Pyridazines. XIII. Synthesis of 6-aryl-5-oxygenated substituted-3(2H)-pyridazinones and evaluation as platelet aggregation inhibitors.

Several 6-aryl-5-oxygenated substituted pyridazinones have been synthesized and evaluated in vitro for inhibition of platelet aggregation induced by adenosine 5'-diphosphate (ADP), thrombin and collagen. All the tested compounds (except 8 and 9) inhibited platelet aggregation in a dose-dependent manner. The IC50 of the most active substance, compound 2b, was around 60 microM against ADP and collagen as inducers. The inhibition of platelet aggregation caused by test compounds was dependent on the level of oxidation of the function at the 5-position, with the order of IC50 values being R-OH (2a, b, 5) < R-CHO (6, 7) < < R-COOH (8, 9). None of the tested compounds increased the intracellular levels of cAMP, indicating a lack of inhibitory activity on cAMP phosphodiesterase (PDE III) in intact cells. These results suggest that the group present at the 5 position of 6-aryl-5-substituted pyridazinones determines the platelet aggregation-inhibitory activity, and that a mechanism other than PDE inhibition is responsible for this effect.