New positron emission tomography (PET) radioligand for imaging σ-1 receptors in living subjects

Abstract

σ-1 receptor (S1R) radioligands have the potential to detect and monitor various neurological diseases. Herein we report the synthesis, radiofluorination, and evaluation of a new S1R ligand 6-(3-fluoropropyl)-3-(2-(azepan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one ([(18)F]FTC-146, [(18)F]13). [(18)F]13 was synthesized by nucleophilic fluorination, affording a product with >99% radiochemical purity (RCP) and specific activity (SA) of 2.6 ± 1.2 Ci/μmol (n = 13) at end of synthesis (EOS). Positron emission tomography (PET) and ex vivo autoradiography studies of [(18)F]13 in mice showed high uptake of the radioligand in S1R rich regions of the brain. Pretreatment with 1 mg/kg haloperidol (2), nonradioactive 13, or BD1047 (18) reduced the binding of [(18)F]13 in the brain at 60 min by 80%, 82%, and 81%, respectively, suggesting that [(18)F]13 accumulation in mouse brain represents specific binding to S1Rs. These results indicate that [(18)F]13 is a promising candidate radiotracer for further evaluation as a tool for studying S1Rs in living subjects.

Figures

Figure 1

Selected Sigma-1 receptor (S1R) ligands and radioligands.

Figure 2

Cell uptake results for A) [18F]13 and B) [3H]1. Uptake of [18F]13 and [3H]1 was determined in control CHO cells and CHO cells transfected with Sigma-1 receptor (S1R) cDNA following incubation for either 30 or 120 min. Results are expressed as counts per minute (CPM) recorded in a sample from a particular well/CPM recorded in medium/amount of protein (μg) present in a sample from that well.

Figure 3

Western blot analysis of Sigma-1 receptor (S1R) expression in control CHO cells, CHO cells transfected with S1R cDNA, and a positive control cell line (JAR cells) known to contain S1R protein. Cell lysates (50 μg of protein) were subjected to gel electrophoresis followed by immunoblot analysis with S1R specific antibody S 18 (400:1). Lane 1: control CHO cells (transfected with empty S1R vector); lane 2: CHO cells over-expressing S1R (transfected with vector containing S1R cDNA); and lane 3: positive control cell lysate for S1R as supplied by Santa Cruz Biotech (JAR cells). Blot was also stained for α-tubulin as a protein loading control.

Figure 4

Time activity curves (TACs) from mouse positron emission tomography (PET) studies. TACs represent accumulation of [18F]13 in whole mouse brain as a function of time for baseline (n=3), pre block with 2 (n=3), pre block with 13 (n=3), and pre block with 18 (n=3). Baseline studies involved iv administration of [18F]13 (95-125 μCi), whereas blocking studies involved pre-treatment of mice with either 2 (1 mg/kg), 13 (1 mg/kg) or 18 (1 mg/kg) 10 min prior to iv administration of [18F]13 (95-125 μCi).

Figure 5

Sagittal mouse PET images and ex vivo autoradiography of sagittal brain sections (12 μm) obtained 60 min post administration of [18F]13. Sections used for autoradiography were stained with nissl for anatomical correlation (right). Baseline study involved injection of radiotracer only ([18F]13, 200 μCi), whereas the blocked study involved pre-treatment with known Sigma-1 receptor ligand 18 (1 mg/kg) 10 min prior to radiotracer administration. PET images (5 min static scans) were acquired just prior to perfusing mice and harvesting brain tissue for autoradiography. White dotted lines indicate location of mouse brain in sagittal PET images. Cb = Cerebellum, Ctx = Cortex, FN = Facial Nucleus, H = Hippocampus, Mb = Midbrain, Ob = Olfactory Bulb.

Figure 6

Graph depicting the percentage of intact radiotracer ([18F]13) in mouse blood, liver, and brain at 30 and 60 min post injection.

Scheme 1

Scheme 2

Radiosynthesis of [18F]13