Triazole-Based Radioligands for PET of P2X₇R: Syntheses, Conformational Studies, and Preliminary Autoradiographic...

The P2X₇ receptor is an emerging target for molecular imaging of inflammation in the brain and peripheral tissues. In this work, we focus on five triazole-based ligands with high affinity and selectivity for P2X₇ receptors (JNJ-64413739, JNJ-55308942, AM-10, AM-12, and AM-15), which are amenable to autoradiography and positron emission tomography (PET) imaging. We studied the phenomenon of conformational and rotational changes of these molecules by NMR and ab initio calculations. The reaction of ligands AM-10 and AM-12 with [¹⁸F]-fluoride resulted in an isotopic exchange on the pyrimidine ring, leaving the halogen atoms on the acyl moieties intact. The reaction yielded [ ¹⁸ F]-AM-10 with a radiochemical yield as high as 27% and a molar activity as high as 152 GBq/μmol. Quantitative autoradiography with [ ¹⁸ F]-AM-10 in sagittal mouse brain cryostat sections indicated a maximum specific binding (B _max) of 15.8 ± 2.8 pmol/g of wet weight and a dissociation constant (K _D) of 16.6 ± 5.1 nM. Thus, we present the first synthesis of [ ¹⁸ F]-AM-10 by isotopic exchange and confirm its specific binding at mouse brain P2X₇ receptors, which should warrant its use in animal and human PET investigations.