Synthesis and Evaluation of¹⁸F-FE-PEO in Rodents: An¹⁸F-Labeled Full Agonist for Opioid Receptor Imaging

Abstract

We have investigated the opioid receptor (OR) agonist (20R)-4,5-a-epoxy-6-(2-18 F-fluoroethoxy)-3-hydroxy-a,17-dimethyl-a-(2-phenyleth-1-yl)-6,14-ethenomorphinan-7-methanol ( 18 F-FE-PEO) as a candidate OR PET ligand. This tracer is attractive because it combines 18 F labeling, is suited to the slow kinetics of high-affinity ligands, and has agonist binding, which has been shown to be more sensitive to changes in OR occupation than is antagonist binding. Methods: Agonist potency and off-target binding were investigated in vitro, and autoradiographic studies on rat brain sections were used to assess binding patterns. Quantification of the tracer in vivo was investigated using small-animal PET in rats with blood sampling. Results: 18 F-FE-PEO was obtained by direct nucleophilic radiofluorination and subsequent deprotection with a yield of 28% 6 15%, a specific activity of 52-224 MBq/nmol, and a radiochemical purity of more than 97% (90 min from end of bombardment). In vitro studies showed it to be a full agonist ligand, which selectively binds to OR with high affinity, although it is not selective to a single OR subtype (inhibition constant, 0.4-1.6 nM across OR subtypes). Autoradiography binding patterns were consistent with the known distribution of OR, although nondisplaceable signal typically constituted one third of the signal in OR-dense regions. Although metabolites were present in blood (;40% of plasma radioactivity was nonparent 3 h after injection), no significant metabolite fraction was found in brain tissue, aiding PET quantification. A plasma input 2-tissue-compartment model provided good fits to the PET data, and regional distribution volumes from the latter correlated well with those from Logan plot analysis (r 2 5 0.98). The cerebellum had the lowest distribution volume, but the time-activity curve data could not be adequately fitted with a 1-tissue-compartment model. Reference tissue models using the cerebellum as the reference region did not provide good fits to the data, so blood-based kinetic analysis is recommended. Conclusion:As the first 18 F-labeled OR agonist ligand, 18 F-FE-PEO is a useful addition to the existing OR ligand portfolio.

Pharmacological Characterization of Low-to-Moderate Affinity Opioid Receptor Agonists and Brain Imaging with 18F-Labeled Derivatives in Rats

“…Additionally, the time course of degradation of the radiotracers was determined by radio-HPLC analysis of the plasma samples from rats that were taken at different time points after tracer injection. 7 , 13 A nonlinear curve fit of the measured data was applied to the plasma activity curve to determine the metabolite-corrected plasma input function of the radiotracers ( Figure 5 B,F). No polar radiometabolites were detected in brain tissue up to a time point of 45 min p.i.…”

“… 5 − 8 Our approach was motivated by ongoing discussions on the mechanism of displacement of the PET ligand from the OR receptor by endogeneous ligands and drug molecules. 7 − 9 …”

A Survey of Molecular Imaging of Opioid Receptors

“…This procedure gave [ 18 F]FE-PEO ( 17 ) in an isolated preparative yield of 35 ± 8% with a molar activity of 55–130 GBq/μmol. In 2013, a research group of the University of Cambridge [99] investigated [ 18 F]FE-PEO ( 17 ) as a candidate OR PET-ligand, obtained by an automated cGMP-compliant method the [ 18 F]FE-PEO at 28 ± 15% yield and 52–224 GBq/μmol molar activity. In 2014, Schoultz et al [63] reported the synthesis and biological evaluation of three structurally-related 6- O -(2-[ 18 F]fluoroethyl)-6- O -desmethy-orvinols, i.e.…”

Discovery of a Lead Brain‐Penetrating Gonadotropin‐Releasing Hormone Receptor Antagonist with Saturable Binding in Brain

“…Both radiotracers had a radiochemical purity > 95 % at the time of injection. [22] Animal experiments were conducted in accordance with national Norwegian regulations on the use of laboratory animals in research (FOR-2015-06-18-761), decision regarding the use of animals in procedures -FOTS ID 12701, establishment number 016: UiO-Institutt for medisinske basalfag (Domus Medica). Processed by the Norwegian Food Safety Authority (NFSA), 28.…”

“…The Eppendorf vial was centrifuged for further 5 minutes. The supernatant was injected directly into the analytic HPLC [22] Other studies of GnRHÀ R antagonist have shown low brain penetration. [14] However, in our study both tracers enter the brain in a tenfold higher concentration as shown in Figure 1.…”