus ventriculoencephalitis.

PET and SPECT Imaging of ALS: An Educational Review

Amyotrophic lateral sclerosis (ALS) is a disease leading to progressive motor degeneration and ultimately death. It is a complex disease that can take a significantly long time to be diagnosed, as other similar pathological conditions must be ruled out for a definite diagnosis of ALS. Noninvasive imaging of ALS has shed light on disease pathology and altered biochemistry in the ALS brain. Other than magnetic resonance imaging (MRI), two types of functional imaging, positron emission tomography (PET) and single photon emission computed tomography (SPECT), have provided valuable data about what happens in the brain of ALS patients compared to healthy controls. PET imaging has revealed a specific pattern of brain metabolism through [18F]FDG, while other radiotracers have uncovered neuroinflammation, changes in neuronal density, and protein aggregation. SPECT imaging has shown a general decrease in regional cerebral blood flow (rCBF) in ALS patients. This educational review summarizes the current state of ALS imaging with various PET and SPECT radiopharmaceuticals to better understand the pathophysiology of ALS.

High sensitivity of asymmetric 18F-THK5351 PET abnormality in patients with corticobasal syndrome

Corticobasal syndrome (CBS) is characterized by symptoms related to the asymmetric involvement of the cerebral cortex and basal ganglia. However, early detection of asymmetric imaging abnormalities can be challenging. Previous studies reported asymmetric ¹⁸F-THK5351 PET abnormalities in CBS patients, but the sensitivity for detecting such abnormalities in larger patient samples, including early-stage cases, remains unclear. Patients clinically diagnosed with CBS were recruited. All patients displayed asymmetric symptoms in the cerebral cortex and basal ganglia. Asymmetric THK5351 PET abnormalities were determined through visual assessment. Brain MRI, perfusion SPECT, and dopamine transporter (DAT) SPECT results were retrospectively reviewed. The 15 patients had a median age of 72 years (59-86 years) and a disease duration of 2 years (0.5-7 years). Four patients met the probable and 11 met the possible CBS criteria according to Armstrong criteria at the time of PET examination. All patients, including early-stage cases, exhibited asymmetric tracer uptake contralateral to their symptom-dominant side in the cerebral cortex/subcortical white matter and striatum (100%). The sensitivity for detecting asymmetric imaging abnormalities contralateral to the symptom-dominant side was 86.7% for brain MRI, 81.8% for perfusion SPECT, and 90% for DAT SPECT. White matter volume reduction was observed in the subcortical region of the precentral gyrus with increased THK5351 uptake, occurring significantly more frequently than gray matter volume reduction. THK5351 PET may be a sensitive imaging technique for detecting asymmetric CBS pathologies, including those in early stages.

18 F-THK5351 PET Can Evaluate Tumor Extension in Intravascular Large B-Cell Lymphoma : Comparison With 11C-Methionine PET and 18F-FDG PET

ABSTRACT

A 79-year-old man presenting with gait disturbance and cognitive decline was diagnosed with intravascular large B-cell lymphoma (IVLBCL) by random skin biopsy. Some IVLBCL lesions were identified by PET examinations using 11 C-methionine, 18 F-FDG, and 18 F-THK5351. 11 C-methionine and 18 F-FDG uptake, which likely reflects the presence of the lymphoma cells themselves, increased clearly in the left putamen but weakly in the left deep white matter. 18 F-THK5351 uptake increased in all lesions, likely reflecting perivascular astrogliosis caused by IVLBCL. Hence, 18 F-THK5351 PET can evaluate tumor extension in IVLBCL lesions where 11 C-methionine and 18 F-FDG PET may fail in its visualization.

Brain PET Imaging of 11C-Methionine, 18F-FDG, and 18F-THK5351 in a Case of Lymphomatoid Granulomatosis

ABSTRACT

A 52-year-old woman complained of upper respiratory symptoms and subsequently developed Wallenberg syndrome. Chest CT and brain MRI revealed multiple nodular lesions in the lungs and brain. She was pathologically diagnosed with low-grade lymphomatoid granulomatosis by lung biopsy. Brain PET examinations using 11C-methionine, 18F-FDG, and 18F-THK5351 were performed. Uptake of 11C-methionine and 18F-FDG was slightly increased in some lesions, likely reflecting the degree of inflammatory cell infiltration. 18F-THK5351 uptake was significantly increased in all lesions, likely reflecting the degree of reactive astrogliosis. This case illustrates the utility of PET studies for diagnosing lymphomatoid granulomatosis and provides insight into its pathophysiology.

Distribution Pattern of the Monoamine Oxidase B Ligand, 18F-THK5351, in the Healthy Brain

BACKGROUND

18F-THK5351 PET estimates the concentrations of monoamine oxidase B (MAO-B) that are preferentially located in astrocytes and can be used to visualize and quantify ongoing astrogliosis. To study images of astrogliosis in neurological disorders, it is essential to understand the detailed binding sites of 18F-THK5351 as the MAO-B ligand under normal conditions. In this study, we examined the detailed distribution pattern of 18F-THK5351 in the healthy brain by comparing 18F-THK5351 uptake between subjects taking and not taking the MAO-B inhibitor.

METHODS

Ten healthy controls (HCs: 67.4 [SD, 15.1] years) and 4 patients with Parkinson disease taking the MAO-B inhibitor underwent 18F-THK5351 PET. The uptake ratio index (URI) was defined to quantify 18F-THK5351 uptake, using the cerebellum as a reference region. The cerebellar URI was set to zero.

RESULTS

For HCs, regions with URI ≥1 were preferentially observed in the following order: the striatum, globus pallidus, thalamus, hypothalamus, amygdala, periaqueductal gray, substantia nigra, medulla, hippocampus, and pons. The peak URI values in the corresponding regions were 2.93, 2.47, 2.12, 2.04, 1.84, 1.68, 1.67, 1.37, 1.20, and 1.11, respectively. For all patients with Parkinson disease taking the MAO-B inhibitor, the URI values in all these regions were significantly decreased (Z score >2) and were reduced from 60.4% to 99.9%, compared with HCs.

CONCLUSIONS

This study presented the detailed distribution pattern of 18F-THK5351 in HCs and suggests that 18F-THK5351 uptake largely reflects MAO-B concentrations under normal conditions.

Astrocytes in Neurodegeneration: Inspiration From Genetics

Despite the discovery of numerous molecules and pathologies, the pathophysiology of various neurodegenerative diseases remains unknown. Genetics participates in the pathogenesis of neurodegeneration. Neural dysfunction, which is thought to be a cell-autonomous mechanism, is insufficient to explain the development of neurodegenerative disease, implying that other cells surrounding or related to neurons, such as glial cells, are involved in the pathogenesis. As the primary component of glial cells, astrocytes play a variety of roles in the maintenance of physiological functions in neurons and other glial cells. The pathophysiology of neurodegeneration is also influenced by reactive astrogliosis in response to central nervous system (CNS) injuries. Furthermore, those risk-gene variants identified in neurodegenerations are involved in astrocyte activation and senescence. In this review, we summarized the relationships between gene variants and astrocytes in four neurodegenerative diseases, including Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Parkinson’s disease (PD), and provided insights into the implications of astrocytes in the neurodegenerations.

Imaging of Reactive Astrogliosis by Positron Emission Tomography

Many neurodegenerative diseases are neuropathologically characterized by neuronal loss, gliosis, and the deposition of misfolded proteins such as β-amyloid (Aβ) plaques and tau tangles in Alzheimer’s disease (AD). In postmortem AD brains, reactive astrocytes and activated microglia are observed surrounding Aβ plaques and tau tangles. These activated glial cells secrete pro-inflammatory cytokines and reactive oxygen species, which may contribute to neurodegeneration. Therefore, in vivo imaging of glial response by positron emission tomography (PET) combined with Aβ and tau PET would provide new insights to better understand the disease process, as well as aid in the differential diagnosis, and monitoring glial response disease-specific therapeutics. There are two promising targets proposed for imaging reactive astrogliosis: monoamine oxidase-B (MAO-B) and imidazoline2 binding site (I2BS), which are predominantly expressed in the mitochondrial membranes of astrocytes and are upregulated in various neurodegenerative conditions. PET tracers targeting these two MAO-B and I2BS have been evaluated in humans. [18F]THK-5351, which was originally designed to target tau aggregates in AD, showed high affinity for MAO-B and clearly visualized reactive astrocytes in progressive supranuclear palsy (PSP). However, the lack of selectivity of [18F]THK-5351 binding to both MAO-B and tau, severely limits its clinical utility as a biomarker. Recently, [18F]SMBT-1 was developed as a selective and reversible MAO-B PET tracer via compound optimization of [18F]THK-5351. In this review, we summarize the strategy underlying molecular imaging of reactive astrogliosis and clinical studies using MAO-B and I2BS PET tracers.

PET imaging of reactive astrocytes in neurological disorders

The reactive astrocytes manifest molecular, structural, and functional remodeling in injury, infection, or diseases of the CNS, which play a critical role in the pathological mechanism of neurological diseases. A growing need exists for dependable approach to better characterize the activation of astrocyte in vivo. As an advanced molecular imaging technology, positron emission tomography (PET) has the potential for visualizing biological activities at the cellular levels. In the review, we summarized the PET visualization strategies for reactive astrocytes and discussed the applications of astrocyte PET imaging in neurological diseases. Future studies are needed to pay more attention to the development of specific imaging agents for astrocytes and further improve our exploration of reactive astrocytes in various diseases.