18F-GE-180: a novel TSPO radiotracer compared to 11C-R-PK11195 in a preclinical model of stroke

PURPOSE: 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. METHODS: 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). RESULTS: 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 (BPND = 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. CONCLUSION: 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

[18F]-FLT Positron Emission Tomography Can Be Used to Image the Response of Sensitive Tumors to PI3-Kinase Inhibition with the Novel Agent GDC-0941

The phosphoinositide 3-kinase (PI3K) pathway is deregulated in a range of cancers, and several targeted inhibitors are entering the clinic. This study aimed to investigate whether the positron emission tomography tracer 3′-deoxy-3′-[ 18 F]fluorothymidine ([ 18 F]-FLT) is suitable to mark the effect of the novel PI3K inhibitor GDC-0941, which has entered phase II clinical trial. CBA nude mice bearing U87 glioma and HCT1 16 colorectal xenografts were imaged at baseline with [ 18 F]-FLT and at acute (18 hours) and chronic (186 hours) time points after twice-daily administration of GDC-0941 (50 mg/kg) or vehicle. Tumor uptake normalized to blood pool was calculated, and tissue was analyzed at sacrifice for PI3K pathway inhibition and thymidine kinase (TK1) expression. Uptake of [ 18 F]-FLT was also assessed in tumors inducibly overexpressing a dominant-negative form of the PI3K p85 subunit p85α, as well as HCT116 liver metastases after GDC-0941 therapy. GDC-0941 treatment induced tumor stasis in U87 xenografts, whereas inhibition of HCT116 tumors was more variable. Tumor uptake of [ 18 F]-FLT was significantly reduced following GDC-0941 dosing in responsive tumors at the acute time point and correlated with pharmacodynamic markers of PI3K signaling inhibition and significant reduction in TK1 expression in U87, but not HCT116, tumors. Reduction of PI3K signaling via expression of Δp85α significantly reduced tumor growth and [ 18 F]-FLT uptake, as did treatment of HCT116 liver metastases with GDC-0941. These results indicate that [ 18 F]-FLT is a str ong candidate for the noninvasive measurement of GDC-0941 action. ©2013 American Association for Cancer Research

[18F]DPA-714: Direct Comparison with [11C]PK11195 in a Model of Cerebral Ischemia in Rats

PURPOSE: Neuroinflammation is involved in several brain disorders and can be monitored through expression of the translocator protein 18 kDa (TSPO) on activated microglia. In recent years, several new PET radioligands for TSPO have been evaluated in disease models. [(18)F]DPA-714 is a TSPO radiotracer with great promise; however results vary between different experimental models of neuroinflammation. To further examine the potential of [(18)F]DPA-714, it was compared directly to [(11)C]PK11195 in experimental cerebral ischaemia in rats. METHODS: Under anaesthesia, the middle cerebral artery of adult rats was occluded for 60 min using the filament model. Rats were allowed recovery for 5 to 7 days before one hour dynamic PET scans with [(11)C]PK11195 and/or [(18)F]DPA-714 under anaesthesia. RESULTS: Uptake of [(11)C]PK11195 vs [(18)F]DPA-714 in the ischemic lesion was similar (core/contralateral ratio: 2.84±0.67 vs 2.28±0.34 respectively), but severity of the brain ischemia and hence ligand uptake in the lesion appeared to vary greatly between animals scanned with [(11)C]PK11195 or with [(18)F]DPA-714. To solve this issue of inter-individual variability, we performed a direct comparison of [(11)C]PK11195 and [(18)F]DPA-714 by scanning the same animals sequentially with both tracers within 24 h. In this direct comparison, the core/contralateral ratio (3.35±1.21 vs 4.66±2.50 for [(11)C]PK11195 vs [(18)F]DPA-714 respectively) showed a significantly better signal-to-noise ratio (1.6 (1.3–1.9, 95%CI) fold by linear regression) for [(18)F]DPA-714. CONCLUSIONS: In a clinically relevant model of neuroinflammation, uptake for both radiotracers appeared to be similar at first, but a high variability was observed in our model. Therefore, to truly compare tracers in such models, we performed scans with both tracers in the same animals. By doing so, our result demonstrated that [(18)F]DPA-714 displayed a higher signal-to-noise ratio than [(11)C]PK11195. Our results suggest that, with the longer half-life of [(18)F] which facilitates distribution of the tracer across PET centre, [(18)F]DPA-714 is a good alternative for TSPO imaging

Time-activity curves of [¹¹C]PK11195 (A and C) and [¹⁸F]DPA-714 (B and D) in the core of the lesion and the contralateral area (animals with visible infarct only) (A, B: HIDAC PET scanner, single tracer and dual scans pooled; C, D: Inveon PET-CT scanner).

<p>Time-activity curves of [¹¹C]PK11195 (A and C) and [¹⁸F]DPA-714 (B and D) in the core of the lesion and the contralateral area (animals with visible infarct only) (A, B: HIDAC PET scanner, single tracer and dual scans pooled; C, D: Inveon PET-CT scanner).</p

Immunohistochemistry (A), autoradiography (B), and PET images (C) from the same animal post-MCAO.

<p>The black squares on the autoradiographs (<b>B</b>) represent the approximate localisation of the immunohistochemistry (<b>A</b>) on adjacent brain sections from the same animal. Characteristically, astrocytes (GFAP antibody, red) delineated the edge of the infarct and abundant microglial cells (CD11b antibody, green) were found in the core of the lesion (left panel), whereas little or no activated astrocyte or microglia could be observed in the contralateral side (right panel). (<b>B</b>) Representative images of autoradiography performed on rat coronal brain sections incubated with [¹⁸F]DPA-714 (18 nM) alone or in presence of PK11195 or unlabelled DPA-714 (20 µM). (<b>C</b>) [¹¹C]PK11195 and [¹⁸F]DPA-74 PET sum images (20–60 min) co-registered with the MRI template of the same animal at similar coronal level.</p

[¹¹C]PK11195 and [¹⁸F]DPA-714 mean uptake values between 20 and 60 min post-injection (expressed as percentage of injected dose per cm³, mean±SD (A) and Core/contralateral ROIs ratio (B)) of the 7 animals scanned with both tracers successively within 24 h (2 animals with no lesion are not shown on these graphs, although there exclusion did not affect the outcome of the statistical analysis).

<p>* Significantly different from [¹¹C]PK11195 values, # significantly different from the contralateral side of the same tracer (Wilcoxon paired test, p<0.05). (<b>C</b>) Correlation between [¹¹C]PK11195 and [¹⁸F]DPA-714 core/contralateral ROIs ratio (all animals included) (dotted line = 95% confidence interval). (<b>D</b>) On the left of each panel, PET images shown are summed images between 20 and 60 min after injection of [¹¹C]PK11195 (left panel) and [¹⁸F]DPA-714 (right panel) co-registered with the MRI template. The ROIs automatically segmented from the corresponding PET images are shown on the right part of each panel. The ROIs are: infarct core (red; ROI covering the infarct in the MCAO territory and/or with the highest uptake), edge-1 (orange; ROI around the MCAO territory and/or with the 2^nd highest uptake), edge-2 (yellow; ROI around the MCAO territory and/or with the 3^rd highest uptake), contralateral ROI (green; ROI with the lowest uptake) and skull edges (white; 5^th ROI segmented in some animals, located on the edge of the skull, this ROI was not included in the statistical analysis).</p

Distribution of the animals in the experimental groups.

*<p>Sprague-Dawley (SD) rats only.</p>†<p>In bracket, two SD rats showed no significant infarct by immunohistochemistry examination.</p>#<p>Wistar rats only.</p>‡<p>Rats were scanned sequentially with [¹¹C]PK11195 and [¹⁸F]DPA-714; six rats were scanned first with [¹¹C]PK11195 then [¹⁸F]DPA-714 and three rats were scanned first with [¹⁸F]DPA-714 then [¹¹C]PK11195.</p