Tyrosine kinase blockers: new platelet activation inhibitors.

Tyrphostins are low-molecular-weight inhibitors of protein tyrosine kinases. Since tyrosine kinase activity has been shown to be increased during thrombin-induced platelet activation, the effect of tyrphostins on platelet activation was investigated. Tyrphostins inhibited dose-dependently thrombin-induced aggregation and the release reaction, with a maximum effect at 25 microM. Using immunoblots of platelet proteins revealed with an anti-phosphotyrosine antibody, tyrphostins were effective inhibitors of tyrosine phosphorylation elicited by thrombin. Using metabolically 32P-labelled human platelets, tyrphostins also inhibited phosphorylation of p43, the main substrate for protein kinase C, and myosin light chain particularly at short periods of activation. The results suggest that tyrosine kinase activity may play a role in platelet signal transduction involving the protein kinase C pathway, and that tyrphostins represent a new type of anti-aggregative drugs.