The signal transducer and activator of transcription 3 (STAT3) plays a critical role in platelet functions.This study sought to understand the effects of the STAT3 inhibitor SC99 on platelet activation and aggregation.Immunoblotting assays were applied to measure the effects of SC99 on the STAT3 signaling pathway.A ChronoLog aggregometer was used to evaluate platelet aggregation.A flow cytometer was used to evaluate P-selectin expression in the presence of SC99.AlamarBlue and Annexin-Ⅴ staining were used to evaluate platelet viability and apoptosis,respectively.A fluorescence microscope was applied to analyze platelet spreading.SC99 inhibited the phosphorylation of JAK2 and STAT3 in human platelets but had no effects on the phosphorylation of AKT,p65 or Src,all of which are involved in platelet activation.Further studies revealed that SC99 inhibited human platelet aggregation induced by collagen and thrombin in a dose-dependent manner.SC99 inhibited thrombin-induced P-selectin expression and fibrinogen binding to single platelets.Moreover,SC99 inhibited platelet spreading on fibrinogen and clot retraction mediated by outside-in signaling.SC99 inhibited platelet aggregation in mice but it did not significantly prolong the bleeding time.Taken together,the present study revealed that SC99 inhibited platelet activation and aggregation as a STAT3 inhibitor.This agent can be developed as a promisingtreatment for thrombotic disorders.