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Mouse models of Alzheimer's disease (AD) are valuable but do not fully recapitulate human AD pathology, such as spontaneous Tau fibril accumulation and neuronal loss, necessitating the development of new AD models. The transgenic (TG) TgF344-AD rat has been reported to develop age-dependent AD features including neuronal loss and neurofibrillary tangles, despite only expressing APP and PSEN1 mutations, suggesting an improved modelling of AD hallmarks. Alterations in neuronal networks as well as learning performance and cognition tasks have been reported in this model, but none have combined a longitudinal, multimodal approach across multiple centres, which mimics the approaches commonly taken in clinical studies. We therefore aimed to further characterise the progression of AD-like pathology and cognition in the TgF344-AD rat from young-adults (6 months (m)) to mid- (12 m) and advanced-stage (18 m, 25 m) of the disease. Methods: TgF344-AD rats and wild-type (WT) littermates were imaged at 6 m, 12 m and 18 m with [18F]DPA-714 (TSPO, neuroinflammation), [18F]Florbetaben (Aβ) and [18F]ASEM (α7-nicotinic acetylcholine receptor) and with magnetic resonance spectroscopy (MRS) and with (S)-[18F]THK5117 (Tau) at 15 and 25 m. Behaviour tests were also performed at 6 m, 12 m and 18 m. Immunohistochemistry (CD11b, GFAP, Aβ, NeuN, NeuroChrom) and Tau (S)-[18F]THK5117 autoradiography, immunohistochemistry and Western blot were also performed. Results: [18F]DPA-714 positron emission tomography (PET) showed an increase in neuroinflammation in TG vs wildtype animals from 12 m in the hippocampus (+11%), and at the advanced-stage AD in the hippocampus (+12%), the thalamus (+11%) and frontal cortex (+14%). This finding coincided with strong increases in brain microgliosis (CD11b) and astrogliosis (GFAP) at these time-points as assessed by immunohistochemistry. In vivo [18F]ASEM PET revealed an age-dependent increase uptake in the striatum and pallidum/nucleus basalis of Meynert in WT

Mouse models of Alzheimer's disease (AD) are valuable but do not fully recapitulate human AD pathology, such as spontaneous Tau fibril accumulation and neuronal loss, necessitating the development of new AD models. The transgenic (TG) TgF344-AD rat has been reported to develop age-dependent AD features including neuronal loss and neurofibrillary tangles, despite only expressing APP and PSEN1 mutations, suggesting an improved modelling of AD hallmarks. Alterations in neuronal networks as well as learning performance and cognition tasks have been reported in this model, but none have combined a longitudinal, multimodal approach across multiple centres, which mimics the approaches commonly taken in clinical studies. We therefore aimed to further characterise the progression of AD-like pathology and cognition in the TgF344-AD rat from young-adults (6 months (m)) to mid- (12 m) and advanced-stage (18 m, 25 m) of the disease. Methods: TgF344-AD rats and wild-type (WT) littermates were imaged at 6 m, 12 m and 18 m with [18F]DPA-714 (TSPO, neuroinflammation), [18F]Florbetaben (Aβ) and [18F]ASEM (α7-nicotinic acetylcholine receptor) and with magnetic resonance spectroscopy (MRS) and with (S)-[18F]THK5117 (Tau) at 15 and 25 m. Behaviour tests were also performed at 6 m, 12 m and 18 m. Immunohistochemistry (CD11b, GFAP, Aβ, NeuN, NeuroChrom) and Tau (S)-[18F]THK5117 autoradiography, immunohistochemistry and Western blot were also performed. Results: [18F]DPA-714 positron emission tomography (PET) showed an increase in neuroinflammation in TG vs wildtype animals from 12 m in the hippocampus (+11%), and at the advanced-stage AD in the hippocampus (+12%), the thalamus (+11%) and frontal cortex (+14%). This finding coincided with strong increases in brain microgliosis (CD11b) and astrogliosis (GFAP) at these time-points as assessed by immunohistochemistry. In vivo [18F]ASEM PET revealed an age-dependent increase uptake in the striatum and pallidum/nucleus basalis of Meynert in WT

The Suzuki-type cross coupling reaction is a palladium-mediated multistepreaction that has been used to synthesize several11C-labeled tracers for PET.However, the impact of the selected organoborane reagent and reactionmedium on the radiochemical yield (RCY) has not been thoroughly investi-gated. To bridge this gap, we studied the synthesis of 1-[11C]methyl-naphthalene using four different organoborane precursors in reactionsperformed in DMF/water and THF/water. In the synthesis of 1-[11C]methyl-naphthalene, the best radiochemical yields (RCYs), approximately 50%,were obtained with boronic acid and pinacol ester precursors, whereasless than 4% RCY was obtained when performing the reaction with theN-methylimidodiacetic acid boronic ester (MIDA ester) precursor. 1-[11C]methylnaphthalene was obtained in higher yields in almost all syntheses per-formed in THF/water as compared to DMF/water. This observation was in linewith previously reported results for [11C]UCB-J, a tracer for the synaptic vesi-cle glycoprotein 2A (SV2A) receptor, that also was obtained in higher RCYwhen synthesized in THF/water. The same trend was observed with [11C]cetrozole, where the RCY was more than doubled in THF/water compared tothe previously published synthesis performed in DMF. These results suggestthat THF/water could be the preferred reaction medium when producing PETtracers via the Suzuki-type coupling reaction.

The positron emission tomography (PET) radioligand [C-11]UCB-J binds to synaptic vesicle protein 2A (SV2A) and is used to investigate synaptic density in the living brain. Clinical studies have indicated reduced [C-11]UCB-J binding in Alzheimer's disease (AD) and Parkinson's disease (PD) brains compared to healthy controls. Still, it is unknown whether [C-11]UCB-J PET can visualise synaptic loss in mouse models of these disorders. Such models are essential for understanding disease pathology and for evaluating the effects of novel disease-modifying drug candidates. In the present study, synaptic density in transgenic models of AD (ArcSwe) and PD (L61) was stud-ied using [C-11]UCB-J PET. Data were acquired during 60 min after injection, and time-activity curves (TACs) in different brain regions and the left ventricle of the heart were generated based on the dynamic PET images. The [C-11]UCB-J brain concentrations were expressed as standardised uptake value (SUV) over time. The area under the SUV curve (AUC), the ratio of AUC in the brain to that in the heart (AUCbrain/blood), and the volume of distribution (VT) obtained by kinetic modelling using the heart TAC as input were compared between trans-genic and age-matched wild type (WT) mice. The L61 mice displayed 11-13% lower AUCbrain/blood ratio and brain VT generated by kinetic modeling compared to the control WT mice. In general, also transgenic ArcSwe mice tended to show lower [C-11]UCB-J brain exposure than age-matched WT controls, but variation within the different animal groups was high. Older WT mice (18-20 months) showed lower [C-11]UCB-J brain exposure than younger WT mice (8-9 months). Together, these data imply that [C-11]UCB-J PET reflects synaptic density in mouse models of neurodegeneration and that inter-subject variation is large. In addition, the study suggested that model-independent AUCbrain/blood ratio can be used to evaluate [C-11]UCB-J binding as an alternative to full pharmacokinetic modelling.

Antibodies are attractive as radioligands due to their outstanding specificity and high affinity, but their inability to cross the blood–brain barrier (BBB) limits their use for CNS targets. To enhance brain distribution, amyloid-β (Aβ) antibodies were fused to a transferrin receptor (TfR) antibody fragment, enabling receptor mediated transport across the BBB. The aim of this study was to label these bispecific antibodies with fluorine-18 and use them for Aβ PET imaging. Bispecific antibody ligands RmAb158-scFv8D3 and Tribody A2, both targeting Aβ and TfR, were functionalized with trans-cyclooctene (TCO) groups and conjugated with 18F-labeled tetrazines through an inverse electron demand Diels–Alder reaction performed at ambient temperature. 18F-labeling did not affect antibody binding in vitro, and initial brain uptake was high. Conjugates with the first tetrazine variant ([18F]T1) displayed high uptake in bone, indicating extensive defluorination, a problem that was resolved with the second and third tetrazine variants ([18F]T2 and [18F]T3). Although the antibody ligands’ half-life in blood was too long to optimally match the physical half-life of fluorine-18 (t1/2 = 110 min), [18F]T3-Tribody A2 PET seemed to discriminate transgenic mice (tg-ArcSwe) with Aβ deposits from wild-type mice 12 h after injection. This study demonstrates that 18F-labeling of bispecific, brain penetrating antibodies is feasible and, with further optimization, could be used for CNS PET imaging.

Objective To investigate brain amyloid pathology in a dementia-risk population defined as cardiovascular risk factors, aging, and dementia risk (CAIDE) score of at least 6 but with normal cognition and to examine associations between brain amyloid load and cognitive performance and vascular risk factors. Methods A subgroup of 48 individuals from the Finnish Geriatric Intervention Study to Prevent Cognitive Impairment and Disability (FINGER) main study participated in brain C-11-Pittsburgh compound B (PiB)-PET imaging, brain MRI, and neuropsychological assessment at the beginning of the study. Lifestyle/vascular risk factors were determined as body mass index, blood pressure, total and low-density lipoprotein cholesterol, and glucose homeostasis model assessment. White matter lesions were visually rated from MRIs by a semiquantitative Fazekas score. Results Twenty participants (42%) had a positive PiB-PET on visual analysis. The PiB-positive group performed worse in executive functioning tests, included more participants with APOE epsilon 4 allele (50%), and showed slightly better glucose homeostasis compared to PiB-negative participants. PiB-positive and -negative participants did not differ significantly in other cognitive domain scores or other vascular risk factors. There was no significant difference in Fazekas score between the PiB groups. Conclusions The high percentage of PiB-positive participants provides evidence of a successful recruitment process of the at-risk population in the main FINGER intervention trial. The results suggest a possible association between early brain amyloid accumulation and decline in executive functions. APOE epsilon 4 was clearly associated with amyloid positivity, but no other risk factor was found to be associated with positive PiB-PET.

Positron emission tomography (PET) imaging of amyloid-β (Aβ) has become an important component of Alzheimer´s disease (AD) diagnosis. 11C-Pittsburgh compound B (11C -PiB) and other analogs bind to fibrillar Aβ and reflect the load of insoluble deposits. However, levels of nonfibrillar, soluble aggregates of Aβ, appear more dynamic during disease progression and more affected by Aβ reducing treatments. The aim of this study was to compare an antibody-based PET ligand, targeting nonfibrillar Aβ, with 11C-PiB after β-secretase (BACE-1) inhibition in two mouse models of AD at an advanced stage of Aβ pathology. Methods: Transgenic ArcSwe mice (16 months) were treated with the BACE-1 inhibitor NB-360 for 2 months, while another group was kept as controls. A third group was analyzed at the age of 16 months as baseline. Each mouse was PET scanned with 11C-PiB to measure Aβ plaque load followed by an antibody scan with the bispecific radioligand 124I-RmAb158-scFv8D3 to investigate nonfibrillar aggregates of Aβ. The same study design was then applied for another mouse model, AppNL-G-F. In this case, NB-360 treatment was initiated at the age of 8 months and animals were scanned with 11C-PiB-PET and 125I-RmAb158-scFv8D3 single-photon emission computer tomography (SPECT). Brain tissue was isolated after scanning and Aβ levels were assessed with ELISA. Results: 124I-RmAb158-scFv8D3 concentrations measured with PET in hippocampus and thalamus of NB-360 treated ArcSwe mice were similar to those observed in baseline animals and significantly lower than concentrations observed in same-age untreated controls. Reduced 125I-RmAb158-scFv8D3 retention was also observed with SPECT in hippocampus, cortex and cerebellum of NB-360 treated AppNL-G-F mice. Radioligand in vivo concentrations corresponded with postmortem brain tissue analysis of soluble Aβ aggregates. For both models, mice treated with NB-360 did not display a reduced 11C-PiB signal compared to untreated controls, and further, bo

Synaptic alterations in certain brain structures are related to cognitive decline in neurodegeneration and aging. Synaptic loss in many neurodegenerative diseases can be visualized by positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A). However, the use of SV2A PET for studying synaptic changes during aging is not particularly explored. Thus, in the present study, PET ligand [18F]SynVesT-1, which binds to SV2A, was used to investigate synaptic density at different ages in healthy mice. Wild type C57Bl/6 mice divided into three age groups (4-5 months (n = 7), 12-14 months (n = 11), 17-19 months (n = 8)) were PET scanned with [18F]SynVesT-1. Brain retention of [18F]SynVesT-1 expressed as the volume of distribution (VT) was calculated using an image-derived input function. Estimates of VT were derived using either a one-tissue compartment model (1TCM), a two-tissue compartment model (2TCM), or the Logan plot with blood input to find the best-fit model for [18F]SynVesT-1. After the PET scans, tissue sections were immunostained for the detection of SV2A and neuronal markers. We found that [18F]SynVesT-1 data acquired 60 min post intravenously injection and analyzed with 1TCM described the brain pharmacokinetics of the radioligand in mice well. [18F]SynVesT-1 brain retention was lower in the oldest group of mice, indicating a decrease in synaptic density in this age group. However, no gradual age-dependent decrease in synaptic density at a region-specific level was observed. Immunostaining indicated that SV2A expression and neuron numbers were similar across all three age groups. In general, these data obtained in healthy aging mice are consistent with previous findings in humans where synaptic density appeared stable during aging up to a certain age, after which a small decrease is observed.