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In early Alzheimer's dementia, there is a need for PET biomarkers of disease progression with close associations to cognitive dysfunction that may aid to predict further cognitive decline and neurodegeneration. Amyloid biomarkers are not suitable for that purpose. The α4β2 nicotinic acetylcholine receptors (α4β2-nAChRs) are widely abundant in the human brain. As neuromodulators they play an important role in cognitive functions such as attention, learning and memory. Post-mortem studies reported lower expression of α4β2-nAChRs in more advanced Alzheimer's dementia. However, there is ongoing controversy whether α4β2-nAChRs are reduced in early Alzheimer's dementia. Therefore, using the recently developed α4β2-nAChR-specific radioligand (-)-18F-flubatine and PET, we aimed to quantify the α4β2-nAChR availability and its relationship to specific cognitive dysfunction in mild Alzheimer's dementia. Fourteen non-smoking patients with mild Alzheimer's dementia, drug-naïve for cholinesterase therapy, were compared with 15 non-smoking healthy controls matched for age, sex and education by applying (-)-18F-flubatine PET together with a neuropsychological test battery. The one-tissue compartment model and Logan plot method with arterial input function were used for kinetic analysis to obtain the total distribution volume (VT) as the primary, and the specific binding part of the distribution volume (VS) as the secondary quantitative outcome measure of α4β2-nAChR availability. VS was determined by using a pseudo-reference region. Correlations between VT within relevant brain regions and Z-scores of five cognitive functions (episodic memory, executive function/working memory, attention, language, visuospatial function) were calculated. VT (and VS) were applied for between-group comparisons. Volume of interest and statistical parametric mapping analyses were carried out. Analyses revealed that in patients with mild Alzheimer's dementia compared to healthy controls, there was significantly lower VT, especially within the hippocampus, fronto-temporal cortices, and basal forebrain, which was similar to comparisons of VS. VT decline in Alzheimer's dementia was associated with distinct domains of impaired cognitive functioning, especially episodic memory and executive function/working memory. Using (-)-18F-flubatine PET in patients with mild Alzheimer's dementia, we show for the first time a cholinergic α4β2-nAChR deficiency mainly present within the basal forebrain-cortical and septohippocampal cholinergic projections and a relationship between lower α4β2-nAChR availability and impairment of distinct cognitive domains, notably episodic memory and executive function/working memory. This shows the potential of (-)-18F-flubatine as PET biomarker of cholinergic α4β2-nAChR dysfunction and specific cognitive decline. Thus, if validated by longitudinal PET studies, (-)-18F-flubatine might become a PET biomarker of progression of neurodegeneration in Alzheimer's dementia. [ABSTRACT FROM AUTHOR]

The vesicular acetylcholine transporter (VAChT) is an important target for in vivo imaging of neurodegenerative processes using positron emission tomography (PET). So far the development of VAChT PET radioligands is based on the single known lead compound vesamicol. In this study we investigated a recently published spiroindoline based compound class (Sluder et al., 2012), which was suggested to have potential in the development of VAChT ligands. Therefore, we synthesized a small series of N , N -substituted spiro[indoline-3,4′-piperidine] derivatives and determined their in vitro binding affinities toward the VAChT. In order to investigate the selectivity, the off-target binding toward σ 1 and σ 2 receptors was determined. The compounds possessed VAChT affinities with K i values in the range of 39–376 nM. Binding affinities toward the σ 1 and σ 2 receptors are in a similar range indicating that the strong structural difference between the spiroindolines and vesamicol did not improve the selectivity. The observed potential to additionally bind to σ receptors let us assume that the herein investigated spiroindolines are not suitable to replace vesamicol as lead compound for the development of VAChT ligands. [ABSTRACT FROM AUTHOR]

Purpose: The α nicotinic acetylcholine receptor (nAChR) is implicated in many neuropsychiatric disorders, making it an important target for positron emission tomography (PET) imaging. The first aim of this work was to compare two α nAChRs PET radioligands, [F]ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-([F]fluorodibenzo[ b,d]thiophene 5,5-dioxide) and [F]DBT-10 (7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-([F]fluorodibenzo[ b,d]thiophene 5,5-dioxide), in nonhuman primates. The second aim was to assess further the quantification and test-retest variability of [F]ASEM in humans. Methods: PET scans with high specific activity [F]ASEM or [F]DBT-10 were acquired in three rhesus monkeys (one male, two female), and the kinetic properties of these radiotracers were compared. Additional [F]ASEM PET scans with blocking doses of nicotine, varenicline, and cold ASEM were acquired separately in two animals. Next, six human subjects (five male, one female) were imaged with [F]ASEM PET for 180 min, and arterial sampling was used to measure the parent input function. Different modeling approaches were compared to identify the optimal analysis method and scan duration for quantification of [F]ASEM distribution volume ( V ). In addition, retest scans were acquired in four subjects (three male, one female), and the test-retest variability of V was assessed. Results: In the rhesus monkey brain [F]ASEM and [F]DBT-10 exhibited highly similar kinetic profiles. Dose-dependent blockade of [F]ASEM binding was observed, while administration of either nicotine or varenicline did not change [F]ASEM V . [F]ASEM was selected for further validation because it has been used in humans. Accurate quantification of [F]ASEM V in humans was achieved using multilinear analysis with at least 90 min of data acquisition, resulting in V values ranging from 19.6 ± 2.5 mL/cm in cerebellum to 25.9 ± 2.9 mL/cm in thalamus. Test-retest variability of V was 11.7 ± 9.8%. Conclusions: These results confirm [F]ASEM as a suitable radiotracer for the imaging and quantification of α nAChRs in humans. [ABSTRACT FROM AUTHOR]

Purpose: Positron emission tomography (PET) radioligands specific to α nicotinic acetylcholine receptors (nAChRs) afford in vivo imaging of this receptor for neuropathologies such as Alzheimer's disease, schizophrenia, and substance abuse. This work aims to characterize the kinetic properties of an α-nAChR-specific radioligand, 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-[F]-fluorodibenzo[b,d]thiophene 5,5-dioxide ([F]DBT-10), in nonhuman primates. Methods: [F]DBT-10 was produced via nucleophilic substitution of the nitro-precursor. Four Macaca mulatta subjects were imaged with [F]DBT-10 PET, with measurement of [F]DBT-10 parent concentrations and metabolism in arterial plasma. Baseline PET scans were acquired for all subjects. Following one scan, ex vivo analysis of brain tissue was performed to inspect for radiolabeled metabolites in brain. Three blocking scans with 0.69 and 1.24 mg/kg of the α-nAChR-specific ligand ASEM were also acquired to assess dose-dependent blockade of [F]DBT-10 binding. Kinetic analysis of PET data was performed using the metabolite-corrected input function to calculate the parent fraction corrected total distribution volume ( V/ f). Results: [F]DBT-10 was produced within 90 min at high specific activities of 428 ± 436 GBq/μmol at end of synthesis. Metabolism of [F]DBT-10 varied across subjects, stabilizing by 120 min post-injection at parent fractions of 15-55 %. Uptake of [F]DBT-10 in brain occurred rapidly, reaching peak standardized uptake values (SUVs) of 2.9-3.7 within 30 min. The plasma-free fraction was 18.8 ± 3.4 %. No evidence for radiolabeled [F]DBT-10 metabolites was found in ex vivo brain tissue samples. Kinetic analysis of PET data was best described by the two-tissue compartment model. Estimated V/ f values were 193-376 ml/cm across regions, with regional rank order of thalamus > frontal cortex > striatum > hippocampus > occipital cortex > cerebellum > pons. Dose-dependent blockade of [F]DBT-10 binding by structural analog ASEM was observed throughout the brain, and occupancy plots yielded a V/ f estimate of 20 ± 16 ml/cm. Conclusion: These results demonstrate suitable kinetic properties of [F]DBT-10 for in vivo quantification of α-nAChR binding in nonhuman primates. [ABSTRACT FROM AUTHOR]

Phosphodiesterase 10A (PDE10A) is a key enzyme of intracellular signal transduction which is involved in the regulation of neurotransmission. The molecular imaging of PDE10A by PET is expected to allow a better understanding of physiological and pathological processes related to PDE10A expression and function in the brain. The aim of this study was to develop a new 18F-labeled PDE10A ligand based on a 6,7-dimethoxy- 4-pyrrolidinylquinazoline and to evaluate its properties in biodistribution studies. Nucleophilic substitution of the 7-tosyloxy-analogue led to the 7-[18F]fluoroethoxy-derivative [18F]IV with radiochemical yields of 25% ± 9% (n = 9), high radiochemical purity of ≥99% and specific activities of 110-1,100 GBq/μmol. [18F]IV showed moderate PDE10A affinity (KD,PDE10A = 14 nM) and high metabolic stability in the brain of female CD-1 mice, wherein the radioligand entered rapidly with a peak uptake of 2.3% ID/g in striatum at 5 min p.i. However, ex vivo autoradiographic and in vivo blocking studies revealed no target specific accumulation and demonstrated [18F]IV to be inapplicable for imaging PDE10A with PET. [ABSTRACT FROM AUTHOR]

Purpose: To conduct a quantitative PET assessment of the specific binding sites in the brain of juvenile pigs for [F]NS10743, a novel diazabicyclononane derivative targeting α7 nicotinic acetylcholine receptors (α7 nAChRs). Methods: Dynamic PET recordings were made in isoflurane-anaesthetized juvenile pigs during 120 min after administration of [F]NS10743 under baseline conditions ( n = 3) and after blocking of the α7 nAChR with NS6740 (3 mg·kg bolus + 1 mg·kg·h continuous infusion; n = 3). Arterial plasma samples were collected for determining the input function of the unmetabolized tracer. Kinetic analysis of regional brain time-radioactivity curves was performed, and parametric maps were calculated relative to arterial input. Results: Plasma [F]NS10743 passed readily into the brain, with peak uptake occurring in α7 nAChR-expressing brain regions such as the colliculi, thalamus, temporal lobe and hippocampus. The highest SUV was approximately 2.3, whereas the lowest uptake was in the olfactory bulb (SUV 1.53 ± 0.32). Administration of NS6740 significantly decreased [F]NS10743 binding late in the emission recording throughout the brain, except in the olfactory bulb, which was therefore chosen as reference region for calculation of BP. The baseline BP ranged from 0.39 ± 0.08 in the cerebellum to 0.76 ± 0.07 in the temporal lobe. Pretreatment and constant infusion with NS6740 significantly reduced the BP in regions with high [F]NS10743 binding (temporal lobe −29%, p = 0.01; midbrain: −35%, p = 0.02), without significantly altering the BP in low binding regions (cerebellum: −16%, p = 0.2). Conclusion: This study confirms the potential of [F]NS10743 as a target-specific radiotracer for the molecular imaging of central α7 nAChRs by PET. [ABSTRACT FROM AUTHOR]

Purpose: Neuroimaging of σ receptors in the human brain has been proposed for the investigation of the pathophysiology of neurodegenerative and psychiatric diseases. However, there is a lack of suitable F-labelled PET radioligands for that purpose. Methods: The selective σ receptor ligand [F]fluspidine (1′-benzyl-3-(2-[F]fluoroethyl)-3H-spiro[[2]benzofuran-1,4′-piperidine]) was synthesized by nucleophilic F substitution of the tosyl precursor. In vitro receptor binding affinity and selectivity were assessed by radioligand competition in tissue homogenate and autoradiographic approaches. In female CD-1 mice, in vivo properties of [F]fluspidine were evaluated by ex vivo brain section imaging and organ distribution of intravenously administered radiotracer. Target specificity was validated by organ distribution of [F]fluspidine after treatment with 1 mg/kg i.p. of the σ receptor antagonist haloperidol or the emopamil binding protein (EBP) inhibitor tamoxifen. In vitro metabolic stability and in vivo metabolism were investigated by LC-MS and radio-HPLC analysis. Results: [F]Fluspidine was obtained with a radiochemical yield of 35-45%, a radiochemical purity of ≥ 99.6% and a specific activity of 150-350 GBq/μmol ( n = 6) within a total synthesis time of 90-120 min. In vitro, fluspidine bound specifically and with high affinity to σ receptors ( K = 0.59 nM). In mice, [F]fluspidine rapidly accumulated in brain with uptake values of 3.9 and 4.7%ID/g and brain to blood ratios of 7 and 13 at 5 and 30 min after intravenous application of the radiotracer, respectively. By ex vivo autoradiography of brain slices, resemblance between binding site occupancy of [F]fluspidine and the expression of σ receptors was shown. The radiotracer uptake in the brain as well as in peripheral σ receptor expressing organs was significantly inhibited by haloperidol but not by tamoxifen. Incubation with rat liver microsomes led to a fast biotransformation of fluspidine. After an incubation period of 30 min only 13% of the parent compound was left. Seven metabolites were identified by HPLC-UV and LC-MS techniques. However, [F]fluspidine showed a higher metabolic stability in vivo. In plasma samples ∼ 94% of parent compound remained at 30 min and ∼ 67% at 60 min post-injection. Only one major radiometabolite was detected. None of the radiometabolites crossed the blood-brain barrier. Conclusion: [F]Fluspidine demonstrated favourable target affinity and specificity as well as metabolic stability both in vitro and in animal experiments. The in vivo properties of [F]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of σ receptors in vivo. [ABSTRACT FROM AUTHOR]

The outstanding diversity of cellular properties mediated by neuronal and nonneuronal α7 nicotinic acetylcholine receptors (α7 nAChR) points to the diagnostic potential of quantitative nuclear molecular imaging of α7 nAChR in neurology and oncology. It was our goal to radiolabel the α7 nAChR agonist 4-[5-(4-fluoro-phenyl)-[1,3,4]oxadiazol-2-yl]-1,4-diaza-bicyclo[3.2.2]nonane (NS10743) and to assess the selectivity of [18F]NS10743 binding site occupancy in animal experiments. [18F]NS10743 was synthesized by nucleophilic substitution of the nitro precursor. In vitro receptor affinity and selectivity were assessed by radioligand competition and autoradiography. The radiotracer properties were evaluated in female CD-1 mice by brain autoradiography and organ distribution. Target specificity was validated after treatment with SSR180711 (10 mg/kg, intraperitoneal), and metabolic stability was investigated using radio-HPLC. The specific activity of [18F]NS10743 exceeded 150 GBq/μmol at a radiochemical purity >99%. In vitro, NS10743 and [18F]NS10743 showed high affinity and specificity towards α7 nAChR. The brain permeation of [18F]NS10743 was fast and sufficient with values of 4.83 and 1.60% injected dose per gram and brain to plasma ratios of 3.83 and 2.05 at 5 and 60 min after radiotracer administration. Brain autoradiography and organ distribution showed target-specific accumulation of [18F]NS10743 in brain substructures and various α7 nAChR-expressing organs. The radiotracer showed a high metabolic stability in vivo with a single polar radiometabolite, which did not cross the blood–brain barrier. The good in vitro and in vivo features of [18F]NS10743 make this radioligand a promising candidate for quantitative in vivo imaging of α7 nAChR expression and encourage further investigations. [ABSTRACT FROM AUTHOR]

A2A adenosine receptors (A2A-AR) have a cardio-protective function upon ischemia and reperfusion, but on the other hand, their stimulation could lead to arrhythmias. Our aim was to investigate the potential use of the PET radiotracer [18F]FLUDA to non-invasively determine the A2A-AR availability for diagnosis of the A2AR status. Therefore, we compared mice with cardiomyocyte-specific overexpression of the human A2A-AR (A2A-AR TG) with the respective wild type (WT). We determined: (1) the functional impact of the selective A2AR ligand FLUDA on the contractile function of atrial mouse samples, (2) the binding parameters (Bmax and KD) of [18F]FLUDA on mouse and human atrial tissue samples by autoradiographic studies, and (3) investigated the in vivo uptake of the radiotracer by dynamic PET imaging in A2A-AR TG and WT. After A2A-AR stimulation by the A2A-AR agonist CGS 21680 in isolated atrial preparations, antagonistic effects of FLUDA were found in A2A-AR-TG animals but not in WT. Radiolabelled [18F]FLUDA exhibited a KD of 5.9 ± 1.6 nM and a Bmax of 455 ± 78 fmol/mg protein in cardiac samples of A2A-AR TG, whereas in WT, as well as in human atrial preparations, only low specific binding was found. Dynamic PET studies revealed a significantly higher initial uptake of [18F]FLUDA into the myocardium of A2A-AR TG compared to WT. The hA2A-AR-specific binding of [18F]FLUDA in vivo was verified by pre-administration of the highly affine A2AAR-specific antagonist istradefylline. Conclusion: [18F]FLUDA is a promising PET probe for the non-invasive assessment of the A2A-AR as a marker for pathologies linked to an increased A2A-AR density in the heart, as shown in patients with heart failure. [ABSTRACT FROM AUTHOR]

Abstract: A series of fluorinated diphenylchalcogen derivatives, possessing a sulfur or an oxygen bridge, has been prepared with the aim to get a suitable radiotracer to image the SERT in vivo using positron emission tomography (PET). The compounds were synthesized and assayed toward the serotonin (SERT), dopamine (DAT), and norepinephrine (NET) transporters. Among the developed series, five compounds display a high SERT affinity (K i: 0.27–2.91nM range) and can be labeled either with carbon-11 or fluorine-18. [Copyright &y& Elsevier]

The adenosine A2B receptor has been proposed as a novel therapeutic target in cancer, as its expression is drastically elevated in several tumors and cancer cells. Noninvasive molecular imaging via positron emission tomography (PET) would allow the in vivo quantification of this receptor in pathological processes and most likely enable the identification and clinical monitoring of respective cancer therapies. On the basis of a bicyclic pyridopyrimidine-2,4-dione core structure, the new adenosine A2B receptor ligand 9 was synthesized, containing a 2-fluoropyridine moiety suitable for labeling with the short-lived PET radionuclide fluorine-18. Compound 9 showed a high binding affinity for the human A2B receptor (Ki(A2B) = 2.51 nM), along with high selectivities versus the A1, A2A, and A3 receptor subtypes. Therefore, it was radiofluorinated via nucleophilic aromatic substitution of the corresponding nitro precursor using [18F]F-/K2.2.2./K2CO3 in DMSO at 120 °C. Metabolic studies of [18F]9 in mice revealed about 60% of radiotracer intact in plasma at 30 minutes p.i. A preliminary PET study in healthy mice showed an overall biodistribution of [18F]9, corresponding to the known ubiquitous but low expression of the A2B receptor. Consequently, [18F]9 represents a novel PET radiotracer with high affinity and selectivity toward the adenosine A2B receptor and a suitable in vivo profile. Subsequent studies are envisaged to investigate the applicability of [18F]9 to detect alterations in the receptor density in certain cancer-related disease models. [ABSTRACT FROM AUTHOR]

To verify vesamicol as lead structure in the development of radioligands for imaging of VAChT in the brain by PET, we systematically modified this molecule and investigated four different groups of derivatives. Structural changes were conducted in all three ring systems A, B, and C resulting in a library of different vesamicol analogs. Based on their in vitro binding affinity toward VAChT as well as σ 1 and σ 2 receptors, we performed a structure-affinity relationship (SAR) study regarding both affinity and selectivity. The compounds possessed VAChT affinities in the range of 1.32 nM (benzovesamicol) to >10 μM and selectivity factors from 0.1 to 73 regarding σ 1 and σ 2 receptors, respectively. We could confirm the exceptional position of benzovesamicols as most affine VAChT ligands. However, we also observed that most of the compounds with high VAChT affinity demonstrated considerable affinity in particular to the σ 1 receptor. Finally, none of the various vesamicol analogs in all four groups showed an in vitro binding profile suitable for specific VAChT imaging in the brain. [ABSTRACT FROM AUTHOR]

Abstract: Neuroimaging of GABAA receptors offers the potential for a better diagnosis of diseases related to a dysfunction of the GABAergic neurotransmission. A series of potent fluorinated analogues of the pyrazolopyrimidine Indiplon has been synthesized and evaluated in vitro as potential agents for imaging the GABAA receptor by means of positron emission tomography (PET). The most promising compound N-(3-fluoropropyl)-N-[3-[3-(thiophene-2-carbonyl)-pyrazolo[1,5-a]pyrimidin-7-yl]-phenyl]-acetamide (5b) showed an IC50 value of 2.78±0.63nM comparable to the lead compound Indiplon (IC50 3.29±0.37nM), thus making it an interesting candidate for further investigations. In addition to the fluorinated reference compounds, suitable precursors for 18F-radiolabelling studies have been synthesized. [Copyright &y& Elsevier]

[Display omitted] • Synthesis of fluorinated and methoxylated benzothiazole derivatives. • Biological evaluation of compounds for their h CB 1 and h CB 2 binding affinity. • Identification of extremely potent and selective CB 2 receptor ligands. • In vitro metabolic studies of 15 and 21 reveal the superior stability of 21 in HLM. • 21 emerges as a credible lead that can be further exploited for radiolabeling. The upregulation of the CB 2 receptors in neuroinflammation and cancer and their potential visualization with PET (positron emission tomography) could provide a valuable diagnostic and therapy-monitoring tool in such disorders. However, the availability of reliable CB 2 -selective imaging probes is still lacking in clinical practice. We have recently identified a benzothiazole-2-ylidine amide hit (6a) as a highly potent CB 2 ligand. With the aim of enhancing its CB 2 over CB 1 selectivity and introducing structural sites suitable for radiolabeling, we herein describe the development of fluorinated and methoxylated benzothiazole derivatives endowed with extremely high CB 2 binding affinity and an exclusive selectivity to the CB 2 receptor. Compounds 14 , 15 , 18 , 19 , 21 , 24 and 25 displayed subnanomolar CB 2 K i values (ranging from 0.16 nM to 0.68 nM) and interestingly, all of the synthesized compounds completely lacked affinity at the CB 1 receptor (K i > 10,000 nM for all compounds), indicating their remarkably high CB 2 over CB 1 selectivity factors. The fluorinated analogs, 15 and 21 , were evaluated for their in vitro metabolic stability in mouse and human liver microsomes (MLM and HLM). Both 15 and 21 displayed an exceptionally high stability (98% and 91% intact compounds, respectively) after 60 min incubation with MLM. Contrastingly, a 5- and 2.8-fold lower stability was demonstrated for compounds 15 and 21 , respectively, upon incubation with HLM for 60 min. Taken together, our data present extremely potent and selective CB 2 ligands as credible leads that can be further exploited for 18F- or 11C-radiolabeling and utilization as PET tracers. [ABSTRACT FROM AUTHOR]

By structure–activity relationship studies on the tilorone scaffold, the ‘one armed’ substituted dibenzothiophenes and the fluoren-9-ones were identified as the most potential α 7 nAChR ligands. While the suitability of dibenzothiophene derivatives as PET tracers is recognized, the potential of fluoren-9-ones is insufficiently investigated. We herein report on a series of fluoren-9-one based derivatives targeting α 7 nAChR with compounds 8a and 8c possessing the highest affinity and selectivity. Accordingly, with [ 18 F] 8a and [ 18 F] 8c we designed and initially evaluated the first fluoren-9-one derived α 7 nAChR selective PET ligands. A future application of these radioligands is facilitated by the herein presented successful implementation of fully automated radiosynthesis. [ABSTRACT FROM AUTHOR]

α4β2* nicotinic receptors (α4β2* nAChRs) could provide a biomarker in neuropsychiatric disorders (e.g., Alzheimer's and Parkinson's diseases, depressive disorders, and nicotine addiction). However, there is a lack of α4β2* nAChR specific PET radioligands with kinetics fast enough to enable quantification of nAChR within a reasonable time frame. Following on from promising preclinical results, the aim of the present study was to evaluate for the first time in humans the novel PET radioligand (−)-[ 18 F]Flubatine, formerly known as (−)-[ 18 F]NCFHEB, as a tool for α4β2* nAChR imaging and in vivo quantification. Dynamic PET emission recordings lasting 270 min were acquired on an ECAT EXACT HR + scanner in 12 healthy male non-smoking subjects (71.0 ± 5.0 years) following the intravenous injection of 353.7 ± 9.4 MBq of (−)-[ 18 F]Flubatine. Individual magnetic resonance imaging (MRI) was performed for co-registration. PET frames were motion-corrected, before the kinetics in 29 brain regions were characterized using 1- and 2-tissue compartment models (1TCM, 2TCM). Given the low amounts of metabolite present in plasma, we tested arterial input functions with and without metabolite corrections. In addition, pixel-based graphical analysis (Logan plot) was used. The model's goodness of fit, with and without metabolite correction was assessed by Akaike's information criterion. Model parameters of interest were the total distribution volume V T (mL/cm 3 ), and the binding potential BP ND relative to the corpus callosum, which served as a reference region. The tracer proved to have high stability in vivo, with 90% of the plasma radioactivity remaining as untransformed parent compound at 90 min, fast brain kinetics with rapid uptake and equilibration between free and receptor-bound tracer. Adequate fits of brain TACs were obtained with the 1TCM. V T could be reliably estimated within 90 min for all regions investigated, and within 30 min for low-binding regions such as the cerebral cortex. The rank order of V T by region corresponded well with the known distribution of α4β2* receptors ( V T [thalamus] 27.4 ± 3.8, V T [putamen] 12.7 ± 0.9, V T [frontal cortex] 10.0 ± 0.8, and V T [corpus callosum] 6.3 ± 0.8). The BP ND , which is a parameter of α4β2* nAChR availability, was 3.41 ± 0.79 for the thalamus, 1.04 ± 0.25 for the putamen and 0.61 ± 0.23 for the frontal cortex, indicating high specific tracer binding. Use of the arterial input function without metabolite correction resulted in a 10% underestimation in V T , and was without important biasing effects on BP ND . Altogether, kinetics and imaging properties of (−)-[ 18 F]Flubatine appear favorable and suggest that (−)-[ 18 F]Flubatine is a very suitable and clinically applicable PET tracer for in vivo imaging of α4β2* nAChRs in neuropsychiatric disorders. [ABSTRACT FROM AUTHOR]

Several spirocyclic piperidine derivatives were designed and synthesized as σ 1 receptor ligands. In vitro competition binding assays showed that the fluoroalkoxy analogues with small substituents possessed high affinity towards σ 1 receptors and subtype selectivity. Particularly for ligand 1′-((6-(2-fluoroethoxy)pyridin-3-yl)methyl)-3 H -spiro[2-benzofuran-1,4′-piperidine] ( 2 ), high σ 1 receptor affinity ( K i = 2.30 nM) and high σ 1 /σ 2 subtype selectivity (142-fold) as well as high σ 1 /VAChT selectivity (234-fold) were observed. [ 18 F] 2 was synthesized using an efficient one-pot, two-step reaction method in a home-made automated synthesis module, with an overall isolated radiochemical yield of 8–10%, a radiochemical purity of higher than 99%, and specific activity of 56–78 GBq/μmol. Biodistribution studies of [ 18 F] 2 in ICR mice indicated high initial brain uptake and a relatively fast washout. Administration of haloperidol, compound 1 and different concentrations of SA4503 (3, 5, or 10 μmol/kg) 5 min prior to injection of [ 18 F] 2 significantly decreased the accumulation of radiotracer in organs known to contain σ 1 receptors. Ex vivo autoradiography in Sprague–Dawley rats demonstrated high accumulation of radiotracer in brain areas with high expression of σ 1 receptors. These encouraging results prove that [ 18 F] 2 is a suitable candidate for σ 1 receptor imaging with PET in humans. [ABSTRACT FROM AUTHOR]

Introduction: (−)-[18 F]Flubatine is a PET tracer with high affinity and selectivity for the nicotinic acetylcholine α4β2 receptor subtype. A clinical trial assessing the availability of this subtype of nAChRs was performed. From a total participant number of 21 Alzheimer’s disease (AD) patients and 20 healthy controls (HCs), the following parameters were determined: plasma protein binding, metabolism and activity distribution between plasma and whole blood. Methods: Plasma protein binding and fraction of unchanged parent compound were assessed by ultracentrifugation and HPLC, respectively. The distribution of radioactivity (parent compound+metabolites) between plasma and whole blood was determined ex vivo at different time-points after injection by gamma counting after separation of whole blood by centrifugation into the cellular and non-cellular components. In additional experiments in vitro, tracer distribution between these blood components was assessed for up to 90min. Results: A fraction of 15%±2% of (−)-[18 F]Flubatine was found to be bound to plasma proteins. Metabolic degradation of (−)-[18 F]Flubatine was very low, resulting in almost 90% unchanged parent compound at 90min p.i. with no significant difference between AD and HC. The radioactivity distribution between plasma and whole blood changed in vivo only slightly over time from 0.82±0.03 at 3min p.i. to 0.87±0.03 at 270min p.i. indicating the contribution of only a small amount of metabolites. In vitro studies revealed that (−)-[18 F]Flubatine was instantaneously distributed between cellular and non-cellular blood parts. Discussion: (−)-[18 F]Flubatine exhibits very favourable characteristics for a PET radiotracer such as slow metabolic degradation and moderate plasma protein binding. Equilibrium of radioactivity distribution between plasma and whole blood is reached instantaneously and remains almost constant over time allowing both convenient sample handling and facilitated fractional blood volume contribution assessment. [ABSTRACT FROM AUTHOR]

Abstract: Both enantiomers of the epibatidine analogue flubatine display high affinity towards the α4β2 nicotinic acetylcholine receptor (nAChR) in vitro, accompanied by negligible interactions with diverse off-target proteins. Extended single dose toxicity studies in rodent indicated a NOEL (No Observed Effect Level) of 6.2μg/kg for (−)-flubatine and 1.55μg/kg for (+)-flubatine. We developed syntheses for both flubatine enantiomers and their corresponding precursors for radiolabeling. The newly synthesized trimethylammonium precursors allowed for highly efficient 18F-radiolabelling in radiochemical yields >60% and specific activities >750GBq/μmol, thus making the radioligands practical for clinical investigation. [Copyright &y& Elsevier]

Abstract: Aminocyclohexyl indoles bind with high affinity and specificity toward the serotonin transporter (SERT). Based on this structural lead, we designed fluoroalkoxydihydroisoquinoline-cyclohexyl indole carbonitriles for future application as 18F-labeled tracers for SERT imaging by PET. Six compounds, three pairs of cis- and trans-isomer derivatives, respectively, were synthesized and evaluated in vitro. The chemistry of the new compounds, their affinity and specificity data, the general route to the phenolic precursor for labeling, and the successful 18F-fluoroalkylation of one pair of compounds are described herein. [Copyright &y& Elsevier]