Synthesis and in vitro evaluation of novel compounds and discovery of a promising iodine-125 radioligand for purinergic P2X7 receptor (P2X7R)

The purinergic P2X ligand-gated ion channel 7 receptor (P2X7R) plays a critical role in various inflammatory processes and other diseases. Fast determination of compounds P2X7R binding potency and discovery of a promise PET radiotracer for imaging P2X7R require a P2X7R suitable radioligand for radioactive competitive binding assay. Herein, we designed and synthesized thirteen new P2X7R ligands and determined the in vitro binding potency. The fluorescence screening assay identified the iodide compound 1c with high potency and specificity toward P2X7R with an IC₅₀ of 0.25 ± 0.05 nM. Therefore, 1c was ¹²⁵I-labeled to afford [¹²⁵I]1c with a good radiochemical yield (44 ± 12 %, n = 3) and high radiochemical purity (>95 %). Radioligand saturation binding assay showed that [¹²⁵I]1c specifically bound to human P2X7R with high affinity (K_d = 1.68 nM and B_max = 94 fmol/mg). A radioactive high throughput binding assay using [¹²⁵I]1c for our new compounds demonstrated that the imidazole compounds 1b, 1c, and 1d exhibited high inhibition for >70 %, while the analogues of GSK314181A exhibited low inhibition for <35 %. In addition, our radioligand competitive binding assays using [¹²⁵I]1c demonstrated that 1b, 1c, and 1d have high potency with IC₅₀ values of 7.91 ± 0.22, 7.06 ± 1.68, and 7.16 ± 0.41 nM toward P2X7R, respectively.Together, compounds 1b, 1c, and 1d are highly potent for P2X7R, and [¹²⁵I]1c has great potential to be a radioligand for screening P2X7R binding potency of the new compounds and investigating the P2X7R expression in animal models of human disease.