1-(4-[18F]Fluorobenzyl)-4-((tetrahydrofuran-2-yl)methyl)piperazine: A novel radiotracer for mapping sigma-1 receptors in the living brain

Objectives: The sigma-1 receptors (Sig-1R) represent a distinct class of intracellular “ligand-operated receptor chaperones”.1 Increasing evidence suggests that the sigma-1 receptors are involved in various human diseases including depression, schizophrenia, drug addiction, Alzheimer’s disease, and neuroinflammation.2 Herein we report the design, synthesis and biological evaluation of 1-(4-[18F]fluorobenzyl)-4-((tetrahydrofuran-2-yl)methyl)piperazine ([18F]1) as a potent PET imaging probe for mapping sigma-1 receptors in the living brain.

Methods: Among a new series of disubstituted piperazine derivatives with binding selectivity for Sig-1R, compound 1 was identified as a candidate for radiolabeling. [18F]1 was synthesized using a one-pot, two-step labeling procedure (Scheme 1). The biological properties of the radioligand were determined in biodistribution and inhibition studies in ICR mice. Effect of P-glycoprotein (P-gp) on brain uptake, and the in vivo metabolic stability of the ligand were also investigated.

Results: In vitro competition binding assays showed that compound 1 exhibited nanomolar affinity for σ1 receptors (Ki (σ1) = 3.70 ± 0.02 nM) and good subtype selectivity (Ki (σ2) = 213.4 ± 13.4 nM; Ki (σ2)/Ki (σ1) = 58). [18F]1 was prepared in 20-30% isolated radiochemical yields with radiochemical purity of >95% and specific activity of 54-86 GBq/μmol (n = 3). The log D value of [18F]1 was determined to be 0.76 ± 0.01. Biodistribution studies in mice revealed high initial brain uptake of [18F]1 with 12.75 ± 0.79 %ID/g at 2 min after injection. The brain-to-blood ratios of [18F]1 were high (22, 21, 20 and 14, respectively, at 15, 30, 60 and 120 min after injection). Administration of the selective Sig-1R ligand SA4503 (5 mol/kg, 0.1 mL, iv) at 5 min prior to injection of [18F]1 significantly reduced the radiotracer uptake in brain (by 79%, 88% and 86%, respectively, at 15, 30 and 60 min after injection) and other organs known to express sigma-1 receptors, suggesting binding specificity of [18F]1 to sigma-1 receptors in vivo. Administration of the P-gp inhibitor cyclosporine A (50 mg/kg, 0.1 mL, iv) before tracer injection slightly increased the uptake of radiotracer both in the brain and the blood at 2 min after injection (saline,0.83 ± 0.07% ID/g in the blood, 10.11 ± 1.0 % ID/g in the brain; cyclosporine A, 0.89 ± 0.03% ID/g in the blood, 10.86 ± 0.69 % ID/g in the brain). But the difference between the control and blocking groups was small and not significant, suggesting that [18F]1 is not a substrate for P-gp. At 30 min after injection, the intact parent tracer [18F]1 accounted for 98% of the radioactivity (n = 2) in the mouse brain, indicating no entry of radioactive metabolites into the brain.

Conclusions: These results suggest that [18F]1 displayed high uptake levels and specific binding in brain. Further investigation is warranted to determine the imaging characteristics of this novel radiotracer, and to assess its potential to image Sig-1R in the living brain.

Acknowledgements: Supported by NSFC (21471019).

References: [1] Hayashi, T.; Su, T.-P. Cell 2007, 131, 596. [2] Maurice, T.; Su, T.-P. Pharmacol. Ther. 2009, 124, 195.