Comparative Evaluation of Three TSPO PET Radiotracers in a LPS-Induced Model of Mild Neuroinflammation in Rats

A Single Intraperitoneal Injection of Endotoxin Changes Glial Cells in Rats as Revealed by Positron Emission Tomography Using [11C]PK11195

Purpose

Intracranial administration of lipopolysaccharide (LPS) is known to elicit a rapid innate immune response, activate glial cells in the brain, and induce depression-like behavior. However, no study has focused on the changes in glial cells induced by intraperitoneal injection of LPS in vivo.

Methods

Ten adult male Fischer F344 rats underwent [¹¹C]PK11195 PET before and 2 days after intraperitoneal injection of LPS to evaluate the changes in glial cells. The difference in standardized uptake values (SUV) of [¹¹C]PK11195 between before and after injection was determined.

Results

There was a cluster of brain regions that showed significant reductions in SUV. This cluster included the bilateral striata and bilateral frontal regions, especially the somatosensory areas.

Conclusions

Changes in activity of glial cells induced by the intraperitoneal injection of LPS were detected in vivo by [¹¹C]PK11195 PET. Intraperitoneal injection of LPS is known to induce depression, and further studies with [¹¹C]PK11195 PET would clarify the relationships between neuroinflammation and depression.

TSPO imaging using the novel PET ligand [18F]GE-180: quantification approaches in patients with multiple sclerosis

Background

PET ligands targeting the translocator protein (TSPO) represent promising tools to visualise neuroinflammation. Here, we analysed parameters obtained in dynamic and static PET images using the novel TSPO ligand [¹⁸F]GE-180 in patients with relapsing remitting multiple sclerosis (RRMS) and an approach for semi-quantitative assessment of this disease in clinical routine.

Seventeen dynamic [¹⁸F]GE-180 PET scans of RRMS patients were evaluated (90 min). A pseudo-reference region (PRR) was defined after identification of the least disease-affected brain area by voxel-based comparison with six healthy controls (HC) and upon exclusion of voxels suspected of being affected in static 60–90 min p.i. images. Standardised uptake value ratios (SUVR) obtained from static images normalised to PRR were correlated to the distribution volume ratios (DVR) derived from dynamic data with Logan reference tissue model.

Results

Group comparison with HC revealed white matter and thalamus as most affected regions. Fewest differences were found in grey matter, and normalisation to frontal cortex (FC) yielded the greatest reduction in variability of healthy grey and white matter. Hence, FC corrected for affected voxels was chosen as PRR, leading to time-activity curves of FC which were congruent to HC data (SUV_60–90 0.37, U test P = 0.42). SUVR showed a very strong correlation with DVR (Pearson ρ > 0.9). Focal MS lesions exhibited a high SUVR (range, 1.3–3.2).

Conclusions

This comparison with parameters from dynamic data suggests that SUVR normalised to corrected frontal cortex as PRR is suitable for the quantification of [¹⁸F]GE-180 uptake in lesions and different brain regions of RRMS patients. This efficient diagnostic protocol based on static [¹⁸F]GE-180 PET scans acquired 60–90 min p.i. allows the semi-quantitative assessment of neuroinflammation in RRMS patients in clinical routine.

KRICT-9 inhibits neuroinflammation, amyloidogenesis and memory loss in Alzheimer's disease models

Alzheimer's disease (AD) is one of the most common forms of dementia and is characterized by neuroinflammation and amyloidogenesis. Here we investigated the effects of KRICT-9 on neuroinflammation and amyloidogenesis in in vitro and in vivo AD models. We found that KRICT-9 decreased lipopolysaccharide (LPS)-induced inflammation in microglial BV-2 cells and astrocytes while reducing nitric oxide generation and expression of inflammatory marker proteins (iNOS and COX-2) as well as APP, BACE1, C99, Iba-1, and GFAP. KRICT-9 also inhibited β-secretase. Pull-down assays and docking model analyses indicated that KRICT-9 binds to the DNA binding domain of signal transducer and activator of transcription 3 (STAT3). KRICT-9 also decreased β-secretase activity and Aβ levels in tissues from LPS-induced mice brains, and it reversed memory impairment in mice. These experiments demonstrated that KRICT-9 protects against LPS-induced neuroinflammation and amyloidogenesis by inhibiting STAT3 activity. This suggests KRICT-9 or KRICT-9-inspired reagents could be used as therapeutic agents to treat AD.

Reduction of PK11195 uptake observed in multiple sclerosis lesions after natalizumab initiation

OBJECTIVE

The objective of this study is to longitudinally analyze the uptake of [¹¹C]PK11195-PET in multiple sclerosis patients after 3 and 6 months of natalizumab treatment.

METHODS

Eighteen MS patients, starting treatment with monocloncal anti-VLA-4, were enrolled in a longitudinal PK-PET study. PK uptake was quantified by volume of distribution (VT) calculation using image-derived input function at baseline, 3 and 6 months. Pharmacokinetic quantification was done using a segmented MRI, and selected areas included white matter, gadolinium enhancing lesions, non-enhancing lesions, cortical grey matter and thalamus. VTs of lesions were calculated in reference to each patient's white matter (VT ratio=VTr), to consider physiologic variability.

RESULTS

Test-retest variability was stable for healthy control (HC). Quantification of PK uptake was completed in 18 patients, and baseline uptake was compared to 6-month uptake. After the start of natalizumab VTr significantly decreased in 13 individual enhancing lesions present within 5 patients (p=0.001). Moreover, VTr of the sum of non-enhancing lesions showed a moderate decrease (p=0.03). No longitudinal changes were detected in normal appearing white matter, the thalamus and cortical grey matter.

CONCLUSION

A reduction in PK11195 uptake was observed in both enhancing and chronic lesions after the start of natalizumab. PK11195 PET can be used as tool to assess the longitudinal change in MS lesions.

[18F]GE-180 PET Detects Reduced Microglia Activation After LM11A-31 Therapy in a Mouse Model of Alzheimer's Disease

Microglial activation is a key pathological feature of Alzheimer's disease (AD). PET imaging of translocator protein 18 kDa (TSPO) is a strategy to detect microglial activation in vivo. Here we assessed flutriciclamide ([18F]GE-180), a new second-generation TSPO-PET radiotracer, for its ability to monitor response to LM11A-31, a novel AD therapeutic in clinical trials. AD mice displaying pathology were treated orally with LM11A-31 for 3 months. Subsequent [18F]GE-180-PET imaging revealed significantly lower signal in cortex and hippocampus of LM11A-31-treated AD mice compared to those treated with vehicle, corresponding with decreased levels of TSPO immunostaining and microglial Iba1 immunostaining. In addition to detecting decreased microglial activation following LM11A-31 treatment, [18F]GE-180 identified activated microglia in AD mice with greater sensitivity than another second-generation TSPO radiotracer, [18F]PBR06. Together, these data demonstrate the promise of [18F]GE-180 as a potentially sensitive tool for tracking neuroinflammation in AD mice and for monitoring therapeutic modulation of microglial activation.

PET imaging of cannabinoid type 2 receptors with [11C]A-836339 did not evidence changes following neuroinflammation in rats

Cannabinoid type 2 receptors (CB2R) have emerged as promising targets for the diagnosis and therapy of brain pathologies. However, no suitable radiotracers for accurate CB2R mapping have been found to date, limiting the investigation of the CB2 receptor expression using positron emission tomography (PET) imaging. In this work, we report the evaluation of the in vivo expression of CB2R with [¹¹C]A-836339 PET after cerebral ischemia and in two rat models of neuroinflammation, first by intrastriatal LPS and then by AMPA injection. PET images and in vitro autoradiography showed a lack of specific [¹¹C]A-836339 uptake in these animal models demonstrating the limitation of this radiotracer to image CB2 receptor under neuroinflammatory conditions. Further, using immunohistochemistry, the CB2 receptor displayed a modest expression increase after cerebral ischemia, LPS and AMPA models. Finally, [¹⁸F]DPA-714-PET and immunohistochemistry demonstrated decreased neuroinflammation by a selective CB2R agonist, JWH133. Taken together, these findings suggest that [¹¹C]A-836339 is not a suitable radiotracer to monitor in vivo CB2R expression by using PET imaging. Future studies will have to investigate alternative radiotracers that could provide an accurate binding to CB2 receptors following brain inflammation.