Specific inhibition of 40 K protein phosphorylation in platelet by a new antithrombotic agent, d-indobufen

Indobufen, (+)2[p-oxo-2-isoindolinyl)-phenyl] butyric acid exhibited unique inhibitory pattern of platelet function with suppression of protein phosphorylation. In collagen-induced activation, d-indobufen potently inhibited aggregation and release reaction in a dose-dependent manner (IC50:0.49 microM). This compound delayed the onset of aggregation and release reaction induced by arachidonic acid without reducing the magnitude of final reactions. When platelets were stimulated with thrombin, indobufen did not inhibit the aggregation significantly up to 100 microM. d-Indobufen suppressed platelet 40 kDa protein phosphorylation in collagen-stimulated platelets and delayed the onset of the phosphorylation in arachidonic acid-stimulated platelets. However, d-indobufen did not directly inhibited protein kinases in platelets, in vitro. These findings suggest that the inhibitory actions of d-indobufen may be due to its effects on the initiation process of 40 kDa protein phosphorylation.