Synthesis, pharmacological activity and structure affinity relationships of spirocyclic σ(1) receptor ligands with a (2-fluoroethyl) residue in 3-position

[18F]Fluspidine-A PET Tracer for Imaging of σ1 Receptors in the Central Nervous System

σ1 receptors play a crucial role in various neurological and neurodegenerative diseases including pain, psychosis, Alzheimer's disease, and depression. Spirocyclic piperidines represent a promising class of potent σ1 receptor ligands. The relationship between structural modifications and σ1 receptor affinity and selectivity over σ2 receptors led to the 2-fluoroethyl derivative fluspidine (2, Ki = 0.59 nM). Enantiomerically pure (S)-configured fluspidine ((S)-2) was prepared by the enantioselective reduction of the α,β-unsaturated ester 23 with NaBH4 and the enantiomerically pure co-catalyst (S,S)-24. The pharmacokinetic properties of both fluspidine enantiomers (R)-2 and (S)-2 were analyzed in vitro. Molecular dynamics simulations revealed very similar interactions of both fluspidine enantiomers with the σ1 receptor protein, with a strong ionic interaction between the protonated amino moiety of the piperidine ring and the COO- moiety of glutamate 172. The 18F-labeled radiotracers (S)-[18F]2 and (R)-[18F]2 were synthesized in automated syntheses using a TRACERlab FX FN synthesis module. High radiochemical yields and radiochemical purity were achieved. Radiometabolites were not found in the brains of mice, piglets, and rhesus monkeys. While both enantiomers revealed similar initial brain uptake, the slow washout of (R)-[18F]2 indicated a kind of irreversible binding. In the first clinical trial, (S)-[18F]2 was used to visualize σ1 receptors in the brains of patients with major depressive disorder (MDD). This study revealed an increased density of σ1 receptors in cortico-striato-(para)limbic brain regions of MDD patients. The increased density of σ1 receptors correlated with the severity of the depressive symptoms. In an occupancy study with the PET tracer (S)-[18F]2, the selective binding of pridopidine at σ1 receptors in the brain of healthy volunteers and HD patients was shown.

Emerging Trends in Copper-Promoted Radical-Involved C-O Bond Formations

The C-O bond is ubiquitous in biologically active molecules, pharmaceutical agents, and functional materials, thereby making it an important functional group. Consequently, the development of C-O bond-forming reactions using catalytic strategies has become an increasingly important research topic in organic synthesis because more conventional methods involving strong base and acid have many limitations. In contrast to the ionic-pathway-based methods, copper-promoted radical-mediated C-O bond formation is experiencing a surge in research interest owing to a renaissance in free-radical chemistry and photoredox catalysis. This Perspective highlights and appraises state-of-the-art techniques in this burgeoning research field. The contents are organized according to the different reaction types and working models.

Propellanes as Rigid Scaffolds for the Stereodefined Attachment of σ-Pharmacophoric Structural Elements to Achieve σ Affinity

Following the concept of conformationally restriction of ligands to achieve high receptor affinity, we exploited the propellane system as rigid scaffold allowing the stereodefined attachment of various substituents. Three types of ligands were designed, synthesized and pharmacologically evaluated as σ₁ receptor ligands. Propellanes with (1) a 2-methoxy-5-methylphenylcarbamate group at the "left" five-membered ring and various amino groups on the "right" side; (2) benzylamino or analogous amino moieties on the "right" side and various substituents at the left five-membered ring and (3) various urea derivatives at one five-membered ring were investigated. The benzylamino substituted carbamate syn,syn-4a showed the highest σ₁ affinity within the group of four stereoisomers emphasizing the importance of the stereochemistry. The cyclohexylmethylamine 18 without further substituents at the propellane scaffold revealed unexpectedly high σ₁ affinity (K_i = 34 nM) confirming the relevance of the bioisosteric replacement of the benzylamino moiety by the cyclohexylmethylamino moiety. Reduction of the distance between the basic amino moiety and the "left" hydrophobic region by incorporation of the amino moiety into the propellane scaffold resulted in azapropellanes with particular high σ₁ affinity. As shown for the propellanamine 18, removal of the carbamate moiety increased the σ₁ affinity of 9a (K_i = 17 nM) considerably. Replacement of the basic amino moiety by H-bond forming urea did not lead to potent σ ligands. According to molecular dynamics simulations, both azapropellanes anti-5 and 9a as well as propellane 18 adopt binding poses at the σ₁ receptor, which result in energetic values correlating well with their different σ₁ affinities. The affinity of the ligands is enthalpy driven. The additional interactions of the carbamate moiety of anti-5 with the σ₁ receptor protein cannot compensate the suboptimal orientations of the rigid propellane and its N-benzyl moiety within the σ₁ receptor-binding pocket, which explains the higher σ₁ affinity of the unsubstituted azapropellane 9a.

Oxidative Kinetic Resolution of Cyclic Benzylic Ethers

A manganese-catalyzed oxidative kinetic resolution of cyclic benzylic ethers through asymmetric C(sp³ )-H oxidation is reported. The practical approach is applicable to a wide range of 1,3-dihydroisobenzofurans bearing diverse functional groups and substituent patterns at the α position with extremely efficient enantiodiscrimination. The generality of the strategy was further demonstrated by efficient oxidative kinetic resolution of another type of five-membered cyclic benzylic ether, 2,3-dihydrobenzofurans, and six-membered 6H-benzo[c]chromenes. Direct late-stage oxidative kinetic resolution of bioactive molecules that are otherwise difficult to access was further explored.

Short and Atom-Economic Enantioselective Synthesis of the σ1-Receptor Ligands (S)- and (R)-Fluspidine-Important Tools for Positron Emission Tomography Studies

Aryl bromides 2a and 2b bearing an alkynyl substituent in the o-position reacted with n-butyllithium and 1-benzylpiperidin-4-one in a one-pot Domino reaction to form ester 3 and aldehyde 5, respectively. Enantiomeric alcohols (R)-8 and (S)-8 were obtained by conjugate NaBH4 reduction of α,β-unsaturated ester 3 in the presence of chiral cocomplexes (R,R)-10 and (S,S)-10. Starting from orthoester 2a, the precursors (R)-8 and (S)-8 for the synthesis of fluspidine enantiomers (R)-1/[18F](R)-1 and (S)-1/[18F](S)-1 were obtained in only two reaction steps without additional steps for N-protection in an atom-economic manner in 95.6% ee and 97.2% ee, respectively.

In vitro and in vivo Human Metabolism of (S)-[18F]Fluspidine - A Radioligand for Imaging σ1 Receptors With Positron Emission Tomography (PET)

(S)-[¹⁸F]fluspidine ((S)-[¹⁸F]1) has recently been explored for positron emission tomography (PET) imaging of sigma-1 receptors in humans. In the current report, we have used plasma samples of healthy volunteers to investigate the radiometabolites of (S)-[¹⁸F]1 and elucidate their structures with LC-MS/MS. For the latter purpose additional in vitro studies were conducted by incubation of (S)-[¹⁸F]1 and (S)-1 with human liver microsomes (HLM). In vitro metabolites were characterized by interpretation of MS/MS fragmentation patterns from collision-induced dissociation or by use of reference compounds. Thereby, structures of corresponding radio-HPLC-detected radiometabolites, both in vitro and in vivo (human), could be identified. By incubation with HLM, mainly debenzylation and hydroxylation occurred, beside further mono- and di-oxygenations. The product hydroxylated at the fluoroethyl side chain was glucuronidated. Plasma samples (10, 20, 30 min p.i., n = 5-6), obtained from human subjects receiving 250–300 MBq (S)-[¹⁸F]1 showed 97.2, 95.4, and 91.0% of unchanged radioligand, respectively. In urine samples (90 min p.i.) the fraction of unchanged radioligand was only 2.6% and three major radiometabolites were detected. The one with the highest percentage, also found in plasma, matched the glucuronide formed in vitro. Only a small amount of debenzylated metabolite was detected. In conclusion, our metabolic study, in particular the high fractions of unchanged radioligand in plasma, confirms the suitability of (S)-[¹⁸F]1 as PET radioligand for sigma-1 receptor imaging.

Synthesis of Phthalans Via Copper-Catalyzed Enantioselective Cyclization/Carboetherification of 2-Vinylbenzyl Alcohols

Enantiomerically enriched phthalans were synthesized efficiently via an enantioselective copper-catalyzed alkene carboetherification reaction. In this reaction, 2-vinylbenzyl alcohols enantioselectively cyclize then couple with vinylarenes. The utility of the method was demonstrated by the enantioselective synthesis of ( R)-fluspidine, a σ₁ receptor ligand.

Sigma-2 receptor: past, present and perspectives on multiple therapeutic exploitations

Identification of sigma-2 receptor (sig-2R) has been controversial. Nevertheless, interest in sig-2R is high for its overexpression in tumors and potentials in oncology. Additionally, sig-2R antagonists inhibit Aβ binding at neurons, blocking the cognitive impairments of Alzheimer's disease. The most representative classes of sig-2R ligands are herein treated with focus on compounds that served to study sig-2R biology and to produce sig-2R: fluorescent ligands; multifunctional anticancer agents; and targeting nanoparticles. Although fluorescent ligands serve as ‘green’ pharmacological tools, sig-2R-multifunctional conjugates and sig-2R-targeted nanoparticles show how sig-2R targeting increases the activity of anticancer drugs in tumors with reduced toxicity. Altogether, this review draws a picture of the multiple approaches of sig-2R ligands in cancer therapy and as Alzheimer's disease modifying disease agents.

Bridging from Brain to Tumor Imaging: (S)-(-)- and (R)-(+)-[18F]Fluspidine for Investigation of Sigma-1 Receptors in Tumor-Bearing Mice

Sigma-1 receptors (Sig1R) are highly expressed in various human cancer cells and hence imaging of this target with positron emission tomography (PET) can contribute to a better understanding of tumor pathophysiology and support the development of antineoplastic drugs. Two Sig1R-specific radiolabeled enantiomers (S)-(-)- and (R)-(+)-[18F]fluspidine were investigated in several tumor cell lines including melanoma, squamous cell/epidermoid carcinoma, prostate carcinoma, and glioblastoma. Dynamic PET scans were performed in mice to investigate the suitability of both radiotracers for tumor imaging. The Sig1R expression in the respective tumors was confirmed by Western blot. Rather low radiotracer uptake was found in heterotopically (subcutaneously) implanted tumors. Therefore, a brain tumor model (U87-MG) with orthotopic implantation was chosen to investigate the suitability of the two Sig1R radiotracers for brain tumor imaging. High tumor uptake as well as a favorable tumor-to-background ratio was found. These results suggest that Sig1R PET imaging of brain tumors with [18F]fluspidine could be possible. Further studies with this tumor model will be performed to confirm specific binding and the integrity of the blood-brain barrier (BBB).

Solid-phase organic synthesis of chiral, non-racemic 1,2,4-trisubstituted 1,4-diazepanes with high σ1 receptor affinity

The aim of this work was to transfer the established chiral-pool synthesis of 1,2,4-trisubstituted 1,4-diazepanes in solution on the solid phase. For this purpose, (S)-configured amino acids, (S)-alanine, and (S)-leucine, with a small methyl and a larger isobutyl moiety were attached to the solid support 9 by reductive amination. After five reaction steps on the solid support, the 1,4-diazepanes (S)-19a,b were cleaved off and reductively alkylated to afford the 1,2,4-trisubstituted 1,4-diazepanes (S)-20a and (S)-21b, respectively. Both compounds show high σ1 affinity and selectivity over the σ2 subtype.

Thiazole-Based σ1 Receptor Ligands: Diversity by Late-Stage C-H Arylation of Thiazoles, Structure-Affinity and Selectivity Relationships, and Molecular Interactions

Spirocyclic thiophene derivatives represent promising σ1 ligands with high σ1 affinity and selectivity over the σ2 subtype. To increase ligand efficiency, the thiophene ring was replaced bioisosterically by a thiazole ring, and the pyran ring was opened. Late-stage diversification by regioselective C-H arylation of thiazoles 9 a-c resulted in a set of 53 compounds with high diversity. This set of compounds was analyzed with respect to σ1 affinity, σ1 /σ2 selectivity, lipophilicity (logD7.4 ), lipophilicity-corrected ligand efficiency (LELP), and molecular target interactions. The most promising candidates were pyridyl-substituted thiazole derivatives 33 c (2-(1-benzyl-4-ethoxypiperidin-4-yl)-5-(pyridin-3-yl)thiazole) and 34 c (2-(1-benzyl-4-ethoxypiperidin-4-yl)-5-(pyridin-4-yl)thiazole), possessing low-nanomolar σ1 affinity (Ki =1.3 and 1.9 nm), high σ1 /σ2 selectivity (>1500-fold), low lipophilicity (logD7.4 =1.8) and very good ligand efficiency (LELP=5.5), indicating promising pharmacodynamics and pharmacokinetics. Molecular simulation studies, including docking and deconvolution of the free binding energy into its major components, led to decreased hydrophobic stabilization of pyridyl derivatives 33 c and 34 c, which was compensated by lower desolvation energy.

Fluorinated PET Tracers for Molecular Imaging of σ1 Receptors in the Central Nervous System

At first the role of σ1 receptors in various neurological, psychiatric and neurodegenerative disorders is discussed. In the second part, the principle of positron emission tomography (PET ) is described and the known fluorinated PET tracers for labeling of σ1 receptors are presented. The third part focuses on fluoroalkyl substituted spirocyclic PET tracers, which represent the most promising class of fluorinated PET tracers reported so far. The homologous fluoroalkyl derivatives 12-15 show high σ1 affinity (K i = 0.59-1.4 nM) and high selectivity over the σ2 subtype (408-1331-fold). The enantiomers of the fluoroethyl derivative fluspidine 13 were prepared and pharmacologically characterized. Whereas the (S)-configured enantiomer (S)-13 (K i = 2.3 nM) is 4-fold less active than the (R)-enantiomer (R)-13 (K i = 0.57 nM), (S)-13 is metabolically more stable. The interactions of (S)-13 and (R)-13 with the σ1 receptor were analyzed at the molecular level using the 3D homology model. In an automated radiosynthesis [18F](S)-13 and [18F](R)-13 were prepared by nucleophilic substitution of the tosylates (S)-17 and (R)-17 with K[18F]F in high radiochemical yield, high radiochemical purity and short reaction time. Application of both enantiomers [18F](S)-13 and [18F](R)-13 to mice and piglets led to fast uptake into the brain, but [18F](R)-13 did not show washout from the brain indicating a quasi-irreversible binding. Both radiotracers [18F](S)-13 and [18F](R)-13 were able to label regions in the mouse and piglet brain with high σ1 receptor density. The specific binding of the enantiomeric tracers [18F](S)-13 and [18F](R)-13 could be replaced by the selective σ1 ligand SA4503.

Medicinal Chemistry of σ1 Receptor Ligands: Pharmacophore Models, Synthesis, Structure Affinity Relationships, and Pharmacological Applications

In the first part of this chapter, we summarize the various pharmacophore models for σ1 receptor ligands. Common to all of them is a basic amine flanked by two hydrophobic regions, representing the pharmacophoric elements. The development of computer-based models like the 3D homology model is described as well as the first crystal structure of the σ1 receptor. The second part focuses on the synthesis and biological properties of different σ1 receptor ligands, identified as 1-9. Monocyclic piperazines 1 and bicyclic piperazines 2 and 3 were developed as cytotoxic compounds, thus the IC50 values of cell growth and survival inhibition studies are given for all derivatives. The mechanism of cell survival inhibition, induction of time-dependent apoptosis, of compound ent-2a is discussed. Experimentally determined σ1 affinity shows good correlation with the results from molecular dynamics simulations based on a 3D homology model. Spirocyclic compounds 4 and 5 represent well-established σ1 receptor ligands. The homologous fluoroalkyl derivatives 4 have favorable pharmacological properties for use as fluorinated PET tracers. The (S)-configured fluoroethyl substituted compound (S)-4b is under investigation as PET tracer for imaging of σ1 receptors in the brain of patients affected by major depression. 1,3-Dioxanes 6c and 6d display a very potent σ1 antagonist profile and the racemic 1,3-dioxane 6c has high anti-allodynic activity at low doses. The arylpropenylamines 7 are very potent σ1 receptor ligands with high σ1/σ2 selectivity. The top compound 7g acts as an agonist as defined by its ability to potentiate neurite outgrowth at low concentrations. Among the morpholinoethoxypyrazoles 8, 8c (known as S1RA) reveals the most promising pharmacokinetic and physicochemical properties. Due to its good safety profile, 8c is currently being investigated in a phase II clinical trial for the treatment of neuropathic pain. The most potent ligand 9e of 3,4-dihydro-2(1H)-quinolones 9 shows promising anti-nociceptive activity in the formalin test.

Comparison of in Silico, Electrochemical, in Vitro and in Vivo Metabolism of a Homologous Series of (Radio)fluorinated σ1 Receptor Ligands Designed for Positron Emission Tomography

The imaging of σ₁ receptors in the brain by fluorinated radiotracers will be used for the validation of σ₁ receptors as drug targets as well as for differential diagnosis of diseases in the central nervous system. The biotransformation of four homologous fluorinated PET tracers 1'-benzyl-3-(ω-fluoromethyl to ω-fluorobutyl)-3H-spiro[2]benzofuran-1,4'-piperidine] ([¹⁸ F]1-4) was investigated. In silico studies using fast metabolizer (FAME) software, electrochemical oxidations, in vitro studies with rat liver microsomes, and in vivo metabolism studies after application of the PET tracers [¹⁸ F]1-4 to mice were performed. Combined liquid chromatography and mass spectrometry (HPLC-MS) analysis allowed structural identification of non-radioactive metabolites. Radio-HPLC and radio-TLC provided information about the presence of unchanged parent radiotracers and their radiometabolites. Radiometabolites were not found in the brain after application of [¹⁸ F]2-4, but liver, plasma, and urine samples contained several radiometabolites. Less than 2 % of the injected dose of [¹⁸ F]4 reached the brain, rendering [¹⁸ F]4 less appropriate as a PET tracer than [¹⁸ F]2 and [¹⁸ F]3. Compounds [¹⁸ F]2 and [¹⁸ F]3 possess the most promising properties for imaging of σ₁ receptors in the brain. High σ₁ affinity (K_i =0.59 nm), low lipophilicity (logD_7.4 =2.57), high brain penetration (4.6 % of injected dose after 30 min), and the absence of radiometabolites in the brain favor the fluoroethyl derivative [¹⁸ F]2 slightly over the fluoropropyl derivative [¹⁸ F]3 for human use.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Distinctive in vivo kinetics of the new σ1 receptor ligands (R)-(+)- and (S)-(-)-18F-fluspidine in porcine brain

Because of their involvement in growth and survival signaling cascades, the σ(1) receptors (σ(1)Rs) represent a novel target for the treatment of cancer and several brain diseases such as depression and neurodegeneration. From a series of σ1R-specific (18)F-fluoroalkylated spirocyclic piperidines, we have chosen (18)F-fluspidine for detailed investigation of the in vivo kinetics of the (R)-(+)- and (S)-(-)-enantiomers to identify their potential for imaging in humans.

METHODS

Enantiopure tosylate precursors for radiolabeling were obtained using chiral preparative high-performance liquid chromatography and used for radiosynthesis of both (18)F-fluspidine enantiomers by nucleophilic substitution with K-(18)F-F-Kryptofix 222-carbonate complex in a synthesis module. Brain pharmacokinetics were investigated by dynamic PET studies in piglets under baseline and blocking conditions using the highly selective σ1R agonist SA4503. Standardized uptake values (SUVs) were calculated for 24 MR-defined brain regions. Total distribution volume (V(T)) and binding potentials (k3'/k4) of (S)-(-)- and (R)-(+)-(18)F-fluspidine were estimated. Furthermore, V(T) values were estimated by graphical analysis using Logan plots.

RESULTS

The (S)- and (R)-tosylates were obtained in excellent enantiomeric purities (>98% and >96% enantiomeric excess, respectively). (S)-(-)- and (R)-(+)-(18)F-fluspidine were synthesized within approximately 70 min (radiochemical yield, 35%-45%; specific activity, 650-870 GBq/μmol; radiochemical purity, >99%). Both radiotracers displayed different brain uptake kinetics. Although the initial brain uptake was similar, the SUV at the end of the study differed significantly (P < 0.05), with (R)-(+)-(18)F-fluspidine showing about 60%-150% higher values. Administration of SA4503 reduced SUV almost equally for both radiotracers by approximately 65%. Furthermore, k(3)' was significantly decreased under blocking conditions in almost all regions ((S)-(-)-(18)F-fluspidine, -90%-95%; (R)-(+)-(18)F-fluspidine, -70%-90%) whereas effects on k(4) differed according to the particular brain region. V(T) estimated by both graphical analysis using Logan plots and full nonlinear kinetic analysis revealed significant inhibition for both radiotracers under blocking conditions.

CONCLUSION

Both (S)-(-)- and (R)-(+)-(18)F-fluspidine appear to be suitable for σ1R imaging in humans. The different pharmacokinetics of (S)-(-)-(18)F-fluspidine and (R)-(+)-(18)F-fluspidine may have the potential for application in the diagnostics of different pathologic conditions.

The development of radiotracers for imaging sigma (σ) receptors in the central nervous system (CNS) using positron emission tomography (PET)

Sigma (σ) receptors are unique mammalian proteins, distributed in the central nervous system and elsewhere, which are increasingly implicated in the pathophysiology of virtually all major central nervous system disorders. The heterogeneous but wide distribution of σ1 in the brain has prompted the development of selective radiotracers for imaging these sites using positron emission tomography (PET). To date, some 50 carbon-11-labelled and fluorine-18-labelled candidate PET radioligands targeting σ receptors have been reported. The historical development of selective σ1 receptor ligands as potential PET imaging agents, as well as the radiochemistry and application of the most recently developed examples, is described herein.

Asymmetric Synthesis of Spirocyclic 2-Benzopyrans for Positron Emission Tomography of σ1 Receptors in the Brain

Sharpless asymmetric dihydroxylation of styrene derivative 6 afforded chiral triols (R)-7 and (S)-7, which were cyclized with tosyl chloride in the presence of Bu₂SnO to provide 2-benzopyrans (R)-4 and (S)-4 with high regioselectivity. The additional hydroxy moiety in the 4-position was exploited for the introduction of various substituents. Williamson ether synthesis and replacement of the Boc protective group with a benzyl moiety led to potent σ₁ ligands with high σ₁/σ₂-selectivity. With exception of the ethoxy derivative 16, the (R)-configured enantiomers represent eutomers with eudismic ratios of up to 29 for the ester (R)-18. The methyl ether (R)-15 represents the most potent σ₁ ligand of this series of compounds, with a K_i value of 1.2 nM and an eudismic ratio of 7. Tosylate (R)-21 was used as precursor for the radiosynthesis of [¹⁸F]-(R)-20, which was available by nucleophilic substitution with K[¹⁸F]F K222 carbonate complex. The radiochemical yield of [¹⁸F]-(R)-20 was 18%–20%, the radiochemical purity greater than 97% and the specific radioactivity 175–300 GBq/µmol. Although radiometabolites were detected in plasma, urine and liver samples, radiometabolites were not found in brain samples. After 30 min, the uptake of the radiotracer in the brain was 3.4% of injected dose per gram of tissue and could be reduced by coadministration of the σ₁ antagonist haloperidol. [¹⁸F]-(R)-20 was able to label those regions of the brain, which were reported to have high density of σ₁ receptors.

A 96-well filtration method for radioligand binding analysis of σ receptor ligands

σ receptors represent a potential drug target for numerous therapeutic indications including cancer, depression, psychostimulant abuse, and stroke. Most published radioligand binding studies for σ receptors utilize a low throughput method employing a "cell harvester." Higher throughput methods are required to facilitate efficient screening of large numbers of novel compounds. In this study, a series of reference compounds was analyzed with a new medium-throughput 96-well filtration method and the results were compared to those obtained using the conventional cell harvester-based method. The 96-well assay utilized rat liver membranes for the determination of both known σ receptor subtypes (σ(1) and σ(2)) because this tissue contains high densities of both subtypes and fulfills criteria required for reliable use with the 96-well format. The new method gave comparable K(i) values for reference ligands analyzed in parallel with samples prepared in rat brain membranes and processed on the traditional cell harvester. For σ(1) receptors, equivalent affinity values were observed for both methods/tissues. For σ(2) receptors, approximately 2-fold higher affinities were observed for most compounds in liver, as compared to brain membranes, but excellent correlation with brain-derived values was maintained. To further demonstrate the utility of the new method it was used to screen a novel series of 2(3H)-benzothiazolone compounds, resulting in the identification of several analogues with nanomolar affinity and greater than 50-fold specificity for σ(1) versus σ(2) receptors.

Characterization of ligand binding to the σ(1) receptor in a human tumor cell line (RPMI 8226) and establishment of a competitive receptor binding assay

The standard assay for the determination of σ(1) receptor affinities of novel compounds is a competitive binding assay using [(3)H]-(+)-pentazocine as radioligand and membrane preparations from guinea pig brain. Herein, a novel competitive binding assay was developed employing the hematopoietic cell line of human multiple myeloma (RPMI 8226), which expresses a large amount of the human σ(1) receptor. Membrane fragments of RPMI 8226 cells were prepared and characterized. A Western blot analysis confirmed the high density of σ(1) receptors in this cell line. Assay conditions were carefully optimized leading to an incubation period of 120 min, an incubation temperature of 37°C, and receptor material for each well was prepared from 300,000 cells. It was shown that a large excess (10 μM) of (+)-pentazocine, haloperidol, and di-o-tolylguanidine provided the same results during determination of the nonspecific binding. Saturation experiments with the radioligand [(3)H]-(+)-pentazocine led to a K(d)-value of 36±0.3 nM and a B(max)-value of 477±7 fmol/mg protein. These data resulted in approximately 122,000 σ(1) binding sites per cell. The assay was validated by using six known σ(1) ligands and eight σ(1) ligands prepared in our lab. The K(i)-values determined with RPMI 8226-derived receptor material are in good accordance with the K(i)-values obtained with guinea pig brain membrane preparations. Compared with guinea pig brain preparations, the RPMI 8226-derived receptor material represents a better standardized receptor material with a high density of human σ(1) receptors.

Molecular imaging of σ receptors: synthesis and evaluation of the potent σ1 selective radioligand [18F]fluspidine

PURPOSE

Neuroimaging of σ(1) 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 (18)F-labelled PET radioligands for that purpose.

METHODS

The selective σ(1) receptor ligand [(18)F]fluspidine (1'-benzyl-3-(2-[(18)F]fluoroethyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]) was synthesized by nucleophilic (18)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 [(18)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 [(18)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(n) and radio-HPLC analysis.

RESULTS

[(18)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(1) receptors (K (i) = 0.59 nM). In mice, [(18)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 [(18)F]fluspidine and the expression of σ(1) receptors was shown. The radiotracer uptake in the brain as well as in peripheral σ(1) 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(n) techniques. However, [(18)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

[(18)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 [(18)F]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of σ(1) receptors in vivo.