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. 2012;2(1):14-28.
Epub 2011 Dec 10.

Synthesis, radiolabelling and in vitro and in vivo evaluation of a novel fluorinated ABP688 derivative for the PET imaging of metabotropic glutamate receptor subtype 5

Selena Milicevic Sephton  1 , Patrick Dennler, Dominique S Leutwiler, Linjing Mu, Cindy A Wanger-Baumann, Roger Schibli, Stefanie D Krämer, Simon M Ametamey
Affiliations

Synthesis, radiolabelling and in vitro and in vivo evaluation of a novel fluorinated ABP688 derivative for the PET imaging of metabotropic glutamate receptor subtype 5

Selena Milicevic Sephton et al. Am J Nucl Med Mol Imaging. 2012.

Abstract

(E)-3-(Pyridin-2-ylethynyl)cyclohex-2-enone O-(2-(3-(18)F-fluoropropoxy)ethyl) oxime ([(18)F]-PSS223) was evaluated in vitro and in vivo to establish its potential as a PET tracer for imaging metabotropic glutamate receptor subtype 5 (mGluR5). [(18)F]-PSS223 was obtained in 20% decay corrected radiochemical yield whereas the non-radioactive PSS223 was accomplished in 70% chemical yield in a S(N)2 reaction of common intermediate mesylate 8 with potassium fluoride. The in vitro binding affinity of [(18)F]-PSS223 was measured directly in a Scatchard assay to give K(d) = 3.34 ± 2.05 nM. [(18)F]-PSS223 was stable in PBS and rat plasma but was significantly metabolized by rat liver microsomal enzymes, but to a lesser extent by human liver microsomes. Within 60 min, 90% and 20% of [(18)F]-PSS223 was metabolized by rat and human microsome enzymes, respectively. In vitro autoradiography on horizontal rat brain slices showed heterogeneous distribution of [(18)F]-PSS223 with the highest accumulation in brain regions where mGluR5 is highly expressed (hippocampus, striatum and cortex). Autoradiography in vitro under blockade conditions with ABP688 confirmed the high specificity of [(18)F]-PSS223 for mGluR5. Under the same blocking conditions but using the mGluR1 antagonist, JNJ16259685, no blockade was observed demonstrating the selectivity of [(18)F]-PSS223 for mGluR5 over mGluR1. Despite favourable in vitro properties of [(18)F]-PSS223, a clear-cut visualization of mGluR5-rich brain regions in vivo in rats was not possible mainly due to a fast clearance from the brain and low metabolic stability of [(18)F]-PSS223.

Keywords: PET imaging; [11C]-ABP688; [18F]-FDEGPECO; [18F]-PSS223; autoradiography; mGluR5; microsome enzymes.

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Figures

Figure 1
Figure 1
Structures of MPEP and selected PET tracers for imaging of mGluR5
Figure 2
Figure 2
Syntheses of PSS223 and [18F]-PSS223 from key intermediate 8 via SN2 reaction.
Figure 3
Figure 3
A. Displacement of [3H]-ABP688 by PSS223. [3H]-ABP688 (total 2 nM) binding (B) as a function of the PSS223 concentration. Symbols represent mean±SD of one experiment performed in triplicate. Non-linear fitting (red line) resulted in IC50 14 nM and corresponding Ki 6 nM; B. Scatchard plot analysis of the specific binding of [18F]-PSS223 to rat brain membranes. Symbols, specific binding (total binding minus binding in the presence of 100 μM MMPEP) of one representative experiment; red line, linear regression.
Figure 4
Figure 4
In vitro autoradiography with rat brain slices. A. Incubation with 1 nM [18F]-PSS223 showing heterogeneous distribution with highest expression in striatum, hippocampus and cortex; B. Brain slices incubated with 1 nM [18F]-PSS223 and 100 nM ABP688; C. Brain slices incubated with 1 nM [18F]-PSS223 and 100 nM mGluR1 antagonist JNJ16259685. Blue, green, orange, red, dark red indicate radioactivity from low to high. Columns represent same experiment performed in duplicate.
Figure 5
Figure 5
A. Metabolism of [18F]-PSS223 by rat liver microsomes. Green symbols, [18F]-PSS223; blue symbols, polar 18F-radiometabolite; lines, fitted exponential functions; B. Metabolism of [18F]-PSS223 by human liver microsomes. Green symbols, [18F]-PSS223; blue and red symbols, polar 18F-radiometabolites.
Figure 6
Figure 6
A. PET dynamic scan of the head of a male Wistar rat injected with [18F]-PSS223. Transverse planes through the head summarized for the indicated time windows. White, CT; B. Time activity curves of [18F]-PSS223 uptake in a male Wistar rat in different brain regions showing increasing uptake in jaw due to rapid defluorination.
Figure 7
Figure 7
Rat brain PET images of [11C]-ABP688, [18F]-FDEGPECO and [18F]-PSS223. Planes (crosshairs) and maximal intensity projections (MIP) averaged from 2 to 45 min after tracer injection.
Figure 8
Figure 8
Likely mechanism of defluorination of [18F]-PSS223 involves cytochrome P450-catalyzed C-oxygenation. The aldehyde 9 is oxidized or reduced to the respective carboxylic acid or alcohol while [18F]-fluoride is accumulated in bone.
See this image and copyright information in PMC

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