Tiina Saanijoki

Siglec-9 is a novel leukocyte ligand for vascular adhesion protein-1 and can be used in PET imaging of inflammation and cancer

Leukocyte migration to sites of inflammation is regulated by several endothelial adhesion molecules. Vascular adhesion protein-1 (VAP-1) is unique among the homing-associated molecules as it is both an enzyme that oxidizes primary amines and an adhesin. Although granulocytes can bind to endothelium via a VAP-1-dependent manner, the counter-receptor(s) on this leukocyte population is(are) not known. Here we used a phage display approach and identified Siglec-9 as a candidate ligand on granulocytes. The binding between Siglec-9 and VAP-1 was confirmed by in vitro and ex vivo adhesion assays. The interaction sites between VAP-1 and Siglec-9 were identified by molecular modeling and confirmed by further binding assays with mutated proteins. Although the binding takes place in the enzymatic groove of VAP-1, it is only partially dependent on the enzymatic activity of VAP-1. In positron emission tomography, the ⁶⁸Gallium-labeled peptide of Siglec-9 specifically detected VAP-1 in vasculature at sites of inflammation and cancer. Thus, the peptide binding to the enzymatic groove of VAP-1 can be used for imaging conditions, such as inflammation and cancer.

64Cu- and 68Ga-labelled [Nle(14),Lys(40)(Ahx-NODAGA)NH2]-exendin-4 for pancreatic beta cell imaging in rats.

PURPOSE Glucagon-like peptide-1 receptor (GLP-1R) is a molecular target for imaging of pancreatic beta cells. We compared the ability of [Nle(14),Lys(40)(Ahx-NODAGA-(64)Cu)NH2]-exendin-4 ([(64)Cu]NODAGA-exendin-4) and [Nle(14),Lys(40)(Ahx-NODAGA-(68)Ga)NH2]-exendin-4 ([(68)Ga]NODAGA-exendin-4) to detect native pancreatic islets in rodents. PROCEDURES The stability, lipophilicity and affinity of the radiotracers to the GLP-1R were determined in vitro. The biodistribution of the tracers was assessed using autoradiography, ex vivo biodistribution and PET imaging. Estimates for human radiation dosimetry were calculated. RESULTS We found GLP-1R-specific labelling of pancreatic islets. However, the pancreas could not be visualised in PET images. The highest uptake of the tracers was observed in the kidneys. Effective dose estimates for [(64)Cu]NODAGA-exendin-4 and [(68)Ga]NODAGA-exendin-4 were 0.144 and 0.012 mSv/MBq, respectively. CONCLUSION [(64)Cu]NODAGA-exendin-4 might be more effective for labelling islets than [(68)Ga]NODAGA-exendin-4. This is probably due to the lower specific radioactivity of [(68)Ga]NODAGA-exendin-4 compared to [(64)Cu]NODAGA-exendin-4. The radiation dose in the kidneys may limit the use of [(64)Cu]NODAGA-exendin-4 as a clinical tracer.

Evaluation of 68Ga-labeled tracers for PET imaging of myocardial perfusion in pigs☆

PURPOSE We evaluated four potential gallium-68 (68Ga)-labeled tracers for positron emission tomography (PET) imaging of myocardial perfusion in comparison with oxygen-15-labeled water ([15O]water) in healthy pigs. Four hexadentate salicylaldimine ligands derived from bis(3-aminopropyl)ethylenediamine (BAPEN) that showed promise in previous rat experiments were selected for this study. METHODS Following an evaluation of myocardial blood flow with [15O]water PET, the pigs (total n=14) underwent a dynamic 90-min PET study with one of four 68Ga-labeled BAPEN derivatives (n=3-5 per tracer) either at rest or under adenosine stress. Serial arterial blood samples were collected during the imaging for the measurements of total radioactivity, radiometabolites, plasma protein binding and blood-to-plasma ratio for the 68Ga chelates. Time-activity curves of the left ventricular blood pool and myocardium were derived from PET images, and metabolite-corrected arterial input function was used for kinetic modeling. Also, ex vivo biodistribution of 68Ga radioactivity was analyzed. RESULTS All four 68Ga tracers showed undesirably slow myocardial accumulation over time, but their in vivo stability, clearance from blood and the kinetics of the myocardium uptake varied. [68Ga][Ga-(sal)2BAPDMEN]1+ showed the highest myocardial uptake in PET images and tissue samples (myocardium-to-blood ratio 7.63±1.89, myocardium-to-lung ratio 3.03±0.33 and myocardium-to-liver ratio 1.80±0.82). However, there was no correlation between the myocardial perfusion measured with [15O]water and the net uptake rates or K1 values of the 68Ga chelates. CONCLUSION Our results revealed that myocardial accumulation of the 68Ga chelates proposed for myocardial perfusion imaging with PET was slow and not determined by myocardial perfusion in a large animal model. These findings suggest that the studied tracers are not suitable for clinical imaging of myocardial perfusion.

Preclinical Evaluation of a Radioiodinated Fully Human Antibody for In Vivo Imaging of Vascular Adhesion Protein-1–Positive Vasculature in Inflammation

Vascular adhesion protein-1 (VAP-1) is an endothelial glycoprotein mediating leukocyte trafficking from blood to sites of inflammation. BTT-1023 is a fully human monoclonal anti-VAP-1 antibody developed to treat inflammatory diseases. In this study, we preclinically evaluated radioiodinated BTT-1023 for inflammation imaging. Methods: Rabbits were intravenously injected with radioiodinated BTT-1023. Distribution and pharmacokinetics were assessed by PET/CT up to 72 h after injection. Human radiation dose estimates for 124I-BTT-1023 were extrapolated. Additionally, rabbits with chemically induced synovitis were imaged with 123I-BTT-1023 SPECT/CT. Results: Radioiodinated BTT-1023 cleared rapidly from blood circulation and distributed to liver and thyroid. Inflamed joints were delineated by SPECT/CT. The estimated human effective dose due to 124I-BTT-1023 was 0.55 mSv/MBq, if blockage of thyroid uptake is assumed. Conclusion: The radioiodinated BTT-1023 was able to detect mild inflammation in vivo. Clinical 124I-BTT-1023 PET studies with injected radioactivity of 0.5–0.7 MBq/kg may be justified.

Opioid Release after High-Intensity Interval Training in Healthy Human Subjects

Central opioidergic mechanisms may modulate the positive effects of physical exercise such as mood elevation and stress reduction. How exercise intensity and concomitant effective changes affect central opioidergic responses is unknown. We studied the effects of acute physical exercise on the cerebral μ-opioid receptors (MOR) of 22 healthy recreationally active males using positron emission tomography (PET) and the MOR-selective radioligand [11C]carfentanil. MOR binding was measured in three conditions on separate days: after a 60-min aerobic moderate-intensity exercise session, after a high-intensity interval training (HIIT) session, and after rest. Mood was measured repeatedly throughout the experiment. HIIT significantly decreased MOR binding selectively in the frontolimbic regions involved in pain, reward, and emotional processing (thalamus, insula, orbitofrontal cortex, hippocampus, and anterior cingulate cortex). Decreased binding correlated with increased negative emotionality. Moderate-intensity exercise did not change MOR binding, although increased euphoria correlated with decreased receptor binding. These observations, consistent with endogenous opioid release, highlight the role of the μ-opioid system in mediating affective responses to high-intensity training as opposed to recreational moderate physical exercise.

μ-opioid receptor system mediates reward processing in humans

The endogenous μ-opioid receptor (MOR) system regulates motivational and hedonic processing. We tested directly whether individual differences in MOR are associated with neural reward responses to food pictures in humans. We scanned 33 non-obese individuals with positron emission tomography (PET) using the MOR-specific radioligand [11C]carfentanil. During a functional magnetic resonance imaging (fMRI) scan, the subjects viewed pictures of appetizing versus bland foods to elicit reward responses. MOR availability was measured in key components of the reward and emotion circuits and used to predict BOLD-fMRI responses to foods. Viewing palatable versus bland foods activates regions involved in homeostatic and reward processing, such as amygdala, ventral striatum, and hypothalamus. MOR availability in the reward and emotion circuit is negatively associated with the fMRI reward responses. Variation in MOR availability may explain why some people feel an urge to eat when encountering food cues, increasing risk for weight gain and obesity.μ-opioid signalling has a known role in the response to various rewarding stimuli, including pleasant foods. Here, Nummenmaa et al. show using PET and fMRI that individual differences in brain μ-opioid receptor density predict the strength of the neural response to highly palatable foods in humans

Affective Adaptation to Repeated SIT and MICT Protocols in Insulin-Resistant Subjects

Introduction The aim of this study was to investigate affective responses to repeated sessions of sprint interval training (SIT) in comparison with moderate-intensity continuous training (MICT) in insulin-resistant subjects. Methods Twenty-six insulin-resistant adults (age, 49 (4) yr; 10 women) were randomized into SIT (n = 13) or MICT (n = 13) groups. Subjects completed six supervised training sessions within 2 wk (SIT session, 4–6 × 30 s all-out cycling/4-min recovery; MICT session, 40–60 min at 60% peak work load). Perceived exertion, stress, and affective state were assessed with questionnaires before, during and after each training session. Results Perceived exertion, displeasure, and arousal were higher during the SIT compared with MICT sessions (all P < 0.01). These, however, alleviated similarly in response to SIT and MICT over the 6 d of training (all P < 0.05). SIT versus MICT exercise increased perceived stress and decreased positive affect and feeling of satisfaction acutely after exercise especially in the beginning of the intervention (all P < 0.05). These negative responses declined significantly during the training period: perceived stress and positive activation were no longer different between the training groups after the third, and satisfaction after the fifth training session (P > 0.05). Conclusions The perceptual and affective responses are more negative both during and acutely after SIT compared with MICT in untrained insulin-resistant adults. These responses, however, show significant improvements already within six training sessions, indicating rapid positive affective and physiological adaptations to continual exercise training, both SIT and MICT. These findings suggest that even very intense SIT is mentally tolerable alternative for untrained people with insulin resistance.

Aerobic exercise modulates anticipatory reward processing via the μ-opioid receptor system

Physical exercise modulates food reward and helps control body weight. The endogenous µ‐opioid receptor (MOR) system is involved in rewarding aspects of both food and physical exercise, yet interaction between endogenous opioid release following exercise and anticipatory food reward remains unresolved. Here we tested whether exercise‐induced opioid release correlates with increased anticipatory reward processing in humans. We scanned 24 healthy lean men after rest and after a 1 h session of aerobic exercise with positron emission tomography (PET) using MOR‐selective radioligand [11C]carfentanil. After both PET scans, the subjects underwent a functional magnetic resonance imaging (fMRI) experiment where they viewed pictures of palatable versus nonpalatable foods to trigger anticipatory food reward responses. Exercise‐induced changes in MOR binding in key regions of reward circuit (amygdala, thalamus, ventral and dorsal striatum, and orbitofrontal and cingulate cortices) were used to predict the changes in anticipatory reward responses in fMRI. Exercise‐induced changes in MOR binding correlated negatively with the exercise‐induced changes in neural anticipatory food reward responses in orbitofrontal and cingulate cortices, insula, ventral striatum, amygdala, and thalamus: higher exercise‐induced opioid release predicted higher brain responses to palatable versus nonpalatable foods. We conclude that MOR activation following exercise may contribute to the considerable interindividual variation in food craving and consumption after exercise, which might promote compensatory eating and compromise weight control.