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Nicotine addiction is a major public health problem, resulting in primary glutamatergic dysfunction. We measured the glutamate receptor binding in the human brain and provided direct evidence for the abnormal glutamate system in smokers. Because antagonism of the metabotropic glutamate receptor 5 (mGluR5) reduced nicotine self-administration in rats and mice, mGluR5 is suggested to be involved in nicotine addiction. mGluR5 receptor binding specifically to an allosteric site was observed by using positron emission tomography with [ 11 C]ABP688. We found a marked global reduction (20.6%; P < 0.0001) in the mGluR5 distribution volume ratio (DVR) in the gray matter of 14 smokers. The most prominent reductions were found in the bilateral medial orbitofrontal cortex. Compared with 14 nonsmokers, 14 ex-smokers had global reductions in the average gray matter mGluR5 DVR (11.5%; P < 0.005), and there was a significant difference in average gray matter mGluR5 DVR between smokers and ex-smokers (9.2%; P < 0.01). Clinical variables reflecting current nicotine consumption, dependence and abstinence were not correlated with mGluR5 DVR. This decrease in mGluR5 receptor binding may be an adaptation to chronic increases in glutamate induced by chronic nicotine administration, and the decreased down-regulation seen in the ex-smokers could be due to incomplete recovery of the receptors, especially because the ex-smokers were abstinent for only 25 wk on average. These results encourage the development and testing of drugs against addiction that directly target the glutamatergic system.

The compound Ro 19-6327, N-(2-aminoethyl)-5-chloropyridine-2-carboxamide, is known to inhibit reversibly and site specifically the enzyme monoamine oxidase B (MAO-B). The 123 I-labelled iodo-analogue N-(2-aminoethyl)-5-iodopyridine-2-carboxamide (Ro 43-0463) was investigated successfully in human volunteers by means of SPET (Single Photon Emission Tomography). We developed therefore the synthesis and radiolabelling of the corresponding fluoro-analogue N-(2-aminoethyl)-5-fluoropyridine-2-carboxamide with 18 F in order to carry out PET (Positron Emission Tomography) investigations of MAO-B related neuropsychiatric diseases. For this purpose two synthetic approaches leading to the electrophilic and the nucleophilic methods of 18 F radiolabelling were undertaken. The nucleophilic approach appeared to be superior when factors such as precursor synthesis, beam time, specific activity and radiochemical purity of the product are considered.

Epidemiological studies have shown an association between maternal overnutrition and increased risk of the progeny for the development of obesity as well as psychiatric disorders. Animal studies have shown results regarding maternal high-fat diet (HFD) and a greater risk of the offspring to develop obesity. However, it still remains unknown whether maternal HFD can program the central reward system in such a way that it will imprint long-term changes that will predispose the offspring to addictive-like behaviors that may lead to obesity. We exposed female dams to either laboratory chow or HFD for a period of 9 weeks: 3 weeks before conception, during gestation and lactation. Offspring born to either control or HFD-exposed dams were examined in behavioral, neurochemical, neuroanatomical, metabolic and positron emission tomography (PET) scan tests. Our results demonstrate that HFD offspring compared with controls consume more alcohol, exhibit increased sensitivity to amphetamine and show greater conditioned place preference to cocaine. In addition, maternal HFD leads to increased preference to sucrose as well as to HFD while leaving the general feeding behavior intact. The hedonic behavioral alterations are accompanied by reduction of striatal dopamine and by increased dopamine 2 receptors in the same brain region as evaluated by post-mortem neurochemical, immunohistochemical as well as PET analyses. Taken together, our data suggest that maternal overnutrition predisposes the offspring to develop hedonic-like behaviors to both drugs of abuse as well as palatable foods and that these types of behaviors may share common neuronal underlying mechanisms that can lead to obesity.

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.

The folate receptor (FR) is upregulated in various cancer types (FR-α isoform) and in activated macrophages (FR-β isoform) which are involved in inflammatory and autoimmune diseases, but its expression in healthy tissues and organs is highly restricted to only a few sites (e.g kidneys). Therefore, the FR is a promising target for imaging and therapy of cancer and inflammation using folate-based radiopharmaceuticals. Herein, we report the synthesis and evaluation of a novel folic acid conjugate with improved properties suitable for positron emission tomography (PET). [(18)F]-fluoro-deoxy-glucose folate ([(18)F]3) was synthesized based on the click chemistry approach using 2-deoxy-2-[(18)F]fluoroglucopyranosyl azide and a folate alkyne derivative. The novel radiotracer [(18)F]3 was produced in good radiochemical yields (25% d.c.) and high specific radioactivity (90 GBq/μmol). Compared to previously published (18)F-folic acid derivatives, an increase in hydrophilicity was achieved by using a glucose entity as a prosthetic group. Biodistribution and PET imaging studies in KB tumor-bearing mice showed a high and specific uptake of the radiotracer in FR-positive tumors (10.03 ± 1.12%ID/g, 60 min p.i.) and kidneys (42.94 ± 2.04%ID/g, 60 min p.i.). FR-unspecific accumulation of radioactivity was only found in the liver (9.49 ± 1.13%ID/g, 60 min p.i.) and gallbladder (17.59 ± 7.22%ID/g, 60 min p.i.). No radiometabolites were detected in blood, urine, and liver tissue up to 30 min after injection of [(18)F]3. [(18)F]-fluoro-deoxy-glucose-folate ([(18)F]3) is thus a promising PET radioligand for imaging FR-positive tumors.

UNLABELLED Large neutral l amino acids are substrates of system L amino acid transporters. The level of one of these LAT1 is increased in many tumors. Aromatic l amino acids may also be substrates of aromatic l amino acid decarboxylase (AADC) the level of which is enhanced in endocrine tumors. Increased amino acid uptake and subsequent decarboxylation result in the intracellular accumulation of the amino acid and its decarboxylation product. (18)F and (11)C labeled neutral aromatic amino acids such as l 34 dihydroxy 6 (18)F fluorophenylalanine ((18)F FDOPA) and 5 hydroxy l [ß (11)C]tryptophan are thus successfully used in PET to image endocrine tumors. However 5 hydroxy l [ß (11)C]tryptophan has a relatively short physical half life (20 min). In this work we evaluated the in vitro and in vivo characteristics of the (18)F labeled tryptophan analog 5 (2 (18)F fluoroethoxy) l tryptophan ((18)F l FEHTP) as a PET probe for tumor imaging. METHODS (18)F l FEHTP was synthesized by no carrier added (18)F fluorination of 5 hydroxy l tryptophan. In vitro cell uptake and efflux of (18)F l FEHTP and (18)F FDOPA were studied with NCI H69 endocrine small cell lung cancer cells PC 3 pseudoendocrine prostate cancer cells and MDA MB 231 exocrine breast cancer cells. Small animal PET was performed with the respective xenograft bearing mice. Tissues were analyzed for potential metabolites. RESULTS (18)F l FEHTP specific activity and radiochemical purity were 50 150 GBq/µmol and greater than 95 respectively. In vitro cell uptake of (18)F l FEHTP was between 48 and 113 of added radioactivity per milligram of protein within 60 min at 37°C and was blocked by greater than 95 in all tested cell lines by the LAT1/2 inhibitor 2 amino 2 norboranecarboxylic acid. (18)F FDOPA uptake ranged from 26 to 53/mg. PET studies revealed similar xenograft to reference tissue ratios for (18)F l FEHTP and (18)F FDOPA at 30 45 min after injection. In contrast to the (18)F FDOPA PET results pretreatment with the AADC inhibitor S carbidopa did not affect the (18)F l FEHTP PET results. No decarboxylation products of (18)F l FEHTP were detected in the xenograft homogenates. CONCLUSION (18)F l FEHTP accumulates in endocrine and nonendocrine tumor models via LAT1 transport but is not decarboxylated by AADC. (18)F l FEHTP may thus serve as a PET probe for tumor imaging and quantification of tumor LAT1 activity. These findings are of interest in view of the ongoing evaluation of LAT1 substrates and inhibitors for cancer therapy.

Introduction We report on the synthesis, radiolabeling, in vitro and in vivo characterization of N -Me-[ 18 F]FHBT (6-(3-[ 18 F]fluoro-2-(hydroxymethyl)propyl)-1,5-dimethylpyrimidin-2,4(1 H ,3 H )-dione), a C-6-substituted N -1-methylated pyrimidine derivative as a reporter probe for imaging herpes simplex virus type 1 thymidine kinase (HSV1-TK) expression. Methods N -Me-[ 18 F]FHBT was synthesized via a standard nucleophilic substitution reaction followed by acidic cleavage of the methoxytrityl protecting group. Cell uptake was studied in vitro with control HEK293 (human embryonic kidney cells) and HEK293 cells stably transfected with nonmutant HSV1-tk (HEK293TK+ cells). Positron emission tomography (PET) imaging and biodistribution studies of N -Me-[ 18 F]FHBT or [ 18 F]FHBG were performed in nude mice bearing xenografts of HEK293 control and TK+ cells. Results N -Me-[ 18 F]FHBT was obtained in a two-step reaction in an overall maximal radiochemical yield (decay-corrected) of 5% and a radiochemical purity >96%. The tracer uptake in HSV1-TK containing HEK293TK+ cells was 14.5 times (at 30 min) and 55.4 times (at 240 min) higher than in control HEK293 cells. In mice, N -Me-[ 18 F]FHBT and [ 18 F]FHBG accumulated significantly and exhibited similar radioactivity levels in the HEK293TK+ xenografts; however, standardized uptake values ratios between HEK293TK+ and HEK293 control xenografts were higher for [ 18 F]FHBG than for N -Me-[ 18 F]FHBT. Both tracers showed high gall bladder and abdominal activity. Conclusion The biological evaluations demonstrated the feasibility of using N -methylated C-6-substituted pyrimidine derivative N -Me-[ 18 F]FHBT as a PET radiotracer for monitoring HSV1-TK expression in vivo.

The histamine H(3) receptor (H(3)R) 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 H(3)R 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 (18)F-labeled high-affinity H(3)R antagonist (18)F-ST889. The radiosynthesis was accomplished via nucleophilic substitution of the mesylate leaving group with a radiochemical yield of 8-20%, radiochemical purity99%, and specific radioactivity65 GBq/μmol. (18)F-ST889 exhibited high in vivo stability and rather low lipophilicity (logD(7.4)=0.35 ± 0.09). In vitro autoradiography showed specific binding in H(3)R-rich brain regions such as striatum and cortex. However, in vivo PET imaging of the rat brain with (18)F-ST889 was not successful. Possible reasons are discussed.

Activation of astrocytic metabotropic glutamate receptor 5 (mGluR5) is postulated to elicit calcium transients, triggering a chain of events that ultimately regulates cerebral blood flow by changing the tone of smooth muscle cells of nearby arterioles. Using concurrent in vivo optical imaging and determination of receptor occupancy with 11C-ABP688, we report here that blocking ∼80% of mGluR5 in vivo does not affect transient hemodynamic responses on brief whisker stimulations while transiently reducing neuronal activity as measured by voltage-sensitive dye imaging. Our results show that mechanisms other than activation of mGluR5 are required to trigger the initial hemodynamic response in normal physiological conditions.

Molecular modeling and phosphorylation assay in vitro were employed to select a novel unsaturated 1, 3-dihydroxyisobutenyl thymine derivative 6 as ligand for HSV-1 TK which may be of interest as lead for the development of an positron emission tomography (PET) imaging agent. Compound 6 was successfully prepared using modified approaches. A significant improvement over the syntheses involving pathways A and B (1% and 3% overall yield, respectively), was observed using synthetic route C (14% overall yield).

Novel C-6 substituted pyrimidine derivatives are good substrates of herpes simplex virus type 1 thymidine kinase (HSV1-TK). Enzyme kinetic experiments showed that our lead compound, N-methyl DHBT (N-methyl-6-(1,3-dihydroxyisobutyl) thymine; N-Me DHBT), is phosphorylated at a similar rate compared to "gold standard" 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine, FHBG, (K(m) = 10 ± 0.3 μM; k(cat) = 0.036 ± 0.015 sec(-1)). Additionally, it does not show cytotoxic properties on B16F1 cells up to a concentration of 10 mM. The x-ray analysis of the crystal structures of HSV1-TK with N-Me DHBT and of HSV1-TK with the fluorinated derivative N-Me FHBT confirmed the binding mode predicted by docking studies and their substrate characteristics. Moreover, the crystal structure of HSV1-TK with N-Me DHBT revealed an additional water-mediated H-bond interesting for the design of further analogues.

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a number of human tumors and has been targeted with radiolabeled bombesin analogues for the diagnosis and therapy of these cancers. Seven bombesin analogues containing various linkers and peptide sequences were designed, synthesized, radiolabeled with (18)F, and characterized in vitro and in vivo as potential PET imaging agents. Binding studies displayed nanomolar binding affinities toward human GRPR for all synthesized bombesin analogues. Two high-affinity peptide candidates 6b (K(i) = 0.7 nM) and 7b (K(i) = 0.1 nM) were chosen for further in vivo evaluation. Both tracers revealed specific uptake in GRPR-expressing PC-3 tumors and the pancreas. Compared to [(18)F]6b, compound [(18)F]7b was characterized by superior tumor uptake, higher specificity of tracer uptake, and more favorable tumor-to-nontarget ratios. In vivo PET imaging allowed for the visualization of PC-3 tumor in nude mice suggesting that [(18)F]7b is a promising PET tracer candidate for the diagnosis of GRPR-positive tumors in humans.

Four novel thiazole containing ABP688 derivatives were synthesized and evaluated for their binding affinity towards the metabotropic glutamate receptor subtype 5 (mGluR5). ( E )-3-((2-(Fluoromethyl)thiazol-4-yl)ethynyl)cyclohex-2-enone O -methyl oxime (FTECMO), the ligand with the highest binding affinity ( K i = 5.5 ± 1.1 nM), was labeled with fluorine-18. [ 18 F]-FTECMO displayed optimal lipophilicity (log D pH7.4 = 1.6 ± 0.2) and high stability in rat and human plasma as well as sufficient stability in rat liver microsomes. In vitro autoradiography with [ 18 F]-FTECMO revealed a heterogeneous and displaceable binding in mGluR5-rich brain regions. PET imaging with [ 18 F]-FTECMO in Wistar rats, however, showed low brain uptake. Uptake of radioactivity into the skull was observed suggesting in vivo defluorination. Thus, although [ 18 F]-FTECMO is an excellent ligand for the detection of mGluR5 in vitro, its in vivo characteristics are not optimal for the imaging of mGluR5 in rats in vivo.

Although many radiolabelled compounds for PET imaging have been developed so far, only a few have reached the status of a clinically established and routinely used PET radiopharmaceutical. At the early stage of development, a reasonable medical indication is obviously fundamental for a PET radiopharmaceutical to be further considered as clinically relevant. However, besides a favourable in vivo behaviour and appropriate imaging characteristics, certain criteria have to be fulfilled, such as a fast, straightforward and reliable radiosynthesis; an assured stability of the label as well as of the compound itself and a good availability of a suitable precursor. In particular, the ease and reliability of the radiochemistry is critical as the radiopharmaceutical needs to be available on demand in sufficient amounts. The precursors play the decisive role in the radiochemical approach as they specify the radiosynthetic route. Furthermore, the accessibility of the appropriate precursors is important for the applicability of radiosynthesis. Today, most precursors of the commonly used PET radiopharmaceuticals are commercially available and provided as approved medical products by suppliers such as ABX – advanced biochemical products GmbH Germany [1].

5,6-Disubstituted pyrimidine derivatives (3-20) were prepared by intramolecular cyclization reaction of alpha-(1-carbamyliminomethylene)-gamma-butyrolactone (2) with sodium ethoxide and subsequent chemical transformation of 2-hydroxy group in C-5 side chain as well as lithiation reaction for introduction of acyclic side chain at C-6. All compounds were characterized by (1)H NMR, (13)C NMR and mass spectra. Structures of compounds 4, 7 and 14 were unambiguously confirmed by X-ray crystal structural analysis. Supramolecular structures of these three compounds differ significantly. Two N-H...O and one C-H...O hydrogen bonds in 4 form three-dimensional network. One O-H...N hydrogen bond and one pi...pi interaction self-assemble the molecules of 7 into sheets. In supramolecular aggregation of 14, only pi...pi stacking interactions participate, so forming chains. The compounds were evaluated for their cytostatic activities against human malignant cell lines. Of all tested compounds, 2,4-dimethoxy-5-methoxytritylethylpyrimidine (9) and 2,4-dichloro-5-chloroethylpyrimidine (14) exhibited the most prominent inhibitory effects. Furthermore, compound 14 showed marked activity against human colon carcinoma (IC(50)=0.4microM).

The aim of this study was to evaluate if hypoxia in feline fibrosarcomas can be detected. This was done using positron emission tomography (PET), two hypoxia tracers and polarographic pO(2) measurements. Of the seven cats included, five received [(18)F]-fluoromisonidazole and two 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide. Perfusion was evaluated with [(15)O]-H(2)O (n = 4) and with contrast-enhanced power Doppler ultrasonography (n = 5). Hypoxia was detected in three cats. Polarographic pO(2) measurements did not confirm PET results. In the ultrasonographic evaluation, low vascularity and low perfusion were seen with a peripheral vascular pattern and no perfusion in the centre of the tumour. This was in contrast to the [(15)O]-H(2)O scans, where central perfusion of the tumour was also found. In conclusion, it appears that hypoxia exists in this tumour type. The presence of tumour necrosis and heterogeneous hypoxia patterns in these tumours may explain the found discrepancies between the applied techniques.

Silicon chemistry has only recently been discovered by radiochemists as a straightforward tool for the introduction of 18 F into biomolecules for positron emission tomography (PET) imaging. 18 F-labeled PET tracers must be stable towards defluorination under physiological conditions, but it is known that the hydrolytic stability of the silicon-fluorine bond is determined by the nature of the substituents on silicon. In the presented study we performed an extensive investigation on the hydrolytic stability of various synthesized organofluorosilane model compounds. By means of density functional theory (DFT) methods a theoretical model of organofluorosilane hydrolysis, which correlates with the experimentally determined hydrolytic half-lives, is developed. The calculation of the difference of Si-F bond lengths between the optimized structures of the starting material A and the intermediate structure C allows the estimation of the hydrolytic stability of newly designed compounds. This model permits the facilitated development of improved building blocks for the synthesis of novel 18 F-silyl-modified biomolecules for PET imaging.

With the aim of reducing analysis time of radiopharmaceuticals, especially for carbon-11 and fluorine-18, radio-high performance liquid chromatography (radio-HPLC) is the analysis method of choice. Faster and more sensitive analytic methods are needed. Recently, ultra performance liquid chromatography (UPLC®) has become an accepted analysis method but has not yet been established in radiopharmacy. This study demonstrates with the established positron emission tomography (PET) tracer [18F]fluoromisonidazole (FMISO) the applicability of using UPLC® in combination with low volume radio flow cell for a fast, sensitive and efficient analytic method. The developed UPLC® method showed a sharp and high radio signal. The total analysis time was thus reduced from 15 to 2 min.

The gastrin-releasing peptide receptor (GRPr) is overexpressed on various human tumors. The goal of our study was the synthesis of new 18F-labeled bombesin analogues for the PET imaging of GRPr expression in prostate tumor using a silicon-based one-step n. c. a. radiolabeling method. The silicon-containing building blocks were efficiently coupled to the N-terminus of the peptides via solid-phase synthesis. Radiolabeling of the obtained peptide precursors proceeded smoothly under acidic conditions (34-85% conversion). Using the di-tert-butyl silyl building block as labeling moiety, products containing a hydrolytically stable 18F-label were obtained. In in vitro receptor binding experiments 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-NMeGly-His-Sta-Leu-NH 2 ( 4b, IC50 = 22.9 nM) displayed a 12-fold higher binding affinity than 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-Gly-His(3Me)-Sta-Leu-NH2 ( 3b, IC50 = 276.6 nM), and 4b was therefore chosen for further evaluation. In vitro and ex vivo metabolite studies of [18F]4b showed no significant degradation. In biodistribution experiments, tumor uptake of [18F]4b was low and unspecific, whereas the GRPr-rich pancreas revealed a high and specific accumulation of the radiotracer. This study demonstrates the applicability of our silicon-based one-step n. c. a. radiolabeling method for the synthesis of new 18F-labeled bombesin derivatives. This innovative approach represents a general, straightforward access to radiolabeled peptides as PET imaging probes.

European Journal of Nuclear Medicine and Molecular Imaging, 35 (4), ISSN:1619-7070, ISSN:1619-7089

Reaction of 3-(Dimethylamino)-2H-azirines with 1,3-Oxazolidine-2-thione to 3-(2-Hydroxyethyl)-2- thiohydantoins The reaction of 3-(dimethylamino)-2H-azirines 1 and 1,3-oxazolidine-2-thione (6), in MeCN at room temperature, yields, after hydrolytic workup, 3-(2-hydroxyethyl)-2-thiohydantoins 7 (Scheme 2). In the case of the spirocyclic 1c, crystallization of the crude reaction mixture leads to spiro [cyclopentane-1, 7′(7′aH)-imidazo [4, 3-b] oxazole] -5′-thione 8c. The mechanism is discussed.

Reaction of 3-(Dimethylamino)-2H-azirines with 1,3-Benzoxazole-2(3H)-thione The reaction of 3-(dimethylamino)-2H-azirines 2 with 1,3-benzoxazole-2(3H)-thione (5), which can be considered as NH-acidic heterocycle (pKaca. 7.3), in MeCN at room temperature, leads to 3-(2-hydroxyphenyl)-2-thiohydantoins 6 and thiourea derivatives of type 7 (Scheme 2). A reaction mechanism for the formation of the products via the crucial zwitterionic intermediate A′ is suggested. This intermediate was trapped by methylation with Mel and hydrolysis to give 9 (Scheme 4). Under normal reaction conditions, A′ undergoes a ring opening to B which is hydrolyzed during workup to yield 6 or rearranges to give the thiourea 7. A reasonable intermediate of the latter transformation is the isothiocyanate E (Scheme 3) which also could be trapped by morpholine. In i-PrOH at 55–65° 2a and 5 react to yield a mixture of 6a, 2-(isopropylthio)-1,3-benzoxazole (12), and the thioamide 13 (Scheme 5). A mechanism for the surprising alkylation of 5via the intermediate 2-amino-2-alkoxyaziridine Fis proposed. Again via an aziridine, e.g. H (Scheme 6), the formation of 13 can be explained.

3-(6-Methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-O-11C-methyl-oxime (11C-ABP688), a noncompetitive and highly selective antagonist for the metabotropic glutamate receptor subtype 5 (mGluR5), was evaluated for its potential as a PET agent.Six healthy male volunteers (mean age, 25 y; range, 21-33 y) were studied. Brain perfusion (15O-H2O) was measured immediately before each 11C-ABP688 PET scan. For anatomic coregistration, T1-weighted MRI was performed on each subject. Arterial blood samples for the determination of the arterial input curve were obtained at predefined time points, and 11C-ABP688 uptake was assessed quantitatively using a 2-tissue-compartment model.An initial rapid uptake of radioactivity followed by a gradual clearance from all examined brain regions was observed. Relatively high radioactivity concentrations were observed in mGluR5-rich brain regions such as the anterior cingulate, medial temporal lobe, amygdala, caudate, and putamen, whereas radioactivity uptake in the cerebellum and white matter, regions known to contain low densities of mGluR5, was low. Specific distribution volume as an outcome measure of mGluR5 density in the various brain regions ranged from 5.45 +/- 1.47 (anterior cingulate) to 1.91 +/- 0.32 (cerebellum), and the rank order of the corresponding specific distribution volumes of 11C-ABP688 in cortical regions was temporalfrontaloccipitalparietal. The metabolism of 11C-ABP688 in plasma was rapid; at 60 min after injection, 25% +/- 0.03% of radioactivity measured in the plasma of healthy volunteers was intact parent compound.The results of these studies indicate that 11C-ABP688 has suitable characteristics and is a promising PET ligand for imaging mGluR5 distribution in humans. Furthermore, it could be of great value for the selection of appropriate doses of clinically relevant candidate drugs that bind to mGluR5 and for PET studies of patients with psychiatric and neurologic disorders.

Purpose: We examined the tumor-targeting and therapeutic effects of 67Cu-labeled single amino acid mutant forms of anti-L1 monoclonal antibody chCE7 in nude mice with orthotopically implanted SKOV3ip human ovarian carcinoma cells.Experimental Design: For radioimmunotherapy, chCE7 antibodies with a mutation of histidine 310 to alanine (chCE7H310A) and a mutation of asparagine 297 to glutamine (chCE7agl) were generated to achieve more rapid blood clearance. Biodistributions of 67Cu-4-(1,4,8,11-tetraazacyclotetradec-1-yl)-methyl benzoic acid tetrachloride (CPTA)–labeled mutant antibodies were measured in nude mice bearing SKOV3ip human ovarian cancer metastases. The effects of single i.v. injections of 67Cu-chCE7agl alone on tumor reduction and survival were investigated. In addition, a combination of low-dose 67Cu-radioimmunotherapy with unlabeled anti-L1 antibody L1-11A on survival was investigated.Results: 67Cu-CPTA-chCE7agl showed high (up to 49% ID/g) and persistent (up to 168 h) uptake in SKOV3ip metastases, with low levels in normal tissues. 67Cu-CPTA-chCE7H310A revealed a shorter half-life in the blood and a lower tumor uptake and retention. A single low dose of 4 MBq of 67Cu-chCE7agl reduced tumor growth but did not prolong survival significantly, whereas a single 10.5 MBq dose of 67Cu-chCE7agl reduced tumor growth and prolonged survival significantly. The combination of unlabeled monoclonal antibody L1-11A with a subtherapeutic dose of 67Cu-radioimmunotherapy also prolonged survival significantly.Conclusion: The results show improved pharmacokinetics and biodistributions as well as the therapeutic effect of the 67Cu-labeled single amino acid mutant chCE7agl. Therapeutic data indicate, for the first time, the feasibility of combining anti–L1-directed growth inhibition and 67Cu-radioimmunotherapy, thereby increasing the efficiency of antibody treatment of metastatic ovarian carcinoma.

(11)C-ABP688 is a new PET ligand to assess the subtype 5 metabotropic glutamate receptor (mGlu(5)). The purpose of this study was to evaluate different methods for the analysis of human (11)C-ABP688 data acquired from 6 healthy, young volunteers.The methods were a 1-tissue-compartment model (K(1), k(2)''), a 2-tissue-compartment model (K1-k4), and the noncompartmental method developed by Logan. Parameters related to receptor density were the total distribution volume (DV), DV'' (= K(1)/k(2)'', 1 tissue compartment); specific DV, DV(C2) (= K(1)/k(2)' x k(3)'/k(4), 2 tissue compartments); and DV(tot) for the noncompartmental method.The 1-tissue-compartment model was too simple to adequately fit the data. DV(C2) calculated with the 2-tissue-compartment model ranged from 5.45 +/- 1.47 (anterior cingulate) to 1.91 +/- 0.32 (cerebellum). The corresponding values for DV(tot), calculated with the 2-tissue-compartment model and the Logan method (in parentheses), were 6.57 +/- 1.45 (6.35 +/- 1.32) and 2.93 +/- 0.53 (2.48 +/- 0.40). There was no clear evidence of a region devoid of mGlu(5) receptors. The first-pass extraction fraction exceeded 95%. The minimal scan duration to obtain stable results was estimated to be 45 min.(11)C-ABP688 displays favorable kinetics for assessing mGlu(5) receptors. For tracer kinetic modeling, 2-tissue-compartment models are clearly superior to models with only 1 tissue compartment. In comparison to the compartmental models, the Logan method is equally useful if only DV(tot) values are required and fast pixelwise parametric maps are desired. The lack of regions devoid of receptors limits the use of reference region methods that do not require arterial blood sampling. Another advantage of the tracer is the fast kinetics that allow for relatively short acquisitions.

Folic acid was linked regioselectively through its alpha- and gamma-carboxyl groups to 4-fluorobenzylamine (FBA), and the alpha- and gamma-FBA-folate regioisomers were evaluated for their ability to bind to folate receptor-positive cells. The 18F-labeled alpha/gamma-FBA-folate counterpart was examined for in vivo tumor targeting efficiency in nude mice bearing folate receptor-positive tumor cells.18F-alpha/gamma-FBA-folate was prepared in a 4-step reaction sequence starting from folic acid. The relative binding affinities of the alpha- and gamma-FBA-folates to the folate receptor with respect to parent folic acid were determined in cultured KB-31 cells (nasopharyngeal epidermal carcinoma cell line) overexpressing the folate receptor using 3H-folic acid. Tumor accumulation of the 18F-labeled alpha/gamma-FBA-folate and 18F-FDG was analyzed in vivo by high-resolution PET. Biodistribution and PET studies were performed under baseline and blockage conditions.The radiochemical yield of the coupling step ranged from 15% to 44%, and the maximum specific radioactivity was 24 GBq/micromol. The in vitro binding affinities of the alpha- and gamma-isomers and folic acid were 71, 62, and 41 nmol/L, respectively. PET revealed heterogeneous uptake of the radioligand, with the highest activity concentrations found in the tumor rim. In contrast, 18F-FDG uptake in a nude mouse bearing KB-31 folate receptor-positive tumors was negligible. Radioligand uptake in tumors at 125 min after injection amounted to 6.56% of the injected dose per gram of tissue (%ID/g) in control animals, whereas radioactivity accumulation in the tumors of folic acid-treated animals was significantly reduced by more than 80%-to 1.07 %ID/g (P = 0.001).This new 18F-labeled folic acid derivative is a promising tool for PET imaging of folate receptor-positive tumors.

In this study we assessed the new glutamatergic ligand (11)C-ABP688 with regard to the following characteristics: (A) brain distribution, (B) first pass extraction fraction, (C) suitable model to describe tracer kinetics and (D) specificity for the mGlu5 receptor. These parameters were assessed using autoradiography and a beta-scintillator positioned in the striatum. The study included 13 male rats. In 2 animals cerebral blood flow was measured using H(2)(15)O. The (11)C-ABP688 data were analyzed using compartmental modeling. A two-tissue compartment model turned out to fit the data more adequately (parameters: K(1), k(2)('), k(3)('), k(4), total distribution volume DV(tot)=K(1)/k(2)(') (1+k(3)(')/k(4)) than a one-tissue compartment model. The autoradiographic studies revealed high uptake in hippocampus, striatum and cortex and low accumulation in thalamus and cerebellum. The uptake was markedly reduced following blockade with the mGlu5 antagonist M-MPEP. The first pass extraction fraction exceeded 85%. Baseline DV(tot) was 15.16+/-2.67 ml plasma/ml tissue and decreased by 56, 67 and 72% following blockade with 1, 2 and 6 mg/kg M-MPEP, respectively. These results show that (11)C-ABP688 is a promising PET ligand for the quantification of mGlu5 receptors in humans and animals. It readily crosses the blood-brain barrier and binds with high specificity to the mGlu5 receptor. The study furthermore demonstrates the usefulness of a beta-scintillator, if necessary in connection with autoradiography, to evaluate new receptor tracers.

(11)C-ABP688 (3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-O-(11)C-methyl-oxime), a noncompetitive and highly selective antagonist for the metabotropic glutamate receptor subtype 5 (mGluR5), was evaluated for its potential as a PET agent.ABP688 was radiolabeled with (11)C by reacting (11)C-methyl iodide with the sodium salt of desmethyl-ABP688 (3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone oxime). The affinity of (11)C-ABP688 for mGluR5 was determined by Scatchard analysis using rat whole-brain membranes (without cerebellum). Ex vivo autoradiography, biodistribution, and PET studies with (11)C-ABP688 were performed on rats, wild-type mice, and mGluR5-knock-out mice.The overall synthesis time was 45-50 min from the end of radionuclide production. (11)C-ABP688 was obtained in good radiochemical yield (35% +/- 8%, n = 17, decay corrected), and the specific radioactivity was 150 +/- 50 GBq/mumol (n = 17) at the end of the synthesis. Scatchard analysis revealed a single high-affinity binding site with a dissociation constant of 1.7 +/- 0.2 nmol/L and a maximum number of binding sites of 231 +/- 18 fmol/mg of protein. Ex vivo autoradiography in wild-type mice and rats showed a heterogeneous distribution pattern consistent with the known distribution of mGluR5 in the brain, with the highest uptake in hippocampus, striatum, and cortex. Blocking studies by coinjection of (11)C-ABP688 and unlabeled 2-methyl-6-(3-methoxyphenyl)ethynyl-pyridine (1 mg/kg), an antagonist for mGluR5, revealed up to 80% specific binding in rat brain. In mGluR5-knock-out mouse brain, a homogeneous and markedly reduced accumulation of (11)C-ABP688 was observed. PET studies on rats and mice using a small-animal PET scanner also demonstrated radioactivity uptake in the brain regions known to be rich in mGluR5. In contrast, radioactivity uptake in mGluR5-knock-out mice was fairly uniform, substantiating the specificity of (11)C-ABP688 binding to mGluR5.(11)C-ABP688 is a selective tracer for imaging mGluR5 in vivo in rodents and may offer a future tool for imaging mGluR5 in humans using PET.

The synthesis and biological evaluation of "6-(1, 3-dihydroxyisobutyl)thymine" (DHBT ; 1), which corresponds to 6-[3-hydroxy-2-(hydroxymethyl)propyl]-5-methylpyrimidine-2, 4(1H, 3H)-dione, is reported. DHBT (1) was designed as a new substrate for herpes simplex virus type-1 thymidine kinase (HSV1 TK). The compound was found to be exclusively phosphorylated by HSV1 TK, and to exhibit good binding affinity (Ki = 35.3 ± ; ; 1.3 μ M). Cell-proliferation assays with HSV1-TK-transduced human osteosarcoma cells (143B-TK+-HSV1-WT) and with both human-thymidine-kinase-1-negative (143B-TK– ) and non-transduced parental (MG-63) cells indicate that 1 is less cytotoxic than the standard drug Ganciclovir. Thus, DHBT (1) represents a promising precursor of a nontoxic reporter probe for the monitoring of HSV1 TK gene expression by means of positron-emission tomography (PET).

15 pages, 3 figures, 1 table.-- et al., Objective— Current imaging modalities of atherosclerosis mainly visualize plaque morphology. Valuable insight into plaque biology was achieved by visualizing enhanced metabolism in plaque-derived macrophages using 18F-fluorodeoxyglucose (18F-FDG). Similarly, enhanced uptake of 18F-fluorocholine (18F-FCH) was associated with macrophages surrounding an abscess. As macrophages are important determinants of plaque vulnerability, we tested 18F-FCH for plaque imaging. Methods and Results— We injected 18F-FCH (n=5) or 18F-FDG (n=5) intravenously into atherosclerotic apolipoprotein E-deficient mice. En face measurements of aortae isolated 20 minutes after 18F-FCH injections demonstrated an excellent correlation between fat stainings and autoradiographies (r=0.842, P, This work was funded in part by grants from the European Union G5RD-CT-2001–00532 and by Bundesamt für Bildung und Wissenschaft 02.0057 (C.M.M., T.F.L.), the Hartmann-Müller Foundation (C.M.M., T.v.L.), the Swiss National Science Foundation 3100-068118 (T.F.L., C.M.M.), PP00A-68835 & 3100-68386.02 (P.A.K.), the Swiss Heart Foundation (P.A.K., A.B., C.M.M.), Sassella and Olga Mayenberg Foundation (A.B.), the Spanish MSC grant FIS C03-08 (J.C.L., A.R.d.M.), as well as support by Abbott/Jomed, Germany (C.M.M). We thank Tibor Cservenvàk and Rolf Hesselmann for radiotracer production.

Purpose: The L1 cell adhesion protein is overexpressed in tumors, such as neuroblastomas, renal cell carcinomas, ovarian carcinomas, and endometrial carcinomas, and represents a target for tumor diagnosis and therapy with anti-L1-CAM antibody chCE7. Divalent fragments of this internalizing antibody labeled with 67/64Cu and 177Lu were evaluated to establish a chCE7 antibody fragment for radioimmunotherapy and positron emission tomography imaging, which combines high-yield production with improved clearance and biodistribution properties. Experimental Design: chCE7F(ab′)2 fragments were produced in high amounts (0.2 g/L) in HEK-293 cells, substituted with the peptide-linked tetraazamacrocycle 3-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate-triglycyl-l-p-isothiocyanato-phenylalanine, and labeled with 67Cu and 177Lu. In vivo bioevaluation involved measuring kinetics of tumor and tissue uptake in nude mice with SK-N-BE2c xenografts and NanoPET (Oxford Positron Systems, Oxford, United Kingdom) imaging with 64Cu-3-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate-triglycine-chCE7F(ab′)2. Results: The 177Lu- and 67Cu-labeled immunoconjugates reached maximal tumor accumulation at 24 hours after injection with similar levels of 12%ID/g to 14%ID/g. Blood levels dropped to 1.0%ID/g for the 177Lu fragment and 2.3%ID/g for the 67Cu fragment at 24 hours. The most striking difference concerned radioactivity present in the kidneys, being 34.5%ID/g for the 177Lu fragment and 16.0%ID/g for the 67Cu fragment at 24 hours. Positron emission tomography imaging allowed clear visualization of s.c. xenografts and peritoneal metastases and a detailed assessment of whole-body tracer distribution. Conclusions: 67/64Cu- and 177Lu-labeled recombinant chCE7F(ab′)2 revealed suitable in vivo characteristics for tumor imaging and therapy but displayed higher kidney uptake than the intact monoclonal antibody. The 67Cu- and 177Lu-labeled immunoconjugates showed different in vivo behavior, with 67/64Cu-3-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate-triglycine-F(ab′)2 appearing as the more favorable conjugate due to superior tumor/kidney ratios.

Our aim was to determine the influence of antibiotic treatment using ceftriaxone on [18F]-fluorodeoxyglucose (FDG) uptake in experimental soft tissue infections. PET scans were performed in two groups (treated n=4; non-treated n=4) at days 3, 5, and 6 after inoculation of the infection. Additional autoradiography was performed in four animals at day 7 and in three animals at day 11. The difference of FDG uptake on day 5 (after three days of antibiotic treatment) between both groups proved to be significant (df=6; T=2.52; p=0.045). FDG uptake determined at the other days did not reveal significant difference between the two groups. It seems to be possible that the effect of antibiotic treatment on FDG uptake is less evident than reported for therapy monitoring of cancer treatment. The change of FDG uptake over time in treated and untreated infections is complex and further in vivo experiments have to be initiated to investigate the potential value of clinical FDG PET in therapy monitoring of infection.

We describe a new method for the asymmetric synthesis of [(18)F]fluorinated aromatic alpha-amino acids (FAA) under phase transfer conditions using achiral glycine derivative NiPBPGly and (S)-NOBIN as a novel substrate/catalyst pair. The key alkylation step proceeds under mild conditions. Substituted [(18)F]fluorobenzylbromides were prepared using nucleophilic [(18)F]fluoride and were used as alkylation agents. Two important FAA, 2-[(18)F]fluoro-L-tyrosine (2-FTYR) and 6-[(18)F]fluoro-L-3,4-dihydroxyphenylalanine (6-FDOPA), were synthesized with an ee of 92 and 96%, respectively. The total synthesis time was 110-120 min and radiochemical yields (d.c.) were 25+/-6% for 2-FTYR and 16+/-5% for 6-FDOPA.

The novel, dedicated small animal PET tomograph, quad-HIDAC, offers submillimeter resolution in instrumental characterization experiments. The aim of this study was to establish the tomograph's utility in a biologic application and to demonstrate the feasibility of rapid dynamic neuroreceptor imaging in mice.We used the well-established, high-affinity dopamine D(2) receptor PET ligand (18)F-fallypride for imaging striatal D(2) receptors in NMRI mice. Dynamic PET data were acquired using the quad-HIDAC tomograph and subject to 2 different kinetic modeling approaches. The cerebellum, a brain region devoid of D(2) receptors, was chosen as a reference region for kinetic modeling.The resolution of the quad-HIDAC camera allowed clear visualization of the left and right mouse striatum with high target-to-nontarget signal ratios. The sensitivity of the tomograph permitted the generation of time-activity curves with initial time frames of 120 s. PET experiments acquiring data for 150 min demonstrated that the binding potential of (18)F-fallypride could be fitted robustly with both reference tissue models for scan durations ofor=40 min. Voxel-wise modeling resulted in parametric maps of high quality. The values for the binding potential in the striatum reached approximately 14, consistent with striatum-to-cerebellum ratios extracted from regional time-activity curves. Comparison of in vivo PET imaging results with ex vivo postmortem tissue sampling analyses indicated discrepancies in signal intensity, possibly resulting from scatter and random background in the cerebellum region of interest and leading to an overestimation of cerebellar activity concentrations and degradation of striatum-to-cerebellum ratios in PET experiments. Intraperitoneal injection of the unlabeled D(2) receptor antagonist haloperidol 30 min before intravenous injection of (18)F-fallypride blocked tracer accumulation in the striatum by95%.The quad-HIDAC camera represents a powerful tool for future dynamic neuroreceptor PET studies in mice and rats under numerous pharmacologic or pathophysiologic conditions.

Synthesis of pyrimidine derivatives with a side-chain attached to the C-6 of pyrimidine ring (6-14) is reported. Target compounds 8 and 12 were subjected to in vitro phosphorylation tests, determination of their binding affinities to herpes simplex virus (HSV-1) thymidine kinase (TK) and catalytic turnover constants. Fluorinated pyrimidine derivative 12 (40 microM) exhibited better binding affinity for HSV-1 TK than acyclovir (ACV, 170 microM) and ganciclovir (GCV, 48 microM). Catalytic turnover constant (k(cat)) of 12 (0.08 s(-1)) was close to the k(cat) values of ACV (0.10 s(-1)) and GCV (0.10 s(-1)). Furthermore, compounds 8 and 12 showed no cytotoxic effects in HSV-1 TK-transduced and non-transduced cell lines. Besides, compounds 8 and 12 did not exhibit antiviral or cytostatic activities against several viruses and malignant tumor cell lines that were evaluated. The new fluorinated pyrimidine derivative 16 that is phosphorylated by HSV-1 TK could be developed as non-toxic PET-tracer molecule. Thus, 18F labelling of the precursor 14 was performed by nucleophilic substitution using [18F] tetrabutylammonium fluoride as the fluorinating reagent.

The application of positron emission tomography (PET) technology to small animals such as mice and rats requires dedicated scanners, which have been developed in recent years. Two types are commercially available so far: the microPET (Fig. 1a) marketed by Concorde Microsystems (Knoxville, USA), and the quad-HIDAC system (Fig. 1b) developed by Oxford Positron Systems (Weston-on-the-Green, UK). Similar to clinical PET tomographs, the microPET camera is based on scintillation crystals arranged in a detector ring (Chatziioannou et al. 1999). The microPET system uses lutetium oxyorthosilicate as scintillation crystals coupled by optical fibers to multichannel photomultiplier tubes. The scanner has a volumetric resolution of approximately 6 mm3 in the center of the field-of-view, which extends 10 cm axially. The camera’s raw detection efficiency is 2.4 %.

Brain edema significantly contributes to the clinical course of human brain tumor patients. There is evidence that an enlargement of the extracellular space (ECS) is involved in the development of brain edema. Although T2-weighted magnetic resonance (T2-MR) images represent brain edema by its increased water content, they do not differentiate ECS enlargement from increased intracellular water content.On the basis of the known distribution of bromide in the ECS, we used (76)Br-bromide and PET to measure the regional ECS in 9 brain tumor patients. Transport rate constants and the distribution volume (DV) of (76)Br-bromide in normal brain and tumor were derived from dynamic PET scans and the measured (76)Br-bromide concentration in arterial plasma. We evaluated different models regarding their reliability in estimating the ECS.Assuming that the DV of (76)Br-bromide represents the ECS, robust estimates were possible for all investigated regions. In normal brain, ECS was within a narrow range-for example, occipital lobe, 19.9% +/- 3.1%-and was lower in 2 dexamethasone-treated patients compared with untreated patients. In 7 of 9 tumors, increased ECS ranged between 43.8% and 61.1%. ECS increases were confined to the tumor mass and did not extend into peritumoral edematous brain. Two patients with large hyperintense lesions according to T2-MR images showed normal ECS values within the lesion.(76)Br-Bromide PET allows a quantitative measurement of the ECS in brain edema and in normal brain. The discrepancies between lesions shown by T2-MRI and regional ECS enlargement as measured with PET challenge the concept of tumor-induced brain edema.

In order to optimize radiocopper labeling of anti-L1-CAM antibody chCE7, five bifunctional copper chelators were synthesized and characterized (CPTA-N-hydoxysuccinimide, DO3A-L-p-isothiocyanato-phenylalanine, DOTA-PA-L-p-isocyanato-phenylalanine, DOTA-glycyl-L-p-isocyanato-phenylalanine and DOTA-triglycyl-L-p-isocyanato-phenylalanine). Substitution with more than 11 chelators per antibody molecule was found to influence immunoreactivity and biodistributions of 67 Cu-MAb chCE7 significantly. CPTA-labeled antibody achieved the best tumor to normal tissue ratios when biodistributions of the different 67 Cu-chCE7 conjugates were assessed in tumor-bearing mice. High resolution PET imaging with 64 Cu-CPTA-labeled MAb chCE7 showed uptake in lymph nodes and heterogeneous distribution in tumor xenografts.

European Journal of Nuclear Medicine and Molecular Imaging, 30 (7), ISSN:1619-7070, ISSN:1619-7089

A new benzomorphane derivative, [11C]methyl-BIII277CL, was evaluated as a potential radiotracer for visualizing the PCP-binding site of the N-methyl-D-aspartate (NMDA) receptor by positron emission tomography (PET). Methyl-BIII277CL was prepared by reacting the desmethyl compound (BIII277CL) with dimethylsulfate. The pharmacological profile of methyl-BIII277CL was determined by in vitro receptor-screening assays. At a concentration of 100 nM, methyl-BIII277CL showed a significant interaction with the PCP-binding site of the NMDA receptor (79% inhibition of specific binding) and the sigma-binding site (46% inhibition). In displacement assays using mice cortical membranes, methyl-BIII277CL displayed a high affinity at the PCP-binding site of the NMDA receptor (Ki = 49 +/- 14 nmol/L) and a 130-fold lower interaction with the sigma1-binding site (Ki = 6.35 +/- 0.26 micromol/L). For saturation experiments and in vivo studies, methyl-BIII277CL was radiolabeled with 11C at the O-position of the desmethyl precursor (BIII277CL) using [11C]methyliodide with a specific activity of 35-70 GBq/micromol at the end of synthesis (EOS). In saturation assays using rat whole brain membranes [11C]methyl-BIII277CL showed a Kd of 6 +/- 1 nmol/L and a Bmax of 670 +/- 154 fmol/mg protein. Biodistribution and PET studies in rats and pigs, however, indicated a lack of specific binding and unfavorable pharmacokinetics. Kinetic modeling using the 1-tissue compartment model demonstrated for [11C]methyl-BIII277CL a low distribution volume (Dv = 0.98 mL/mL(tissue)) and very high values for the kinetic parameters K1 and k2 (K1 = 0.36 mL/mL(tissue)/min and k2 = 0.37min(-1)) in pig cortex. [11C]methyl-BIII277CL, due to the lack of specificity in vivo, may not be a candidate for imaging the PCP-binding site of the NMDA receptor.

Previous studies in mice and PET investigations in a Rhesus monkey showed that the regional uptake of 18 F-memantine could be blocked by pharmacological doses of memantine and (+)-MK-801. In the present study, the binding characteristics of 18 F-memantine was examined in five healthy volunteers. In humans, 18 F-memantine was homogeneously distributed in gray matter i.e. cortex and basal ganglia regions, as well as the cerebellum. No radioactive metabolites were detected in plasma during the time-frame of the PET studies. The uptake of 18 F-memantine in receptor-rich regions such as striatum and frontal cortex could be well described by a 1-tissue compartment model. The DV″ values of all gray matter regions were similar and ranged from 15 to 20 ml/ml. The white matter showed lower DV″ values of 15 ± 1.4 ml/ml. These results suggest that 18 F-memantine distribution in human brain does not reflect the regional NMDA receptor concentration, and therefore, this radioligand is not suitable for the PET imaging of the NMDA receptors.