Isolation and functional characterization of DNA-derived aptamers that act as thrombin inhibitors in human platelets and coagulation assays.

Aptamer sequences were isolated from defibrotide, a single-stranded, commercial DNA preparation and studied for thrombin inhibitory activity. Three different aptamers were identified, sequenced and their biological activity was determined in platelet aggregation and coagulation assays. All aptamers were potent inhibitors of thrombin-induced platelet aggregation and thromboxane formation and prolonged the thrombin time in human plasma. There was no effect of any of these compounds when a thromboxane mimetic (U 46.1619), collagen or thrombin activating peptide (TRAP-6) were used as agonists, excluding a nonspecific binding of the compounds to the thrombin receptor. The data suggest that thrombin-inhibitory aptamers are present in the mammalian genome and may constitute an endogenous antithrombin system.