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

Purpose 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.

Methods The selective σ1 receptor ligand [¹⁸F]fluspidine (1'-Benzyl-3-(2-[¹⁸F]fluoroethyl)-3Hspiro[[2]benzofuran-1,4'-piperidine]) was synthesised 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.

Results [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 60min p.i. Only one major radiometabolite was detected. None of the radiometabolites did cross the blood-brain-barrier.
Conclusion [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.