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Abstract Background Status epilepticus (SE) is proposed to lead to an age-dependent acute activation of a repertoire of inflammatory processes, which may contribute to neuronal damage in the hippocampus. The extent and temporal profiles of activation of these processes are well known in the adult brain, but less so in the developing brain. We have now further elucidated to what extent inflammation is activated by SE by investigating the acute expression of several cytokines and subacute glial reactivity in the postnatal rat hippocampus. Methods SE was induced by an intraperitoneal (i.p.) injection of kainic acid (KA) in 9- and 21-day-old (P9 and P21) rats. The mRNA expression of interleukin-1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), interleukin-10 (IL-10), matrix metalloproteinase-9 (MMP-9), glial-derived neurotrophic factor (GDNF), interferon gamma (IFN-γ), and transforming growth factor-beta 1 (TGF-β1) were measured from 4 h up to 3 days after KA injection with real-time quantitative PCR (qPCR). IL-1β protein expression was studied with ELISA, GFAP expression with western blotting, and microglial and astrocyte morphology with immunohistochemistry 3 days after SE. Results SE increased mRNA expression of IL-1β, TNF-α and IL-10 mRNA in hippocampus of both P9 and P21 rats, their induction being more rapid and pronounced in P21 than in P9 rats. MMP-9 expression was augmented similarly in both age groups and GDNF expression augmented only in P21 rats, whereas neither IFN-γ nor TGF-β1 expression was induced in either age group. Microglia and astrocytes exhibited activated morphology in the hippocampus of P21 rats, but not in P9 rats 3 d after SE. Microglial activation was most pronounced in the CA1 region and also detected in the basomedial amygdala. Conclusion Our results suggest that SE provokes an age-specific cytokine expression in the acute phase, and age-specific glial cell activation in the subacute phase as verified now in the postnatal rat hippocampus. In the juvenile hippocampus, transient increases in cytokine mRNA expression after SE, in contrast to prolonged glial reactivity and region-specific microglial activity after SE, suggest that the inflammatory response is changed from a fulminant and general initial phase to a more moderate and specific subacute response.

Hypoxic-ischemic encephalopathy due to insufficient oxygen delivery to brain tissue is a leading cause of death or severe morbidity in neonates. The early recognition of the most severely affected individuals remains a clinical challenge. We hypothesized that hypoxic-ischemic injury can be detected using PET radiotracers for hypoxia ([ 18 F]EF5), glucose metabolism ([ 18 F]FDG), and inflammation ([ 18 F]F-DPA)., Methods: A preclinical model of neonatal hypoxic-ischemic brain injury was made in 9-d-old rat pups by permanent ligation of the left common carotid artery followed by hypoxia (8% oxygen and 92% nitrogen) for 120 min. In vivo PET imaging was performed immediately after injury induction or at different timepoints up to 21 d later. After imaging, ex vivo brain autoradiography was performed. Brain sections were stained with cresyl violet to evaluate the extent of the brain injury and to correlate it with [ 18 F]FDG uptake., Results: PET imaging revealed that all three of the radiotracers tested had significant uptake in the injured brain hemisphere. Ex vivo autoradiography revealed high [ 18 F]EF5 uptake in the hypoxic hemisphere immediately after the injury (P < 0.0001), decreasing to baseline even 1 d postinjury. [ 18 F]FDG uptake was highest in the injured hemisphere on the day of injury (P < 0.0001), whereas [ 18 F]F-DPA uptake was evident after 4 d (P = 0.029), peaking 7 d postinjury (P < 0.0001), and remained significant 21 d after the injury. Targeted evaluation demonstrated that [ 18 F]FDG uptake measured by in vivo imaging 1 d postinjury correlated positively with the brain volume loss detected 21 d later (r = 0.72, P = 0.028)., Conclusion: Neonatal hypoxic-ischemic brain injury can be detected using PET imaging. Different types of radiotracers illustrate distinct phases of hypoxic brain damage. PET may be a new useful technique, worthy of being explored for clinical use, to predict and evaluate the course of the injury., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Clustering time activity curves of PET images have been used to separate clinically relevant areas of the brain or tumours. However, PET image segmentation in multiorgan level is much less studied due to the available total-body data being limited to animal studies. Now, the new PET scanners providing the opportunity to acquire total-body PET scans also from humans are becoming more common, which opens plenty of new clinically interesting opportunities. Therefore, organ-level segmentation of PET images has important applications, yet it lacks sufficient research. In this proof of concept study, we evaluate if the previously used segmentation approaches are suitable for segmenting dynamic human total-body PET images in organ level. Our focus is on general-purpose unsupervised methods that are independent of external data and can be used for all tracers, organisms, and health conditions. Additional anatomical image modalities, such as CT or MRI, are not used, but the segmentation is done purely based on the dynamic PET images. The tested methods are commonly used building blocks of the more sophisticated methods rather than final methods as such, and our goal is to evaluate if these basic tools are suited for the arising human total-body PET image segmentation. First, we excluded methods that were computationally too demanding for the large datasets from human total-body PET scanners. These criteria filtered out most of the commonly used approaches, leaving only two clustering methods, k -means and Gaussian mixture model (GMM), for further analyses. We combined k -means with two different preprocessing approaches, namely, principal component analysis (PCA) and independent component analysis (ICA). Then, we selected a suitable number of clusters using 10 images. Finally, we tested how well the usable approaches segment the remaining PET images in organ level, highlight the best approaches together with their limitations, and discuss how further research could tackle the observed shortcomings. In this study, we utilised 40 total-body [ 18 F] fluorodeoxyglucose PET images of rats to mimic the coming large human PET images and a few actual human total-body images to ensure that our conclusions from the rat data generalise to the human data. Our results show that ICA combined with k -means has weaker performance than the other two computationally usable approaches and that certain organs are easier to segment than others. While GMM performed sufficiently, it was by far the slowest one among the tested approaches, making k -means combined with PCA the most promising candidate for further development. However, even with the best methods, the mean Jaccard index was slightly below 0.5 for the easiest tested organ and below 0.2 for the most challenging organ. Thus, we conclude that there is a lack of accurate and computationally light general-purpose segmentation method that can analyse dynamic total-body PET images., Competing Interests: The authors declare that they have no conflicts of interest., (Copyright © 2023 Maria K. Jaakkola et al.)

Introduction: [ 18 F]FMTEB, along with other tracers, was developed as a promising PET radioligand for imaging metabotropic glutamate receptor subtype 5 (mGluR5). Despite favorable preliminary results, it has not been used further for studies of mGluR5. This paper presents an in-depth preclinical evaluation of [ 18 F]FMTEB in healthy Sprague Dawley rats., Methods: [ 18 F]FMTEB was synthesized from a boronic ester precursor using copper-mediated fluorination. In vivo PET imaging was performed on six rats, of which three were pre-treated with a high affinity mGluR5 receptor antagonist. An additional 18 rats were used for ex vivo experiments for metabolite analyses in plasma, brain and urine, and for biodistribution and ex vivo brain autoradiography at different time points., Results: [ 18 F]FMTEB was synthesized in adequate radiochemical yield and a molar activity of 154 ± 64 GBq/μmol. Both in vivo imaging and ex vivo brain autoradiography showed high specificity for mGluR5, and the blocking experiments showed a clear decrease in radioactivity in mGluR5-rich brain areas. Metabolite analyses confirmed fast metabolism of the tracer in plasma. The percentage of parent compound in brain tissue exceeded 90 % up to 90 min after injection., Conclusion: [ 18 F]FMTEB produced via copper-mediated 18 F-fluorination fulfilled the requirements for preclinical evaluation in rats. The absence of specific uptake in cerebellum and absence of defluorination of the tracer allowed cerebellum to be used as a reference tissue. Due to the fast kinetics in rats, the region-to-cerebellum ratios equilibrated within 30 min. These results prove [ 18 F]FMTEB to be a good candidate for mapping mGluR5 in rat brain and a suitable alternative to [ 18 F]FPEB., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could appear to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier Inc.)

Background: Hijacking the transferrin receptor (TfR) is an effective strategy to transport amyloid-beta (Aβ) immuno-positron emission tomography (immunoPET) ligands across the blood-brain barrier (BBB). Such ligands are more sensitive and specific than small-molecule ligands at detecting Aβ pathology in mouse models of Alzheimer's disease (AD). This study aimed to determine if this strategy would be as sensitive in rats and to assess how TfR affinity affects BBB transport of bispecific immunoPET radioligands., Methods: Two affinity variants of the rat TfR antibody, OX26, were chemically conjugated to a F(ab') 2 fragment of the anti-Aβ antibody, bapineuzumab (Bapi), to generate two bispecific fusion proteins: OX26 5 -F(ab') 2 -Bapi and OX26 76 -F(ab') 2 -Bapi. Pharmacokinetic analyses were performed 4 h and 70 h post-injection of radioiodinated fusion proteins in wild-type (WT) rats. [ 124 I]I-OX26 5 -F(ab') 2 -Bapi was administered to TgF344-AD and WT rats for in vivo PET imaging. Ex vivo distribution of injected [ 124 I]I-OX26 5 -F(ab') 2 -Bapi and Aβ pathology were assessed., Results: More [ 125 I]I-OX26 5 -F(ab') 2 -Bapi was taken up into the brain 4 h post-administration than [ 124 I]I-OX26 76 -F(ab') 2 -Bapi. [ 124 I]I-OX26 5 -F(ab') 2 -Bapi PET visualized Aβ pathology with significantly higher signals in the TgF344-AD rats than in the WT littermates without Aβ pathology. The PET signals significantly correlated with Aβ levels in AD animals., Conclusion: Affinity to TfR affects how efficiently a TfR-targeting bispecific fusion protein will cross the BBB, such that the higher-affinity bispecific fusion protein crossed the BBB more efficiently. Furthermore, bispecific immunoPET imaging of brain Aβ pathology using TfR-mediated transport provides good imaging contrast between TgF344-AD and WT rats, suggesting that this immunoPET strategy has the potential to be translated to higher species., (© 2022. The Author(s).)

Background: Many malignant tumours have increased TSPO expression, which has been related to a poor prognosis. TSPO-PET tracers have not comprehensively been evaluated in peripherally located tumours. This study aimed to evaluate whether N,N-diethyl-2-(2-(4-([ 18 F]fluoro)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide ([ 18 F]F-DPA) can reflect radiotherapy (RT)-induced changes in TSPO activity in head and neck squamous cell carcinoma (HNSCC)., Methods: RT was used to induce inflammatory responses in HNSCC xenografts and cells. [ 18 F]F-DPA uptake was measured in vivo in non-irradiated and irradiated tumours, followed by ex vivo biodistribution, autoradiography, and radiometabolite analysis. In vitro studies were performed in parental and TSPO-silenced (TSPO siRNA) cells. TSPO protein and mRNA expression, as well as tumour-associated macrophages (TAMs), were also assessed., Results: In vivo imaging and ex vivo measurement revealed significantly higher [ 18 F]F-DPA uptake in irradiated, compared to non-irradiated tumours. In vitro labelling studies with cells confirmed this finding, whereas no effect of RT on [ 18 F]F-DPA uptake was detected in TSPO siRNA cells. Radiometabolite analysis showed that the amount of unchanged [ 18 F]F-DPA in tumours was 95%, also after irradiation. PK11195 pre-treatment reduced the tumour-to-blood ratio of [ 18 F]F-DPA by 73% in xenografts and by 88% in cells. TSPO protein and mRNA levels increased after RT, but were highly variable. The proportion of M1/M2 TAMs decreased after RT, whereas the proportion of monocytes and migratory monocytes/macrophages increased., Conclusions: [ 18 F]F-DPA can detect changes in TSPO expression levels after RT in HNSCC, which does not seem to reflect inflammation. Further studies are however needed to clarify the physiological mechanisms regulated by TSPO after RT.

Unlabelled: In vivo imaging of N-methyl-d-aspartate (NMDA) glutamate receptor and γ-aminobutyric acid (GABA)-A receptor during progression of brain pathology is challenging because of the lack of imaging tracers with high affinity and specificity., Methods: We monitored changes in NMDA receptor and GABA-A receptor in a clinically relevant model of traumatic brain injury (TBI) induced by lateral fluid percussion in adult rats, using 2 new ligands for PET: (18)F-GE-179 for the open/active state of the NMDA receptor ion channel and (18)F-GE-194 for GABA-A receptor. Ex vivo brain autoradiography of radioligands was performed at subacute (5-6 d) and chronic (40-42 d) time points after TBI., Results: At 5-6 d after TBI, (18)F-GE-179 binding was higher in the cortical lesion area, in the lesion core, and in the hippocampus than in the corresponding contralateral regions; this increase was probably related to increased permeability of the blood-brain barrier. At 40-42 d after TBI, (18)F-GE-179 binding was significantly higher in the medial cortex, in the corpus callosum, and in the thalamus than in the corresponding contralateral regions. Five to 6 days after TBI, (18)F-GE-194 binding was significantly higher in the lesion core and significantly lower in the ipsilateral thalamus. By 40-42 d after TBI, the reduction in (18)F-GE-194 binding extended to the cortical lesion, including the perilesional cortex around the lesion core. The reduction in thalamic binding was more extensive at 40-42 d than at 5-6 d after TBI, suggesting a progressive decrease in thalamic GABA-A receptor density. Immunohistochemistry against GABA-A α1 subunit revealed a similar decrease to (18)F-GE-194 binding, particularly during the chronic phase., Conclusion: Our data support the validity of novel (18)F-GE-179 and (18)F-GE-194 radioligands for the detection of changes in active NMDA receptor and GABA-A receptor in the injured brain. These tools are useful for follow-up evaluation of secondary postinjury pathologies., (© 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.)