Assessment of serotonin release capacity in the human brain using dexfenfluramine challenge and [18F]altanserin positron emission tomography.

Although alterations of serotonin (5-HT) system functioning have been proposed for a variety of psychiatric disorders, a direct method quantitatively assessing 5-HT release capacity in the living human brain is still lacking. Therefore, we evaluated a novel method to assess 5-HT release capacity in vivo using dexfenfluramine challenge and [(18)F]altanserin positron emission tomography (PET). Thirteen healthy male subjects received placebo and single oral doses of 40 mg (n = 6) or 60 mg (n = 7) of the potent 5-HT releaser dexfenfluramine separated by an interval of 14 days. Three further subjects received placebo on both days. Two hours after placebo/drug administration, 250 MBq of the 5-HT(2A) receptor selective PET-radiotracer [(18)F]altanserin was administered intravenously as a 30s bolus. Dynamic PET data were subsequently acquired over 90 min. Moreover, arterial blood samples were drawn for measurement of total activity and metabolite correction of the input function. Dexfenfluramine as well as cortisol and prolactin plasma concentration-time profiles was quantitatively determined. Tracer distribution volumes for five volumes-of-interest (prefrontal and occipital cortex, insula, thalamus, caudatum) were calculated by the Logan plot and a 2-tissue compartment model. Dexfenfluramine dose-dependently decreased the total distribution volume of [(18)F]altanserin in cortical regions independent of the PET modeling approach. Cortisol and prolactin plasma concentrations were dose-dependently increased by dexfenfluramine. The decrease in cortical [(18)F]altanserin receptor binding under dexfenfluramine was correlated with the increase of plasma prolactin. These data suggest that the combination of a dexfenfluramine-induced 5-HT release and subsequent assessment of 5-HT(2A) receptor availability with [(18)F]altanserin PET is suitable to measure cortical 5-HT release capacity in the human brain.

Guidance on current good radiopharmacy practice (cGRPP) for the small-scale preparation of radiopharmaceuticals.

This guidance is meant as a guidance to Part B of the EANM "Guidelines on Good Radiopharmacy Practice (GRPP)" issued by the Radiopharmacy Committee of the EANM (see www.eanm.org ), covering the small-scale "in house" preparation of radiopharmaceuticals which are not kit procedures. The aim is to provide more detailed and practice-oriented guidance to those who are involved in the small-scale preparation of, for example, PET, therapeutic or other radiopharmaceuticals which are not intended for commercial purposes or distribution.

Homology modeling and dynamics of the extracellular domain of rat and human neuronal nicotinic acetylcholine receptor subtypes alpha4beta2 and alpha7.

In recent years, it has become clear that the neuronal nicotinic acetylcholine receptor (nAChR) is a valid target in the treatment of a variety of diseases, including Alzheimer's disease, anxiety, and nicotine addiction. As with most membrane proteins, information on the three-dimensional (3D) structure of nAChR is limited to data from electron microscopy, at a resolution that makes the application of structure-based design approaches to develop specific ligands difficult. Based on a high-resolution crystal structure of AChBP, homology models of the extracellular domain of the neuronal rat and human nAChR subtypes alpha4beta2 and alpha7 (the subtypes most abundant in brain) were built, and their stability assessed with molecular dynamics (MD). All models built showed conformational stability over time, confirming the quality of the starting 3D model. Lipophilicity and electrostatic potential studies performed on the rat and human alpha4beta2 and alpha7 nicotinic models were compared to AChBP, revealing the importance of the hydrophobic aromatic pocket and the critical role of the alpha-subunit Trp-the homolog of AChBP-Trp 143-for ligand binding. The models presented provide a valuable framework for the structure-based design of specific alpha4beta2 nAChR subtype ligands aimed at improving therapeutic and diagnostic applications.

Automated no-carrier-added synthesis of [1-11C]-labeled D- and L-enantiomers of lactic acid.

The first purely chemical method for automated no-carrier-added synthesis of [1-(11)C]-labeled d(R)- and l(S)-2-hydroxypropanoic acid (lactic acid) was developed for experimental neurophysiology studies and position emission tomography (PET) diagnosis. Starting from sodium 1-hydroxyethanesulfonate and [(11)C]HCN (trapped as [(11)C]KCN) the intermediate dl-(R,S)-[1-(11)C]-2-hydroxypropanenitrile was prepared. Its rapid acid hydrolysis gave dl-(R,S)-[1-(11)C]lactic acid, which was isolated by preparative reversed phase HPLC and automatically injected on a second preparative C(18) HPLC column coated with a chiral selector, where both [1-(11)C]lactic acid enantiomers were separated by chiral ligand-exchange chromatography. Two novel chiral selectors for HPLC enantiomeric separation of alpha-hydroxy acids, namely d(R)- or l(S)-2-amino-3-methyl-3-(5-phenylpentylsulfanyl)-butanoic acid were utilized for the preparative HPLC separation of the [1-(11)C]lactic acid enantiomers. The preparation of the selectors and the coating procedure for the manufacturing of the preparative chiral HPLC columns are described. A highly efficient trap for [(11)C]HCN is presented. The whole radiosynthesis is automated, takes about 45 min and leads to more than 80% decay corrected overall radiochemical yield of each enantiomer (up to 2.5 GBq) with over 99% radiochemical, chemical and enantiomeric purity. The specific activity at the end of the synthesis is about 400 GBq/micromol.

Uptake of 18F-fluorocholine, 18F-fluoro-ethyl-L: -tyrosine and 18F-fluoro-2-deoxyglucose in F98 gliomas in the rat.

INTRODUCTION: The positron emission tomography (PET) tracers (18)F-fluoro-ethyl-L: -tyrosine (FET), (18)F-fluorocholine (N,N-dimethyl-N-[(18)F]fluoromethyl-2-hydroxyethylammonium (FCH]) and (18)F-fluoro-2-deoxyglucose (FDG) are used in the diagnosis of brain tumours. The aim of this study was threefold: (a) to assess the uptake of the different tracers in the F98 rat glioma, (b) to evaluate the impact of blood-brain barrier (BBB) disruption and microvessel density (MVD) on tracer uptake and (c) to compare the uptake in the tumours to that in the radiation injuries (induced by proton irradiation of healthy rats) of our previous study. METHODS: F98 gliomas were induced in 26 rats. The uptake of FET, FCH and FDG was measured using autoradiography and correlated with histology, disruption of the BBB and MVD. RESULTS: The mean FET, FCH and FDG standardised uptake values (SUVs) in the tumour and the contralateral normal cortex (in parentheses) were 4.19+/-0.86 (1.32+/-0.26), 2.98+/-0.58 (0.51+/-0.11) and 11.02+/-3.84 (4.76+/-1.77) respectively. MVD was significantly correlated only with FCH uptake. There was a trend towards a negative correlation between the degree of BBB disruption and FCH uptake and a trend towards a positive correlation with FET uptake. The ratio of the uptake in tumours to that in the radiation injuries was 1.97 (FCH), 2.71 (FET) and 2.37 (FDG). CONCLUSION: MVD displayed a significant effect only on FCH uptake. The degree of BBB disruption seems to affect the accumulation of FET and FCH, but not FDG. Mean tumour uptake for all tracers was significantly higher than the accumulation in radiation injuries.

Synthesis and binding studies of epibatidine analogues as ligands for the nicotinic acetylcholine receptors.

Neuronal nicotinic acetylcholine receptors (nAChRs) are transmembrane ligand-gated ion channels. Recent research demonstrated that selective nAChR ligands may have therapeutic potential in a number of CNS diseases and disorders. The alkaloid epibatidine is a highly potent non-opioid analgesic and nAChR agonist, but too toxic to be a useful ligand. To develop ligands selective for distinct nAChR subtypes and with reduced toxicity, a series of epibatidine and homoepibatidine analogues were synthesized. (+/-)-8-Methyl-3-(pyridin-3-yl)-8-azabicyclo[3,2,1]oct-2-ene, showed high affinity towards alpha4beta2 (Ki=2 nM), subtype selectivity (alpha4beta2/alpha7 affinity ratio>100) and relatively low toxicity in mice and can be labeled with 11C and 18F as positron emission tomography (PET) tracers for imaging of nAChRs.

Ligand selectivity for the acetylcholine binding site of the rat alpha4beta2 and alpha3beta4 nicotinic subtypes investigated by molecular docking.

The homology models of the extracellular domains of the neuronal alpha4beta2 (pdb code: 1ole) and ganglionic alpha3beta4 (pdb code: 1olf) rat nicotinic acetylcholine receptor (nAChR) subtypes were refined and energetically minimized. In this work, a series of nAChR ligands (1-15) were docked into the modeled binding cavity of both receptors. High-affinity, toxic ligands such as epibatidine (1) and dechloroepibatidine (2) docked into cluster 1 with the charged tertiary amino group, forming a pi-cation interaction with Trp 147 on the (+) side of the alpha4 subunit and establishing a characteristic H-bond with the Lys 77 on the (-) side of the beta2 subunit. The nontoxic ligands such as 33bMet (3), (S)-A-85380 (4), and acetylcholine (6) docked into cluster 2 with the same pi-cation interaction but with the rest of the molecule occupying a different moiety of the binding pocket. Molecular docking into the alpha3beta4 subtype showed that both enantiomers of 1 (1a and 1b) are representative templates for ligands with affinity toward this ganglionic nAChR subtype. The ranking scores of the docked molecules confirm the existence of structure-dependent subtype selectivity and shed light on the design of specific and selective alpha4beta2 nAChR subtype ligands.

Fluorocholine PET/CT in patients with prostate cancer: initial experience.

Institutional review board approval and written informed consent were obtained. Patients with newly diagnosed prostate cancer and patients suspected of having recurrent prostate cancer were prospectively evaluated with fluorine 18 fluorocholine (FCH) combined in-line positron emission tomography (PET) and computed tomography (CT). In 19 patients (mean age, 67 years +/- 8; range, 57-85 years), standardized uptake values of FCH in 17 different tissues were determined by using volumes of interest. In nine patients evaluated at initial staging, histologic findings of the resected prostate were compared to FCH uptake. Only small variations of physiologic tracer accumulation were measured in all organs but the kidneys. Differentiation of benign hyperplasia from cancerous prostate lesions was not possible with FCH PET/CT. However, in patients with recurrent prostate cancer, FCH PET/CT is a promising imaging modality for detecting local recurrence and lymph node metastases.

Uptake of 18F-fluorocholine, 18F-fluoroethyl-L-tyrosine, and 18F-FDG in acute cerebral radiation injury in the rat: implications for separation of radiation necrosis from tumor recurrence.

UNLABELLED: Differentiation between posttherapy radiation necrosis and recurrent tumor in humans with brain tumor is still a difficult diagnostic task. The new PET tracers (18)F-fluoro-ethyl-l-tyrosine (FET) and (18)F-fluorocholine (N,N-dimethyl-N-(18)F-fluoromethyl-2-hydroxyethylammonium [FCH]) have shown promise for improving diagnostic accuracy. This study assessed uptake of these tracers in experimental radiation injury. METHODS: In a first model, circumscribed lesions were induced in the cortex of 35 rats using proton irradiation of 150 or 250 Gy. After radiation injury developed, uptake of (18)F-FET, (18)F-FCH, and (18)F-FDG was measured using autoradiography and correlated with histology and disruption of the blood-brain barrier as determined with Evans blue. In a second model, uptake of the tracers was assessed in acute cryolesions, which are characterized by the absence of inflammatory cells. RESULTS: Mean (18)F-FET, (18)F-FCH, and (18)F-FDG standardized uptake values in the most active part of the radiation lesion and the contralateral normal cortex (in parentheses) were 2.27 +/- 0.46 (1.42 +/- 0.23), 2.52 +/- 0.42 (0.61 +/- 0.12), and 6.21 +/- 1.19 (4.35 +/- 0.47). The degree of uptake of (18)F-FCH and (18)F-FDG correlated with the density of macrophages. In cryolesions, (18)F-FET uptake was similar to that in radiation lesions, and (18)F-FCH uptake was significantly reduced. CONCLUSION: Comparison of tracer accumulation in cryolesions and radiation injuries demonstrates that (18)F-FET uptake is most likely due to a disruption of the blood-brain barrier alone, whereas (18)F-FCH is additionally trapped by macrophages. Uptake of both tracers in the radiation injuries is generally lower than the published uptake in tumors, suggesting that (18)F-FET and (18)F-FCH are promising tracers for separating radiation necrosis from tumor recurrence. However, the comparability of our data with the literature is limited by factors such as different species and acquisition protocols and modalities. Thus, more studies are needed to settle this issue. Nevertheless, (18)F-FCH and (18)F-FET seem superior to (18)F-FDG for this purpose.

A-186253, a specific antagonist of the alpha 4 beta 2 nAChRs: its properties and potential to study brain nicotinic acetylcholine receptors.

Imaging the living brain and the distribution of the ligand gated channels that participate in the neurotransmission is one of the challenges that is hoped to bring new insights for the treatment of neurological diseases. Herein, we probed a new nicotinic derivative, A-186253 as a potential molecule to discriminate with high resolution the different neuronal nicotinic receptor subtypes that are expressed in distinct brain areas. Binding with a high affinity of 440 pM at the major brain alpha4beta2 receptor subtype and presenting an excellent safety margin, properties of the A-186253 were thoroughly evaluated. While autoradiography confirmed its specificity for the alpha4beta2 subtype, functional investigations revealed for short exposures a broader spectrum of action at receptors including the ganglionic alpha3beta4 and the homomeric alpha7 subtypes. Specificity was, however, observed at alpha4beta2 when receptors were exposed for several minutes with low concentration of the A-186253. In view of these promising results, the A-186253 was radiolabeled and tested in positron emission tomography on rats and pigs. Despite the high selectivity observed in vitro, the A-186253 displayed a complex binding profile and little displacement by the agonist cytisine. While the A-186253 can be valuable to discriminate receptor subtypes, improvements of this molecule must be brought for in vivo measurements.

Constant-infusion H(2)15O PET and acetazolamide challenge in the assessment of cerebral perfusion status.

UNLABELLED: Assessing the baseline perfusion and perfusion reserve after acetazolamide (ACZ) challenge is a common method for the evaluation of patients with cerebrovascular disease. Most previous studies using H(2)(15)O PET applied the bolus injection technique. There is considerable discrepancy regarding the optimal time point of imaging after ACZ injection. The purpose of this study was to continuously monitor cerebral blood flow (CBF) after ACZ using constant-infusion H(2)(15)O PET. METHODS: Four patients with stenoses of an internal carotid artery and 6 with moyamoya disease were studied. H(2)(15)O was continuously infused, and data were recorded in 1-min frames. After equilibration of H(2)(15)O, 5 min of baseline data were acquired, and then 1 g of ACZ was administered intravenously and data collection continued for 10-22 min. Arterial blood was continuously drawn for absolute quantification of CBF. RESULTS: The arterial (15)O concentration remained generally stable during scanning, and the cerebellar blood flow fluctuations of the 5 baseline scans were small. The scan-to-scan difference was 6% (difference of 2 successive scans/mean). In the nonpathologic areas, the increase in CBF started 1-2 min after administration of ACZ. The largest fraction of the increase occurred from 0 to 10 min. The ratio of CBF in pathologic areas to CBF in cerebellum showed an initial decrease that stabilized after 5 min. CONCLUSION: A continuous-infusion protocol is a viable alternative to single bolus injections for the assessment of cerebral perfusion status. Such a protocol is advantageous when the time course of CBF after an intervention is not known. With continuous monitoring, the optimal time point for evaluation of a certain parameter can be chosen post hoc. Furthermore, the time course of CBF itself may allow the definition of new parameters for evaluating perfusion status in cerebrovascular patients, both for assessment before a revascularization procedure and for follow-up. A limitation of the present study is the relatively small number of patients with each type of cerebrovascular disease and the lack of healthy subjects.

18F-choline in experimental soft tissue infection assessed with autoradiography and high-resolution PET.

For each oncological tracer it is important to know the uptake in non-tumorous lesions. The purpose of this study was to measure the accumulation of fluorine-18 choline (FCH), a promising agent for the evaluation of certain tumour types, in infectious tissue. Unilateral thigh muscle abscesses were induced in five rats by intramuscular injection of 0.1 ml of a bacterial suspension ( Staphylococcus aureus, 1.2 x 10(9) CFU/ml). In all animals, FCH accumulation was measured with high-resolution positron emission tomography (PET) on day 6. Autoradiography of the abscess and ipsilateral healthy muscle was performed on day 7 (three animals) and day 11 (two animals) and correlated with histology. In addition, (18)F-fluorodeoxyglucose (FDG) PET was performed on day 5. Increased FCH uptake was noted in specific layers of the abscess wall which contained an infiltrate of mainly granulocytes on day 7 and mainly macrophages on day 11. The autoradiographic standardised uptake values in the most active part of the abscess wall were 2.99 on day 7 ( n=3) and 4.05 on day 11 ( n=2). In healthy muscle the corresponding values were 0.99 and 0.64. The abscesses were clearly visualised on the FCH and FDG PET images. In conclusion, this study demonstrated avid FCH accumulation in inflammatory tissue, which limits the specificity of FCH for tumour detection. Future studies are now needed to determine the degree of this limitation in human cancer patients.

Functional imaging of physiological processes by positron emission tomography.

Tracer technology makes it possible to observe physiological and biochemical processes in the living organism in a dynamic mode. Positron emission tomography adds the use of chemically unchanged biomolecules and of quantification. This opens up fascinating possibilities for both fundamental research and routine diagnostic applications.

(18)F-FDG and (18)F-FET uptake in experimental soft tissue infection.

The aim of this study was to compare the uptake of (18)F-fluoroethyl- L-tyrosine ((18)F-FET) with that of (18)F-fluorodeoxyglucose ((18)F-FDG) in activated inflammatory white blood cells. Unilateral thigh muscle abscesses were induced in 11 rats by intramuscular inoculation of 0.1 ml of a bacterial suspension ( S. aureus, 1.2 x 10(9) CFU/ml). Four animals were intraperitoneally injected with 130-180 MBq (18)F-FDG, four with 140-170 MBq (18)F-FET and three with a mixture of 140-170 MBq (18)F-FET and 1.8 MBq (14)C-deoxyglucose. Autoradiography (10 microm slice thickness) of the abscess and the contralateral muscle was performed and detailed spatial correlation of autoradiography and histopathology (haematoxylin-eosin staining) was obtained. Regions of interest were placed on the abscess wall and the grey values (digitised image intensities) measured were converted to kBq/cc per kBq injected activity per gram (SUV). Areas with increased (18)F-FDG uptake corresponded to cellular inflammatory infiltrates mainly consisting of granulocytes. The SUV was calculated to be 4.08+/-0.65 (mean+/-SD). The uptake of (18)F-FET in activated white blood cells was not increased: the SUV of the abscess wall, at 0.74+/-0.14, was even below that of contralateral muscle. The low uptake of (18)F-FET in non-neoplastic inflammatory cells promises a higher specificity for the detection of tumour cells than is achieved with (18)F-FDG, since the immunological host response will not be labelled and inflammation can be excluded.