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Novel sigma-2 (σ2) receptor ligands with benzimidazolone and 5,6-dimethoxyisoindoline as pharmacophores were designed and synthesized. Compound 4exhibited low nanomolar affinity for the σ2receptors (Ki(σ2) = 2.30 nM) and high subtype selectivity (Ki(σ1)/Ki(σ2) > 1500). Radioligand [18F]4was prepared in radiochemical yields of 18 ± 7%, with >99% radiochemical purity and molar activity of 244 ± 136 GBq/μmol. Biodistribution and blocking studies in mice and small animal PET/CT imaging in rats indicated highly specific binding of [18F]4in organs known to express the σ2receptors. Small animal PET/CT imaging with [18F]4showed clear visualization of the tumors in subcutaneous A549 lung cancer and U87MG glioma xenografts, and intracranial orthotopic U87MG glioma models. Co-administration of CM398 with [18F]4significantly reduced activity uptake in the tumors, indicating that [18F]4specifically binds to the σ2receptors expressed in A549 and U87MG xenografts.

The development of cannabinoid receptor type 2 (CB2R) radioligands for positron emission tomography (PET) imaging was intensively explored. To overcome the low metabolic stability and simultaneously increase the binding affinity of known CB2R radioligands, a carborane moiety was used as a bioisostere. Here we report the synthesis and characterization of carborane-based 1,8-naphthyridinones and thiazoles as novel CB2R ligands. All tested compounds showed low nanomolar CB2R affinity, with (Z)-N-[3-(4-fluorobutyl)-4,5-dimethylthiazole-2(3H)-ylidene]-(1,7-dicarba-closo-dodecaboranyl)-carboxamide (LUZ5) exhibiting the highest affinity (0.8 nM). Compound [18F]LUZ5-d8was obtained with an automated radiosynthesizer in high radiochemical yield and purity. In vivoevaluation revealed the improved metabolic stability of [18F]LUZ5-d8compared to that of [18F]JHU94620. PET experiments in rats revealed high uptake in spleen and low uptake in brain. Thus, the introduction of a carborane moiety is an appropriate tool for modifying literature-known CB2R ligands and gaining access to a new class of high-affinity CB2R ligands, while the in vivopharmacology still needs to be addressed.

Sigma (σ) receptors represent attractive targets for the development of potential agents for the treatment of several disorders, including Alzheimer's disease and neuropathic pain. In the search for multitarget small molecules (MSMs) against such disorders, we have re-discovered chromenones as new affine σ1/σ2ligands. 6-(4-(Piperidin-1-yl)butoxy)-4H-chromen-4-one (7), a previously identified MSM with potent dual-target activities against acetylcholinesterase and monoamine oxidase B, also exhibited σ1/σ2affinity. 6-(3-(Azepan-1-yl)propoxy)-4H-chromen-4-one (20) showed a Kivalue for σ1of 27.2 nM (selectivity (σ1/σ2) = 28), combining the desired σ1receptor affinity with a dual inhibitory capacity against both acetyl- and butyrylcholinesterase. 6-((5-Morpholinopentyl)oxy)-4H-chromen-4-one (12) was almost equipotent to S1RA, an established σ1receptor antagonist.

Direct interaction of the sigma-1 (σ1) receptor, an endoplasmic reticulum chaperone located in close vicinity to the mitochondrion, with a variety of proteins involved in essential processes regulating proliferation, survival, and death of cells, indicates a role of this protein in tumor biology. Since tumor therapies address precisely these processes to stop the growth of tumor cells, the σ1 receptor could be a suitable modulator of the effectiveness of selected therapies. Recent initial studies have shown not only anti-proliferative effects of ligands targeting this protein, but also modulating effects in both chemotherapy and radiotherapy. However, in this regard the influence of functional expression of the σ1receptor has not yet been fully clarified. The purpose of this pilot experiment was to investigate the role of σ1 receptor on cellular radiosensitivity in an in vitromodel. Therefore, clonogenic assays were performed to assess the susceptibility of HEK293 cells, stably transfected with human σ1 receptor, towards irradiation (X-ray) in comparison to non-transfected cells. Moreover, irradiation combined with pharmacological treatment should prove whether agonistic and antagonistic ligand binding to σ1 receptor influences the effectiveness of radiation treatment. The data obtained are not fully conclusive by indicating, on the one hand, an involvement of σ1 receptor in radiation-induced effects along with pharmacological effects independent from the σ1 receptor level, on the other hand, suggesting limitations of the model used herein. Consequently, subsequent work will focus on the investigation of tumor cells with different receptor densities.

Contilisant, a permeable, antioxidant, and neuroprotectant agent, showing high nM affinity at H3R and excellent inhibition of the monoamine oxidases and cholinesterases, is an affine and selective S1R agonist in the nanomolar range, based on the binding affinity and functional experiment, a result confirmed by molecular modeling. In addition, contilisant significantly restores the cognitive deficit induced by Aβ1–42in the radial maze assay in an in vivo Alzheimer’s disease test, comparing very favorably with donepezil.

Novel ligands with the 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline or 5,6-dimethoxyisoindoline pharmacophore were designed and synthesized for evaluation of their structure–activity relationship to the sigma-2 (σ2) receptor and developed as suitable PET radioligands. Compound 1was found to possess nanomolar affinity (Ki(σ1) = 2.57 nM) for the σ2receptor, high subtype selectivity (>2000-fold), and high selectivity over 40 other receptors and transporters. Radioligand [18F]1was prepared with radiochemical yield of 37–54%, > 99% radiochemical purity, and molar activity of 107–189 GBq/μmol. Biodistribution and blocking studies in mice and micro-PET/CT imaging of [18F]1in rats indicated excellent binding specificity to the σ2receptors in vivo. Micro-PET/CT imaging of [18F]1in the U87MG glioma xenograft model demonstrated clear tumor visualization with high tumor uptake and tumor-to-background ratio. Co-injection with CM398 (5 μmol/kg) led to a remarkable reduction of tumor uptake (80%, 60–70 min), indicating high specific binding of [18F]1in U87MG glioma xenografts.

The development of cannabinoid receptor type 2 (CB2R) PET radioligands has been intensively explored due to the pronounced CB2R upregulation under various pathological conditions. Herein, we report on the synthesis of a series of CB2R affine fluorinated indole-2-carboxamide ligands. Compound RM365 was selected for PET radiotracer development due to its high CB2R affinity (Ki= 2.1 nM) and selectivity over CB1R (factor > 300). Preliminary in vitroevaluation of [18F]RM365 indicated species differences in the binding to CB2R (KDof 2.32 nM for the hCB2R vs KD> 10,000 nM for the rCB2R). Metabolism studies in mice revealed a high in vivo stability of [18F]RM365. PET imaging in a rat model of local hCB2R(D80N) overexpression in the brain demonstrates the ability of [18F]RM365 to reach and selectively label the hCB2R(D80N) with a high signal-to-background ratio. Thus, [18F]RM365 is a very promising PET radioligand for the imaging of upregulated hCB2R expression under pathological conditions.

Molecular imaging of brain structures by highly sensitive non-invasive techniques offers unique possibilities in the understanding of physiological and pathological processes in the central nervous system. In particular, the quantitative analysis by positron emission tomography (PET) of α7 nicotinic acetylcholine receptors (α7 nAChR), which are involved in different signalling pathways in the brain, is assumed to provide important information on the relation between receptor dysfunction and the pathogeneses of neuropsychiatric brain diseases, but the applicability of this imaging approach is still hampered due to insufficient imaging agents. This paper presents the recent efforts made to develop PET radiotracers targeting α7 nAChR as well as the current state of the evaluation of the most promising radiolabelled compounds in animal models and humans.

Radiolabeled vesamicol analogs are promising candidates as ligands for the vesicular acetylcholine transporter (VAChT) to enable in vivoimaging of early cholinergic degenerations in brain. The 4‐fluorobenzoyl‐substituted azaspirovesamicol derivative FBASV is one out of six novel vesamicol analogs and demonstrated most appropriate in vitrobinding data. 18F‐radiolabeling was performed by microwave‐assisted nucleophilic aromatic substitution of the corresponding nitro precursor and two methods were developed for the purification of [18F]FBASV. Utilizing method A, the remaining nitro precursor was reduced to its corresponding amine, which was separated via semi‐preparative HPLC on a conventional RP column. In method B a phenyl column was used for the direct separation of [18F]FBASV and its nitro precursor, resulting in a change of the elution order and better separation parameters. Thus, [18F]FBASV was synthesized with a RCY of 16–18%, a specific activity >300 GBq/µmol, and a radiochemical purity of >99.5% suitable for future in vivostudies. Copyright © 2011 John Wiley & Sons, Ltd.

The cannabinoid receptor type 2 (CB2R) is an attractive target for the diagnosis and therapy of neurodegenerative diseases and cancer. In this study, we aimed at the development of a novel 18F-labeled radioligand starting from the structure of the known naphthyrid-2-one CB2R ligands. Compound 28(LU13) was identified with the highest binding affinity and selectivity versus CB1R (CB2RKi= 0.6 nM; CB1RKi/CB2RKi> 1000) and was selected for radiolabeling with fluorine-18 and biological characterization. The new radioligand [18F]LU13showed high CB2R affinity in vitro as well as high metabolic stability in vivo. PET imaging with [18F]LU13in a rat model of vector-based/-related hCB2R overexpression in the striatum revealed a high signal-to-background ratio. Thus, [18F]LU13is a novel and highly promising PET radioligand for the imaging of upregulated CB2R expression under pathological conditions in the brain.

Racemic [18F]FBFP ([18F]1) proved to be a potent σ1receptor radiotracer with superior imaging properties. The pure enantiomers of unlabeled compounds (S)- and (R)-1and the corresponding iodonium ylide precursors were synthesized and characterized. The two enantiomers (S)-1and (R)-1exhibited comparable high affinity for σ1receptors and selectivity over σ2receptors. The Ca2+fluorescence assay indicated that (R)-1behaved as an antagonist and (S)-1as an agonist for σ1receptors. The 18F-labeled enantiomers (S)- and (R)-[18F]1were obtained in > 99% enantiomeric purity from the corresponding enantiopure iodonium ylide precursors with radiochemical yield of 24.4% ± 2.6% and molar activity of 86 – 214 GBq/μmol. In ICR mice both (S)- and (R)-[18F]1displayed comparable high brain uptake, brain-to-blood ratio, in vivostability and binding specificity in the brain and peripheral organs. In micro-positron emission tomography (PET) imaging studies in rats, (S)-[18F]1exhibited faster clearance from the brain than (R)-[18F]1, indicating different brain kinetics of the two enantiomers. Both (S)- and (R)-[18F]1warrant further evaluation in primates to translate a single enantiomer with more suitable kinetics for imaging the σ1receptors in humans.

The involvement of gamma amino butyric acid (GABA) receptors in a variety of neurological and psychiatric diseases has promoted the development and use of radiolabelled benzodiazepines (BZ) for brain imaging by PET. However, these radioligands are unable to distinguish between the various subtypes of GABAA receptors. Novel non‐BZ such as the pyrazolo‐pyrimidine indiplon proved to be selective for the α1‐subunit of the GABAA receptor. Here, we describe the syntheses of four novel 18F‐labelled indiplon derivatives. Radiosyntheses were performed via n.c.a. 18F‐nucleophilic substitution starting from the tosyl, bromo, and 4‐nitrobenzoyl precursors to obtain fluorine substituted N‐alkylamide side chain derivatives of indiplon, followed by multistep purification using semi‐preparative high‐performance liquid chromatography and solid phase extraction. Tosyl and bromo precursors were converted into 18F‐labelled indiplon derivatives with good and reproducible radiochemical yield (RCY) (35–70%, decay corrected), high radiochemical purity (≥98.5%), and high specific activity ( > 150 GBq/µmol). By contrast, a low RCY (5–10%) and specific activity (10–15 GBq/µmol) were achieved for the 4‐nitrobenzoyl precursor. Copyright © 2008 John Wiley & Sons, Ltd.

The goal of this study is to develop a novel 99mTc‐labeled σ receptor imaging agent. Potassium 4‐(cyclohexylpiperazin‐1‐yl)‐dithioformate, 2, and the corresponding rhenium complex, ReN‐2, were synthesized and characterized. ReN‐2 possessed moderate affinity toward σ1 (Ki = 1.94 ± 0.60 µmol/L) and σ2 (Ki = 2.83 ± 1.39 µmol/L) receptors. The radiolabeled complex 99mTcN‐2 was prepared in high yield (> 95%) through the [99mTcN]int2+ precursor and characterized by HPLC. 99mTcN‐2 was found to be a lipophilic and neutral complex with good stability. The biodistribution in tumor‐bearing mice showed that 99mTcN‐2 had good tumor uptake (2.12 ± 0.01 %ID/g at 2 h p.i.) and moderate brain uptake (0.27 ± 0.05 %ID/g at 2 h p.i.). After blocking with haloperidol, the uptakes by tumor and brain were lower than control. The results indicated that the complex has specific binding to the σ receptors in vivo. Further structural modifications of this complex are needed to obtain 99mTc‐based σ receptor imaging agents with high affinity and subtype selectivity. Copyright © 2007 John Wiley & Sons, Ltd.

The development of renal interstitial fibrosis (RIF) is related to the expression and excretion of cytokines and growth factors. Thus, we investigated the time course of mRNA expression of cytokines known as causative factors in a model of RIF in rats before and on day 10 after unilateral ureteral obstruction (UUO), when first signs of fibrosis were visible, as well as during progressive RIF. UUO causes a fivefold increase in mRNA expression of monocyte chemoattractant protein 1 15 days after surgery as compared with contralateral kidneys. The level remains elevated about three-fold up to day 25. The mRNA of the fibrogenic cytokine transforming growth factor beta 1 (TGF-β1) is increased two- to threefold during the time course, whereas the mRNAs of platelet-derived growth factor B chain (PDGF-B) and its receptor beta (PDGF-Rβ) increase after UUO, reaching their maxima on days 10–15. PDGF-B mRNA increase up to day 15, marking the onset of fibrosis, and decreases thereafter, whereas the expression of the PDGF-Rβ mRNA remains elevated more than threefold over the entire study period. Incubation of cultured renal fibroblasts with TGF-β1 and/or PDGF-B suggests that their specific action on cell growth and proliferation is maintained even when they are used in combination. The sustained elevation of TGF-β1 and PDGF-B/PDGF-Rβ mRNA levels confirms the assumption of a particular involvement of these cytokines in the pathogenesis of RIF. The mRNA expression of the gap junctional protein connexin 43 in ureteral ligated kidneys is increased sixfold already 5 days after UUO. In this way, the increased connexin 43 mRNA levels indicate a possible function in the remodeling of the kidney tissue after tubular damage and fibrosis.