Synthesis, radiolabeling, in vitro and in vivo evaluation of [¹⁸F]-FPECMO as a positron emission tomography radioligand for imaging the metabotropic glutamate receptor subtype 5

Abstract 

Introduction

[¹⁸F]-(E)-3-((6-Fluoropyridin-2-yl)ethynyl)cyclohex-2-enone O-methyl oxime ([¹⁸F]-FPECMO) is a novel derivative of [¹¹C]-ABP688. [¹⁸F]-FPECMO was characterized as a PET imaging agent for the metabotropic glutamate receptor subtype 5 (mGluR5).

Methods

[¹⁸F]-FPECMO was synthesized in a one-step reaction sequence by reacting [¹⁸F]-KF-K₂₂₂ complex with (E)-3-((6-bromopyridin-2-yl)ethynyl)cyclohex-2-enone O-methyl oxime in dry DMSO. The in vitro affinity of FPECMO was determined by displacement assays using rat whole brain homogenates (without cerebellum) and the mGluR5-specific radioligand [³H]-M-MPEP. Further in vitro characterization involved metabolite studies, lipophilicity determination and autoradiographical analyses of brain slices. In vivo evaluation was performed by postmortem biodistribution studies and PET experiments using Sprague-Dawley rats.

Results

The radiochemical yield after semipreparative HPLC was 35±7% and specific activity was >240 GBq/μmol. [¹⁸F]-FPECMO exhibited optimal lipophilicity (logD=2.1) and high metabolic stability in vitro. Displacement studies revealed a K_i value of 3.6±0.7 nM for FPECMO. Biodistribution studies and ex vivo autoradiography showed highest radioactivity accumulation in mGluR5-rich brain regions such as the striatum and hippocampus. Co-injection of [¹⁸F]-FPECMO and ABP688 (1 mg/kg body weight), an mGluR5 antagonist, showed 40% specific binding in the striatum, hippocampus and cortex, regions known to contain high densities of the mGluR5. PET imaging, however, did not allow the visualization of mGluR5-rich brain regions in the rat brain due to a fast washout of [¹⁸F]-FPECMO from mGluR5-expressing tissues and rapid defluorination.

Conclusions

[¹⁸F]-FPECMO showed significant potential for the detection of mGluR5 in vitro; however, its in vivo characteristics are not optimal for a clear-cut visualization of the mGluR5 in rats.

Keywords: Positron emission tomography, Metabotropic glutamate receptor subtype 5, FPECMO, Radiosynthesis