Four novel thiazole containing ABP688 derivatives were synthesized and evaluated for their binding affinity towards the metabotropic glutamate receptor subtype 5 (mGluR5). (E)-3-((2-(Fluoromethyl)thiazol-4-yl)ethynyl)cyclohex-2-enone O-methyl oxime (FTECMO), the ligand with the highest binding affinity (K(i)=5.5+/-1.1 nM), was labeled with fluorine-18. [(18)F]-FTECMO displayed optimal lipophilicity (log D(pH7.4)=1.6+/-0.2) and high stability in rat and human plasma as well as sufficient stability in rat liver microsomes. In vitro autoradiography with [(18)F]-FTECMO revealed a heterogeneous and displaceable binding in mGluR5-rich brain regions. PET imaging with [(18)F]-FTECMO in Wistar rats, however, showed low brain uptake. Uptake of radioactivity into the skull was observed suggesting in vivo defluorination. Thus, although [(18)F]-FTECMO is an excellent ligand for the detection of mGluR5 in vitro, its in vivo characteristics are not optimal for the imaging of mGluR5 in rats in vivo.
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