It remains unclear how different translocator protein (TSPO) ligands reflect the spatial extent of astrocyte or microglial activation in various neuroinflammatory conditions. Here, we use a reproducible lipopolysaccharide (LPS)-induced model of acute central nervous system inflammation to compare the binding performance of a new TSPO ligand (18)F-GE-180 with (11)C-(R)-PK11195. Using immunohistochemistry, we also explore the ability of the TSPO ligands to detect activated microglial cells and astrocytes. Lewis rats (n = 30) were microinjected with LPS (1 or 10 μg) or saline (1 μL) into the left striatum. The animals were imaged in vivo at 16 h after the injection using PET radiotracers (18)F-GE-180 or (11)C-(R)-PK11195 (n = 3 in each group) and were killed afterward for autoradiography of the brain. Immunohistochemical assessment of OX-42 and glial fibrillary acidic protein (GFAP) was performed to identify activated microglial cells and reactive astrocytes. In vivo PET imaging revealed an increase in the ipsilateral TSPO binding, compared with binding in the contralateral hemisphere, after the microinjection of 10 μg of LPS. No increase was observed with vehicle. By autoradiography, the TSPO radiotracer binding potential in the injected hemisphere was increased after striatal injection of 1 or 10 μg of LPS. However, the significant increase was observed only when using (18)F-GE-180. The area of CD11b-expressing microglial cells extended beyond that of enhanced GFAP staining and mapped more closely to the extent of (18)F-GE-180 binding than to (11)C-(R)-PK11195 binding. The signal from either PET ligand was significantly increased in regions of increased GFAP immunoreactivity and OX-42 colocalization, meaning that the presence of both activated microglia and astrocytes in a given area leads to increased binding of the TSPO radiotracers. (18)F-GE-180 is able to reveal sites of activated microglia in both gray and white matter. However, the signal is increased by the presence of activated astrocytes. Therefore, (18)F-GE-180 is a promising new fluorinated longer-half-life tracer that reveals the presence of activated microglia in a manner that is superior to (11)C-(R)-PK11195 due to the higher binding potential observed for this ligand.

Neuroinflammation plays a critical role in various neuropathological conditions, and hence there is renewed interest in the translocator protein (TSPO) as a biomarker of microglial activation and macrophage infiltration in the brain. This is reflected in the large amount of research conducted seeking to replace the prototypical PET radiotracer ¹¹C-R-PK11195 with a TSPO ligand with higher performance. Here we report the in vivo preclinical investigation of the novel TSPO tracer ¹⁸F-GE-180 in a rat model of stroke. Focal cerebral ischaemia was induced in Wistar rats by 60-min occlusion of the middle cerebral artery (MCAO). Brain damage was assessed 24 h after MCAO by T2 MRI. Rats were scanned with ¹¹C-R-PK11195 and ¹⁸F-GE-180 5 or 6 days after MCAO. Specificity of binding was confirmed by injection of unlabelled R-PK11195 or GE-180 20 min after injection of ¹⁸F-GE-180. In vivo data were confirmed by ex vivo immunohistochemistry for microglial (CD11b) and astrocytic biomarkers (GFAP). ¹⁸F-GE-180 uptake was 24 % higher in the core of the ischaemic lesion and 18 % lower in the contralateral healthy tissue than that of ¹¹C-R-PK11195 uptake (1.5 ± 0.2-fold higher signal to noise ratio). We confirmed this finding using the simplified reference tissue model (BP_ND = 3.5 ± 0.4 and 2.4 ± 0.5 for ¹⁸F-GE-180 and ¹¹C-R-PK11195, respectively, with R ₁ = 1). Injection of unlabelled R-PK11195 or GE-180 20 min after injection of ¹⁸F-GE-180 significantly displaced ¹⁸F-GE-180 (69 ± 5 % and 63 ± 4 %, respectively). Specificity of the binding was also confirmed by in vitro autoradiography, and the location and presence of activated microglia and infiltrated macrophages were confirmed by immunohistochemistry. The in vivo binding characteristics of ¹⁸F-GE-180 demonstrate a better signal to noise ratio than ¹¹C-R-PK11195 due to both a better signal in the lesion and lower nonspecific binding in healthy tissue. These results provide evidence that ¹⁸F-GE-180 is a strong candidate to replace ¹¹C-R-PK11195.

Alzheimer's disease (AD) is the most common cause of dementia. Neuroinflammation appears to play an important role in AD pathogenesis. Ligands of the 18 kDa translocator protein (TSPO), a marker for activated microglia, have been used as positron emission tomography (PET) tracers to reflect neuroinflammation in humans and mouse models. Here, we used the novel TSPO-targeted PET tracer¹⁸F-GE180 (flutriciclamide) to investigate differences in neuroinflammation between young and old WT and APP/PS1dE9 transgenic (Tg) mice.In vivoPET scans revealed an overt age-dependent elevation in whole-brain uptake of¹⁸F-GE180 in both WT and Tg mice, and a significant increase in whole-brain uptake of¹⁸F-GE180 (peak-uptake and retention) in old Tg mice compared with young Tg mice and all WT mice. Similarly, the¹⁸F-GE180 binding potential in hippocampus was highest to lowest in old Tg > old WT > young Tg > young WT mice using MRI coregistration.Ex vivoPET and autoradiography analysis further confirmed ourin vivoPET results: enhanced uptake and specific binding (SUV_75%) of¹⁸F-GE180 in hippocampus and cortex was highest in old Tg mice followed by old WT, young Tg, and finally young WT mice.¹⁸F-GE180 specificity was confirmed by anin vivocold tracer competition study. We also examined¹⁸F-GE180 metabolites in 4-month-old WT mice and found that, although total radioactivity declined over 2 h, of the remaining radioactivity, ∼90% was due to parent¹⁸F-GE180. In conclusion,¹⁸F-GE180 PET scans may be useful for longitudinal monitoring of neuroinflammation during AD progression and treatment.SIGNIFICANCE STATEMENTMicroglial activation, a player in Alzheimer's disease (AD) pathogenesis, is thought to reflect neuroinflammation. Usingin vivomicroPET imaging with a novel TSPO radioligand,¹⁸F-GE180, we detected significantly enhanced neuroinflammation during normal aging in WT mice and in response to AD-associated pathology in APP/PS1dE9 Tg mice, an AD mouse model. Increased uptake and specific binding of¹⁸F-GE180 in whole brain and hippocampus were confirmed byex vivoPET and autoradiography. The binding specificity and stability of¹⁸F-GE180 was further confirmed by a cold tracer competition study and a metabolite study, respectively. Therefore,¹⁸F-GE180 PET imaging may be useful for longitudinal monitoring of neuroinflammation during AD progression and treatment and may also be useful for other neurodegenerative diseases.

Despite numerous evidences for neurotoxicity of overexpressed alpha-synuclein, a protective function was suggested for endogenous alpha-synuclein and other members of the synuclein family. This protective role is most important for and evident in presynaptic terminals, where synucleins are normally accumulated. However, mice lacking synucleins display no adverse phenotype. In particular, no significant changes in striatal dopamine metabolism and only subtle deficit of dopaminergic neurons in the substantia nigra were found in juvenile or adult mice. To assess whether aging and synuclein deficiency may have additive detrimental effect on the nigrostriatal system, we studied dopaminergic neurons of the substantia nigra and their striatal synapses in 24-26-month-old alpha-synuclein and gamma-synuclein null mutant mice. Significant approximately 36% reduction of the striatal dopamine was found in aging alpha-synuclein, but not gamma-synuclein null mutant mice when compared to age-matching wild type mice. This was accompanied by the reduction of TH-positive fibers in the striatum and decrease of striatal levels of TH and DAT. However, no progressive loss of TH-positive neurons was revealed in the substantia nigra of synuclein-deficient aging animals. Our results are consistent with a hypothesis that alpha-synuclein is important for normal function and integrity of synapses, and suggest that in the aging nervous system dysfunction of this protein could become a predisposition factor for the development of nigrostriatal pathology.