Search

Radiochemical conversion is an important term to be included in the "Consensus nomenclature rules for radiopharmaceutical chemistry". Radiochemical conversion should be used to define reaction efficiency by measuring the transformation of components in a crude reaction mixture at a given time, whereas radiochemical yield is better suited to define the efficiency of an entire reaction process including, for example, separation, isolation, filtration, and formulation. (C) 2020 Elsevier Inc. All rights reserved.

The class of tetrahydro-1H-3-benzazepines was systematically modified in 1-, 3- and 7-position. In particular, a F-atom was introduced in β- or γ-position of the 4-phenylbutyl side chain in 3-position. Ligands with the F-atom in γ-position possess higher GluN2B affinity than analogs bearing the F-atom in β-position. This effect was attributed to the reduced basicity of β-fluoro amines. 3-Benzazepines with a benzylic OH moiety show moderate GluN2B affinity, but considerable selectivity over the σ2 receptor. However, removal of the benzylic OH moiety led to increased GluN2B affinity, but reduced GluN2B/σ2 selectivity. With respect to GluN2B affinity the phenol 17b with a γ-fluorophenylbutyl moiety in 3-position represents the most interesting fluorinated ligand (Ki(GluN2B) = 16 nM). Most of the synthesized ligands reveal either similar GluN2B and σ1 affinity or higher σ1 affinity than GluN2B affinity. The methyl ether 16b shows high σ1 affinity (Ki(σ1) = 6.6 nM) and high selectivity over a broad panel of receptors and transporters. The high antiallodynic activity in the mouse capsaicin assay proved the σ1 antagonistic activity of 16b.

Recently, the eutomers of highly potent GluN2B selective NMDA receptor antagonists with 3-benzazepine scaffold were identified. Herein, pharmacokinetic properties regarding lipophilicity, plasma protein binding (PPB) and metabolism are analyzed. The logD7.4 values of 1.68 for phenol 1 and 2.46 for methyl ether 2 are in a very good range for CNS agents. A very similar logD7.4 value was recorded for the prototypical GluN2B antagonist ifenprodil (logD7.4 = 1.49). The herein developed high performance affinity chromatography (HPAC) method using human serum albumin as stationary phase led to PPB of 3-benzazepines (R)-1-3 and (S)-1-3 of 76–98%. Upon incubation with mouse liver microsomes, (R)-1-3 and (S)-1-3 showed moderate to high metabolic stability. The (R)-configured eutomers turned out to be metabolically more stable than their (S)-configured distomers. During phase I metabolism of 3-benzazepines 1-3 hydroxylations at both aromatic rings, the aliphatic side chain and the seven-membered ring were observed. O-demethylation of methyl ether (S)-2 was faster than O-demethylation of its enantiomer (R)-2. In phase I biotransformation the phenol eutomer (R)-1 showed comparable stability as ifenprodil. In phase II biotransformation, glucuronidation of the phenolic (only 1) and benzylic hydroxy groups was observed. Both enantiomers formed the same type of metabolites, respectively, but in different amounts. Whereas, the benzylic hydroxy group of (R)-2 was glucuronidated preferably, predominant benzylic glucuronidation of (S)-3 was detected. Mouse liver microsomes produced the glucuronide of phenol 1 (main metabolite) in larger amounts than rat liver microsomes.

Functional imaging of glutamate receptors using PET imaging modality can be used to study numerous CNS disorders and also to select appropriate doses of clinically relevant glutamate-receptor-targeting candidate drugs. Great strides have been made in developing PET imaging probes for the non-invasive detection of glutamate receptors in the brain. This review highlights recent progress made towards the development of glutamatergic PET imaging agents. Focus is placed on PET imaging probes that have been labelled with either carbon-11 or fluorine-18.

Introduction Endarterectomized human atherosclerotic plaques are a valuable basis for gene expression studies to disclose novel imaging biomarkers and therapeutic targets, such as the cannabinoid receptor type 2 (CB2). In this work, CB2 is expressed on activated immune cells, which are abundant in inflamed plaques. We evaluated the CB2-specific radiotracer [ 11 C]RS-016 for imaging vascular inflammation in human and mouse atherosclerotic lesions. Methods The differential gene expression of microscopically classified human carotid plaques was evaluated using quantitative polymerase chain reaction. In addition, CB2 expression levels in human plaques were investigated by in vitro autoradiography. As an appropriate animal model we used apolipoprotein E knockout mice (ApoE KO) with shear stress-induced atherosclerosis to evaluate CB2 levels in vivo. Positron emission tomography (PET) was performed with both the CB2 radioligand [ 11 C]RS-016 and the metabolic radiotracer [ 18 F]fluorodeoxyglucose ([ 18 F]FDG) at various time points. Retrospectively, carotids were dissected for histopathology and gene expression analysis. Results We identified 28 human genes differentially expressed in atherosclerotic plaques compared to normal arteries of which 12 were upregulated preferentially in vulnerable plaques. The latter group included members of matrix metalloproteinase family and the T-lymphocyte activation antigens CD80 and CD86. CB2 was upregulated by 2-fold in human atherosclerotic plaques correlating with CD68 expression levels. Specific in vitro binding of [ 11 C]RS-016 was predominantly observed to plaques. In vivo PET imaging of ApoE KO mice revealed accumulation of [ 11 C]RS-016 and [ 18 F]FDG in atherosclerotic plaques. Development of advanced plaques with elevated CB2 and CD68 levels were found in vitro in ApoE KO mice resembling human vulnerable plaques. Conclusion We identified human genes associated with plaque vulnerability, which potentially could serve as novel imaging or therapeutic targets. The CB2-specific radiotracer [ 11 C]RS-016 detected human plaques by in vitro autoradiography and accumulated in vivo in plaques of ApoE KO mice, however not exclusively in vulnerable plaques.

Chronic stress leads to changes in energy status and is a major risk factor for depression, with common symptoms of reductions in body weight and effortful motivation for reward. Indeed, stress-induced disturbed energy status could be a major aetio-pathogenic factor for depression. Improved understanding of these putative inter-relationships requires animal model studies of effects of stress on both peripheral and central energy-status measures and determinants. Here we conducted a study in mice fed on a standard low-fat diet and exposed to either 15-day chronic social stress (CSS) or control handling (CON). Relative to CON mice, CSS mice had attenuated body weight maintenance/gain despite consuming the same amount of food and expending the same amount of energy at any given body weight. The low weight of CSS mice was associated with less white and brown adipose tissues, and with a high respiratory exchange ratio consistent with increased dependence on glucose as energy substrate. Basal plasma insulin was low in CSS mice and exogenous glucose challenge resulted in a relatively prolonged elevation of blood glucose. With regard to hunger and satiety hormones, respectively, CSS mice had higher levels of acylated ghrelin in plasma and of ghrelin receptor gene expression in ventromedial hypothalamus and lower levels of plasma leptin, relative to CON mice. However, whilst CSS mice displayed this constellation of peripheral changes consistent with increases in energy need and glucose utilization relative to CON mice, they also displayed attenuated uptake of [18F]FDG in brain tissue specifically. Reduced brain glucose utilization in CSS mice could contribute to the reduced effortful motivation for reward in the form of sweet-tasting food that we have reported previously for CSS mice. It will now be important to utilize this model to further understanding of the mechanisms via which chronic stress can increase energy need but decrease brain glucose utilization and how this relates to regional and cellular changes in neural circuits for reward processing relevant to depression.

Introduction Both subtypes of sigma (σ) receptors, σ 1 and σ 2 , are over-expressed in many cancers with σ 2 proposed as a biomarker of tumor proliferation. We are interested in developing a high affinity selective σ 2 radioligand for in vivo monitoring of proliferative status of solid tumors and response to anti-cancer therapies. 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28) represents one of the lead candidates in the development of σ receptor ligands for therapeutic and diagnostic applications. However, the utility of PB28 is limited due to its relatively high lipophilicity. Methods A more hydrophilic analogue (–)-( S )- 1 was radiolabeled with 11 C via standard O -alkylation. In vitro autoradiography with [ 11 C](–)-( S )- 1 was done using rat brain slices. PET imaging was performed in mice bearing EMT6, C6 or PC-3 tumors after i.v. injection of [ 11 C](–)-( S )- 1 . Results [ 11 C](–)-( S )- 1 was produced in 53%±7% isolated decay-corrected yield with radiochemical and chemical purity over 99% and specific activity greater than 100GBq/μmol. In vitro autoradiography with [ 11 C(–)-( S )- 1 resulted in a heterogeneous binding of the tracer in the rat brain with the highest radioactivity signals in the cortex region followed by cerebellum. This binding was successfully blocked by 10μM of either haloperidol, (+)-( R )- 1 or PB28. For C6 xenografts low target-to-nontarget ratio and high non-specific binding did not allow clear tumor visualization. No accumulation was visible in EMT6 tumor or in PC-3 tumor. Rat and mouse brain uptake was low and homogeneous while stronger signal was detected in the spinal cord. High accumulation of radioactivity was observed in liver and intestine suggesting hepatobiliary clearance. Conclusions Despite excellent in vitro properties, [ 11 C](–)-( S )- 1 did not provide high enough specific binding in vivo and is, therefore, not a useful PET tracer for imaging σ 2 expression in tumors.

Our goal is to develop a highly specific and selective PET brain tracer for imaging CB2 expression in patients with neuroinflammatory diseases. Based on our previous findings on a carbon-11 labeled 4-oxo-quinoline structure, designated KD2, further structural optimizations were performed, which led to the discovery of N-(1-adamantyl)-1-(2-ethoxyethyl)-8-methoxy-4-oxo-1,4-dihydroquinoline-3-carboxamide (RS-016). Compared to KD2, RS-016 exhibits a higher binding affinity towards CB2 (Ki = 0.7 nM) with a selectivity over CB1 of >10,000 and lower lipophilicity (logD7.4 = 2.78). [11C]RS-016 was obtained in ≥99% radiochemical purity and up to 850 GBq/μmol specific radioactivity at the end of synthesis. In vitro autoradiography on rodent spleen tissue showed high specific binding to CB2. [11C]RS-016 was stable in vitro in rodent and human plasma over 40 min, whereas 47% intact compound was found in vivo in rat blood plasma 20 min post injection (p.i.). High specific binding to CB2 was observed in murine spleen tissues and postmortem ALS patient spinal cord tissues in vitro autoradiography, ex vivo biodistribution data confirmed the high and specific uptake of [11C]RS-016 in spleen region in rats. In vivo specificity of [11C]RS-016 could also be shown in brain by PET imaging using a murine neuroinflammation model, which has higher CB2 receptor expression level in the brain induced by lipopolysaccharide (LPS) application.

Aiming at a convenient way toward the synthesis of terminal (fluoroalkyl)silanes, the effect of (diethylamino)sulfur trifluoride (DAST) on some terminal hydroxyalkylsilanes was investigated. Reaction of DAST with a substituted (hydroxyethyl)diphenylsilane led to the formation of the desired (2-fluoroethyl)diphenylsilane in considerable amounts although the competing elimination route was still the favored mechanism. This new reaction was studied under various parameters with the best conditions yielding the substitution product in a ratio of 1:4 over the undesired elimination product. In a different approach trying to optimize the outcome of the reaction, the presence of two bulky 2,4,6-trimethoxyphenyl (TMOP) groups on the silicon atom favored the elimination route.

In the search for an efficient, fluorine-18 labeled amino acid based radiotracer for tumor imaging with positron emission tomography (PET), two new tryptophan analogs were synthesized and characterized in vitro and in vivo. Both are tryptophan alkyl-derivatives, namely 2-(3-[(18)F]fluoropropyl)-DL-tryptophan ([(18)F]2-FPTRP) and 5-(3-[(18)F]fluoro-propyl)-DL-tryptophan ([(18)F]5-FPTRP). Standard reference compounds and precursors were prepared by multi step approaches. Radiosynthesis was achieved by no-carrier-added nucleophilic [(18)F]fluorination in 29-34% decay corrected yields with radiochemical purity over 99%. In vitro cell uptake assays showed that both compounds are substrates for amino acid transport and enter small cell lung cancer cells (NCI-H69) most probably almost exclusively via large neutral amino acids transporter(s) (LAT). Small animal PET imaging with xenograft bearing mice revealed high tumor/background ratios for [(18)F]2-FPTRP comparable to the well established tyrosine analog O-(2-[(18)F]fluroethyl)-L-tyrosine ([(18)F]FET). Radiometabolite studies showed no evidence of involvement of a biotransformation step in tumor accumulation.

σ₂ Receptors are promising biomarkers for cancer diagnosis given the relationship between the proliferative status of tumors and their density. With the aim of contributing to the research of σ₂ receptor Positron Emission Tomography (PET) probes, we developed 2-[3-[6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl]propyl]-3,4-dihydroisoquinolin-1(2H)-one (3), with optimal σ₂ pharmacological properties and appropriate lipophilicity. Hence, 3 served as the lead compound for the development of a series of dihydroisoquinolinones amenable to radiolabeling. Radiosynthesis for compound 26, which displayed the most appropriate σ₂ profile, was developed and σ₂ specific binding for the corresponding [(18)F]-26 was confirmed by in vitro autoradiography on rat brain slices. Despite the excellent in vitro properties, [(18)F]-26 could not successfully image σ₂ receptors in the rat brain in vivo, maybe because of its interaction with P-gp. Nevertheless, [(18)F]-26 may still be worthy of further investigation for the imaging of σ₂ receptors in peripheral tumors devoid of P-gp overexpression.

Sleep deprivation (wake therapy) provides rapid clinical relief in many patients with major depressive disorder (MDD). Changes in glutamatergic neurotransmission may contribute to the antidepressant response, yet the exact underlying mechanisms are unknown. Metabotropic glutamate receptors of subtype 5 (mGluR5) are importantly involved in modulating glutamatergic neurotransmission and neuronal plasticity. The density of these receptors is reduced in the brain of patients with MDD, particularly in brain structures involved in regulating wakefulness and sleep. We hypothesized that prolonged wakefulness would increase mGluR5 availability in human brain.Metabotropic glutamate receptor subtype 5 binding was quantified with positron emission tomography in 22 young healthy men who completed two experimental blocks separated by 1 week. Two positron emission tomography examinations were conducted in randomized, crossover fashion with the highly selective radioligand, ¹¹C-ABP688, once after 9 hours (sleep control) and once after 33 hours (sleep deprivation) of controlled wakefulness. ¹¹C-ABP688 uptake was quantified in 13 volumes of interest with high mGluR5 expression and presumed involvement in sleep-wake regulation.Sleep deprivation induced a global increase in mGluR5 binding when compared with sleep control (p.006). In anterior cingulate cortex, insula, medial temporal lobe, parahippocampal gyrus, striatum, and amygdala, this increase correlated significantly with the sleep deprivation-induced increase in subjective sleepiness.This molecular imaging study demonstrates that cerebral functional mGluR5 availability is increased after a single night without sleep. Given that mGluR5 density is reduced in MDD, further research is warranted to examine whether this mechanism is involved in the potent antidepressant effect of wake therapy.

The histamine H3 receptor (H3R) plays a role in cognition and memory processes and is implicated in different neurological disorders, including Alzheimer’s disease, schizophrenia, and narcolepsy. In vivo studies of the H3R occupancy using a radiolabeled PET tracer would be very useful for CNS drug discovery and development. We report here the radiosynthesis, in vitro and in vivo evaluation of a novel 18F-labeled high-affinity H3R antagonist 18F-ST889. The radiosynthesis was accomplished via nucleophilic substitution of the mesylate leaving group with a radiochemical yield of 8–20%, radiochemical purity >99%, and specific radioactivity >65 GBq/μmol. 18F-ST889 exhibited high in vivo stability and rather low lipophilicity (log D7.4 = 0.35 ± 0.09). In vitro autoradiography showed specific binding in H3R-rich brain regions such as striatum and cortex. However, in vivo PET imaging of the rat brain with 18F-ST889 was not successful. Possible reasons are discussed.

(E)-3-(pyridin-2-ylethynyl)cyclohex-2-enone O-2-(2-(18)F-fluoroethoxy)ethyl oxime, ([(18)F]-FDEGPECO), a novel high affinity radioligand for the metabotropic glutamate receptor subtype 5 (mGluR5) was assessed for its potential as a PET imaging agent. In vitro autoradiography on rat brain slices resulted in a heterogeneous and displaceable binding to mGluR5-rich brain regions. [(18)F]-FDEGPECO showed high stability in rat plasma and brain homogenate as well as in human plasma and microsomes. Good blood-brain barrier passage was predicted from an in vitro transport assay with P-glycoprotein-transfected hMDR1-MDCK cells. In vivo PET imaging on rats revealed specific uptake of radioactivity in the mGluR5-rich brain regions such as hippocampus, striatum and cortex while the cerebellum, a region with low mGluR5-expression, showed negligible uptake. Blockade experiments by co-injection of [(18)F]-FDEGPECO and M-MPEP (6mg/kg), an antagonist for mGluR5, reduced the level of radioactivity in mGluR5-regions to that of the cerebellum, pointing to an effective blockade of specifically bound [(18)F]-FDEGPECO. Postmortem biodistribution studies at 15min p.i. confirmed the distribution pattern observed in PET. HPLC analysis of rat brain extracts indicated that 98.5% and 91% of the total radioactivity were parent compound at 5min and 17min p.i., respectively. Taken together, the high affinity and the high in vivo specificity of [(18)F]-FDEGPECO for mGluR5 in the rat brain as well as the lack of in vivo defluorination make this new [(18)F]-labeled ABP688 derivative a suitable ligand for the preclinical PET imaging of mGluR5. These favorable characteristics warrant further evaluation in humans.

11 C-ABP-688 is a selective tracer for the mGluR5 receptor. Its kinetics is fast and thus favourable for an equilibrium approach to determine receptor-related parameters. The purpose of this study was to test the hypothesis that the pattern of the 11 C-ABP688 uptake using a bolus-plus-infusion (B/I) protocol at early time points corresponds to the perfusion and at a later time point to the total distribution volume. Methods A bolus and a B/I study (1 h each) was performed in five healthy male volunteers. With the B/I protocol, early and late scans were normalized to gray matter, cerebellum and white matter. The same normalization was done on the maps of the total distribution volume (Vt) and K 1 which were calculated in the study with bolus only injection and the Logan method (Vt) and a two-tissue compartment model ( K 1 ). Results There was an excellent correlation close to the identity line between the pattern of the late uptake in the B/I study and Vt of the bolus-only study for all three normalizations. The pattern of the early uptake in the B/I study correlated well with the K 1 maps, but only when normalized to gray matter and cerebellum, not to white matter. Conclusion It is demonstrated that with a B/I protocol the 11 C-ABP688 distribution in late scans reflects the pattern of the total distribution volume and is therefore a measure for the density pattern of mGluR5. The early scans following injection are related to blood flow, although not in a fully quantitative manner. The advantage of the B/I protocol is that no arterial blood sampling is required, which is advantageous in clinical studies.

The direct labeling of biomolecules with fluorine-18 is highly desirable. An option is the ring-opening of an activated aziridine moiety in a biomolecule using 18 F-fluoride. Therefore, a series of aziridine-based model compounds and three aziridine-based biomolecules four aziridine-based model compounds were synthesized and evaluated as potential precursors for a direct one-step radiolabeling with fluorine-18. High to moderate yields of 18 F-incorporation were achieved under mild labeling conditions. The influence of different activating groups, reaction temperature, solvent and base was investigated. The applicability of this method for the direct 18 F-radiolabeling of biomolecules for positron emission tomography (PET) studies is illustrated with examples.

Introduction Fluoroethyl-desmethyl-ABP688 (FE-DABP688) is a novel derivative of the previously described positron emission tomography (PET) ligand 3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone- O -[ 11 C]-methyl-oxime. FE-DABP688 was radiolabeled with fluorine-18 and characterized as a PET imaging agent for the metabotropic glutamate receptor subtype 5 (mGluR5). Methods FE-DABP688 was radiolabeled by reacting 2-[ 18 F]-fluoroethyl tosylate with the sodium salt of 3-(pyridin-2-ylethynyl)-cyclohex-2-enone-oxime in dry DMF. The in vitro affinity of [ 18 F]-FE-DABP688 for mGluR5 was determined by Scatchard analysis of saturation binding data using rat whole-brain membranes (without cerebellum). Further in vitro characterization of the tracer involved plasma stability and lipophilicity testing. In vivo evaluation of [ 18 F]-FE-DABP688 was performed by postmortem biodistribution experiments and PET studies in rats using the dedicated small-animal PET tomograph quad-HIDAC. Results The radiotracer was obtained in good radiochemical yields in an overall synthesis time of 150 min. The radiochemical yield after semipreparative HPLC was 25±8% ( n >7, decay corrected), and specific activity was 30±5 GBq/μmol ( n >7). [ 18 F]-FE-DABP688 exhibited optimal lipophilicity with a log D value of 2.1±0.1 and high plasma stability. Saturation assays of [ 18 F]-FE-DABP688 revealed a single high-affinity binding site with a dissociation constant ( K d ) of 1.6±0.4 nM and a B max value of 119±24 fmol/mg protein. PET scanning indicated radioactivity uptake in mGluR5-rich regions such as the hippocampus, striatum and cortex, while radioactivity accumulation in the cerebellum, a region with negligible mGluR5 density, was significantly lower. Biodistribution studies showed a similar distribution pattern of [ 18 F]-FE-DABP688 binding in the brain. The hippocampus-to-cerebellum and striatum-to-cerebellum ratios were 1.81±0.16 and 1.93±0.36, respectively. Blocking studies using coinjection of [ 18 F]-FE-DABP688 and unlabeled 2-methyl-6-((3-methoxyphenyl)ethynyl)-pyridine (1 mg/kg) revealed more than 45% specific binding in the hippocampus and striatum, thus demonstrating the in vivo specificity of tracer binding. Conclusions [ 18 F]-FE-DABP688 may be a useful PET tracer for imaging mGluR5 in rodents.

Introduction The clinically established positron emission tomography (PET) tracers 6-[ 18 F]-fluoro- l -DOPA ([ 18 F]FDOPA), 6-[ 18 F]-fluoro- l - m -tyrosine ([ 18 F]FMT) and 2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-[ 18 F]-fluoroethyl)-nortropane ([ 18 F]FECNT) serve as markers of presynaptic integrity of dopaminergic nerve terminals in humans. This study describes our efforts to adopt the methodology of human Parkinson's disease (PD) PET studies to mice. Methods The PET imaging characteristics of [ 18 F]FDOPA, [ 18 F]FMT and [ 18 F]FECNT were analyzed in healthy C57BL/6 mice using the dedicated small-animal PET tomograph quad-HIDAC. Furthermore, [ 18 F]FECNT was tested in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. Results [ 18 F]FDOPA and [ 18 F]FMT failed to clearly visualize the mouse striatum, whereas PET experiments using [ 18 F]FECNT proved that the employed methodology is capable of delineating the striatum in mice with exquisite resolution. Moreover, [ 18 F]FECNT PET imaging of healthy and MPTP-lesioned mice demonstrated that the detection and quantification of striatal degeneration in lesioned mice can be accomplished. Conclusions This study shows the feasibility of using [ 18 F]FECNT PET to analyze noninvasively the striatal degeneration in the MPTP mouse model of PD. This methodology can be therefore considered as a viable complement to established in vivo microdialysis and postmortem techniques.

For the electrophilic preparation of 6-[18F]fluoro-L-m-tyrosine ([18F]FMT), a PET tracer for measuring changes in dopaminergic function in movement disorders, a novel precursor, N-(tert-butoxycarbonyl)-3-(tert-butoxycarbonyloxy)-6-trimethylstannnyl-L-phenylalanine ethyl ester, was synthesized in four steps and 26% yield starting from L-m-tyrosine. [18F]FMT produced by two methods at two institutions was comparable in both radiochemical yield, 25-26%, and quality (chemical, enantiomeric, and radiochemical purity and specific activity) as that obtained with the original N-trifluoroacetyl-3-acetyl-6-trimethylstannyl-L-m-tyrosine ethyl ester [18F]FMT precursor.

(D)-7-Iodo-N-(1-phosphonoethyl)-5-aminomethylquinoxaline-2,3 -dione (I-PAMQX), is a potent, in vivo active antagonist acting at the glycine binding site of the NMDA receptor complex. Radioiodinated [131I]I-PAMQX was prepared with good yields and high specific activity from its 7-bromo analogue. Biodistribution studies of [131I]I-PAMQX in mice showed a relatively slow clearance from the blood. The uptake of radioactivity was highest in the kidneys, moderate in the heart, lung, liver and bones, and low in the brain.

The effect of the fluorinated memantine derivative and NMDA receptor antagonist, 1-amino-3-fluoromethyl-5-methyl-adamantane (19F-MEM), at the NMDA receptor ion channel was studied by patch clamp recording. The results showed that 19F-MEM is a moderate NMDA receptor channel blocker. A procedure for the routine preparation of the 18F-labelled analog 18F-MEM has been developed using a two-step reaction sequence. This involves the no-carrier-added nucleophilic radiofluorination of 1-[N-(tert-butyloxy)carbamoyl]-3-(toluenesulfonyloxy)methyl- 5-methyl-adamantane and the subsequent cleavage of the BOC-protecting group using aqueous HCI. The 18F-MEM was obtained in 22 +/- 7% radiochemical yield (decay-corrected to EOB) in a total synthesis time including HPLC purification of 90 min. A biodistribution study after i.v. injection of 18F-MEM in mice showed a fast clearance of radioactivity from blood and relatively high initial uptake in the kidney and in the lung, which gradually decreased with time. The brain uptake was high (up to 3.6% ID/g, 60 min postinjection) with increasing brain-blood ratios: 2.40, 5.10, 6.33, and 9.27 at 5, 30, 60, and 120 min, respectively. The regional accumulation of the radioactivity in the mouse brain was consistent with the known distribution of the PCP recognition site. Preliminary PET evaluation of the radiotracer in a rhesus monkey demonstrated good uptake and prolonged retention in the brain, with a plateau from 35 min onwards p.i. in the NMDA receptor-rich regions (frontal cortex, striata, and temporal cortex). Delineation of the hippocampus, a region known to contain a high density of NMDA receptors, was not possible owing to the resolution of the PET tomograph. The regional brain uptake of 18F-MEM was changed by memantine and by a pharmacological dose of (+)-MK-801, indicating competition for the same binding sites. In a preliminary experiment, haloperidol, a dopamine D2 and sigma receptor antagonist, decreased the binding of 18F-MEM from the brain regions examined, suggesting that binding was also occurring to the sigma recognition sites.

The reversible and highly selective monoamine oxidase B (MAO-B) inhibitor Ro 19-6327, a picolinic acid derivative, was selected for the development of new radiopharmaceuticals, whereby in place of Cl either 123I or 18F was introduced. The respective labelling procedures have been described earlier. In this study, some metabolic properties were investigated. Blood and urine samples were analysed, and halogenated picolinylglycine, a more hydrophilic compound, was identified as the main metabolite. This shows that the amine is oxidised to the respective carboxylate, but the intermediate imine or aldehyde that was proposed earlier could not be detected. First experiments with single photon emission tomography and positron emission tomography (PET) showed that the iodo compound can be used to investigate MAO-B in vivo while the fluoro compound is accumulated in the brain to such a low degree that no PET studies can be performed. We conclude that the main reason for the poor uptake of the fluoro compound is its lower lipophilicity as compared to the iodo compound and, to a lesser degree, its metabolism, which is similar for both compounds.

Nicotine addiction is a major public health problem and increases the risk for various psychiatric conditions, including depression and schizophrenia. On a neurophysiological level, it is associated with aberrant glutamate activity. We recently showed marked global reductions in metabotropic glutamate receptor subtype 5 (mGluR5) binding in smokers and short-term ex-smokers. Now, we examined mGluR5 in long-term ex-smokers (abstinence>1.5y). In addition, we investigated the effects of chronic nicotine exposure (35 weeks in two groups using 4 and 8 mg/l nicotine) on mGluR5 binding in rats. In humans mGluR5 receptor binding was measured with the highly selective mGluR5 radioligand [11C]ABP688. In male Dark Agouti rats we used PET with the novel mGluR5 radiotracer [18F]PSS232. Long-term ex-smokers and individuals who had never smoked showed no differences in mGluR5 binding in any of the brain regions examined. Long-term ex-smokers showed significantly higher mGluR5 binding than recent ex-smokers, most prominently in the frontal cortex (42%) and the thalamus (57%). At follow-up we found a tendency for lower mGluR5 binding in relapsed vs. abstinent ex-smokers. In rats, we demonstrated that nicotine consumption reduces mGluR5 binding, although this effect did not follow a linear dose-response-type relationship. In a series of studies we provided evidence for a downregulation of mGluR5 as a pathological mechanism involved in nicotine dependence and the relapse to smoking. Altogether, these findings suggest that mGluR5 binding is a clinically relevant biomarker for the course of nicotine addiction and a promising target for the discovery of novel drugs against nicotine dependence and other substance-related disorders.

INTRODUCTION AND OBJECTIVES: Skeletal muscle encloses sources of Muscle Precursor Cells (MPCs), which are capable of reconstructing muscle fiber upon injury. MPCs are studied for the treatment of urinary incontinence, also for patients after prostatectomy. However, the safety of injecting these cells in proximity of a potential location of tumor recurrence has not yet been investigated. METHODS: We have injected human MPCs, isolated from muscle biopsies harvested from the rectus abdominis, together with different types of prostate cancer, DU145, PC3 and LnCAP (ATCCLGC Standard) in vivo. Tumor sizes were measured with caliper twice a week for 6 weeks. Lymph node metastasis and tumor phenotypes were analyzed by histology. Tissue physiological activity was assessed by PET scan with F-choline. After harvest tissues were examined by IHC evaluating myogenic differentiation apoptosis and cell cycle arrest of the retrieved tumor. Data was analyzed with SPSS v11 (SPSS Inc, Chicago, IL) by independent samples t-tests or one way ANOVA (p 0.05 is considered significant). RESULTS: When co-injected with MPCs, Prostate carcinoma growth was reduced up to 5 fold (fig.1), while lymph node metastases were reduced from 100% after 6 weeks to only 10%. Lymph node and bone metastasis were identified by PET scan with F-choline (Fig.2) on the animals injected with tumor alone. In contrast, no animal bearing tumor co-injected with MPCs presented positive lymph node or metastasis. Histology demonstrates that MPCs differentiated in vivo developing into organized and functional muscle, while the tumor in its proximity were undergoing apoptosis (Caspase3 positive), cell cycle arrest (p21 positive) and expressing the tumor suppressor (BIN1). CONCLUSIONS: In this study, we report that the use of MPCs in proximity of prostate cancer is safe in vivo. Further, it decreased tumor growth and metastasis formation could be demonstrated. These results suggest that MPCs can be safely injected on the pelvic floor after prostatectomy.

Noninvasive functional imaging of tumors can provide valuable early-response biomarkers, in particular, for targeted chemotherapy. Using various experimental tumor models, we have investigated the ability of positron emission tomography (PET) measurements of 2-deoxy-2-[18F]fluoro-glucose (FDG) and 3′-deoxy-3′-[18F]fluorothymidine (FLT) to detect response to the allosteric mammalian target of rapamycin (mTOR) inhibitor everolimus. Tumor models were declared sensitive (murine melanoma B16/BL6 and human lung H596) or relatively insensitive (human colon HCT116 and cervical KB31), according to the IC50 values (concentration inhibiting cell growth by 50%) for inhibition of proliferation in vitro (1 µM, respectively). Everolimus strongly inhibited growth of the sensitive models in vivo but also significantly inhibited growth of the insensitive models, an effect attributable to its known anti-angiogenic/vascular properties. However, although tumor FDG and FLT uptake was significantly reduced in the sensitive models, it was not affected in the insensitive models, suggesting that endothelial-directed effects could not be detected by these PET tracers. Consistent with this hypothesis, in a well-vascularized orthotopic rat mammary tumor model, other antiangiogenic agents also failed to affect FDG uptake, despite inhibiting tumor growth. In contrast, the cytotoxic patupilone, a microtubule stabilizer, blocked tumor growth, and markedly reduced FDG uptake. These results suggest that FDG/FLT-PET may not be a suitable method for early markers of response to antiangiogenic agents and mTOR inhibitors in which anti-angiogenic/vascular effects predominate because the method could provide false-negative responses. These conclusions warrant clinical testing.