Molecular imaging of σ receptors: synthesis and evaluation of the potent σ1 selective radioligand [18F]fluspidine
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Neuroimaging of σ1 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 18F-labelled PET radioligands for that purpose.
The selective σ1 receptor ligand [18F]fluspidine (1′-benzyl-3-(2-[18F]fluoroethyl)-3H-spiro[benzofuran-1,4′-piperidine]) was synthesized by nucleophilic 18F− 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 [18F]fluspidine were evaluated by ex vivo brain section imaging and organ distribution of intravenously administered radiotracer. Target specificity was validated by organ distribution of [18F]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-MSn and radio-HPLC analysis.
[18F]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 σ1 receptors (Ki = 0.59 nM). In mice, [18F]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 [18F]fluspidine and the expression of σ1 receptors was shown. The radiotracer uptake in the brain as well as in peripheral σ1 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-MSn techniques. However, [18F]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.
[18F]Fluspidine demonstrated favourable target affinity and specificity as well as metabolic stability both in vitro and in animal experiments. The in vivo properties of [18F]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of σ1 receptors in vivo.
KeywordsSigma receptors PET Neurology Oncology Spirobenzofuran Metabolism LC-MSn
- 4.Kitaichi K, Chabot JG, Moebius FF, Flandorfer A, Glossmann H, Quirion R. Expression of the purported sigma(1)(σ1) receptor in the mammalian brain and its possible relevance in deficits induced by antagonism of the NMDA receptor complex as revealed using an antisense strategy. J Chem Neuroanat 2000;20:375–87.CrossRefPubMedGoogle Scholar
- 10.Ishikawa M, Hashimoto K. The role of sigma-1 receptors in the pathophysiology of neuropsychiatric diseases. J Receptor Ligand Channel Res 2010;3:25–36.Google Scholar
- 21.Silve S, Dupuy PH, Labit-Lebouteiller C, Kaghad M, Chalon P, Rahier A, et al. Emopamil-binding protein, a mammalian protein that binds a series of structurally diverse neuroprotective agents, exhibits delta8-delta7 sterol isomerase activity in yeast. J Biol Chem 1996;271:22434–40.CrossRefPubMedGoogle Scholar
- 37.Berardi F, Ferorelli S, Colabufo NA, Leopoldo M, Perrone R, Tortorella V. A multireceptorial binding reinvestigation on an extended class of sigma ligands: N-[omega-(indan-1-yl and tetralin-1-yl)alkyl] derivatives of 3,3-dimethylpiperidine reveal high affinities towards sigma1 and EBP sites. Bioorg Med Chem 2001;9:1325–35.CrossRefPubMedGoogle Scholar
- 39.Große Maestrup E, Wiese C, Schepmann D, Brust P, Wünsch B. Synthesis, pharmacological activity and structure-affinity relationships of spirocyclic σ1 receptor ligands with a (2-fluoroethyl) residue in 3-position. Bioorg Med Chem (in press).Google Scholar
- 56.Junien JL, Su TP. Sigma receptors in the central nervous system and the periphery. In: Itzhak Y, Bowen WD, editors. Sigma receptors. New York: Academic; 1994. p. 21–44.Google Scholar
- 58.Hirata M, Mori T, Soga S, Umeda T, Ohmomo Y. In vivo evaluation of radioiodinated 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)-piperazine derivatives as new ligands for sigma receptor imaging using single photon emission computed tomography. Biol Pharm Bull 2006;29:2009–15.CrossRefPubMedGoogle Scholar
- 60.Parkinson A, Kazmi F, Buckley DB, Yerino P, Ogilvie BW, Paris BL. System-dependent outcomes during the evaluation of drug candidates as inhibitors of cytochrome P450 (CYP) and uridine diphosphate glucuronosyltransferase (UGT) enzymes: human hepatocytes versus liver microsomes versus recombinant enzymes. Drug Metab Pharmacokinet 2010;25:16–27.CrossRefPubMedGoogle Scholar