A-317491, a novel potent and selective nonnucleotide antagonist of P2X3 and P2X2/3 receptors, reduces chronic inflammatory and neuropathic pain in the rat

P2X(3) and P2X(2/3) receptors are highly localized on peripheral and central processes of sensory afferent nerves, and activation of these channels contributes to the pronociceptive effects of ATP. A-317491 is a novel non-nucleotide antagonist of P2X3 and P2X2/3 receptor activation. A-317491 potently blocked recombinant human and rat P2X(3) and P2X(2/3) receptor-mediated calcium flux (K-i = 22-92 nM) and was highly selective (IC50 > 10 muM) over other P2 receptors and other neurotransmitter receptors, ion channels, and enzymes. A-317491 also blocked native P2X3 and P2X2/3 receptors in rat dorsal root ganglion neurons. Blockade of P2X3 containing channels was stereospecific because the R-enantiomer (A-317344) of A-317491 was significantly less active at P2X3 and P2X2/3 receptors. A-317491 dosedependently (ED50 = 30 mumol/kg s.c.) reduced complete Freund's adjuvant-induced thermal hyperalgesia in the rat. A-317491 was most potent (ED50 = 10-15 mumol/kg s.c.) in attenuating both thermal hyperalgesia and mechanical allodynia after chronic nerve constriction injury. The R-enantiomer, A-317344, was inactive in these chronic pain models. Although active in chronic pain models, A-317491 was ineffective (ED50 > 100 [mumol/kg s.c.) in reducing nodception in animal models of acute pain, postoperative pain, and visceral pain. The present data indicate that a potent and selective antagonist of P2X(3) and P2X(2/3) receptors effectively reduces both nerve injury and chronic inflammatory nociception, but P2X(3) and P2X(2/3) receptor activation may not be a major mediator of acute, acute inflammatory, or visceral pain.