Discovery of enantioselective suitability of (R)-(+)- and (S)-(-)-[18F]fluspidine for σ1 receptor imaging


Discovery of enantioselective suitability of (R)-(+)- and (S)-(-)-[18F]fluspidine for σ1 receptor imaging

Deuther-Conrad, W.; Maisonial, A.; Patt, M.; Stittsworth, S.; Becker, G.; Habermann, B.; Holl, K.; Schepmann, D.; Funke, U.; Donat, C. K.; Fischer, S.; Hiller, A.; Wenzel, B.; Steinbach, J.; Wünsch, B.; Lever, S.; Hesse, S.; Sabri, O.; Brust, P.

Objectives: It is widely accepted that σ1 receptors represent a novel biological target for the possible pharmacological treatment of cancer and several brain diseases, e.g. depression and neurodegeneration. From a series of σ1-specific racemic 18F-fluoroalkylated spirocyclic piperidines1-3 we have chosen the superior [18F]fluspidine [1] for detailed investigation of the (R)-(+)- and (S)-(–)-enantiomers (Ki: 0.52 nM; 2.3 nM) to identify their individual potential for disease-related imaging studies in humans.

Methods: The racemic tosylate precursor of [1] was enantioseparated by semi-preparative chiral HPLC on immobilized cellulose-tris-(3,5-dimethylphenyl)-carbamate as stationary phase. Automated radiosynthesis of (R)- and (S)-[1] was accomplished by nucleophilic substitution using K[18F]F-Kryptofix 222-carbonate complex (Figure 1) and organ distribution studies were performed in CD-1 mice to obtain organ-specific time activity data (%ID/g). Brain pharmacokinetics of the two enantiomers was confirmed by dynamic PET studies in pigs. Additionally, the highly selective σ1 receptor agonist SA4503 was used in blocking studies to assess target specificity. SUV values were calculated for 24 MR-defined brain regions. Using a metabolite-corrected plasma input function compartment modelling was applied to estimate the distribution volume of both enantiomers.

Figure 2. PET time-activity curves for (S)- and (R)-[1] in pig brain (baseline and blocking conditions)

Results: Enantiomerically pure (R)- and (S)-tosylate precursors were obtained with high enantiomeric excess of >98 % and >96 %, respectively. (R)- and (S)-[1] were synthesized within ~70 min with RCY of 35-45% (EOS), RCP of >99%, and As of 650 and 870 GBq/µmol, respectively. In mice, both radiotracers readily passed the blood-brain barrier. However, large differences in brain pharmacokinetics of the two enantiomers were found with continuous increase of brain uptake of (R)-[1] (3.57 %ID/g at 5’, 6.01% ID/g at 240’ p.i.) in comparison to (S)-[1] with higher initial brain uptake (4.35 %ID/g at 5’ p.i.) and rapid clearance (1.04% ID/g at 240’ p.i.). Dynamic PET studies in pigs confirmed these enantiomer-related differences in brain uptake kinetics. Under baseline conditions, the initial uptake of (S)-[1] in brain was higher than that of (R)-[1] (e.g. SUVmax, Cerebellum ~3.4 vs. ~2.9). Clearance of (S)-[1] from brain was fast (SUVCerebellum ~1.1 at 95-120’ p.i.) whereas the uptake of (R)-[1] remained close to the initial level (SUVCerebellum ~2.5 at 95-120’ p.i.). In comparison to baseline data, application of σ1 specific SA4503 reduced the uptake of (S)- and (R)-[1] in the target region cerebellum by initially 40% and 15% (SUVmax ~2.0 and ~2.5, respectively) and at later measurement periods significantly by ~80% (SUV ~0.2 and ~0.6 at 95-120’ p.i., respectively). Washout kinetics and SUV values determined under blocking conditions indicate both target specificity of the binding as well as minor nonspecific binding of the two radiotracers.

Conclusions: We successfully developed and validated an automated synthesis of the two enantiomers of [1]. The pharmacokinetics of (S)-[1] as investigated in two different animal models suggests that this radiotracer is most suitable for upcoming studies of depression-related changes in receptor expression in human brain. The irreversible-like binding behaviour of (R)-[1] may have advantages for tumor imaging.
Acknowledgements: Supported by DFG (STE 601/10-2, WU 176/7-2) and NIH (T32 EB004822).
References: [1] Fischer S, et al (2011) Eur J Nucl Med Mol Imaging, 38, 540-551. [2] Maisonial A, et al (2011) ChemMedChem, 6, 1401-1410. [3] Maisonial A, et al (2012) Bioorg Med Chem, 20, 257-269

  • Lecture (Conference)
    The 20th International Symposium on Radiopharmaceutical Sciences - ISRS2013, 12.-17.05.2013, Jeju, South Korea
  • Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 56(2013)1, S55-S55

Permalink: https://www.hzdr.de/publications/Publ-18204
Publ.-Id: 18204