Vasorelaxant and antiplatelet activity of 4,7-dimethyl-1,2,5-oxadiazolo[3,4-d]pyridazine 1,5,6-trioxide: role of soluble guanylate cyclase, nitric oxide and thiols

1 Certain heterocyclic N-oxides are vasodilators and inhibitors of platelet aggregation. The pharmacological activity of the furoxan derivative conden sed with pyridazine di-N-oxide 4,7-dimethyl-1,2,5-oxadiazolo[3,4-d]pyridazi ne 1,5,6-trioxide (FPTO) and the corresponding furazan (FPDO) was studied. 2 FPTO reacted with thiols generating nitrite (NO), S-nitrosoglutathione an d hydroxylamine (nitroxyl) and converted oxyHb to metHb. FPDO did not gener ate detectable amounts of NO-like species but reacted with thiols and oxyHb . 3 FPTO and FPDO haem-dependently stimulated the activity of soluble guanyla te cyclase (sGC) and this stimulation was inhibited by 1H-[1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ) and by 0.1 mM dithiothreitol. 4 FPTO relaxed noradrenaline-precontracted aortic rings and its concentrati on-response curve was biphasic (pIC(50) = 9.03 +/- 0.13 and 5.85 +/- 0.06). FPDO was significantly less potent vasodilator (pIC(50) = 5.19 +/- 0.14). The vasorelaxant activity of FPTO and FPDO was inhibited by ODQ. oxyHb sign ificantly inhibited only FPTO-dependent relaxation. 5 FPTO and FPDO were equipotent inhibitors of ADP-induced platelet aggregat ion (IC50 = 0.63 +/- 0.15 and 0.49 +/- 0.05 mu M, respectively). The antipl atelet activity of FPTO (but not FPDO) was partially suppressed by oxyHb. T he antiaggregatory effects of FPTO and FPDO were only partially blocked by sGC inhibitors. 6 FPTO and FPDO (10-20 mu M) significantly increased cyclic GMP levels in a ortic rings and platelets and this increase was blocked by ODQ. 7 Thus, FPTO can generate NO and, like FPDO, reacts with thiols and haem. T he vasorelaxant activity of FPTO and FPDO is sGC-dependent and a predominan t role is played by NO at FPTO concentrations below 1 mu M. On the contrary , inhibition of platelet aggregation is only partially related to sGC activ ation.