Specific Radioligand Research Articles

PET Imaging of Lung Inflammation with [18F]FEDAC, a Radioligand for Translocator Protein (18 kDa)

PurposeThe translocator protein (18 kDa) (TSPO) is highly expressed on the bronchial and bronchiole epithelium, submucosal glands in intrapulmonary bronchi, pneumocytes and alveolar macrophages in human lung. This study aimed to perform positron emission tomography (PET) imaging of lung inflammation with [18F]FEDAC, a specific TSPO radioligand, and to determine cellular sources enriching TSPO expression in the lung.MethodsAn acute lung injury model was prepared by intratracheal administration of lipopolysaccharide (LPS) to rat. Uptake of radioactivity in the rat lungs was measured with small-animal PET after injection of [18F]FEDAC. Presence of TSPO was examined in the lung tissue using Western blot and immunohistochemical assays.ResultsThe uptake of [18F]FEDAC increased in the lung with the progress of inflammation by treatment with LPS. Pretreatment with a TSPO-selective ligand PK11195 showed a significant decrease in the lung uptake of [18F]FEDAC due to competitive binding to TSPO. TSPO expression was elevated in the inflamed lung section and its level responded to the [18F]FEDAC uptake and severity of inflammation. Increase of TSPO expression was mainly found in the neutrophils and macrophages of inflamed lungs.ConclusionFrom this study we conclude that PET with [18F]FEDAC may be a useful tool for imaging TSPO expression and evaluating progress of lung inflammation. Study on human lung using [18F]FEDAC-PET is promising.

Binding of lurasidone, a novel antipsychotic, to rat 5-HT7 receptor: Analysis by [3H]SB-269970 autoradiography

Lurasidone is a novel antipsychotic agent with high affinity for dopamine D(2) and serotonin 5-HT(7), 5-HT(2A), and 5-HT(1A) receptors. We previously reported that in addition to its antipsychotic action, lurasidone shows beneficial effects on mood and cognition in rats, likely through 5-HT(7) receptor antagonistic actions. In this study, we evaluated binding of lurasidone to 5-HT(7) receptors in the rat brain by autoradiography using [(3)H]SB-269970, a specific radioligand for 5-HT(7) receptors. Brain slices were incubated with 4 nM [(3)H]SB-269970 at room temperature and exposed to imaging plates for 8 weeks before phosphorimager analysis. Using this method, we first investigated 5-HT(7) receptor distribution. We found that 5-HT(7) receptors are abundantly localized in brain limbic structures, including the lateral septum, thalamus, hypothalamus, hippocampus, and amygdala. On the other hand, its distribution was moderate in the cortex and low in the caudate putamen and cerebellum. Secondly, binding of lurasidone, a selective 5-HT(7) receptor antagonist SB-656104-A and an atypical antipsychotic olanzapine to this receptor was examined. Lurasidone, SB-656104-A (10–1000 nM), and olanzapine (100–10,000 nM) showed concentration-dependent inhibition of [(3)H]SB-269970 binding with IC(50) values of 90, 49, and 5200 nM, respectively. Similar inhibitory actions of these drugs were shown in in vitro [(3)H]SB-269970 binding to 5-HT(7) receptors expressed in Chinese hamster ovary cells. Since there was no marked species difference in rat and human 5-HT(7) receptor binding by lurasidone (K(i) = 1.55 and 2.10 nM, respectively), these findings suggest that binding to 5-HT(7) receptors might play some role in its beneficial pharmacological actions in schizophrenic patients.

α4βδ GABA(A) receptors are high-affinity targets for γ-hydroxybutyric acid (GHB).

γ-Hydroxybutyric acid (GHB) binding to brain-specific high-affinity sites is well-established and proposed to explain both physiological and pharmacological actions. However, the mechanistic links between these lines of data are unknown. To identify molecular targets for specific GHB high-affinity binding, we undertook photolinking studies combined with proteomic analyses and identified several GABA(A) receptor subunits as possible candidates. A subsequent functional screening of various recombinant GABA(A) receptors in Xenopus laevis oocytes using the two-electrode voltage clamp technique showed GHB to be a partial agonist at αβδ- but not αβγ-receptors, proving that the δ-subunit is essential for potency and efficacy. GHB showed preference for α4 over α(1,2,6)-subunits and preferably activated α4β1δ (EC(50) = 140 nM) over α4β(2/3)δ (EC(50) = 8.41/1.03 mM). Introduction of a mutation, α4F71L, in α4β1(δ)-receptors completely abolished GHB but not GABA function, indicating nonidentical binding sites. Radioligand binding studies using the specific GHB radioligand [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid showed a 39% reduction (P = 0.0056) in the number of binding sites in α4 KO brain tissue compared with WT controls, corroborating the direct involvement of the α4-subunit in high-affinity GHB binding. Our data link specific GHB forebrain binding sites with α4-containing GABA(A) receptors and postulate a role for extrasynaptic α4δ-containing GABA(A) receptors in GHB pharmacology and physiology. This finding will aid in elucidating the molecular mechanisms behind the proposed function of GHB as a neurotransmitter and its unique therapeutic effects in narcolepsy and alcoholism.

Nonuniform Cardiac Denervation Observed by 11C-meta-Hydroxyephedrine PET in 6-OHDA-Treated Monkeys

Parkinson's disease presents nonmotor complications such as autonomic dysfunction that do not respond to traditional anti-parkinsonian therapies. The lack of established preclinical monkey models of Parkinson's disease with cardiac dysfunction hampers development and testing of new treatments to alleviate or prevent this feature. This study aimed to assess the feasibility of developing a model of cardiac dysautonomia in nonhuman primates and preclinical evaluations tools. Five rhesus monkeys received intravenous injections of 6-hydroxydopamine (total dose: 50 mg/kg). The animals were evaluated before and after with a battery of tests, including positron emission tomography with the norepinephrine analog 11C-meta-hydroxyephedrine. Imaging 1 week after neurotoxin treatment revealed nearly complete loss of specific radioligand uptake. Partial progressive recovery of cardiac uptake found between 1 and 10 weeks remained stable between 10 and 14 weeks. In all five animals, examination of the pattern of uptake (using Logan plot analysis to create distribution volume maps) revealed a persistent region-specific significant loss in the inferior wall of the left ventricle at 10 (P<0.001) and 14 weeks (P<0.01) relative to the anterior wall. Blood levels of dopamine, norepinephrine (P<0.05), epinephrine, and 3,4-dihydroxyphenylacetic acid (P<0.01) were notably decreased after 6-hydroxydopamine at all time points. These results demonstrate that systemic injection of 6-hydroxydopamine in nonhuman primates creates a nonuniform but reproducible pattern of cardiac denervation as well as a persistent loss of circulating catecholamines, supporting the use of this method to further develop a monkey model of cardiac dysautonomia.

11 Early clinical development of amyloid-beta specific PET radioligand [18F]AZD4694

11 Early clinical development of amyloid-beta specific PET radioligand [18F]AZD4694

The sigma-1 receptor antagonist PB212 reduces the Ca2+-release through the inositol (1, 4, 5)-trisphosphate receptor in SK-N-SH cells

Sigma-1 receptors are specifically located at the endoplasmic reticulum–mitochondrion interface, but upon stimulation by ligands or under prolonged cellular stress, they translocate to other areas of the cell. Sigma-1 receptors are involved in the regulation of intracellular [Ca2+] by affecting the Ca2+-influx or the release from intracellular stores. In SK-N-SH cells, we measured the affinity of 4-methyl-1-[4-(6-methoxynaphthalen-1-yl)butyl]piperidine (PB212) at sigma-1 receptor by using a competition binding assay with specific radioligand; we obtained a Ki value=316±19nM. PB212 also showed an antiproliferative effect in SK-N-SH cells (EC50=32±4μM) but had no effect in MCF7 cells, which only express sigma-2 receptor; these findings suggest that PB212 behaves as a sigma-1 receptor antagonist. We have studied the effect of PB212 on Ca2+ homeostasis of the SK-N-SH cell line with the fluorescent probe Fura-2. 100μM PB212 induced a Ca2+-efflux from the endoplasmic reticulum through the inositol (1, 4, 5)-trisphosphate (IP3) receptor. Moreover, [PB212] ranging from 1 to 100μM reduced the Ca2+-response, triggered by carbachol or bradykinin that engage the phospholipase C/IP3 pathway; such a response is generally increased by sigma-1 receptor agonists. On the other hand, PB212 did not reduce the Ca2+-response mediated by IP3 in LoVo cells, which do not express neither sigma-1 nor sigma-2 receptors, and in MCF7 cells. The fact that the activity of the sigma-1 receptor can be experimentally modulated by agonists and antagonists supports the intriguing hypothesis that some endogenous molecules, unknown at the moment, modulate the sigma-1 receptor and its cellular targets.

Synthesis and preliminary evaluation of [18F]-labeled 2-oxoquinoline derivatives for PET imaging of cannabinoid CB2 receptor

The cannabinoid receptor type 2 (CB(2)) is an important target for development of drugs and imaging agents for diseases, such as neuroinflammation, neurodegeneration and cancer. Recently, we reported synthesis and results of in vitro receptor binding of a focused library of fluorinated 2-oxoquinoline derivatives as CB(2) receptor ligands. Some of the compounds demonstrated to be good CB(2)-specific ligands with Ki values in the nanomolar to subnanomolar concentrations; therefore, we pursued the development of their (18)F-labeled analogues that should be useful for positron emission tomography (PET) imaging of CB(2) receptor expression. Here, we report the radiosynthesis of two (18)F-labeled 2-oxoquinoline derivatives and the preliminary in vitro and ex vivo evaluation of one compound as a CB(2)-specific radioligand. 4-[(18)F]fluorobenzyl amine [(18)F]-3 was prepared by radiofluorination of 4-cyano-N,N,N-trimethylanilinium triflate salt followed by reduction with LiAlH(4) and then coupled with acid chlorides 11 and 12 to afford [(18)F]-13 and [(18)F]-14. In vitro CB(2) receptor binding assay was performed using U87 cells transduced with CB(2) and CB(1) receptor. Ex vivo autoradiography was performed with [(18)F]-14 on spleen and on CB(2)- and CB(1)-expressing and wild-type U87 subcutaneous tumors grown in mice. The radiochemical yields of [(18)F]-13 and [(18)F]-14 were 10%-15.0% with an average of 12% (n=10); radiochemical purity was >99% with specific activity 1200 mCi/μmol. The dissociation constant Kd for [(18)F]-14 was 3.4 nM. Ex vivo autoradiography showed accumulation of [(18)F]-14 in the CB(2)-expressing tumor. Two new [(18)F]-labeled CB(2) ligands have been synthesized. Compound [(18)F]-14 appears to be a potential PET imaging agent for the assessment of CB(2) receptor expression; however, poor solubility restrain its use in vivo.

Diagnostic Imaging of Higher Brain Dysfunction in Patients With Adult Moyamoya Disease Using Statistical Imaging Analysis for [&lt;sup&gt;123&lt;/sup&gt;I]Iomazenil Single Photon Emission Computed Tomography

[123I]iomazenil (IMZ) is a specific radioligand for the central benzodiazepine (BZ) receptor that may be useful as a marker of cortical neuron loss after cerebral ischemia using single photon emission computed tomography (SPECT). This study used statistical imaging analysis for IMZ-SPECT to investigate the relationship between higher brain dysfunction and cortical neuron loss in the medial frontal lobes, to establish a confirmatory diagnosis of higher brain dysfunction in patients with adult moyamoya disease. IMZ-SPECT was estimated by three-dimensional stereotactic surface projections (3D-SSP). Cortical neuron loss was analyzed using the stereotactic extraction estimation (SEE) method (level 3: gyrus level) for 3D-SSP Z-score maps (Z-score >2). Extent of pixels with significant reduction of BZ receptor density within the target gyri (i.e. bilateral medial frontal gyri [MFGs] and anterior cingulate gyri [ACGs]) was calculated. In 6 patients with higher brain dysfunction, significant cortical neuron loss was observed in the bilateral MFGs in 4 patients, unilateral MFG in 1 patient, and bilateral ACGs in 2 patients. In 12 patients without higher brain dysfunction, no significant cortical neuron loss was observed in the bilateral MFGs or ACGs, and mild loss was observed in the bilateral MFGs in 2 patients, unilateral MFG in 4 patients, and unilateral ACG in 2 patients. Long-standing mild hemodynamic ischemia in the anterior circulation of patients with adult moyamoya disease could lead to incomplete brain infarction within the medial frontal lobes. Statistical imaging analysis using 3D-SSP and SEE methods for IMZ-SPECT could demonstrate significant cortical neuron loss in the bilateral frontal medial cortices involving MFG and/or ACG which correlate with higher brain dysfunction in patients with adult moyamoya disease.

Occupancy of α7 Nicotinic Acetylcholine Receptors in the Brain by Tropisetron: A Positron Emission Tomography Study Using [11C]CHIBA-1001 in Healthy Human Subjects

ObjectiveAgonists of α7-nicotinic acetylcholine receptors (nAChRs) have been developed as potential therapeutic drugs for neuropsychiatric diseases such as schizophrenia and Alzheimer's disease. Positron emission tomography (PET) is a noninvasive brain imaging technique to measure receptor occupancy in the living human brain. Although much effort has been expended to create specific PET radioligands for α7-nAChRs in the brain, only 4-[11C]methylphenyl-1,4-diazabicyclo[3.2.2.]nonane-4-carboxylate ([11C]CHIBA-1001) is currently available for clinical studies. In contrast, two 5-hydroxytryptamine-3 (5-HT3) receptor antagonists, tropisetron and ondansetron, have been used to treat patients with chemotherapy-induced or postoperative nausea and vomiting. Furthermore, tropisetron, but not ondansetron, possesses high affinity for α7-nAChRs. In the present study, we evaluated the receptor occupancy in the human brain after a single oral administration of tropisetron and ondansetron using [11C]CHIBA-1001 and PET.MethodsTwo serial dynamic PET scans using [11C]CHIBA-1001 in healthy non-smoking male subjects were performed before and after receiving an oral administration of these medications.ResultsA single oral administration of tropisetron, but not ondansetron, decreased the total distribution volume of [11C]CHIBA-1001 in the human brain.ConclusionThis study shows that tropisetron, but not ondansetron, could bind to α7-nAChRs in the human brain after a single oral administration. Therefore, [11C]CHIBA-1001 may be a useful PET radioligand to measure the occupancy of α7-nAChRs in the human brain.

Up-regulation of adenylylcyclases I and II induced by long-term adaptation of rats to morphine fades away 20 days after morphine withdrawal

Background Activation of adenylyl cyclase (AC) by prolonged exposure of mammalian organism to morphine was demonstrated in previous studies of mechanism of action of this drug. However, expression level of individual AC isoforms was not analyzed in crucial cell structure, plasma membrane (PM). Methods Rats were adapted to morphine for 10 days and sacrificed 24 h (group + M10) or 20 days (+ M10/−M20) after the last dose. Control animals were sacrificed in parallel with morphine-treated (groups − M10 and (− M10/−M20)). Percoll®-purified PM were isolated from brain cortex and analyzed by immunoblotting and specific radioligand binding. Results ACI (ACII) was increased 8× (2.5×) in morphine-adapted rats (+ M10) when compared with controls (− M10). Increase of ACI and II by long-term adaptation to increasing doses of morphine represented a specific effect as the amount of ACIII–ACIX, of prototypical PM marker, Na, K-ATPase and of trimeric G protein α and β subunits was unchanged. Increase of ACI and II was not detected in PM isolated from group (+ M10/−M20). Thus, the marked increase of ACI and ACII faded away 20 days since the last dose of morphine. Conclusions We assume that the specific increase in expression level of ACI and ACII in brain cortex of morphine-adapted rats proceeds as a compensatory, homeostatic response to prolonged exposure to inhibitory drug, morphine. General significance Our findings demonstrate that the dramatic and specific change of the crucial component of the opioid receptor cascade in brain cortex, manifested as an increase in PM level of ACI and II, is reversible.

Development of novel N-methyl and N-allyl-substituted oxazolomorphinans and their interaction with opioid receptors

Methods The chemistry involved the design and synthesis of two sets of oxazolomorphinans having the new heteroring anellated to the A-ring of the morphinan backbone. Binding affinities of the newly synthesized compounds at opioid receptors were determined by in vitro competition binding assays using rat brain (μ, δ) and guinea pig brain ( ) membranes and employing [H]DAMGO (μ), [H] [Ile]deltorphin II (δ) and [H]U-69,593 ( ) as specific opioid radioligands. The in vitro pharmacological activities were established using [S]GTPgS functional assays in membranes from Chinese hamster ovary (CHO) cells expressing human opioid receptors.

Synthesis and molecular study of a new NMB analogue (BBN) for imaging prostate cancer

Radiolabeled somatostatin analogues are important tools for the in vivo localization and targeted radionuclide therapy of somatostatin-receptor-positive tumors. The aim of this study was to evaluate a new somatostatin analogue designed for the labeling with 99mTc: [6-hydrazinopyridine-3-carboxylic acid (HYNIC0), 1-Nal3, Thr8]-octreotide ([HYNIC]-NATE), using ethylenediamine-N,N′-diacetic acid (EDDA) and tricine as coligands.Synthesis was preformed on a solid phase using a standard Fmoc strategy. Labeling with 99mTc was performed at 100°C for 10 min using SnCl2 as a reductant. Radiochemical analysis involved ITLC and high-performance liquid chromatography methods. Peptide conjugate affinity was determined in AR4-2J cell membranes. The internalization and externalization rates were studied in sstr2-expressing AR4-2J cells. Biodistribution of radiopeptide was studied in rats bearing the AR4-2J tumor.Radiolabeling was performed at high specific activities, and radiochemical purity was >95%. Peptide conjugate showed high affinity binding for sstr2. The radioligand showed a moderate and specific internalization into AR4-2J cells (14.13±0.61% at 4 h). In animal biodistribution studies, a receptor-specific uptake of radioactivity was observed in somatostatin-receptor-positive organs. After 4 h, uptake in the AR4-2J tumor was 1.33±0.23%ID/g (percentage of injected dose per gram of tissue).These data show that [99mTc/EDDA/tricine/HYNIC]-NATE is a specific radioligand for the somatostatin-receptor-positive tumors and is a suitable candidate for clinical studies.

Interaction of mephedrone with dopamine and serotonin targets in rats

IntroductionWe described a first approach to the pharmacological targets of mephedrone (4-methyl-methcathinone) in rats to establish the basis of the mechanism of action of this drug of abuse. Experimental proceduresWe performed in vitro experiments in isolated synaptosomes or tissue membrane preparations from rat cortex or striatum, studying the effect of mephedrone on monoamine uptake and the displacement of several specific radioligands by this drug. ResultsIn isolated synaptosomes from rat cortex or striatum, mephedrone inhibited the uptake of serotonin (5-HT) with an IC50 value lower than that of dopamine (DA) uptake (IC50=0.31±0.08 and 0.97±0.05μM, respectively). Moreover, mephedrone displaced competitively both [3H]paroxetine and [3H]WIN35428 binding in a concentration-dependent manner (Ki values of 17.55±0.78μM and 1.53±0.47μM, respectively), indicating a greater affinity for DA than for 5-HT membrane transporters. The affinity profile of mephedrone for the 5-HT2 and D2 receptors was assessed by studying [3H]ketanserin and [3H] raclopride binding in rat membranes. Mephedrone showed a greater affinity for the 5-HT2 than for the D2 receptors. DiscussionThese results provide evidence that mephedrone, interacting with 5-HT and DA transporters and receptors must display a similar pattern of other psychoactive drugs such as amphetamine-like compounds.

Glycine receptor internalization by protein kinases activation

Although glycine-induced currents in the central nervous system have been proven to be modulated by protein kinases A (PKA) and C (PKC), the mechanism is not well understood. In order to better comprehend the mechanism involved in this phenomenon, we tested the PKA and PKC activation effect on the specific [(3) H]glycine and [(3) H]strychnine binding to postsynaptic glycine receptor (GlyR) in intact rat retina. The specific binding constituted about 20% of the total radioligand binding. Kinetic analysis of the specific binding exhibited a sigmoidal behavior with three glycine and two strychnine binding sites and affinities of 212 nM for [(3) H]glycine and 50 nM for [(3) H]strychnine. Specific radioligand binding was decreased (60-85%) by PKA and PKC activation, an effect that was blocked by specific kinases inhibitors, as well as by cytochalasin D. GlyR expressed in the plasma membrane decreased about 50% in response to kinases activation, which was consistent with an increase of the receptor in the microsomal fraction when PKA was activated. Moreover, immunoprecipitation studies indicated that these kinases lead to a time-dependent receptor phosphorylation. Our results suggest that in retina, GlyR is cross-regulated by G protein-coupled receptors, activating PKA and PKC.

Preclinical Evaluation of a New Bombesin Analog for Imaging of Gastrin-Releasing Peptide Receptors

Bombesin (BBN) is a peptide showing high affinity for the gastrin-releasing peptide receptor. Tumors such as prostate, small cell lung cancer, breast, gastric, and colon cancer are known to over express receptors to BBN and gastrin-releasing peptide (GRP). The goal of this study was to evaluate a new (67)Ga radiolabeled BBN analog based on the bifunctional chelating ligand DOTA (1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid), which could be used as a tool for diagnosis of GRP receptor-positive tumors. DOTA-GABA-BBN (7-14) NH(2) was synthesized using a standard Fmoc strategy. Labeling with (67)Ga was performed at 95°C for 30 minutes in ammonium acetate buffer (pH = 4.8). Radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was examined in the presence of human serum at 37°C up to 24 hours. The receptor-bound internalization and externalization rates were studied in GRP receptor expressing PC-3 cells. Biodistribution of radiopeptide was studied in nude mice bearing PC-3 tumor. Labeling yield of >90% was obtained corresponding to a specific activity of approximatrly 2.6 MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed a good and specific internalization into PC-3 cells (16.13% ± 0.71% at 4 hours). After 4 hours, a considerable amount of activity (52.42% ± 1.86%) was externalized. In animal biodistribution studies, a receptor-specific uptake of radioactivity was observed in GRP-receptor-positive organs. After 4 hours, the uptake in mouse tumor and pancreas was 1.30% ± 0.18% ID/g (percentage of injected dose per gram of tissue) and 1.21% ± 0.13% ID/g, respectively. These data show that [(67)Ga]-DOTA-GABA-BBN (7-14) NH₂ is a specific radioligand for GRP receptor positive tumors and is a suitable candidate for clinical studies.

Estrogen receptors and gonadal steroids in vulnerability and protection of dopamine neurons in a mouse model of Parkinson’s disease

17β-estradiol is well known to have neuroprotective effects in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. We investigated the neuroprotective contribution of estrogen receptors (ERα and ERβ) against MPTP toxicity by examining the membrane dopamine (DA) transporter (DAT), the vesicular monoamine transporter 2 (VMAT2) and tyrosine hydroxylase (TH) in ER knock out (ERKO) C57Bl/6 male mice compared to their plasma steroid levels. A dose–response to MPTP comparing wild-type (WT) to ERKO mice was studied. WT mice were also compared to ERKO mice pretreated with 17β-estradiol alone and with MPTP. Specific radioligand binding autoradiography and in situ hybridization for DAT, VMAT2 and TH were assayed in the striatum and the substantia nigra (SN). Intact ERKOβ mice had both striatal transporters levels lower than WT and ERKOα mice. MPTP caused a dose-dependant loss of both striatal transporters that correlated with striatal DA concentrations. Compared to WT and ERKOβ mice, ERKOα mice DAT, VMAT2 and TH were affected at lower MPTP doses. In the striatum and SN, ERKOα mice were more vulnerable and 17β-estradiol protected against MPTP toxicity only in WT mice. ERKOα mice blood plasma had higher levels of testosterone, dihydrotestosterone and 3β-diol compared to the plasma of WT and ERKOβ mice. 17β-estradiol treatment increased estradiol plasma levels in all genotypes. Striatal DA concentrations and SN TH mRNA correlated inversely with plasma testosterone and 3β-diol levels. Hence, in male mice the lack of ERα or ERβ altered their basal plasma steroid levels and both striatal DA transporters as well as their susceptibility to MPTP toxicity.

Synthesis, evaluation and metabolic studies of radiotracers containing a 4-(4-[18F]-fluorobenzyl)piperidin-1-yl moiety for the PET imaging of NR2B NMDA receptors

In this study, novel specific PET radioligands containing the 4-(4-fluorobenzyl)piperidine moiety and selectively antagonistic for the NR2B subunit containing NMDA receptors were developed. Two antagonists, RGH-896 (1a) and 4-(4-fluorobenzyl)piperidinyl-1-methyl-2-benzimidazol-5-ol (2a), belonging to two different structural families, were radiolabeled by an aromatic nucleophilic radiofluorination followed by a reduction of the para-position carbonyl function. Radiotracers [18F]1a, [18F]2a or the pattern 4-(4-[18F]-fluorobenzyl)piperidine ([18F]6) demonstrated an identical in vivo behavior with high accumulation of radioactivity in bone and cartilage which would suggest a radiodefluorination of the radiotracers. The identification of metabolites from 6 by LC-MS-MS confirmed the significant degree of defluorination as a result of the in vivo hydroxylation in the benzyl ring. In conclusion, [18F]1a or [18F]2a are not suitable for imaging the NR2B NMDA receptors due to their poor brain penetration. We also argue for a cautious use of the radiolabeled pattern, 4-(4-[18F]-fluorobenzyl)piperidine, to develop PET radiotracers.

P02-357 - In vivo molecular imaging reveals distinct distributions of the serotonin transporter, the major inhibitory and excitatory serotonin receptors

IntroductionBased on evidences in molecular neuroimaging, postmortem and genetic studies, impaired serotonergic neurotransmission has been implicated with affective disorders. Moreover, a growing number of evidences showed strong interrelations within the serotonergic system suggesting a common mechanism in the modulation of receptor and transporter densities.ObjectiveHere we directly investigated the regional expression of the 5-HT1A, 5-HT2A and 5-HTT using PET and the three highly selective and specific radioligands [carbonyl-11C]WAY-100635, [18F]Altanserin and [11C]DASB in healthy subjects.MethodsA total of 55 healthy subjects (5-HT1A: 36 subjects, 18 males, age = 26.0 ± 4.9; 5-HT2A: 19 subjects, 11 males, age = 28.2 ± 5.9; 5-HTT: 8 males, age = 28.12 ± 3.6) were included in this study. Binding potential (BPND) values were quantified according to the AAL parcellation scheme.ResultsBPND values averaged over both hemispheres ranged from 0.40–6.35 for the 5-HT1A receptor; 0.01–2.01 for the 5-HT2A receptor and 0.09–2.05 for the 5-HTT, respectively. There was a specific topological pattern according to the ratio between the 5-HT1A, 5-HT2A receptors and 5-HTT (“fingerprints”).ConclusionsSuch information can be essential for detecting potential local alterations in the ratio between different binding proteins on a network level in pathological conditions.Moreover, these data might provide further insight in area-specific effects of frequently prescribed selective serotonin re-uptake inhibitors (SSRI): 1)due to the distinct local receptor and transporter availability;2)SSRI application alters the postsynaptic receptor expression and thus;3)leads to a modified interaction of inhibitory and exhibitory receptors.

Positron emission tomography imaging in multiple sclerosis-current status and future applications.

Multiple Sclerosis (MS) is traditionally considered as a central nervous system (CNS) white matter inflammatory disease. However, recent studies have focused on the neurodegenerative aspects of the disease, which occur early in the pathological process, providing an opportunity for therapeutic intervention and application of neuroprotective strategies. The relationship between neural inflammation and cell death remains controversial. The recent development of new radiolabelled ligands provides positron emission tomography (PET) imaging with a role for studying early aspects of the MS pathology. We provide an overview of current PET research in MS, particularly focussing on possible applications of new radioligands for studying inflammation and neurodegenerative processes. Pathological aspects of neuroinflammation, axonal degeneration and neuronal repair may be explored in vivo with selective PET tracers. Specific radioligands for the cannabinoid system may be applied in MS research to understand the role of this neurotransmitter system in the pathogenesis of the disease. PET imaging represents a promising tool for elucidating controversial aspects of MS pathology and for the assessment of selective and potentially neuroprotective therapies.

Decreased cerebral α4β2* nicotinic acetylcholine receptor availability in patients with mild cognitive impairment and Alzheimer’s disease assessed with positron emission tomography

Postmortem studies indicate a loss of nicotinic acetylcholine receptor (nAChRs) in Alzheimer's disease (AD). In order to establish whether these changes in the cholinergic system occur at an early stage of AD, we carried out positron emission tomography (PET) with a specific radioligand for the α4β2* nicotinic acetylcholine receptor (α4β2* nAChR) in patients with mild to moderate AD and in patients with amnestic mild cognitive impairment (MCI), who have a high risk to progress to AD. Nine patients with moderate AD, eight patients with MCI and seven age-matched healthy controls underwent 2-[(18)F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2-[(18)F]FA-85380) PET. After coregistration with individual magnetic resonance imaging the binding potential (BP(ND)) of 2-[(18)F]FA-85380 was calculated using either the corpus callosum or the cerebellum as reference regions. PET data were analysed by region of interest analysis and by voxel-based analysis. Both patients with AD and MCI showed a significant reduction in 2-[(18)F]FA-85380 BP(ND) in typical AD-affected brain regions. Thereby, the corpus callosum was identified as the most suitable reference region. The 2-[(18)F]FA-85380 BP(ND) correlated with the severity of cognitive impairment. Only MCI patients that converted to AD in the later course (n = 5) had a reduction in 2-[(18)F]FA-85380 BP(ND). 2-[(18)F]FA-85380 PET appears to be a sensitive and feasible tool for the detection of a reduction in α4β2* nAChRs which seems to be an early event in AD. In addition, 2-[(18)F]FA-85380 PET might give prognostic information about a conversion from MCI to AD.