Design and pharmacological characterization of selective P2-purinoceptor antagonists

Abstract

At least five distinct P2-purinoceptor subtypes have been characterized to date, based on the rank order of potency of several ATP analogues: P2X, P2Y, P2U, P2T and P2Z. However, the characterization of P2-purinoceptor subtypes is hampered by an unavailability of potent, highly selective, competitive antagonists. In the search for selective P2-purinoceptor antagonists, the structure-activity relationships for a series of analogues of pyridoxal-5-phosphate and suramin at P2-purinoceptor subtypes were investigated in our laboratories. Two of these compounds were the subject of a more detailed pharmacological characterization: pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) and the symmetrical 3′-urea of 8-(benzamido)naphthalene-1,3,5-trisulfonic acid (NF023). The results demonstrate that the two parent compounds, pyridoxal-5-phosphate and suramin, do not differentiate between P2X- and P2Y-purinoceptors. In contrast, PPADS and NF023 were found to be selective antagonists of P2X-purinoceptor-mediated responses in several smooth muscle preparations. In addition, PPADS and NF023 were shown to displace competitively the binding of [3H]α,β-methylene ATP to rabbit and rat bladder membranes, respectively, which indicates that these two compounds act directly on P2X-receptors. PPADS and NF023 were ineffective at P2U-purinoceptors in rat mesenteric arterial bed. P2T-purinoceptor-mediated platelet aggregation was only affected by PPADS in concentrations higher than 100 μM. Suramin and NF023 were inhibitors of ecto-ATPase activity in the same concentration range needed for P2-purinoceptor antagonism. In contrast, PPADS was only very weakly active in inhibiting ecto-ATPase activity. AT 100 μM, PPADS and NF023 did not interact with α1-adrenoceptors, adenosine A1- and A2-, histamine H1- and muscarinic M1-, M2- and M3-receptors. In conclusion, PPADS and NF023 are specific P2-purinoceptor antagonists showing a high selectivity for the P2X-subtype. These two compounds may prove to be useful starting points in the synthesis of novel, highly potent and selective antagonists at P2-purinoceptor subtypes.