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

Abstract

Purpose: Over the past 20 years, neuroinflammation (NI) has increasingly been recognised as having an important role in many neurodegenerative diseases, including Alzheimer's disease. As such, being able to image NI non-invasively in patients is critical to monitor pathological processes and potential therapies targeting neuroinflammation. The translocator protein (TSPO) has proven a reliable NI biomarker for positron emission tomography (PET) imaging. However, if TSPO imaging in acute conditions such as stroke provides strong and reliable signals, TSPO imaging in neurodegenerative diseases has proven more challenging. Here, we report results comparing the recently developed TSPO tracers [¹⁸F]GE-180 and [¹⁸F]DPA-714 with (R)-[¹¹C]PK11195 in a rodent model of subtle focal inflammation.

Procedures: Adult male Wistar rats were stereotactically injected with 1 μg lipopolysaccharide in the right striatum. Three days later, animals underwent a 60-min PET scan with (R)-[¹¹C]PK11195 and [¹⁸F]GE-180 (n = 6) or [¹⁸F]DPA-714 (n = 6). Ten animals were scanned with either [¹⁸F]GE-180 (n = 5) or [¹⁸F]DPA-714 (n = 5) only. Kinetic analysis of PET data was performed using the simplified reference tissue model (SRTM) with a contralateral reference region or a novel data-driven input to estimate binding potential BP_ND. Autoradiography and immunohistochemistry were performed to confirm in vivo results.

Results: At 40-60 min post-injection, [¹⁸F]GE-180 dual-scanned animals showed a significantly increased core/contralateral uptake ratio vs. the same animals scanned with (R)-[¹¹C]PK11195 (3.41 ± 1.09 vs. 2.43 ± 0.39, p = 0.03); [¹⁸]DPA-714 did not (2.80 ± 0.69 vs. 2.26 ± 0.41). Kinetic modelling with a contralateral reference region identified significantly higher binding potential (BP_ND) in the core of the LPS injection site with [¹⁸F]GE-180 but not with [¹⁸F]DPA-714 vs. (R)-[¹¹C]PK11195. A cerebellar reference region and novel data-driven input to the SRTM were unable to distinguish differences in tracer BP_ND.

Conclusions: Second-generation TSPO-PET tracers are able to accurately detect mild-level NI. In this model, [¹⁸F]GE-180 shows a higher core/contralateral ratio and BP_ND when compared to (R)-[¹¹C]PK11195, while [¹⁸F]DPA-714 did not.

Keywords: Inflammation; LPS; Preclinical kinetic modelling; Second-generation tracers; TSPO.

Fig. 1.

Chemical structures of TSPO PET radiotracers

Fig. 2.

Average core and contralateral tissue time activity curves (TACs) of LPS-injected animals that underwent dual scans with each tracer are shown. TACs are for a (R)-[¹¹C]PK11195_[ ¹⁸ _F]GE-180, b [¹⁸F]GE-180, c (R)-[¹¹C]PK11195_[ ¹⁸ _F]DPA-714 and d [¹⁸F]DPA-714 over 60 min duration of scans. Core/contralateral ratios indicated on the figures are calculated over the 40–60 min time frame. Dual scan [¹⁸F]GE-180 core/contralateral ratios were significantly higher than e (R)-[¹¹C]PK11195 (p = 0.03), while f [¹⁸F]DPA-714 values were not (Wilcoxon signed rank, p < 0.05). Individual animals are represented by different symbols, which correspond between (R)-[¹¹C]PK11195 and the respective F-18 tracer. Data are presented as mean ± SD

Fig. 3.

Average TACs for cerebellum (black) and contralateral (grey) reference regions for (R)-[¹¹C]PK11195, [¹⁸F]GE-180 and [¹⁸F]DPA-714 in LPS animals. At 40–60 min, the cerebellar uptake (%ID/cm³) was significantly higher for all three tracers (p = 0.001) than the contralateral uptake

Fig. 4.

Co-registered (sum 40–60 min) PET/CT images of representative animals dual-scanned with a and c (R)-[¹¹C]PK11195 followed by either b [¹⁸F]GE-180 (top) or d [¹⁸F]DPA-714 (bottom). Inset top: example of regions defined by automatic segmentation. Inset bottom: [¹⁸F]DPA-714 autoradiographic image showing increased specific uptake in the right striatum

Fig. 5.

a Claudin-5 (×60 magnification) and IgG (×20 magnification) staining in ipsilateral and contralateral striatum for LPS and positive control AMPA-challenged animals. Claudin-5 staining revealed no differences between contralateral and ipsilateral sides in the LPS-challenged animals, while for AMPA-challenged animals, there was visible damage to vessels in the ipsilateral side and some disruption in the contralateral hemisphere. IgG appears normal in ipsilateral and contralateral sides of LPS animals but shows signs of leakage in the ipsilateral side of AMPA animals. b Staining for microglia (CD11b, red) and astrocytes (GFAP, green), merged at ×10 and ×20 (white rectangles) magnification; c NeuN staining at ×20 magnification in core and contralateral striatum