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

Neuroimaging of sigma(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 F-18-labelled PET radioligands for that purpose. The selective sigma(1) receptor ligand [F-18]fluspidine (1'-benzyl-3-(2-[F-18]fluoroethyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]) was synthesized by nucleophilic F-18(-) 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-18]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-18]fluspidine after treatment with 1 mg/kg i.p. of the sigma 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. [F-18]Fluspidine was obtained with a radiochemical yield of 35-45%, a radiochemical purity of a parts per thousand yenaEuro parts per thousand 99.6% and a specific activity of 150-350 GBq/mu mol (n = 6) within a total synthesis time of 90-120 min. In vitro, fluspidine bound specifically and with high affinity to sigma(1) receptors (K (i) = 0.59 nM). In mice, [F-18]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-18]fluspidine and the expression of sigma(1) receptors was shown. The radiotracer uptake in the brain as well as in peripheral sigma(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-MSn techniques. However, [F-18]fluspidine showed a higher metabolic stability in vivo. In plasma samples similar to aEuro parts per thousand 94% of parent compound remained at 30 min and similar to aEuro parts per thousand 67% at 60 min post-injection. Only one major radiometabolite was detected. None of the radiometabolites crossed the blood-brain barrier. [F-18]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-18]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of sigma(1) receptors in vivo.