P2X receptors are cation channels gated by extracellular ATP and related nucleotides. Because of the widespread distribution of P2X receptors and the high subtype diversity, potent and selective antagonists are needed to dissect their roles in intact tissues. Based on suramin as a lead compound, several derivates have been described that block recombinant P2X receptors with orders of magnitude higher potency than suramin. Here we characterized the suramin analogue 4,4',4'',4'''-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid (NF449) with respect to its potency to antagonize ATP or alphabeta-methyleneadenosine 5'-trisphosphate-induced inward currents of homomeric rat P2X(1)-P2X(4) receptors or heteromeric P2X(1 + 5) and P2X(2+3) receptors, respectively. NF449 most potently blocked P2X(1) and P2X(1 + 5) receptors with IC(50) values of 0.3 nM and 0.7 nM, respectively. Three to four orders of magnitude higher NF449 concentrations were required to block homomeric P2X(3) or heteromeric P2X(2 + 3) receptors (IC(50) 1.8 and 0.3 microM, respectively). NF449 was least potent at homomeric P2X(2) receptors (IC(50) 47 microM) and homomeric P2X(4) receptors (IC(50) > 300 microM). Altogether, these results characterize NF449 as the so far most potent and selective antagonist of receptors incorporating the P2X(1) subunit such as the P2X(1) homomer and the P2X(1 + 5) heteromer.