18F-Labelled Alkyl-Substituted Spirocyclic Piperidines – Potential Radiotracers For PET Imaging Of σ1 Receptors

Objectives: Neuroprotective effects mediated by signal transduction via the transmembrane σ1 receptor localised in the endoplasmatic reticulum make this receptor a promising target for novel approaches in the therapy of neurodegenerative diseases. Furthermore, behavioural changes are assumed to be related to alterations in the expression of σ1 receptors mainly expressed in the striatum. Thus, molecular imaging of σ1 receptors of the brain may hold potential in diagnostics and drug development, and we have compared in mice radiotracer properties of a series of new 18F-labelled spirocyclic piperidine derivatives with high affinity and selectivity for σ1 receptors.

Methods: Radiosynthesis of fluoromethyl- ([18F]WMS1850), fluoroethyl- ([18F]fluspidine), fluoropropyl- ([18F]WMS1813), and fluorobutyl-([18F]WMS1847) substituted derivatives was performed by nucleophilic substitution of the corresponding tosylate precursors using K[18F]F-K222-carbonate complex. Organ distribution of radiotracers applied i.v. was determined in female CD-1 mice at 5, 30, 60, and 120 min p.i. Spatial distribution of the radiotracer binding sites was examined by ex vivo brain autoradiography at 45 min p.i. Target specificity was investigated in blocking studies with pre-application of 1 mg/kg of the σ1 receptor ligand haloperidol by assessing the organ distribution of the respective radiotracer at 60 min p.i. The metabolic stability in vivo of each radiotracer was evaluated by radio-TLC and -HPLC analyses of brain, plasma, and urine samples.

Results: The radiotracers were obtained with radiochemical yields of 35-53%, radiochemical purities >98.5%, and specific activities >150 GBq/µmol. All radiotracers readily passed the blood-brain barrier with high brain uptake values at 30 min p.i.: [18F]fluspidine = 4.71 ± 1.39 % ID/g, [18F]WMS1813 = 3.18 ± 0.68 % ID/g, [18F]WMS1850 = 2.65 ± 0.68 %ID/g, and [18F]WMS1847 = 1.78 ± 0.16 %ID/g. High initial radioactivity uptake was also observed in peripheral organs which express σ1 receptors such as spleen, thymus, kidney, and stomach. In brain as well as in these organs the uptake of radioactivity was significantly reduced in mice pre-treated with haloperidol. Distribution patterns of the radiotracer binding sites in brain were resembling for all four radiotracers with [18F]fluspidine possessing the highest target (facial nucleus)-to-nontarget (olfactory bulb) ratio (4.69 at 45 min p.i.). The metabolic stability in vivo was high for all radiotracers (75% parent radiotracer in plasma at 30 min p.i.), and none of the peripherally detected radiometabolites crossed the blood-brain barrier.

Conclusion: Fluoroalkylated spirocyclic piperidines are high affinity ligands for σ1 receptors with high brain uptake, specific binding, and good metabolic stability. Within the herein reported series of 18F-labelled derivatives, the in vivo data identify [18F]fluspidine as the most suitable radiotracer for further development in molecular imaging of σ1 receptors. [18F]fluspidine radiosynthesis is selected for transfer to an automated radiosynthesis module for further preclinical development.