Discovery of Triazolopyrimidine Derivatives as Selective P2X3 Receptor Antagonists Binding to an Unprecedented Allosteric Site as Evidenced by Cryo-Electron Microscopy.

The P2X3 receptor (P2X3R), an ATP-gated cation channel predominantly expressed in C- and Aδ-primary afferent neurons, has been proposed as a drug target for neurological inflammatory diseases, e.g., neuropathic pain, and chronic cough. Aiming to develop novel, selective P2X3R antagonists, tetrazolopyrimidine-based hit compound 9 was optimized through structure-activity relationship studies by modifying the tetrazole core as well as side chain substituents. The optimized antagonist 26a, featuring a cyclopropane-substituted triazolopyrimidine core, displayed potent P2X3R-antagonistic activity (IC50 = 54.9 nM), 20-fold selectivity versus the heteromeric P2X2/3R, and high selectivity versus other P2XR subtypes. Noncompetitive P2X3R blockade was experimentally confirmed by calcium influx assays. Cryo-electron microscopy revealed that 26a stabilizes the P2X3R in its desensitized state, acting as a molecular barrier to prevent ions from accessing the central pore. In vivo studies in a rat neuropathic pain model (spinal nerve ligation) showed dose-dependent antiallodynic effects of 26a, thus presenting a novel, promising lead structure.