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

Resting-state auditory network in tinnitus: a fMRI study

BACKGROUND AND PURPOSE: Positron emission tomography (PET) has the potential to improve our understanding of the preclinical pharmacokinetics and metabolism of therapeutic agents, and is easily translated to clinical studies in humans. However, studies involving proteins radiolabelled with clinically relevant PET isotopes are currently limited. Here we illustrate the potential of PET imaging in a preclinical study of the biodistribution and metabolism of (18)F-labelled IL-1 receptor antagonist ([(18)F]IL-1RA) using a novel [(18)F]-radiolabelling technique. EXPERIMENTAL APPROACH: IL-1RA was radiolabelled by reductive amination on lysine moieties with [(18)F]fluoroacetaldehyde. Sprague-Dawley rats were injected intravenously with [(18)F]IL-1RA and imaged with a PET camera for 2 h. For the study of IL-1RA metabolites by ex vivoγ-counting of samples, rats were killed 20 min, 1 h or 2 h after injection of [(18)F]IL-1RA. KEY RESULTS: [(18)F]IL-1RA distribution into the major organs of interest was as follows: kidneys >> liver > lungs >> brain. In lungs and liver, [(18)F]IL-1RA uptake peaked within 1 min post-injection then decreased rapidly to reach a plateau from 10 min post-injection. In the brain, the uptake exhibited slower pharmacokinetics with a smaller post-injection peak and a plateau from 6 min onward. IL-1RA was rapidly metabolized and these metabolites represented ∼40% of total activity in plasma and ∼80% in urine, 20 min after injection. CONCLUSIONS AND IMPLICATIONS: Preclinical PET imaging is a feasible method of assessing the biodistribution of new biological compounds of therapeutic interest rapidly. The biodistribution of [(18)F]IL-1RA reported here is in agreement with an earlier study suggesting low uptake in the normal brain, with rapid metabolism and excretion via the kidneys

[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

Spatial organization of the nucleus compartmentalizes and regulates the genome

The nucleus must simultaneously orchestrate DNA replication, transcription, splicing, signalling, and directional transport of proteins into the nucleus and RNA out of the nucleus. Yet the nucleus has no internal membranes to compartmentalize these functions as the cytoplasm does. In fact, such compartmentalization would necessarily be detrimental because particular genes at different locations on the linear chromosomes need to be made at different times while others on the same chromosome need to be tightly shut off. Moreover, expressed genes need to be accessible to a feedback mechanism to determine when to modulate transcription. To accommodate these additional needs the nucleus appears to form microdomains by co-assembly of functional complexes. Thus, microdomains can either form around activated regions on a chromosome or regions on a linear chromosome could be fed into such microdomains for activation. Findings that genome encoded regulatory elements such as enhancers can be hundreds of kb and even Mb apart further highlights the need for such a system as these distal elements must come together in the 3D space of the genome for their efficient functioning. While this much is understood, there is much still to be learned about mechanisms that the nucleus uses to regulate the genome and much more to be learned about how these microdomains come into being. As there is no stable structure within the nucleus except for the nuclear envelope, much recent research has been focusing on potential roles of this subnuclear organelle in establishing 3D nuclear architecture and orchestrating the regulation of these various functions