Synthesis and preliminary biological evaluation of O-2((2-[(18)F]fluoroethyl)methylamino)ethyltyrosine ([(18)F]FEMAET) as a potential cationic amino acid PET tracer for tumor imaging.

Amino acid transport is an attractive target for oncologic imaging. Despite a high demand of cancer cells for cationic amino acids, their potential as PET probes remains unexplored. Arginine, in particular, is involved in a number of biosynthetic pathways that significantly influence carcinogenesis and tumor biology. Cationic amino acids are transported by several cationic transport systems including, ATB(0,+) (SLC6A14), which is upregulated in certain human cancers including cervical, colorectal and estrogen receptor-positive breast cancer. In this work, we report the synthesis and preliminary biological evaluation of a new cationic analog of the clinically used PET tumor imaging agent O-(2-[(18)F]fluroethyl)-L-tyrosine ([(18)F]FET), namely O-2((2-[(18)F]fluoroethyl)methylamino)ethyltyrosine ([(18)F]FEMAET). Reference compound and precursor were prepared by multi-step approaches. Radiosynthesis was achieved by no-carrier-added nucleophilic [(18)F]fluorination in 16-20% decay-corrected yields with radiochemical purity >99%. The new tracer showed good stability in vitro and in vivo. Cell uptake assays demonstrated that FEMAET and [(18)F]FEMAET accumulate in prostate cancer (PC-3) and small cell lung cancer cells (NCI-H69), with an energy-dependent mechanism. Small animal PET imaging with NCI-H69 xenograft-bearing mice revealed good tumor visualization comparable to [(18)F]FET and low brain uptake, indicating negligible transport across the blood-brain barrier. In conclusion, the non-natural cationic amino acid PET probe [(18)F]FEMAET accumulates in cancer cells in vitro and in vivo with possible involvement of ATB(0,+).

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.

(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.

Development of [(18)F]-PSS223 as a PET tracer for imaging of metabotropic glutamate receptor subtype 5 (mGluR5).

Involvement of metabotropic glutamate receptor subtype 5 (mGluR5) in physiological and pathophysiological processes in the brain has been demonstrated, and hence mGluR5 has emerged as an important drug target. [(11)C]-ABP688 is clinically the most successful mGluR5 positron emission tomography (PET) tracer to date and it allows visualization and quantification of mGluR5. Due to the short half-life of carbon-11, clinical use of [(11)C]-ABP688 is limited to facilities with an on-site cyclotron and a fluorine-18 (half-life 110 min) analogue would be more practical. Based on the [(11)C]-ABP688 structural motif, a novel derivative [(18)F]-PSS223 was prepared and evaluated as a PET tracer for imaging of mGluR5 in vitro and in vivo. Our results show favourable in vitro binding properties; however rapid defluorination of [(18)F]-PSS223 does not allow visualization of mGluR5 in the rat brain.