[en] We recently introduced a hybrid imaging instrument, PETRUS, based on a combination of positron emission tomography (PET) for molecular imaging, x-ray computed tomography (CT) for anatomical imaging, co-registration and attenuation correction, and ultrafast ultrasound imaging (UUI) for motion-correction, hemodynamic and biomechanical imaging. In order to ensure a precise co-registration of simultaneous PET-UUI acquisitions, ultrasound probes attached to an ultrafast ultrasound scanner are operated in the field of view (FOV) of a small animal PET/CT scanner using a remote-controlled micro-positioner. Here we explore the effect of the presence of ultrasound probes on PET image quality. We compare the performance of PET and image quality with and without the presence of probes in the PET field of view, both in vitro following the NEMA-NU-4-2008 standard protocol, and in vivo in small animals. Overall, deviations in the quality of images acquired with and without the ultrasound probes were under 10% and under 7% for the NEMA protocol and in vivo tests, respectively. Our results demonstrate the capability of the PETRUS device to acquire multimodal images in vivo without significant degradation of image quality. (note)

[en] In recent years there has been an increase in the development of radioligands targeting the 18-kDa translocator protein (TSPO). TSPO expression is well documented in activated microglia and serves as a biomarker for imaging neuroinflammation. In addition, TSPO has also been reported to be overexpressed in a number of cancer cell lines and human tumours including glioma. Here we investigated the use of [¹⁸F]DPA-714, a new TSPO positron emission tomography (PET) radioligand to image glioma in vivo. We studied the uptake of [¹⁸F]DPA-714 in three different rat strains implanted with 9L rat glioma cells: Fischer (F), Wistar (W) and Sprague Dawley (SD) rats. Dynamic [¹⁸F]DPA-714 PET imaging, kinetic modelling of PET data and in vivo displacement studies using unlabelled DPA-714 and PK11195 were performed. Validation of TSPO expression in 9L glioma cell lines and intracranial 9L gliomas were investigated using Western blotting and immunohistochemistry of brain tissue sections. All rats showed significant [¹⁸F]DPA-714 PET accumulation at the site of 9L tumour implantation compared to the contralateral brain hemisphere with a difference in uptake among the three strains (F > W > SD). The radiotracer showed high specificity for TSPO as demonstrated by the significant reduction of [¹⁸F]DPA-714 binding in the tumour after administration of unlabelled DPA-714 or PK11195. TSPO expression was confirmed by Western blotting in 9L cells in vitro and by immunohistochemistry ex vivo. The TSPO radioligand [¹⁸F]DPA-714 can be used for PET imaging of intracranial 9L glioma in different rat strains. This preclinical study demonstrates the feasibility of employing [¹⁸F]DPA-714 as an alternative radiotracer to image human glioma. (orig.)

[en] The tumor microenvironment is highly heterogeneous. For gliomas, the tumor-associated inflammatory response is pivotal to support growth and invasion. Factors of glioma growth, inflammation, and invasion, such as the translocator protein (TSPO) and matrix metalloproteinases (MMP), may serve as specific imaging biomarkers of the glioma microenvironment. In this study, noninvasive imaging by PET with [F-18]DPA-714 (TSPO) and [F-18]BR-351 (MMP) was used for the assessment of localization and quantification of the expression of TSPO and MMP. Imaging was performed in addition to established clinical imaging biomarker of active tumor volume ([F-18]FET) in conjunction with MRI. We hypothesized that each imaging biomarker revealed distinct areas of the heterogeneous glioma tissue in a mouse model of human glioma. Tracers were found to be increased 1.4-to 1.7-fold, with [F-18]FET showing the biggest volume as depicted by a thresholding-based, volumes of interest analysis. Tumor areas, which could not be detected by a single tracer and/or MRI parameter alone, were measured. Specific compartments of [F-18]DPA-714 (14%) and [F-18]BR-351 (11%) volumes along the tumor rim could be identified. [F-18]DPA-714 (TSPO) and [F-18]BR-351 (MMP) matched with histology. Glioma-associated microglia/macrophages (GAM) were identified as TSPO and MMP sources. Multi-tracer and multimodal molecular imaging approaches may allow us to gain important insights into glioma-associated inflammation (GAM, MMP). Moreover, this noninvasive technique enables characterization of the glioma microenvironment with respect to the disease-driving cellular compartments at the various disease stages. (authors)

[en] The key role of neuroinflammation in acute and chronic neurological disorders has stimulated the search for specific radiotracers targeting the peripheral benzodiazepine receptor (PBR)/18 kDa translocator protein (TSPO), a hallmark of neuroinflammation. Here we evaluate the new radiotracer for positron emission tomography (PET) [¹⁸F]PBR111 in a rodent model of acute inflammation and compare it with [¹¹C]CLINME, an ¹¹C-labelled tracer of the same chemical family, and with the isoquinolinic carboxamide [¹¹C]PK11195. We studied radiometabolites by HPLC, in vitro binding by autoradiography and in vivo brain kinetics as well as in vivo specificity of binding using PET imaging. We show that this radiotracer has a high in vitro specificity for PBR/TSPO versus central benzodiazepine receptors, as reflected by the drastic reduction of its binding to target tissue by addition of PK11195 or PBR111, while addition of flumazenil does not affect binding. Only intact [¹⁸F]PBR111 is detected in brain up to 60 min after i.v. injection, and PET imaging shows an increased uptake in the lesion as compared to the contralateral side as early as 6 min after injection. Administration of an excess of PK11195 and PBR111, 20 min after [¹⁸F]PBR111 administration, induces a rapid and complete displacement of [¹⁸F]PBR111 binding from the lesion. Modelling of the PET data using the simplified reference tissue model showed increased binding potential (BP) in comparison to [¹¹C]PK11195. [¹⁸F]PBR111 is a metabolically stable tracer with a high specific in vitro and in vivo binding to TSPO. In addition, considering the longer half-life of ¹⁸F over ¹¹C, these results support [¹⁸F]PBR111 as a promising PET tracer of the PBR/TSPO for neuroinflammation imaging. (orig.)

[en] The hypothesis of this study was that distinct experimental glioblastoma phenotypes resembling human disease can be noninvasively distinguished at various disease stages by imaging in vivo. Cultured spheroids from 2 human glioblastomas were implanted into the brains of nude rats. Glioblastoma growth dynamics were followed by PET using ¹⁸F-FDG, ¹¹C-methyl-L-methionine (¹¹C-MET), and 39-deoxy- 39-¹⁸F-fluoro-thymidine (¹⁸F-FLT) and by MRI at 3-6 wk after implantation. For image validation, parameters were co-registered with immunohistochemical analysis. Two tumor phenotypes (angiogenic and infiltrative) were obtained. The angiogenic phenotype showed high uptake of ¹¹C-MET and ¹⁸F-FLT and relatively low uptake of ¹⁸F-FDG. ¹¹C-MET was an early indicator of vessel remodeling and tumor proliferation. ¹⁸F-FLT uptake correlated to positive Ki67 staining at 6 wk. T1- and T2-weighted MR images displayed clear tumor delineation with strong gadolinium enhancement at 6 wk. The infiltrative phenotype did not accumulate ¹¹C-MET and ¹⁸F-FLT and impaired the ¹⁸F-FDG uptake. In contrast, the Ki67 index showed a high proliferation rate. The extent of the infiltrative tumors could be observed by MRI but with low contrast. For angiogenic glioblastomas, noninvasive assessment of tumor activity corresponds well to immunohistochemical markers, and ¹¹C-MET was more sensitive than ¹⁸F-FLT at detecting early tumor development. In contrast, infiltrative glioblastoma growth in the absence of blood-brain barrier breakdown is difficult to noninvasively follow by existing imaging techniques, and a negative ¹⁸F-FLT PET result does not exclude the presence of proliferating glioma tissue. The angiogenic model may serve as an advanced system to study imaging-guided anti-angiogenic and antiproliferative therapies. (authors)

[en] Resistance to bevacizumab (BEV) in glioblastoma is believed to occur via activation of molecular networks including the mTOR/PI3K pathway. Using an MR/PET molecular imaging biomarker approach, we investigated the response to combining BEV with the mTOR/PI3K inhibitor BEZ235. Tumours were established by orthotopically implanting U87MG-luc2 cells in mice. Animals were treated with BEZ235 and/or BEV, and imaged using diffusion-weighted-MRI, T2-weighted and T2*-weighted before and after administration of superparamagnetic iron oxide contrast agent. Maps for changes in relaxation rates (ΔR2, ΔR2* and apparent diffusion coefficient) were calculated. Vessel size index and microvessel density index were derived. 3'-Deoxy-3'-["1"8F]fluorothymidine (["1"8F]FLT) PET and O-(2-["1"8F]fluoroethyl)-l-tyrosine (["1"8F]FET) PET were further performed and tumour endothelium/proliferation markers assessed by immunohistochemistry. Treatment with BEV resulted in a pronounced decrease in tumour volume (T2-weighted MRI). No additive effect on tumour volume was observed with the BEV/BEZ235 combination compared with BEV monotherapy. The Ki67 proliferation index and ["1"8F]FLT uptake studies were used to support the observations. Using ΔR2* and ΔR2 values, respectively, the BEV/BEZ235 combination significantly reduced tumour microvessel volume in comparison to BEV alone. Decreased microvessel density index was further observed in animals treated with the combination, supported by von Willebrand factor (vWF) immunohistochemistry. ["1"8F]FET uptake was decreased following treatment with BEV alone, but was not further reduced following treatment with the combination. vWF immunohistochemistry analysis showed that the mean tumour vessel size was increased in all cohorts. Assessing MR imaging biomarker parameters together with ["1"8F]FET and ["1"8F]FLT PET provided information on mechanism of action of the drug combination and clues as to potential clinical responses. Following translation to clinical use, treatment with a BEV/BEZ235 combination could reduce peritumoral oedema obviating the requirement for steroids. The use of hypothesis-driven molecular imaging studies facilitates the preclinical evaluation of drug response. Studies of this kind may more accurately predict the clinical potential of the BEV/BEZ235 combination regimen as a novel therapeutic approach in oncology. (orig.)