Comparative investigation of the novel radiotracers (R)- and (S)-[¹⁸F]fluspidine for imaging of σ₁ receptors by PET

Objectives
Sigma1 receptor (σ₁), found in plasma, mitochondrial and endoplasmic reticular membranes and involved in many cellular functions of the endocrine, immune, and nervous systems, is a promising target for novel concepts in diagnostics and therapy of cancer and brain diseases. To support such research by molecular imaging of σ₁, we have comparatively evaluated the ¹⁸F-labelled (R)- and (S)-enantiomers of the σ₁ ligand fluspidine in pigs.

Methods
(R)- and (S)-[¹⁸F]fluspidine were synthesized from enantiopure tosylate precursors with high specific activities (650-870 GBq/µmol) and radiochemical purity (>99%). Brain pharmacokinetics were investigated by dynamic PET studies in anaesthetized pigs under baseline (n=3 per enantiomer) and blocking conditions (n=3 per enantiomer) using σ₁ selective SA4503. SUVs were calculated for 24 MR-defined brain regions. One- and two-tissue compartment modeling was applied to calculate individual rate constants, total distribution volumes (V_T) and binding potentials of (R)- and (S)-[¹⁸F]fluspidine. Furthermore, V_T was estimated from graphical Logan plot analyses. Organ doses (ODs) and effective doses (ED) of both radiotracers were determined in CD-1 mice and an extended single dose toxicity study of (S)-fluspidine was performed in SPRD rats.

Results
(R)- and (S)-[¹⁸F]fluspidine displayed significantly different brain uptake kinetics. Although initial SUVs were similar (R)-[¹⁸F]fluspidine showed higher values (60-150%) at 120 min p.i. Brain distribution in pigs correlated with brain distribution in CD-1 mice (Fig. 1). SA4503 reduced SUVs and k₃ of both radiotracers by ~65% and 70-95%, respectively, in almost all brain regions (Fig. 1B, C), while effects on k₄ differed region-specific. V_T values estimated by full nonlinear kinetic and Logan analyses correlated highly and revealed also a significant decrease in all brain regions for both radiotracers by SA4503. The expected EDs of (R)- and (S)-[¹⁸F]fluspidine to humans are 31 and 28 µSv/MBq, respectively, and up to 620 µg (S)-fluspidine/kg no test item related effects were observed in the toxicity study.

Conclusion
Both [¹⁸F]fluspidine enantiomers appear to be suitable for σ₁ imaging in human brain. The different pharmacokinetics of (R)-[¹⁸F]fluspidine and (S)-[¹⁸F]fluspidine may be translated into diagnostic imaging of different pathological conditions such as degenerative as well as cancerous processes.

Fig. 1: Distribution of [¹⁸F]fluspidine binding sites in brain. (A) Ex vivo autoradiography of a sagittal rat brain slice obtained at 45 min after intravenous injection of [¹⁸F]fluspidine. (B, C) Sagittal brain PET images acquired as summed data from 0-20 min after intravenous injection of piglet with (S)-[¹⁸F]fluspidine under baseline conditions (B) or under blocking with SA4503 (C).