4-nitroquinoline 1-oxide-induced oral epithelial lesions exhibit time- and stage-dependent changes in the tumor immune microenvironment.

Oral tongue squamous cell carcinoma (OTSCC) is the most common cancer of the oral cavity and is associated with high morbidity due to local invasion and lymph node metastasis. Tumor infiltrating lymphocytes (TILs) are associated with good prognosis in oral cancer patients and dictate response to treatment. Ectopic sites for immune activation in tumors, known as tertiary lymphoid structures (TLS), and tumor-associated high-endothelial venules (TA-HEVs), which are specialized lymphocyte recruiting vessels, are associated with a favorable prognosis in OSCC. Why only some tumors support the development of TLS and HEVs is poorly understood. In the current study we explored the infiltration of lymphocyte subsets and the development of TLS and HEVs in oral epithelial lesions using the 4-nitroquinoline 1-oxide (4NQO)-induced mouse model of oral carcinogenesis. We found that the immune response to 4NQO-induced oral epithelial lesions was dominated by T cell subsets. The number of T cells (CD4+, FoxP3+, and CD8+), B cells (B220+) and PNAd+ HEVs increased from the earliest to the latest endpoints. All the immune markers increased with the severity of the dysplasia, while the number of HEVs and B cells further increased in SCCs. HEVs were present already in early-stage lesions, while TLS did not develop at any timepoint. This suggests that the 4NQO model is applicable to study the dynamics of the tumor immune microenvironment at early phases of oral cancer development, including the regulation of TA-HEVs in OTSCC.

Weakly supervised semantic segmentation for MRI: exploring the advantages and disadvantages of class activation maps for biological image segmentation with soft boundaries.

Fully supervised semantic segmentation models require pixel-level annotations that are costly to obtain. As a remedy, weakly supervised semantic segmentation has been proposed, where image-level labels and class activation maps (CAM) can detect discriminative regions for specific class objects. In this paper, we evaluated several CAM methods applied to different convolutional neural networks (CNN) to highlight tissue damage of cod fillets with soft boundaries in MRI. Our results show that different CAM methods produce very different CAM regions, even when applying them to the same CNN model. CAM methods that claim to highlight more of the class object do not necessarily highlight more damaged regions or originate from the same high discriminatory regions, nor do these damaged regions show high agreement across the different CAM methods. Additionally, CAM methods produce damaged regions that do not align with external reference metrics, and even show correlations contrary to what can be expected.

Glioblastoma PET/MRI: kinetic investigation of [18F]rhPSMA-7.3, [18F]FET and [18F]fluciclovine in an orthotopic mouse model of cancer.

PURPOSE: Glioblastoma multiforme (GBM) is the most common glioma and standard therapies can only slightly prolong the survival. Neo-vascularization is a potential target to image tumor microenvironment, as it defines its brain invasion. We investigate [18F]rhPSMA-7.3 with PET/MRI for quantitative imaging of neo-vascularization in GBM bearing mice and human tumor tissue and compare it to [18F]FET and [18F]fluciclovine using PET pharmacokinetic modeling (PKM). METHODS: [18F]rhPSMA-7.3, [18F]FET, and [18F]fluciclovine were i.v. injected with 10.5 ± 3.1 MBq, 8.0 ± 2.2 MBq, 11.5 ± 1.9 MBq (n = 28, GL261-luc2) and up to 90 min PET/MR imaged 21/28 days after surgery. Regions of interest were delineated on T2-weighted MRI for (i) tumor, (ii) brain, and (iii) the inferior vena cava. Time-activity curves were expressed as SUV mean, SUVR and PKM performed using 1-/2-tissue-compartment models (1TCM, 2TCM), Patlak and Logan analysis (LA). Immunofluorescent staining (IFS), western blotting, and autoradiography of tumor tissue were performed for result validation. RESULTS: [18F]rhPSMA-7.3 showed a tumor uptake with a tumor-to-background-ratio (TBR) = 2.1-2.5, in 15-60 min. PKM (2TCM) confirmed higher K1 (0.34/0.08, p = 0.0012) and volume of distribution VT (0.24/0.1, p = 0.0017) in the tumor region compared to the brain. Linearity in LA and similar k3 = 0.6 and k4 = 0.47 (2TCM, tumor, p = ns) indicated reversible binding. K1, an indicator for vascularization, increased (0.1/0.34, 21 to 28 days, p < 0.005). IFS confirmed co-expression of PSMA and tumor vascularization. [18F]fluciclovine showed higher TBR (2.5/1.8, p < 0.001, 60 min) and VS (1.3/0.7, p < 0.05, tumor) compared to [18F]FET and LA indicated reversible binding. VT increased (p < 0.001, tumor, 21 to 28 days) for [18F]FET (0.5-1.4) and [18F]fluciclovine (0.84-1.5). CONCLUSION: [18F]rhPSMA-7.3 showed to be a potential candidate to investigate the tumor microenvironment of GBM. Following PKM, this uptake was associated with tumor vascularization. In contrast to what is known from PSMA-PET in prostate cancer, reversible binding was found for [18F]rhPSMA-7.3 in GBM, contradicting cellular trapping. Finally, [18F]fluciclovine was superior to [18F]FET rendering it more suitable for PET imaging of GBM.

Magnetic resonance imaging for non-invasive measurement of plastic ingestion in marine wildlife.

Monitoring plastic ingestion by marine wildlife is important for both characterizing the extent of plastic pollution in the environment and understanding its effect on species and ecosystems. Current methods to detect plastic in the digestive system of animals are slow and invasive, such that the number of animals that can be screened is limited. In this article, magnetic resonance imaging (MRI) is investigated as a possible technology to perform rapid, non-invasive detection of plastic ingestion. Standard MRI methods were able to directly measure one type of plastic in a fulmar stomach and another type was able to be indirectly detected. In addition to MRI, other standard nuclear magnetic resonance (NMR) measurements were made. Different types of plastic were tested, and distinctive NMR signal characteristics were found in common for each type, allowing them to be distinguished from one another. The NMR results indicate specialized MRI sequences could be used to directly image several types of plastic. Although current commercial MRI technology is not suitable for field use, existing single-sided MRI research systems could be adapted for use outside the laboratory and become an important tool for future monitoring of wild animals.

The marine natural product mimic MPM-1 is cytolytic and induces DAMP release from human cancer cell lines.

Bioprospecting contributes to the discovery of new molecules with anticancer properties. Compounds with cytolytic activity and the ability to induce immunogenic cell death can be administered as intratumoral injections with the aim to activate anti-tumor immune responses by causing the release of tumor antigens as well as damage-associated molecular patterns (DAMPs) from dying cancer cells. In the present study, we report the cytolytic and DAMP-releasing effects of a new natural product mimic termed MPM-1 that was inspired by the marine Eusynstyelamides. We found that MPM-1 rapidly killed cancer cells in vitro by inducing a necrosis-like death, which was accompanied by lysosomal swelling and perturbation of autophagy in HSC-3 (human oral squamous cell carcinoma) cells. MPM-1 also induced release of the DAMPs adenosine triphosphate (ATP) and high mobility group box 1 (HMGB1) from Ramos (B-cell lymphoma) and HSC-3 cells, as well as cell surface expression of calreticulin in HSC-3 cells. This indicates that MPM-1 has the ability to induce immunogenic cell death, further suggesting that it may have potential as a novel anticancer compound.

Diffusion tensor imaging for spatially-resolved characterization of muscle fiber structure in seafood.

The fiber structure of tissue in meat and seafood has a significant impact on their perceived quality. However, quantifiable description of muscle structure is challenging. We investigate diffusion tensor imaging (DTI) magnetic resonance imaging (MRI) as a method to quantitatively describe tissue structure. DTI measures the anisotropy of water molecule diffusion within muscle fibers. A pilot study evaluated three different cod loin samples: one of high-quality, one of medium-quality, and one of poor-quality. DTI parameters such as fractional anisotropy, axial diffusion and radial diffusion showed clear differences between the sample qualities. Changes in the DTI metrics consistent with freezing and thawing damage to the tissue were observed. The DTI maps were compared to T2-weighted images and DTI detected significant details that were not visible in T2-weighted images. Overall, these results indicate that DTI is a promising method for spatially-resolved characterization of tissue structure in seafood and meat.

Leptin counteracts hypothermia in hypothyroidism through its pyrexic effects and by stabilizing serum thyroid hormone levels.

OBJECTIVE: Thyroid hormones (TH) are essential for the homeostatic control of energy metabolism and the regulation of body temperature. The hypothalamic-pituitary-thyroid (HPT) axis is regulated by negative feedback mechanisms, ensuring that TH levels are maintained at a constant level. However, the feedback mechanisms underlying the resetting of the HPT axis regulation in the control of body temperature are still not fully understood. Here, we aimed to determine the thermoregulatory response in hypothyroid mice to different environmental temperatures and the underlying mechanisms. METHODS: Distinct thermogenic challenges were induced in hypothyroid female C57BL/6N and leptin-deficient ob/ob mice through housing at either room temperature or thermoneutrality. The thermogenic and metabolic effects were analyzed through metabolic chambers, 18F-FDG-PET/MRI, infrared thermography, metabolic profiling, histology, gene expression and Western blot analysis. RESULTS: In hypothyroid mice maintained at room temperature, high leptin serum levels induce a pyrexic effect leading to the stabilization of body temperature through brown adipose tissue thermogenesis and white adipose tissue browning. Housing at thermoneutrality leads to the normalization of leptin levels and a reduction of the central temperature set point, resulting in decreased thermogenesis in brown and white adipose tissue and skeletal muscle and a significant decline in body temperature. Furthermore, anapyrexia in hypothyroid leptin-deficient ob/ob mice indicates that besides its pyrexic actions, leptin exerts a stimulatory effect on the HPT axis to stabilize the remaining TH serum levels in hypothyroid mice. CONCLUSION: This study led to the identification of a previously unknown endocrine loop in which leptin acts in concert with the HPT axis to stabilize body temperature in hypothyroid mice.

Pathophysiological Changes in the Enteric Nervous System of Rotenone-Exposed Mice as Early Radiological Markers for Parkinson's Disease.

Parkinson's disease (PD) is known to involve the peripheral nervous system (PNS) and the enteric nervous system (ENS). Functional changes in PNS and ENS appear early in the course of the disease and are responsible for some of the non-motor symptoms observed in PD patients like constipation, that can precede the appearance of motor symptoms by years. Here we analyzed the effect of the pesticide rotenone, a mitochondrial Complex I inhibitor, on the function and neuronal composition of the ENS by measuring intestinal contractility in a tissue bath and by analyzing related protein expression. Our results show that rotenone changes the normal physiological response of the intestine to carbachol, dopamine and electric field stimulation (EFS). Changes in the reaction to EFS seem to be related to the reduction in the cholinergic input but also related to the noradrenergic input, as suggested by the non-adrenergic non-cholinergic (NANC) reaction to the EFS in rotenone-exposed mice. The magnitude and direction of these alterations varies between intestinal regions and exposure times and is associated with an early up-regulation of dopaminergic, cholinergic and adrenergic receptors and an irregular reduction in the amount of enteric neurons in rotenone-exposed mice. The early appearance of these alterations, that start occurring before the substantia nigra is affected in this mouse model, suggests that these alterations could be also observed in patients before the onset of motor symptoms and makes them ideal potential candidates to be used as radiological markers for the detection of Parkinson's disease in its early stages.

Development of 18F-Labeled Radiotracers for PET Imaging of the Adenosine A2A Receptor: Synthesis, Radiolabeling and Preliminary Biological Evaluation.

The adenosine A2A receptor (A2AR) represents a potential therapeutic target for neurodegenerative diseases. Aiming at the development of a positron emission tomography (PET) radiotracer to monitor changes of receptor density and/or occupancy during the A2AR-tailored therapy, we designed a library of fluorinated analogs based on a recently published lead compound (PPY). Among those, the highly affine 4-fluorobenzyl derivate (PPY1; Ki(hA2AR) = 5.3 nM) and the 2-fluorobenzyl derivate (PPY2; Ki(hA2AR) = 2.1 nM) were chosen for 18F-labeling via an alcohol-enhanced copper-mediated procedure starting from the corresponding boronic acid pinacol ester precursors. Investigations of the metabolic stability of [18F]PPY1 and [18F]PPY2 in CD-1 mice by radio-HPLC analysis revealed parent fractions of more than 76% of total activity in the brain. Specific binding of [18F]PPY2 on mice brain slices was demonstrated by in vitro autoradiography. In vivo PET/magnetic resonance imaging (MRI) studies in CD-1 mice revealed a reasonable high initial brain uptake for both radiotracers, followed by a fast clearance.

Preclinical Evaluation of [18F]FACH in Healthy Mice and Piglets: An 18F-Labeled Ligand for Imaging of Monocarboxylate Transporters with PET.

The expression of monocarboxylate transporters (MCTs) is linked to pathophysiological changes in diseases, including cancer, such that MCTs could potentially serve as diagnostic markers or therapeutic targets. We recently developed [18F]FACH as a radiotracer for non-invasive molecular imaging of MCTs by positron emission tomography (PET). The aim of this study was to evaluate further the specificity, metabolic stability, and pharmacokinetics of [18F]FACH in healthy mice and piglets. We measured the [18F]FACH plasma protein binding fractions in mice and piglets and the specific binding in cryosections of murine kidney and lung. The biodistribution of [18F]FACH was evaluated by tissue sampling ex vivo and by dynamic PET/MRI in vivo, with and without pre-treatment by the MCT inhibitor α-CCA-Na or the reference compound, FACH-Na. Additionally, we performed compartmental modelling of the PET signal in kidney cortex and liver. Saturation binding studies in kidney cortex cryosections indicated a KD of 118 ± 12 nM and Bmax of 6.0 pmol/mg wet weight. The specificity of [18F]FACH uptake in the kidney cortex was confirmed in vivo by reductions in AUC0-60min after pre-treatment with α-CCA-Na in mice (-47%) and in piglets (-66%). [18F]FACH was metabolically stable in mouse, but polar radio-metabolites were present in plasma and tissues of piglets. The [18F]FACH binding potential (BPND) in the kidney cortex was approximately 1.3 in mice. The MCT1 specificity of [18F]FACH uptake was confirmed by displacement studies in 4T1 cells. [18F]FACH has suitable properties for the detection of the MCTs in kidney, and thus has potential as a molecular imaging tool for MCT-related pathologies, which should next be assessed in relevant disease models.

Synthesis and Biological Evaluation of a Novel 18F-Labeled Radiotracer for PET Imaging of the Adenosine A2A Receptor.

The adenosine A2A receptor (A2AR) has emerged as a potential non-dopaminergic target for the treatment of Parkinson's disease and, thus, the non-invasive imaging with positron emission tomography (PET) is of utmost importance to monitor the receptor expression and occupancy during an A2AR-tailored therapy. Aiming at the development of a PET radiotracer, we herein report the design of a series of novel fluorinated analogs (TOZ1-TOZ7) based on the structure of the A2AR antagonist tozadenant, and the preclinical evaluation of [18F]TOZ1. Autoradiography proved A2AR-specific in vitro binding of [18F]TOZ1 to striatum of mouse and pig brain. Investigations of the metabolic stability in mice revealed parent fractions of more than 76% and 92% of total activity in plasma and brain samples, respectively. Dynamic PET/magnetic resonance imaging (MRI) studies in mice revealed a brain uptake but no A2AR-specific in vivo binding.

Adenosine/A2B Receptor Signaling Ameliorates the Effects of Aging and Counteracts Obesity.

The combination of aging populations with the obesity pandemic results in an alarming rise in non-communicable diseases. Here, we show that the enigmatic adenosine A2B receptor (A2B) is abundantly expressed in skeletal muscle (SKM) as well as brown adipose tissue (BAT) and might be targeted to counteract age-related muscle atrophy (sarcopenia) as well as obesity. Mice with SKM-specific deletion of A2B exhibited sarcopenia, diminished muscle strength, and reduced energy expenditure (EE), whereas pharmacological A2B activation counteracted these processes. Adipose tissue-specific ablation of A2B exacerbated age-related processes and reduced BAT EE, whereas A2B stimulation ameliorated obesity. In humans, A2B expression correlated with EE in SKM, BAT activity, and abundance of thermogenic adipocytes in white fat. Moreover, A2B agonist treatment increased EE from human adipocytes, myocytes, and muscle explants. Mechanistically, A2B forms heterodimers required for adenosine signaling. Overall, adenosine/A2B signaling links muscle and BAT and has both anti-aging and anti-obesity potential.

Preclinical Incorporation Dosimetry of [18F]FACH-A Novel 18F-Labeled MCT1/MCT4 Lactate Transporter Inhibitor for Imaging Cancer Metabolism with PET.

Overexpression of monocarboxylate transporters (MCTs) has been shown for a variety of human cancers (e.g., colon, brain, breast, and kidney) and inhibition resulted in intracellular lactate accumulation, acidosis, and cell death. Thus, MCTs are promising targets to investigate tumor cancer metabolism with positron emission tomography (PET). Here, the organ doses (ODs) and the effective dose (ED) of the first 18F-labeled MCT1/MCT4 inhibitor were estimated in juvenile pigs. Whole-body dosimetry was performed in three piglets (age: ~6 weeks, weight: ~13-15 kg). The animals were anesthetized and subjected to sequential hybrid Positron Emission Tomography and Computed Tomography (PET/CT) up to 5 h after an intravenous (iv) injection of 156 ± 54 MBq [18F]FACH. All relevant organs were defined by volumes of interest. Exponential curves were fitted to the time-activity data. Time and mass scales were adapted to the human order of magnitude and the ODs calculated using the ICRP 89 adult male phantom with OLINDA 2.1. The ED was calculated using tissue weighting factors as published in Publication 103 of the International Commission of Radiation Protection (ICRP103). The highest organ dose was received by the urinary bladder (62.6 ± 28.9 µSv/MBq), followed by the gall bladder (50.4 ± 37.5 µSv/MBq) and the pancreas (30.5 ± 27.3 µSv/MBq). The highest contribution to the ED was by the urinary bladder (2.5 ± 1.1 µSv/MBq), followed by the red marrow (1.7 ± 0.3 µSv/MBq) and the stomach (1.3 ± 0.4 µSv/MBq). According to this preclinical analysis, the ED to humans is 12.4 µSv/MBq when applying the ICRP103 tissue weighting factors. Taking into account that preclinical dosimetry underestimates the dose to humans by up to 40%, the conversion factor applied for estimation of the ED to humans would rise to 20.6 µSv/MBq. In this case, the ED to humans upon an iv application of ~300 MBq [18F]FACH would be about 6.2 mSv. This risk assessment encourages the translation of [18F]FACH into clinical study phases and the further investigation of its potential as a clinical tool for cancer imaging with PET.

Sigma-1 Receptor Positron Emission Tomography: A New Molecular Imaging Approach Using (S)-(-)-[18F]Fluspidine in Glioblastoma.

Glioblastoma multiforme (GBM) is the most devastating primary brain tumour characterised by infiltrative growth and resistance to therapies. According to recent research, the sigma-1 receptor (sig1R), an endoplasmic reticulum chaperone protein, is involved in signaling pathways assumed to control the proliferation of cancer cells and thus could serve as candidate for molecular characterisation of GBM. To test this hypothesis, we used the clinically applied sig1R-ligand (S)-(-)-[18F]fluspidine in imaging studies in an orthotopic mouse model of GBM (U87-MG) as well as in human GBM tissue. A tumour-specific overexpression of sig1R in the U87-MG model was revealed in vitro by autoradiography. The binding parameters demonstrated target-selective binding according to identical KD values in the tumour area and the contralateral side, but a higher density of sig1R in the tumour. Different kinetic profiles were observed in both areas, with a slower washout in the tumour tissue compared to the contralateral side. The translational relevance of sig1R imaging in oncology is reflected by the autoradiographic detection of tumour-specific expression of sig1R in samples obtained from patients with glioblastoma. Thus, the herein presented data support further research on sig1R in neuro-oncology.

PET Imaging of the Adenosine A2A Receptor in the Rotenone-Based Mouse Model of Parkinson's Disease with [18F]FESCH Synthesized by a Simplified Two-Step One-Pot Radiolabeling Strategy.

The adenosine A2A receptor (A2AR) is regarded as a particularly appropriate target for non-dopaminergic treatment of Parkinson's disease (PD). An increased A2AR availability has been found in the human striatum at early stages of PD and in patients with PD and dyskinesias. The aim of this small animal positron emission tomography/magnetic resonance (PET/MR) imaging study was to investigate whether rotenone-treated mice reflect the aspect of striatal A2AR upregulation in PD. For that purpose, we selected the known A2AR-specific radiotracer [18F]FESCH and developed a simplified two-step one-pot radiosynthesis. PET images showed a high uptake of [18F]FESCH in the mouse striatum. Concomitantly, metabolism studies with [18F]FESCH revealed the presence of a brain-penetrant radiometabolite. In rotenone-treated mice, a slightly higher striatal A2AR binding of [18F]FESCH was found. Nonetheless, the correlation between the increased A2AR levels within the proposed PD animal model remains to be further investigated.

Radiosynthesis and Biological Investigation of a Novel Fluorine-18 Labeled Benzoimidazotriazine- Based Radioligand for the Imaging of Phosphodiesterase 2A with Positron Emission Tomography.

A specific radioligand for the imaging of cyclic nucleotide phosphodiesterase 2A (PDE2A) via positron emission tomography (PET) would be helpful for research on the physiology and disease-related changes in the expression of this enzyme in the brain. In this report, the radiosynthesis of a novel PDE2A radioligand and the subsequent biological evaluation were described. Our prospective compound 1-(2-chloro-5-methoxy phenyl)-8-(2-fluoropyridin-4-yl)-3- methylbenzo[e]imidazo[5,1-c][1,2,4]triazine, benzoimidazotriazine (BIT1) (IC50 PDE2A = 3.33 nM; 16-fold selectivity over PDE10A) was fluorine-18 labeled via aromatic nucleophilic substitution of the corresponding nitro precursor using the K[18F]F-K2.2.2-carbonate complex system. The new radioligand [18F]BIT1 was obtained with a high radiochemical yield (54 ± 2%, n = 3), a high radiochemical purity (≥99%), and high molar activities (155-175 GBq/µmol, n = 3). In vitro autoradiography on pig brain cryosections exhibited a heterogeneous spatial distribution of [18F]BIT1 corresponding to the known pattern of expression of PDE2A. The investigation of in vivo metabolism of [18F]BIT1 in a mouse revealed sufficient metabolic stability. PET studies in mouse exhibited a moderate brain uptake of [18F]BIT1 with a maximum standardized uptake value of ~0.7 at 5 minutes p.i. However, in vivo blocking studies revealed a non-target specific binding of [18F]BIT1. Therefore, further structural modifications are needed to improve target selectivity.

Radiosynthesis and in vivo evaluation of a fluorine-18 labeled pyrazine based radioligand for PET imaging of the adenosine A2B receptor.

On the basis of a pyrazine core structure, three new adenosine A2B receptor ligands (7a-c) were synthesized containing a 2-fluoropyridine moiety suitable for 18F-labeling. Compound 7a was docked into a homology model of the A2B receptor based on X-ray structures of the related A2A receptor, and its interactions with the adenosine binding site were rationalized. Binding affinity data were determined at the four human adenosine receptor subtypes. Despite a rather low selectivity regarding the A1 receptor, 7a was radiolabeled as the most suitable candidate (Ki(A2B) = 4.24 nM) in order to perform in vivo studies in mice with the aim to estimate fundamental pharmacokinetic characteristics of the compound class. Organ distribution studies and a single PET study demonstrated brain uptake of [18F]7a with a standardized uptake value (SUV) of ≈1 at 5 min post injection followed by a fast wash out. Metabolism studies of [18F]7a in mice revealed the formation of a blood-brain barrier penetrable radiometabolite, which could be structurally identified. The results of this study provide an important basis for the design of new derivatives with improved binding properties and metabolic stability in vivo.

Bridging from Brain to Tumor Imaging: (S)-(-)- and (R)-(+)-[18F]Fluspidine for Investigation of Sigma-1 Receptors in Tumor-Bearing Mice.

Sigma-1 receptors (Sig1R) are highly expressed in various human cancer cells and hence imaging of this target with positron emission tomography (PET) can contribute to a better understanding of tumor pathophysiology and support the development of antineoplastic drugs. Two Sig1R-specific radiolabeled enantiomers (S)-(-)- and (R)-(+)-[18F]fluspidine were investigated in several tumor cell lines including melanoma, squamous cell/epidermoid carcinoma, prostate carcinoma, and glioblastoma. Dynamic PET scans were performed in mice to investigate the suitability of both radiotracers for tumor imaging. The Sig1R expression in the respective tumors was confirmed by Western blot. Rather low radiotracer uptake was found in heterotopically (subcutaneously) implanted tumors. Therefore, a brain tumor model (U87-MG) with orthotopic implantation was chosen to investigate the suitability of the two Sig1R radiotracers for brain tumor imaging. High tumor uptake as well as a favorable tumor-to-background ratio was found. These results suggest that Sig1R PET imaging of brain tumors with [18F]fluspidine could be possible. Further studies with this tumor model will be performed to confirm specific binding and the integrity of the blood-brain barrier (BBB).

Investigation of an 18F-labelled Imidazopyridotriazine for Molecular Imaging of Cyclic Nucleotide Phosphodiesterase 2A.

Specific radioligands for in vivo visualization and quantification of cyclic nucleotide phosphodiesterase 2A (PDE2A) by positron emission tomography (PET) are increasingly gaining interest in brain research. Herein we describe the synthesis, the 18F-labelling as well as the biological evaluation of our latest PDE2A (radio-)ligand 9-(5-Butoxy-2-fluorophenyl)-2-(2-([18F])fluoroethoxy)-7-methylimidazo[5,1-c]pyrido[2,3-e][1,2,4]triazine (([18F])TA5). It is the most potent PDE2A ligand out of our series of imidazopyridotriazine-based derivatives so far (IC50 hPDE2A = 3.0 nM; IC50 hPDE10A > 1000 nM). Radiolabelling was performed in a one-step procedure starting from the corresponding tosylate precursor. In vitro autoradiography on rat and pig brain slices displayed a homogenous and non-specific binding of the radioligand. Investigation of stability in vivo by reversed-phase HPLC (RP-HPLC) and micellar liquid chromatography (MLC) analyses of plasma and brain samples obtained from mice revealed a high fraction of one main radiometabolite. Hence, we concluded that [18F]TA5 is not appropriate for molecular imaging of PDE2A neither in vitro nor in vivo. Our ongoing work is focusing on further structurally modified compounds with enhanced metabolic stability.